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The Eulerian-Eulerian (EE) simulation approach + GENeralized TwO Phase (GENTOP) concept and two-fluid three-flow field with droplet were used to simulate the heat and mass transfer. Analysis encompassed the examination of steam and liquid phase distributions, as well as flow pattern transitions. Heat transfer enhancements were quantified using a comprehensive heat transfer factor. The disturbance intensity of the secondary flow was characterized by the absolute vortex flux. Results revealed a correlation between the intensity of the secondary flow and axial length, indicative of progressive fluid disturbance enhancement. Additionally, the maximal motion of the steam phase vortex was observed both near the wall and channel center. The EE+GENTOP concept exhibited superior capability in reproducing multi-scale flow including small-scale dispersed bubbles and large-scale continuous steam (coalescence and break-up). For investigations into the entrainment and deposition of liquid droplets in annular flow, the selection of a two-fluid three-flow field with the droplet model is recommended.

Curved pipes are ubiquitous in several industrial applications and biological systems. The study of flows through these configurations is therefore of significant practical interest. In the present thesis, the modal stability of flows in both toroidal and spatially developing bent pipes is investigated to understand the onset of transition. In addition, turbulence characteristics and large-scale structures in flows through toroidal and helically coiled pipes are analysed. The modal stability of steady flows through toroidal pipes is studied for values of the curvature approaching zero, i.e. the straight pipe, to provide further evidence of the fact that the Hagen–Poiseuille velocity profile is stable to infinitesimal perturbations for any Reynolds number. A novel computational technique is developed for computing the laminar pulsatile flow in a toroidal pipe. A characterisation of this flow for different values of the pulsation frequency and the pulsation amplitude is presented. Floquet theory is then applied to investigate how the stability is affected by the governing parameters, in particular the amplitude and the frequency of the pulsation. The flows through spatially developing bent pipes are also studied for understanding how the transition changes with respect to the toroidal geometry. Both a 90°- and a 180°-bend pipe with the same curvature are investigated. A transition from steady to periodic regime is observed in both cases. The stability analysis reveals that the core of the instability is located at the outer wall of the bend, approximately 15° downstream of the bend inlet, for both pipes. Similar critical Reynolds numbers are also found. Finally, the fully developed turbulent flow in toroidal and helical pipes is considered for studying the effect of torsion on the turbulence characteristics. Both curvature and torsion are found to significantly modify the anisotropy of the flow compared to the case of a straight pipe. Large-scale structures are ext, Krökta rör finns överallt i olika industriella tillämpningar och biologiska system. Att studera flöden genom dessa konfigurationer har därför ett betydande praktiskt intresse. I denna avhandling undersöks den modala stabiliteten hos flöden i både torusformade och rumsutvecklande krökta rör för att förstå övergången till turbulens. Dessutom analyseras turbulensegenskaper och storskaliga strukturer i flöden genom toroidformade och spiralformade rör. Den modala stabiliteten för stadiga flöden genom toroidformade rör studeras för värden på krökningen som närmar sig noll, d.v.s. det raka röret, för att ge ytterligare bevis på det faktum att Hagen–Poiseuille hastighetsprofilen är stabil mot små störningar för alla Reynoldstal. En ny beräkningsteknik har utvecklats för att beräkna det laminära pulserande flödet i ett torusformat rör. En karaktärisering av detta flöde för olika värden på pulsationsfrekvensen och pulsationsamplituden presenteras. Floquet-teori används sedan för att undersöka hur stabiliteten påverkas av de styrande parametrarna, i synnerhet amplituden och frekvensen av pulseringen. Flödena genom rumsutvecklande krökta rör studeras också för att förstå hur turbulens-övergången förändras med avseende på toroidgeometrin. Både ett 90°- och ett 180°-krökt rör med samma krökning undersöks. En övergång från stadigt till periodisk regim observeras i båda fallen. Stabilitetsanalysen avslöjar att instabilitetens kärna är belägen vid krökens yttervägg, cirka 15° nedströms böjinloppet, för båda rören. Liknande kritiska Reynoldstal är också hittad. Slutligen undersöks det fullt utvecklade turbulenta flödet i toroidala och spiralformade rör för att studera effekten av torsion på turbulensegenskaperna. Både krökning och torsion har visat sig signifikant modifiera flödets anisotropi jämfört med fallet med ett rakt rör. Storskaliga strukturer extraheras via proper orthogonal decomposition för att bedöma förekomsten av virvelväxling (swirl switching på engel, QC 230524

Curved pipes are ubiquitous in several industrial applications and biological systems. The study of flows through these configurations is therefore of significant practical interest. In the present thesis, the modal stability of flows in both toroidal and spatially developing bent pipes is investigated to understand the onset of transition. In addition, turbulence characteristics and large-scale structures in flows through toroidal and helically coiled pipes are analysed. The modal stability of steady flows through toroidal pipes is studied for values of the curvature approaching zero, i.e. the straight pipe, to provide further evidence of the fact that the Hagen–Poiseuille velocity profile is stable to infinitesimal perturbations for any Reynolds number. A novel computational technique is developed for computing the laminar pulsatile flow in a toroidal pipe. A characterisation of this flow for different values of the pulsation frequency and the pulsation amplitude is presented. Floquet theory is then applied to investigate how the stability is affected by the governing parameters, in particular the amplitude and the frequency of the pulsation. The flows through spatially developing bent pipes are also studied for understanding how the transition changes with respect to the toroidal geometry. Both a 90°- and a 180°-bend pipe with the same curvature are investigated. A transition from steady to periodic regime is observed in both cases. The stability analysis reveals that the core of the instability is located at the outer wall of the bend, approximately 15° downstream of the bend inlet, for both pipes. Similar critical Reynolds numbers are also found. Finally, the fully developed turbulent flow in toroidal and helical pipes is considered for studying the effect of torsion on the turbulence characteristics. Both curvature and torsion are found to significantly modify the anisotropy of the flow compared to the case of a straight pipe. Large-scale structures are ext, Krökta rör finns överallt i olika industriella tillämpningar och biologiska system. Att studera flöden genom dessa konfigurationer har därför ett betydande praktiskt intresse. I denna avhandling undersöks den modala stabiliteten hos flöden i både torusformade och rumsutvecklande krökta rör för att förstå övergången till turbulens. Dessutom analyseras turbulensegenskaper och storskaliga strukturer i flöden genom toroidformade och spiralformade rör. Den modala stabiliteten för stadiga flöden genom toroidformade rör studeras för värden på krökningen som närmar sig noll, d.v.s. det raka röret, för att ge ytterligare bevis på det faktum att Hagen–Poiseuille hastighetsprofilen är stabil mot små störningar för alla Reynoldstal. En ny beräkningsteknik har utvecklats för att beräkna det laminära pulserande flödet i ett torusformat rör. En karaktärisering av detta flöde för olika värden på pulsationsfrekvensen och pulsationsamplituden presenteras. Floquet-teori används sedan för att undersöka hur stabiliteten påverkas av de styrande parametrarna, i synnerhet amplituden och frekvensen av pulseringen. Flödena genom rumsutvecklande krökta rör studeras också för att förstå hur turbulens-övergången förändras med avseende på toroidgeometrin. Både ett 90°- och ett 180°-krökt rör med samma krökning undersöks. En övergång från stadigt till periodisk regim observeras i båda fallen. Stabilitetsanalysen avslöjar att instabilitetens kärna är belägen vid krökens yttervägg, cirka 15° nedströms böjinloppet, för båda rören. Liknande kritiska Reynoldstal är också hittad. Slutligen undersöks det fullt utvecklade turbulenta flödet i toroidala och spiralformade rör för att studera effekten av torsion på turbulensegenskaperna. Både krökning och torsion har visat sig signifikant modifiera flödets anisotropi jämfört med fallet med ett rakt rör. Storskaliga strukturer extraheras via proper orthogonal decomposition för att bedöma förekomsten av virvelväxling (swirl switching på engel, QC 230524

To enhance the performance of microchannel heat sinks, secondary channels or ribs are oftenused separately. However, the synergistic enhancement of heat dissipation by secondary channels and individually composite with different types of ribs and the factors influencing them are not yet clear. In this study, the numerical simulation software ANSYS Fluent is used for simulation and obtaining the optimal microchannel heat sink structure from diamond, rectangular, back triangular, front triangular and elliptical ribs with secondary channel composite. Firstly, the results show that overall performance of microchannel heat sink with front triangular ribs with secondary channel composite is better than other designs. Secondly, the optimized microchannel heat sink with front triangular ribs and secondary channels outperforms the conventional microchannel heat sink by up to 34% in terms of overall performance when Reynolds number is 200, relative rib width/rib length is 0.633/0.400 and relative secondary channel width is 0.600. Finally, by sensitivity analysis, it is found the effect of relative rib width on heat dissipation is about 60%, the effect of flow rate on pressure drop is about 40%, and the effect of overall performance on relative rib width and relative secondary channel width are about 26% and 25%, respectively.

Curved pipes are ubiquitous in several industrial applications and biological systems. The study of flows through these configurations is therefore of significant practical interest. In the present thesis, the modal stability of flows in both toroidal and spatially developing bent pipes is investigated to understand the onset of transition. In addition, turbulence characteristics and large-scale structures in flows through toroidal and helically coiled pipes are analysed. The modal stability of steady flows through toroidal pipes is studied for values of the curvature approaching zero, i.e. the straight pipe, to provide further evidence of the fact that the Hagen–Poiseuille velocity profile is stable to infinitesimal perturbations for any Reynolds number. A novel computational technique is developed for computing the laminar pulsatile flow in a toroidal pipe. A characterisation of this flow for different values of the pulsation frequency and the pulsation amplitude is presented. Floquet theory is then applied to investigate how the stability is affected by the governing parameters, in particular the amplitude and the frequency of the pulsation. The flows through spatially developing bent pipes are also studied for understanding how the transition changes with respect to the toroidal geometry. Both a 90°- and a 180°-bend pipe with the same curvature are investigated. A transition from steady to periodic regime is observed in both cases. The stability analysis reveals that the core of the instability is located at the outer wall of the bend, approximately 15° downstream of the bend inlet, for both pipes. Similar critical Reynolds numbers are also found. Finally, the fully developed turbulent flow in toroidal and helical pipes is considered for studying the effect of torsion on the turbulence characteristics. Both curvature and torsion are found to significantly modify the anisotropy of the flow compared to the case of a straight pipe. Large-scale structures are ext, Krökta rör finns överallt i olika industriella tillämpningar och biologiska system. Att studera flöden genom dessa konfigurationer har därför ett betydande praktiskt intresse. I denna avhandling undersöks den modala stabiliteten hos flöden i både torusformade och rumsutvecklande krökta rör för att förstå övergången till turbulens. Dessutom analyseras turbulensegenskaper och storskaliga strukturer i flöden genom toroidformade och spiralformade rör. Den modala stabiliteten för stadiga flöden genom toroidformade rör studeras för värden på krökningen som närmar sig noll, d.v.s. det raka röret, för att ge ytterligare bevis på det faktum att Hagen–Poiseuille hastighetsprofilen är stabil mot små störningar för alla Reynoldstal. En ny beräkningsteknik har utvecklats för att beräkna det laminära pulserande flödet i ett torusformat rör. En karaktärisering av detta flöde för olika värden på pulsationsfrekvensen och pulsationsamplituden presenteras. Floquet-teori används sedan för att undersöka hur stabiliteten påverkas av de styrande parametrarna, i synnerhet amplituden och frekvensen av pulseringen. Flödena genom rumsutvecklande krökta rör studeras också för att förstå hur turbulens-övergången förändras med avseende på toroidgeometrin. Både ett 90°- och ett 180°-krökt rör med samma krökning undersöks. En övergång från stadigt till periodisk regim observeras i båda fallen. Stabilitetsanalysen avslöjar att instabilitetens kärna är belägen vid krökens yttervägg, cirka 15° nedströms böjinloppet, för båda rören. Liknande kritiska Reynoldstal är också hittad. Slutligen undersöks det fullt utvecklade turbulenta flödet i toroidala och spiralformade rör för att studera effekten av torsion på turbulensegenskaperna. Både krökning och torsion har visat sig signifikant modifiera flödets anisotropi jämfört med fallet med ett rakt rör. Storskaliga strukturer extraheras via proper orthogonal decomposition för att bedöma förekomsten av virvelväxling (swirl switching på engel, QC 230524

Curved pipes are ubiquitous in several industrial applications and biological systems. The study of flows through these configurations is therefore of significant practical interest. In the present thesis, the modal stability of flows in both toroidal and spatially developing bent pipes is investigated to understand the onset of transition. In addition, turbulence characteristics and large-scale structures in flows through toroidal and helically coiled pipes are analysed. The modal stability of steady flows through toroidal pipes is studied for values of the curvature approaching zero, i.e. the straight pipe, to provide further evidence of the fact that the Hagen–Poiseuille velocity profile is stable to infinitesimal perturbations for any Reynolds number. A novel computational technique is developed for computing the laminar pulsatile flow in a toroidal pipe. A characterisation of this flow for different values of the pulsation frequency and the pulsation amplitude is presented. Floquet theory is then applied to investigate how the stability is affected by the governing parameters, in particular the amplitude and the frequency of the pulsation. The flows through spatially developing bent pipes are also studied for understanding how the transition changes with respect to the toroidal geometry. Both a 90°- and a 180°-bend pipe with the same curvature are investigated. A transition from steady to periodic regime is observed in both cases. The stability analysis reveals that the core of the instability is located at the outer wall of the bend, approximately 15° downstream of the bend inlet, for both pipes. Similar critical Reynolds numbers are also found. Finally, the fully developed turbulent flow in toroidal and helical pipes is considered for studying the effect of torsion on the turbulence characteristics. Both curvature and torsion are found to significantly modify the anisotropy of the flow compared to the case of a straight pipe. Large-scale structures are ext, Krökta rör finns överallt i olika industriella tillämpningar och biologiska system. Att studera flöden genom dessa konfigurationer har därför ett betydande praktiskt intresse. I denna avhandling undersöks den modala stabiliteten hos flöden i både torusformade och rumsutvecklande krökta rör för att förstå övergången till turbulens. Dessutom analyseras turbulensegenskaper och storskaliga strukturer i flöden genom toroidformade och spiralformade rör. Den modala stabiliteten för stadiga flöden genom toroidformade rör studeras för värden på krökningen som närmar sig noll, d.v.s. det raka röret, för att ge ytterligare bevis på det faktum att Hagen–Poiseuille hastighetsprofilen är stabil mot små störningar för alla Reynoldstal. En ny beräkningsteknik har utvecklats för att beräkna det laminära pulserande flödet i ett torusformat rör. En karaktärisering av detta flöde för olika värden på pulsationsfrekvensen och pulsationsamplituden presenteras. Floquet-teori används sedan för att undersöka hur stabiliteten påverkas av de styrande parametrarna, i synnerhet amplituden och frekvensen av pulseringen. Flödena genom rumsutvecklande krökta rör studeras också för att förstå hur turbulens-övergången förändras med avseende på toroidgeometrin. Både ett 90°- och ett 180°-krökt rör med samma krökning undersöks. En övergång från stadigt till periodisk regim observeras i båda fallen. Stabilitetsanalysen avslöjar att instabilitetens kärna är belägen vid krökens yttervägg, cirka 15° nedströms böjinloppet, för båda rören. Liknande kritiska Reynoldstal är också hittad. Slutligen undersöks det fullt utvecklade turbulenta flödet i toroidala och spiralformade rör för att studera effekten av torsion på turbulensegenskaperna. Både krökning och torsion har visat sig signifikant modifiera flödets anisotropi jämfört med fallet med ett rakt rör. Storskaliga strukturer extraheras via proper orthogonal decomposition för att bedöma förekomsten av virvelväxling (swirl switching på engel, QC 230524

Three secondary flows, namely the inward radial flow along the cyclone lid, the downward axial flow along the external surface of the vortex finder, and the radial inward flow below the vortex finder (lip flow) have been studied at a wide range of flow rate 0.22–7.54 LPM using the LES simulations. To evaluate these flows the corresponding methods were originally proposed. The highly significant effect of the Reynolds number on these secondary flows has been described by equations. The main finding is that the magnitude of all secondary flows decrease with increasing Reynolds number. The secondary inward radial flow along the cyclone lid is not constant and reaches its maximum value at the central radial position between the vortex finder external wall and the cyclone wall. The secondary downward axial flow along the external surface of the vortex finder significantly increases at the lowest part of the vortex finder and it is much larger than the secondary flow along the cyclone lid. The lip flow is much larger than the secondary inward radial flow along the cyclone lid, which was assumed in cyclone models to be equal to the lip flow, and the ratio of these two secondary flows is practically independent of the Reynolds number.

The secondary flow generated by an elliptic cylinder oscillating harmonically in a viscous, incompressible fluid is investigated theoretically and experimentally, which is inclined its major axis at an arbitrary angle to the direction of oscillation. The positions of stagnation points where the streaming is toward the cylinder shift on the surface of the body depending on the values of K and ?, where k is the ratio of the length of a minor axis to that of a major one of the elliptic cylinder and ? the angle between the major axis and the direction of oscillation. Moreover, both the direction of the flow toward the cylinder and that of the flow away from the cylinder also depend upon k and ?. The flow pattern of the streaming has symmetry with respect to a turn of ? around the center axis of the cylinder.

This work applies resolvent analysis to incompressible flow through a rectangular duct, in order to identify dominant linear energy-amplification mechanisms present in such flows. In particular, we formulate the resolvent operator from linearizing the Navier–Stokes equations about a two-dimensional base/mean flow. The laminar base flow only has a nonzero streamwise velocity component, while the turbulent case exhibits a secondary mean flow (Prandtl’s secondary flow of the second kind). A singular value decomposition of the resolvent operator allows for the identification of structures corresponding to maximal energy amplification, for specified streamwise wavenumbers and temporal frequencies. Resolvent analysis has been fruitful for analysis of wall-bounded flows with spanwise homogeneity, and here we aim to explore how such methods and findings can extend for a flow in spatial domains of finite spanwise extent. We investigate how linear energy-amplification mechanisms (in particular the response to harmonic forcing) change in magnitude and structure as the aspect ratio (defined as the duct width divided by its height) varies between one (a square duct) and ten. We additionally study the effect that secondary flow has on linear energy-amplification mechanisms, finding that in different regimes it can either enhance or suppress amplification. We further investigate how the secondary flow alters the forcing and response mode shapes leading to maximal linear energy amplification., QC 20230508

The curved shape of the tube causes the flowing fluid to experience centrifugal force. The extent of centrifugal force experienced depends on the local axial velocity of the fluid particle and radius of curvature of the coil. The fluid particles flowing at the core of the pipe have higher velocities than those flowing near to the pipe wall. Thus the fluid particles flowing close to the tube wall experience a lower centrifugal force than the fluid particles flowing in the tube core. This causes the fluid from the core region to be pushed towards the outer wall. This stream bifurcates at the wall and drives the fluid towards the inner wall along the tube periphery, causing generation of counter-rotating vortices called secondary flows whichproduce additional transport of the fluid over the cross section of the pipe. This additional convective transport increases heat transfer and the pressure drop when compared to that in a straight tube

The curved shape of the tube causes the flowing fluid to experience centrifugal force. The extent of centrifugal force experienced depends on the local axial velocity of the fluid particle and radius of curvature of the coil. The fluid particles flowing at the core of the pipe have higher velocities than those flowing near to the pipe wall. Thus the fluid particles flowing close to the tube wall experience a lower centrifugal force than the fluid particles flowing in the tube core. This causes the fluid from the core region to be pushed towards the outer wall. This stream bifurcates at the wall and drives the fluid towards the inner wall along the tube periphery, causing generation of counter-rotating vortices called secondary flows whichproduce additional transport of the fluid over the cross section of the pipe. This additional convective transport increases heat transfer and the pressure drop when compared to that in a straight tube

The curved shape of the tube causes the flowing fluid to experience centrifugal force. The extent of centrifugal force experienced depends on the local axial velocity of the fluid particle and radius of curvature of the coil. The fluid particles flowing at the core of the pipe have higher velocities than those flowing near to the pipe wall. Thus the fluid particles flowing close to the tube wall experience a lower centrifugal force than the fluid particles flowing in the tube core. This causes the fluid from the core region to be pushed towards the outer wall. This stream bifurcates at the wall and drives the fluid towards the inner wall along the tube periphery, causing generation of counter-rotating vortices called secondary flows whichproduce additional transport of the fluid over the cross section of the pipe. This additional convective transport increases heat transfer and the pressure drop when compared to that in a straight tube

The curved shape of the tube causes the flowing fluid to experience centrifugal force. The extent of centrifugal force experienced depends on the local axial velocity of the fluid particle and radius of curvature of the coil. The fluid particles flowing at the core of the pipe have higher velocities than those flowing near to the pipe wall. Thus the fluid particles flowing close to the tube wall experience a lower centrifugal force than the fluid particles flowing in the tube core. This causes the fluid from the core region to be pushed towards the outer wall. This stream bifurcates at the wall and drives the fluid towards the inner wall along the tube periphery, causing generation of counter-rotating vortices called secondary flows whichproduce additional transport of the fluid over the cross section of the pipe. This additional convective transport increases heat transfer and the pressure drop when compared to that in a straight tube

The curved shape of the tube causes the flowing fluid to experience centrifugal force. The extent of centrifugal force experienced depends on the local axial velocity of the fluid particle and radius of curvature of the coil. The fluid particles flowing at the core of the pipe have higher velocities than those flowing near to the pipe wall. Thus the fluid particles flowing close to the tube wall experience a lower centrifugal force than the fluid particles flowing in the tube core. This causes the fluid from the core region to be pushed towards the outer wall. This stream bifurcates at the wall and drives the fluid towards the inner wall along the tube periphery, causing generation of counter-rotating vortices called secondary flows whichproduce additional transport of the fluid over the cross section of the pipe. This additional convective transport increases heat transfer and the pressure drop when compared to that in a straight tube

The curved shape of the tube causes the flowing fluid to experience centrifugal force. The extent of centrifugal force experienced depends on the local axial velocity of the fluid particle and radius of curvature of the coil. The fluid particles flowing at the core of the pipe have higher velocities than those flowing near to the pipe wall. Thus the fluid particles flowing close to the tube wall experience a lower centrifugal force than the fluid particles flowing in the tube core. This causes the fluid from the core region to be pushed towards the outer wall. This stream bifurcates at the wall and drives the fluid towards the inner wall along the tube periphery, causing generation of counter-rotating vortices called secondary flows whichproduce additional transport of the fluid over the cross section of the pipe. This additional convective transport increases heat transfer and the pressure drop when compared to that in a straight tube

The curved shape of the tube causes the flowing fluid to experience centrifugal force. The extent of centrifugal force experienced depends on the local axial velocity of the fluid particle and radius of curvature of the coil. The fluid particles flowing at the core of the pipe have higher velocities than those flowing near to the pipe wall. Thus the fluid particles flowing close to the tube wall experience a lower centrifugal force than the fluid particles flowing in the tube core. This causes the fluid from the core region to be pushed towards the outer wall. This stream bifurcates at the wall and drives the fluid towards the inner wall along the tube periphery, causing generation of counter-rotating vortices called secondary flows whichproduce additional transport of the fluid over the cross section of the pipe. This additional convective transport increases heat transfer and the pressure drop when compared to that in a straight tube

The flow through pipe bends and elbows occurs in a wide range of applications. While many experimental data are available for such flows in the literature, their numerical simulation is less abundant. Here, we present highly-resolved simulations of laminar and turbulent water flow in a 90° pipe bend using Smoothed Particle Hydrodynamics (SPH) methods coupled to a Large-Eddy Simulation (LES) model for turbulence. Direct comparison with available experimental data is provided in terms of streamwise velocity profiles, turbulence intensity profiles and cross-sectional velocity maps at different stations upstream, inside and downstream of the pipe bend. The numerical results are in good agreement with the experimental data. In particular, maximum root-mean-square deviations from the experimental velocity profiles are always less than ~1.4%. Convergence to the experimental measurements of the turbulent fluctuations is achieved by quadrupling the resolution necessary to guarantee convergence of the velocity profiles. At such resolution, the deviations from the experimental data are ~0.8%. In addition, the cross-sectional velocity maps inside and downstream of the bend shows that the experimentally observed details of the secondary flow are also very well predicted by the numerical simulations., This research was funded by the European Union’s Horizon 2020 Programme under the ENERXICO Project grant number 828947 and under the Mexican CONACYT-SENER-Hidrocarburos grant number B-S-69926 and by CONACYT under project number 368., Peer Reviewed, Postprint (published version)

The flow through pipe bends and elbows occurs in a wide range of applications. While many experimental data are available for such flows in the literature, their numerical simulation is less abundant. Here, we present highly-resolved simulations of laminar and turbulent water flow in a 90° pipe bend using Smoothed Particle Hydrodynamics (SPH) methods coupled to a Large-Eddy Simulation (LES) model for turbulence. Direct comparison with available experimental data is provided in terms of streamwise velocity profiles, turbulence intensity profiles and cross-sectional velocity maps at different stations upstream, inside and downstream of the pipe bend. The numerical results are in good agreement with the experimental data. In particular, maximum root-mean-square deviations from the experimental velocity profiles are always less than ~1.4%. Convergence to the experimental measurements of the turbulent fluctuations is achieved by quadrupling the resolution necessary to guarantee convergence of the velocity profiles. At such resolution, the deviations from the experimental data are ~0.8%. In addition, the cross-sectional velocity maps inside and downstream of the bend shows that the experimentally observed details of the secondary flow are also very well predicted by the numerical simulations., This research was funded by the European Union’s Horizon 2020 Programme under the ENERXICO Project grant number 828947 and under the Mexican CONACYT-SENER-Hidrocarburos grant number B-S-69926 and by CONACYT under project number 368., Peer Reviewed, Postprint (published version)

We investigate turbulent flow with highly pulsating axial velocity passing through a duct with both first and second bends. The time-dependent velocity fields downstream of the bends were measured using time-resolved stereo particle image velocimetry for the steady case (Reynolds number Re = 36,700) and the pulsatile case (Re = 37,800 and Womersley number α = 59.1). Proper orthogonal decomposition (POD) of the in-plane velocity data isolates the energetic structures of the secondary flow. The modes downstream of the first bend have a Dean motion (mode 0), single swirl (mode 1), and double swirl (mode 2), which agree with those of previous studies on steady turbulent flow. Downstream of the second bend, additional vortices appear in the modes owing to the secondary flow originating in the first bend. The modal structure of the pulsatile case is virtually the same as that of the steady case. To our knowledge, we are the first to find swirl switching in pulsatile flow, whereas the switching has been reported only for steady cases. We further conduct a time-frequency analysis via wavelet transformation onto the POD time coefficient, showing intermittency in energy of the mode associated with swirl switching., This work was partially supported by a research grant from The Hiroshima University Education and Research Support Foundation.

The division of sediment at river bifurcations is crucial for the morphodynamics of anastamosing rivers and distributary delta channel networks. Many river bifurcations are strongly asymmetric and have a planform where a small channel branches off to the side. Such a configuration is also typical for man-made diversions of water and sediment into canals. At asymmetric bifurcations, the division of sediment is influenced by the secondary current, which is caused by the turning of the flow toward the side. The secondary currents cause especially water from the lower parts of the water column to be diverted into the side branch. As the sediment concentration close to the bottom is high, side branches can receive a disproportionately large fraction of the incoming sediment load, relative to the water discharge. Lateral diversions have been extensively studied with physical and numerical experiments, with the goal to either mitigate or exploit this effect. However, a systematic mathematical analysis of the parameter space has not yet appeared in the literature. Here, we present a comprehensive analysis by way of an idealized model, revealing how the division of sediment is influenced by the width and depth of the branches. We show that the excess of sediment that is diverted into the side branch is lower when the inlet to the side branch is wider and shallower. This may have larger implications for the stability of delta channel networks, as inlets to side branches tend to be locally wider, which likely contributes to their morphological stability.

Auch wenn insbesondere in den Industrieländern der Anteil erneuerbarer Energien zunimmt, kommen zur Elektrizitätserzeugung weiterhin konventionelle Kraftwerke mit Dampfturbinen als Generatorantrieb zum Einsatz. Darüber hinaus finden Dampfturbinen auch bei der regenerativen Stromerzeugung z.B. mittels Biomasse, Geo- oder Solarthermie Anwendung. Um dabei einen effizienten Energiewandlungsprozess zu ermöglichen, ist ein hoher Wirkungsgrad eines der wesentlichen Auslegungskriterien für Dampfturbinen. Im Hochdruckteil von Dampfturbinen tragen die Sekundärströmungen einen wesentlichen Anteil zu den wirkungsgradmindernden Verlusten bei. In Flugtriebwerksturbinen setzen einige Hersteller nicht-achsensymmetrische Seitenwandkonturierung zur Reduzierung der Sekundärströmungen ein. Diese reduzieren den Querdruckgradienten in der Schaufelpassage, die für Sekundärströmungen treibende Kraft. In Dampfturbinen hat sich dieses Gestaltungselement noch nicht bis zur Einsatzreife durchgesetzt. Dies ist einerseits auf die Unterschiede im Auslegungsprozess zwischen Triebwerks- und Dampfturbinen, andererseits auf die bei modernen Triebwerken deutlich höhere aerodynamischen Profilbelastung und damit tendenziell stärkeren Sekundärströmungen zurückzuführen. In der vorliegenden Arbeit werden zwei vom Industriepartner ausgelegte, 7-stufige, für Hochdruckdampfturbinen typische Beschaufelungen sowohl experimentell, als auch mittels CFD untersucht. Die Konfigurationen unterscheiden sich ausschließlich darin, dass eine Beschaufelung eine nicht-achsensymmetrische Seitenwandkontur aufweist, wie sie im Rahmen des konventionellen Auslegungsprozesses ohne zu große Mehraufwände eingebracht werden kann. Die andere Beschaufelung weist keine Konturierung auf und dient als Referenz. Im Rahmen dieser Arbeit werden zunächst Maßnahmen zur Verbesserung der Regel- und Messgenauigkeit vorgestellt, die im Rahmen dieser Arbeit notwendig waren, um den Prüfstand für das Herausmessen der aus anderen Arbeiten zur Seiten, Even though in industrialized countries there is an increasing portion of renewable energies, conventional power plants with steam turbines as generator drive application will still be in use. In addition steam turbines are also used for electricity production from renewable energy sources like due to biomass, solarthermal or geothermal heat. For an efficient energy conversion process, for steam turbines a high efficiency is an essential design criterion. For high pressure turbines secondary flows are one of the most important loss mechanisms. Non-axisymmetric endwall contouring has become a well known method for reducing secondary flow in turbines of aero engines. Due to this geometries the cross passage pressure gradient is reduced, the main force that drives the secondary force. In steam turbines this method has not yet become stablished to industrial maturity.The reasons for that are differences in the design process of steam turbines and turbines for aero engines on the one hand, and on the other hand the stronger secondary flows in aero engines do to a higher profile loading. In this thesis, two seven stage turbines with typical high pressure steam turbine blading and designed by an industrial turbine manufacture are investigated experimentally on a test stand as well as numerically with computational fluid dynamics methods. One of the bladings has non-axisymmetric endwalls.

As a contribution to advancing turbofan engine ground test technology in support of propulsion system integration in modern conceptual aircraft, a novel inlet distortion generator (ScreenVaneTM) was invented. The device simultaneously reproduces combined inlet total pressure and swirl distortion elements in a tailored profile intended to match a defined turbofan engine inlet distortion profile. The device design methodology was intended to be sufficiently generic to be utilized in support of any arbitrary inlet distortion profile yet adequately specific to generate high-fidelity inlet distortion profile simulation. For the current investigation, a specific inlet distortion profile was defined using computational analysis of a conceptual boundary layer ingesting S-duct turbofan engine inlet. The resulting inlet distortion profile, consisting of both total pressure and swirl distortion elements, was used as the objective profile to be matched by the ScreenVane in a turbofan engine ground test facility. A ScreenVane combined inlet total pressure and swirl distortion generator was designed, computationally analyzed, and experimentally validated. The design process involved specifying a total pressure loss screen pattern and organizing a unique arrangement of swirl inducing turning vanes. Computational results indicated that the ScreenVane manufactured distortion profile matched the predicted S-duct turbofan engine inlet manufactured distortion profile with excellent agreement in pattern shape, extent, and intensity. Computational full-field total pressure recovery and swirl angle profiles matched within approximately 1% and 2.5° (RMSD), respectively. Experimental turbofan engine ground test results indicated that the ScreenVane manufactured distortion profile matched the predicted S-duct turbofan engine inlet manufactured distortion profile with excellent agreement in pattern shape, extent, and intensity. Experimental full-field total pressure recovery and swirl angle p

Secondary flow (also termed as stagnation flow, dead space, recirculation zone, and vortex) is rheological phenomenon occurring during flow of polymer melts through abrupt contraction channels as result of flow separation from solid boundary leading to accelerating flow regime with recirculating material in corners. Polymer melt captured in secondary flow slowly rotates in direction opposite to main flow direction and simultaneously moves in third direction through helical motion. This may first reduce flow stability and second increase residence time initiating highly undesirable thermal degradation of polymer melt. Since the first visual experimental observation performed by Tordella and preliminary theoretical prediction made by Langlois and Rivlin at the end of the 1950s, this phenomenon represents one of the most fundamental rheological problems ever with many practical and theoretical impacts discussed here. This comprehensive review written in historical perspective summarizes key factors (Newtonian viscosity, shear thinning, viscoelasticity, flow geometry, and extensional viscosity) influencing secondary entry flows for polymer melts and provides deep and critical discussion of the most important experimental and theoretical works on this topic (such as branched low-density polyethylene, LDPE, linear low-density polyethylene, LLDPE, high-density polyethylene, HDPE, polystyrene, PS, isotactic polypropylene, PP, polymethyl methacrylate, PMMA, polyamide, Nylon PA 66, or polybutadiene, BR). © 2018, © 2018 Taylor & Francis Group, LLC.

The present work is aimed at evaluating the contribution to the secondary flow in duct flow with square and rectangular cross section from three-dimensional coherent structures, defined as intense Reynolds-stress events. The contribution to a certain mean quantity is defined as the ensemble average over the detected coherent structures, weighted with their own occupied volume fraction. Our analysis unveils that the contribution to the cross-stream components of the mean velocity is either very similar to the same contribution in channel flow, or almost negligible in respect to the contribution from the portion of the domain not occupied by coherent structures. These results suggest that the most intense events are not directly responsible for the secondary flow., QC 20200618

The present work is aimed at evaluating the contribution to the secondary flow in duct flow with square and rectangular cross section from three-dimensional coherent structures, defined as intense Reynolds-stress events. The contribution to a certain mean quantity is defined as the ensemble average over the detected coherent structures, weighted with their own occupied volume fraction. Our analysis unveils that the contribution to the cross-stream components of the mean velocity is either very similar to the same contribution in channel flow, or almost negligible in respect to the contribution from the portion of the domain not occupied by coherent structures. These results suggest that the most intense events are not directly responsible for the secondary flow., QC 20211110

In this work we extend the vorticity-flux approach, proposed by Brown and Roshko (2012) for the analysis of turbulent shear layers and wakes, to the study of secondary flows of Prandtl's second kind. To this end, we assess direct numerical simulations (DNSs) of turbulent flow through sinusoidal channels (Vidal et al., 2018a) at bulk Reynolds numbers Re h = 2500 and 5000, and with various wall wave parameters, leading to a range of secondary flow intensities. We find that the fluctuating vorticity-flux difference (w'omega(y)') over bar (+) - (v'omega(z)') over bar (+) is closely connected to the in-plane cross-flow, in particular the large negative values present around the wall peak, which enhance the transport of near-wall momentum towards the channel core. The tilting of sweep events at the wall valley is also connected to the secondary flow magnitude, and is associated with positive values of the fluctuating vorticity-flux difference. Furthermore, conditionally averaged fields show that, unlike what is observed in channels with flat walls, the behavior in the vorticity-flux field at the peak is mostly due to Q1 and Q4 events, which essentially tilt momentum towards the peak., QC 20200103

Inertial microfluidic particle manipulation technology, which can manipulate the target particle entirely relying on the microchannel characteristic geometry and intrinsic hydrodynamic effects, has attracted great attention due to the fascinating advantages such as high-throughput, simplicity, precise manipulation and low cost. As a passive microfluidic technology, inertial microfluidics can precisely focus, separate or trap particles in the continuous and high-throughput manner without the extra requirement of external field for functionalities, and is very promising for a wide range of industrial, biomedical and clinical applications. In the regime of inertial microfluidics, both the non-negligible inertia and viscosity of fluid introduce two important inertial effects, including the inertial migration and the secondary flow. The inertial migration in the straight microchannel normally contributes to multiple equilibrium positions, which causes the difficulty of efficient particle focusing and separation. It is found that the secondary flow can remedy this congenital deficiency through its hydrodynamic effects, and has been widely adopted to reduce the number of the equilibrium position and greatly improve the particle manipulation performance in inertial microfluidic devices...

Unsteady viscous incompressible flows in a spherical layer due to an increase in the rotation velocity of the inner sphere with constant acceleration are investigated. The acceleration starts at the Reynolds numbers Re corresponding to a stationary flow and ends at Re higher than the stability limit of the stationary flow, whereupon the rotation velocity of the inner sphere remains constant. The outer sphere is fixed and the spherical layer thickness is equal to the inner sphere radius. The inner sphere acceleration effect is studied on both the formation of one of two possible secondary-flow structures after the acceleration has been stopped, namely, traveling azimuthal waves with wavenumbers of 3 or 4, and on the change in the flow structure during the action of the acceleration. It is shown that not only an increase in the acceleration but also a decrease in Re corresponding to the acceleration onset can lead to an increase in the deviation of the instantaneous velocity profiles from their stationary values and can be accompanied by a variation in the secondary flow wavenumber. © Published under licence by IOP Publishing Ltd.

Secondary flow (also termed as stagnation flow, dead space, recirculation zone, and vortex) is rheological phenomenon occurring during flow of polymer melts through abrupt contraction channels as result of flow separation from solid boundary leading to accelerating flow regime with recirculating material in corners. Polymer melt captured in secondary flow slowly rotates in direction opposite to main flow direction and simultaneously moves in third direction through helical motion. This may first reduce flow stability and second increase residence time initiating highly undesirable thermal degradation of polymer melt. Since the first visual experimental observation performed by Tordella and preliminary theoretical prediction made by Langlois and Rivlin at the end of the 1950s, this phenomenon represents one of the most fundamental rheological problems ever with many practical and theoretical impacts discussed here. This comprehensive review written in historical perspective summarizes key factors (Newtonian viscosity, shear thinning, viscoelasticity, flow geometry, and extensional viscosity) influencing secondary entry flows for polymer melts and provides deep and critical discussion of the most important experimental and theoretical works on this topic (such as branched low-density polyethylene, LDPE, linear low-density polyethylene, LLDPE, high-density polyethylene, HDPE, polystyrene, PS, isotactic polypropylene, PP, polymethyl methacrylate, PMMA, polyamide, Nylon PA 66, or polybutadiene, BR). © 2018, © 2018 Taylor & Francis Group, LLC.

In this paper, a comprehensive numerical study is presented for the fully developed two-dimensional flow of viscous incompressible fluid through a curved rectangular duct with different aspect ratios 2 and 3 for a constant curvature 14δ=0.1"> . Unsteady solutions are obtained by using a spectral method and covering a wide range of Dean number 14100≤Dn≤1000"> and the Grashof number 141000≤Gr≤2000"> . The outer wall of the duct is heated while the inner wall is cooled. The main concern of this study is to find out the unsteady flow behavior i.e whether the unsteady flow is steady-state, periodic, multi-periodic or chaotic, if the Dean number or the Grashof number is changed. For the aspect ratio 2, it is found that the unsteady flow is a steady-state solution for 14Dn=100"> and 14Gr=100, 500, 1500, 2000"> but periodic at 14Dn=100"> and 14Gr=1000. "> If the Dean number is increased i.e. at 14=500"> , it is found the unsteady flow is periodic at 14Gr=1000, 1500 "> but chaotic at 14Gr=100, 500, 2000."> If the Dean number is increased further i.e. at 14 Dn=1000"> , the unsteady flow becomes chaotic for any value of Gr in the range. For the aspect ratio 3, however, it is found that the unsteady flow is a steady-state solution for 14 Dn=100"> at 14 Gr=100 "> and 14 Gr=2000 "> but periodic at 14Dn=100"> and 14Gr=500,1000,1500"> . If the Dean number is increased i.e. at Dn = 500 and 1000, the unsteady flow becomes chaotic for any value of Gr in the range. Contours of secondary flow patterns and temperature profiles are also obtained, and it is found that the unsteady flow consists of a single-, two-, three- , four-, five-, six-, seven- and eight-vortex solutions. It is also found that the chaotic flow enhances heat transfer more significantly than the steady-state or periodic solutions as the Dean number are increased.

In this paper, a comprehensive numerical study is presented for the fully developed two-dimensional flow of viscous incompressible fluid through a curved rectangular duct with different aspect ratios 2 and 3 for a constant curvature 14δ=0.1"> . Unsteady solutions are obtained by using a spectral method and covering a wide range of Dean number 14100≤Dn≤1000"> and the Grashof number 141000≤Gr≤2000"> . The outer wall of the duct is heated while the inner wall is cooled. The main concern of this study is to find out the unsteady flow behavior i.e whether the unsteady flow is steady-state, periodic, multi-periodic or chaotic, if the Dean number or the Grashof number is changed. For the aspect ratio 2, it is found that the unsteady flow is a steady-state solution for 14Dn=100"> and 14Gr=100, 500, 1500, 2000"> but periodic at 14Dn=100"> and 14Gr=1000. "> If the Dean number is increased i.e. at 14=500"> , it is found the unsteady flow is periodic at 14Gr=1000, 1500 "> but chaotic at 14Gr=100, 500, 2000."> If the Dean number is increased further i.e. at 14 Dn=1000"> , the unsteady flow becomes chaotic for any value of Gr in the range. For the aspect ratio 3, however, it is found that the unsteady flow is a steady-state solution for 14 Dn=100"> at 14 Gr=100 "> and 14 Gr=2000 "> but periodic at 14Dn=100"> and 14Gr=500,1000,1500"> . If the Dean number is increased i.e. at Dn = 500 and 1000, the unsteady flow becomes chaotic for any value of Gr in the range. Contours of secondary flow patterns and temperature profiles are also obtained, and it is found that the unsteady flow consists of a single-, two-, three- , four-, five-, six-, seven- and eight-vortex solutions. It is also found that the chaotic flow enhances heat transfer more significantly than the steady-state or periodic solutions as the Dean number are increased.

The threat of global warming made the increase of efficiency a crucial objective for future power plants and aero engines. As secondary flow is a major source of aerodynamic losses in turbines, its mitigation is critical to enable efficient designs. However, the suppression of secondary flow formation is challenging due to the complexity of the flow field and to the variability of its features according to each cascade characteristics. One solution is the use of three- dimensional endwall fence, to reduce secondary flow losses. Three-dimensional fences can reduce the secondary flow penetration, increase the pitch-wise uniformity of vorticity, reduce the kinetic energy of the crossflow and decrease the mixing out of the vortices downstream the blade passage. At the same time, the endwall fence entails an increment of wetted area and viscous losses, making the design of its shape a trade-off problem. The design of such devices must be tailored to the specific turbine and is usually obtained via lengthy and time-consuming experimental campaigns. Shape optimization can greatly enhance the design process of these components as it provides a way to investigate the fluid dynamic performance of a multitude of fence geometries at affordable computational cost. This paper presents a numerical methodology to automate the endwall fence design; a surrogate-based optimization is applied to minimize the total pressure losses of a steam turbine cascade. The results of the numerical analysis indicate a reduction of 48% of the secondary kinetic energy leading to a decrease of 1.96% in total pressure loss of the optimized design with respect to the baseline geometry without endwall fence. The partition of the passage vorticity led to a weakening of the secondary flow strength and consequently to reduced downstream mixing losses. A wind tunnel validation of the improvement has been finally carried out, confirming the success of the methodology., Aerospace Engineering | Flight Performance and Propulsion

This thesis presents a new methodology for representing the effect of spiral flow on the bed shear stress magnitude and direction, using a low-vertical-resolution three-dimensional model. Furthermore, it discusses the performance of the new methodology in comparison with conventional high-vertical-resolution three-dimensional and depth-averaged models.

In this paper, a comprehensive numerical study is presented for the fully developed two-dimensional flow of viscous incompressible fluid through a curved rectangular duct with different aspect ratios 2 and 3 for a constant curvature 14δ=0.1"> . Unsteady solutions are obtained by using a spectral method and covering a wide range of Dean number 14100≤Dn≤1000"> and the Grashof number 141000≤Gr≤2000"> . The outer wall of the duct is heated while the inner wall is cooled. The main concern of this study is to find out the unsteady flow behavior i.e whether the unsteady flow is steady-state, periodic, multi-periodic or chaotic, if the Dean number or the Grashof number is changed. For the aspect ratio 2, it is found that the unsteady flow is a steady-state solution for 14Dn=100"> and 14Gr=100, 500, 1500, 2000"> but periodic at 14Dn=100"> and 14Gr=1000. "> If the Dean number is increased i.e. at 14=500"> , it is found the unsteady flow is periodic at 14Gr=1000, 1500 "> but chaotic at 14Gr=100, 500, 2000."> If the Dean number is increased further i.e. at 14 Dn=1000"> , the unsteady flow becomes chaotic for any value of Gr in the range. For the aspect ratio 3, however, it is found that the unsteady flow is a steady-state solution for 14 Dn=100"> at 14 Gr=100 "> and 14 Gr=2000 "> but periodic at 14Dn=100"> and 14Gr=500,1000,1500"> . If the Dean number is increased i.e. at Dn = 500 and 1000, the unsteady flow becomes chaotic for any value of Gr in the range. Contours of secondary flow patterns and temperature profiles are also obtained, and it is found that the unsteady flow consists of a single-, two-, three- , four-, five-, six-, seven- and eight-vortex solutions. It is also found that the chaotic flow enhances heat transfer more significantly than the steady-state or periodic solutions as the Dean number are increased.

In this paper, a comprehensive numerical study is presented for the fully developed two-dimensional flow of viscous incompressible fluid through a curved rectangular duct with different aspect ratios 2 and 3 for a constant curvature 14δ=0.1"> . Unsteady solutions are obtained by using a spectral method and covering a wide range of Dean number 14100≤Dn≤1000"> and the Grashof number 141000≤Gr≤2000"> . The outer wall of the duct is heated while the inner wall is cooled. The main concern of this study is to find out the unsteady flow behavior i.e whether the unsteady flow is steady-state, periodic, multi-periodic or chaotic, if the Dean number or the Grashof number is changed. For the aspect ratio 2, it is found that the unsteady flow is a steady-state solution for 14Dn=100"> and 14Gr=100, 500, 1500, 2000"> but periodic at 14Dn=100"> and 14Gr=1000. "> If the Dean number is increased i.e. at 14=500"> , it is found the unsteady flow is periodic at 14Gr=1000, 1500 "> but chaotic at 14Gr=100, 500, 2000."> If the Dean number is increased further i.e. at 14 Dn=1000"> , the unsteady flow becomes chaotic for any value of Gr in the range. For the aspect ratio 3, however, it is found that the unsteady flow is a steady-state solution for 14 Dn=100"> at 14 Gr=100 "> and 14 Gr=2000 "> but periodic at 14Dn=100"> and 14Gr=500,1000,1500"> . If the Dean number is increased i.e. at Dn = 500 and 1000, the unsteady flow becomes chaotic for any value of Gr in the range. Contours of secondary flow patterns and temperature profiles are also obtained, and it is found that the unsteady flow consists of a single-, two-, three- , four-, five-, six-, seven- and eight-vortex solutions. It is also found that the chaotic flow enhances heat transfer more significantly than the steady-state or periodic solutions as the Dean number are increased.

In this paper, a comprehensive numerical study is presented for the fully developed two-dimensional flow of viscous incompressible fluid through a curved rectangular duct with different aspect ratios 2 and 3 for a constant curvature 14δ=0.1"> . Unsteady solutions are obtained by using a spectral method and covering a wide range of Dean number 14100≤Dn≤1000"> and the Grashof number 141000≤Gr≤2000"> . The outer wall of the duct is heated while the inner wall is cooled. The main concern of this study is to find out the unsteady flow behavior i.e whether the unsteady flow is steady-state, periodic, multi-periodic or chaotic, if the Dean number or the Grashof number is changed. For the aspect ratio 2, it is found that the unsteady flow is a steady-state solution for 14Dn=100"> and 14Gr=100, 500, 1500, 2000"> but periodic at 14Dn=100"> and 14Gr=1000. "> If the Dean number is increased i.e. at 14=500"> , it is found the unsteady flow is periodic at 14Gr=1000, 1500 "> but chaotic at 14Gr=100, 500, 2000."> If the Dean number is increased further i.e. at 14 Dn=1000"> , the unsteady flow becomes chaotic for any value of Gr in the range. For the aspect ratio 3, however, it is found that the unsteady flow is a steady-state solution for 14 Dn=100"> at 14 Gr=100 "> and 14 Gr=2000 "> but periodic at 14Dn=100"> and 14Gr=500,1000,1500"> . If the Dean number is increased i.e. at Dn = 500 and 1000, the unsteady flow becomes chaotic for any value of Gr in the range. Contours of secondary flow patterns and temperature profiles are also obtained, and it is found that the unsteady flow consists of a single-, two-, three- , four-, five-, six-, seven- and eight-vortex solutions. It is also found that the chaotic flow enhances heat transfer more significantly than the steady-state or periodic solutions as the Dean number are increased.

In the present study we perform direct numerical simulations (DNSs) of fully-developed turbulent rectangular ducts with semi-cylindrical side-walls at Re-t,Re- c similar or equal to 180 with width-to-height ratios of 3 and 5. The friction Reynolds number Re-tau,Re- (c) is based on the centerplane friction velocity and the half-height of the duct. The results are compared with the corresponding duct cases with straight side-walls (Vinuesa et al., 2014), and also with spanwise-periodic channel and pipe flows. We focus on the influence of the semi-cylindrical side-walls on the mean cross-stream secondary flow and on further characterizing the mechanisms that produce it. The role of the secondary and primary Reynolds-shear stresses in the production of the secondary flow is analyzed by means of quadrant analysis and conditional averaging. Unexpectedly, the ducts with semi-cylindrical side-walls exhibit higher cross-flow rates and their secondary vortices relocate near the transition point between the straight and curved walls. This behavior is associated to the statistically preferential arrangement of sweeping events entering through the curved wall and ejections arising from the adjacent straight wall. Therefore, the configuration with minimum secondary flow corresponds to the duct with straight side-walls and sharp corners. Consequences on experimental facilities and comparisons between experiments and various numerical and theoretical models are discussed revealing the uniqueness of pipe flow., QC 20180907

This thesis presents a new methodology for representing the effect of spiral flow on the bed shear stress magnitude and direction, using a low-vertical-resolution three-dimensional model. Furthermore, it discusses the performance of the new methodology in comparison with conventional high-vertical-resolution three-dimensional and depth-averaged models.

The threat of global warming made the increase of efficiency a crucial objective for future power plants and aero engines. As secondary flow is a major source of aerodynamic losses in turbines, its mitigation is critical to enable efficient designs. However, the suppression of secondary flow formation is challenging due to the complexity of the flow field and to the variability of its features according to each cascade characteristics. One solution is the use of three- dimensional endwall fence, to reduce secondary flow losses. Three-dimensional fences can reduce the secondary flow penetration, increase the pitch-wise uniformity of vorticity, reduce the kinetic energy of the crossflow and decrease the mixing out of the vortices downstream the blade passage. At the same time, the endwall fence entails an increment of wetted area and viscous losses, making the design of its shape a trade-off problem. The design of such devices must be tailored to the specific turbine and is usually obtained via lengthy and time-consuming experimental campaigns. Shape optimization can greatly enhance the design process of these components as it provides a way to investigate the fluid dynamic performance of a multitude of fence geometries at affordable computational cost. This paper presents a numerical methodology to automate the endwall fence design; a surrogate-based optimization is applied to minimize the total pressure losses of a steam turbine cascade. The results of the numerical analysis indicate a reduction of 48% of the secondary kinetic energy leading to a decrease of 1.96% in total pressure loss of the optimized design with respect to the baseline geometry without endwall fence. The partition of the passage vorticity led to a weakening of the secondary flow strength and consequently to reduced downstream mixing losses. A wind tunnel validation of the improvement has been finally carried out, confirming the success of the methodology., Aerospace Engineering | Flight Performance and Propulsion

This experiment investigated the effects of a non-axisymmetric endwall contour and upstream purge flow on the secondary flow of an inlet guide vane. Three cases were tested in a transonic wind tunnel with an exit Mach number of 0.93-a flat endwall with no upstream purge flow, the same flat endwall with upstream purge flow, and a 3D contoured endwall with upstream purge flow. All cases had a backward-facing step upstream of the vanes. Five-hole probe measurements were taken 0.2, 0.4, and 0.6 Cx downstream of the vane row trailing edge, and were used to calculate loss coefficient, secondary velocity, and secondary kinetic energy. Additionally, surface static pressure measurements were taken to determine the vane loading at 4% spanwise position. Surface oil flow visualizations were performed to analyze the flow qualitatively. No statistically significant differences were found between the three cases in mass averaged downstream measurements. The contoured endwall redistributed losses, rather than making an improvement distinguishable beyond experimental uncertainty. Flow visualization found that the passage vortex penetrated further in the spanwise direction into the passage for the contoured endwall (compared to the flat endwall), and stayed closer to the endwall with a blowing ratio of 1.5 with a flat endwall (compared to no blowing with flat endwall). This was corroborated by the five hole probe results.

The fairway connecting Holwerd and Nes forms the main transport route between Friesland and Ameland (the Dutch Wadden Sea). It consists of several tidal channels that funnel the majority of the tidal currents propagating through the tidal basin. Over the past decades, several channel bends have expanded leading to a fairway length-increase of about 1 km, and the channel in front of Holwerd has become subject to sedimentation. Since 1990 this channel was dredged and the volume of dredged material has increased exponentially. A relation between dredging activities, channel migration and frequent delays of the ferry has been suggested. The main goal of this thesis is therefore to explain the difference in evolution between the non-natural (intervened) and natural tidal meanders in the Ameland basin. To achieve this goal, the evolution of geographic channel dimensions are analyzed by assessing bathymetric data and the outcome is linked to the physics by means of hydrodynamic modeling. Channels have been classified based on their locations and dimensions. Three meandering stages have been identified that describe the basic channel behavior. The first meandering stage is the developed meander, characterized by a clear sinusoidal shape and with flood chutes. In this stage, the depth at a cross-over is approximately two times smaller than at the channel section with a maximum curvature (the channel top), where the width profile is funnel shaped. The developed, variable meander has similar characteristics, but differs by a lower correlation in width and depth, and closer to the tidal watershed the ebb channel location varies more in time. The third stage is the undeveloped meander, which is characterized by small and relatively straight channels, the absence of flood chutes, and a rather constant width- and depth-profile along the channel. In this study, the channels are evaluated that have similar meandering features and distance from the Ameland inlet as the dredged channe, Civil Engineering and Geosciences, Hydraulic Engineering, Coastal Engineering

In the present study we perform direct numerical simulations (DNSs) of fully-developed turbulent square ducts with round corners at Reτ,c ∼ 180 and 360, and rectangular ducts of width-toheight ratios of 3 and 5 with rounded side walls at Reτ,c ∼ 180. The friction Reynolds number Reτ,c is based on the centerplane friction velocity and the half-height of the duct. The results are compared with the corresponding duct cases with 90° corners. We focus on the influence of the rounding on the mean cross-stream secondary flow and on further characterizing the mechanisms that produce it. Unexpectedly, the rounded ducts exhibit higher cross-flow rates and their secondary vortices relocate near the transition point between the straight and curved walls. This behavior is associated to the statistically preferential arrangement of sweeping events entering through the curved wall, which trigger an ejection on the adjacent straight wall. We have yet to find effective modifications to the corners or transverse ends of a rectangular duct that would render better rigorous modeling of two-dimensional channel flows., QC 20200630

The spanwise oscillation of channel walls is known to substantially reduce the skin-friction drag in turbulent channel flows. In order to understand the limitations of this flow control approach when applied in ducts, direct numerical simulations of controlled turbulent duct flows with an aspect ratio of A R = 3 are performed. In contrast to channel flows, the spanwise extension of the duct is limited. Therefore, the spanwise wall oscillation either directly interacts with the duct side walls or its spatial extent is limited to a certain region of the duct. The present results show that this spanwise limitation of the oscillating region strongly diminishes the drag reduction potential of the control technique. We propose a simple model that allows estimating the achievable drag reduction rates in duct flows as a function of the width of the duct and the spanwise extent of the controlled region., QC 20180103

The current work aims to make a foundation for an engineering design of a cyclone gasifier to be able not only to predict its flow field with a suitable accuracy but also to investigate a large number of design alternatives with limited computer resources. A good single-phase flow model that can form the basis in an Euler-Lagrange model for multi-phase flow is also necessary for modelling the reacting flow inside a cyclone gasifier. The present paper provides an objective comparison between several popular turbulence modelling options including standard k-ε and SST with curvature corrections, SSG-RSM and LES Smagorinsky models, for the single-phase flow inside cyclone separators/gasifiers that can serve as a guide for further work on the reacting multi-phase flow inside cyclone gasifiers and similar devices. A detailed comparison between the models and experimental data for the mean velocity and fluctuating parts of the velocity profiles are presented. Furthermore, the capabilities of the turbulence models to capture the physical phenomena present in a cyclone gasifier that affects the design process are investigated., Validerad;2017;Nivå 1;2017-11-10 (andbra)

The current work aims to make a foundation for an engineering design of a cyclone gasifier to be able not only to predict its flow field with a suitable accuracy but also to investigate a large number of design alternatives with limited computer resources. A good single-phase flow model that can form the basis in an Euler-Lagrange model for multi-phase flow is also necessary for modelling the reacting flow inside a cyclone gasifier. The present paper provides an objective comparison between several popular turbulence modelling options including standard k-ε and SST with curvature corrections, SSG-RSM and LES Smagorinsky models, for the single-phase flow inside cyclone separators/gasifiers that can serve as a guide for further work on the reacting multi-phase flow inside cyclone gasifiers and similar devices. A detailed comparison between the models and experimental data for the mean velocity and fluctuating parts of the velocity profiles are presented. Furthermore, the capabilities of the turbulence models to capture the physical phenomena present in a cyclone gasifier that affects the design process are investigated., Validerad;2017;Nivå 1;2017-11-10 (andbra)

The fairway connecting Holwerd and Nes forms the main transport route between Friesland and Ameland (the Dutch Wadden Sea). It consists of several tidal channels that funnel the majority of the tidal currents propagating through the tidal basin. Over the past decades, several channel bends have expanded leading to a fairway length-increase of about 1 km, and the channel in front of Holwerd has become subject to sedimentation. Since 1990 this channel was dredged and the volume of dredged material has increased exponentially. A relation between dredging activities, channel migration and frequent delays of the ferry has been suggested. The main goal of this thesis is therefore to explain the difference in evolution between the non-natural (intervened) and natural tidal meanders in the Ameland basin. To achieve this goal, the evolution of geographic channel dimensions are analyzed by assessing bathymetric data and the outcome is linked to the physics by means of hydrodynamic modeling. Channels have been classified based on their locations and dimensions. Three meandering stages have been identified that describe the basic channel behavior. The first meandering stage is the developed meander, characterized by a clear sinusoidal shape and with flood chutes. In this stage, the depth at a cross-over is approximately two times smaller than at the channel section with a maximum curvature (the channel top), where the width profile is funnel shaped. The developed, variable meander has similar characteristics, but differs by a lower correlation in width and depth, and closer to the tidal watershed the ebb channel location varies more in time. The third stage is the undeveloped meander, which is characterized by small and relatively straight channels, the absence of flood chutes, and a rather constant width- and depth-profile along the channel. In this study, the channels are evaluated that have similar meandering features and distance from the Ameland inlet as the dredged channe, Civil Engineering and Geosciences, Hydraulic Engineering, Coastal Engineering

Next generation aircraft design focuses on highly integrated airframe/engine architectures that exploit advantages in system level efficiency and performance. One such design concept incorporates boundary layer ingestion which locates the turbofan engine inlet near enough to the lifting surface of the aircraft skin that the boundary layer is ingested and reenergized. This process reduces overall aircraft drag and associated required thrust, resulting in fuel savings and decreased emissions; however, boundary layer ingestion also creates unique challenges for the turbofan engines operating in less than optimal inlet flow conditions. The engine inlet flow profiles predicted from boundary layer ingesting aircraft architectures contain complex distortions that affect the engine operability, durability, efficiency, and performance. One component of these complex distortion profiles is off-axial secondary flow, commonly referred to as swirl. As a means to investigate the interactions of swirl distortion with turbofan engines, an experiment was designed to measure distorted flow profiles in an operating turbofan research engine. Three-dimensional flow properties were measured at discrete planes immediately upstream and immediately downstream of the fan rotor, isolating the component for analysis. Constant speed tests were conducted under clean and distorted test conditions. For clean tests, a straight cylindrical inlet duct was attached to the fan case; for distorted tests, a StreamVane swirl distortion generator was inserted into the inlet duct. The StreamVane was designed to induce a swirl distortion matching results of computation fluid dynamics models of a conceptual blended wing body aircraft at a plane upstream of the fan. The swirl distortion was then free to develop naturally within the inlet duct before being ingested by the engine. Results from the investigation revealed that the generated swirl profile developed, mixed, and dissipated in the inlet duct upstream

The dependence of curvature-induced secondary flow on the curvature ratio H/R (H is the average flow depth and R is the centreline radius of curvature), the Froude number Fr, and the dimensionless roughness coefficient Cf was systematically investigated in a series of 25 experiments in a laboratory flume. The investigated H/R values were (±0.005) 0.050, 0.073, 0.112 and 0.151, the Froude numbers were (±0.03) 0.10, 0.23, 0.32, 0.40 and 0.50, and roughness heights of the sediment bed were 0.002 m and 0.067 m. With width-to-depth ratios of 4.9 to 17.1, these experiments are more representative of small and medium-size rivers than the largest rivers on earth. Commonly used parameterizations for secondary flow predict the normalized magnitude to grow linearly with H/R, and to decrease with increasing bed roughness. This behaviour was only observed in mildly curved flows. The normalized magnitude was found to grow less than linearly with H/R in moderately curved flows, and to reach a maximum value in strongly curved flow. This confirms observations by Blanckaert (2009), who called this phenomenon the saturation of the secondary flow. He attributed the less than linear growth to nonlinear interactions between the primary and the secondary flows. For high values of H/R, secondary flow was found to be stronger over the very rough gravel bed than over the smoother sand bed. This is explained by the nonlinear interactions between the primary and secondary flows that are less pronounced over the rougher bed. The experiments did not reveal any dependence of the secondary flow on Fr. Predictions of the magnitude of the secondary flow with the nonlinear model of Blanckaert and de Vriend (2003, 2010) agreed very well with the experimental data over the entire range of investigated experimental conditions. Nonlinear effects are always important when the parameter Cf-0.25H/R is larger than an indicative value of 0.1, and negligible when this parameter is smaller than 0.05. These valu

In this study, a flat-plate channel configured with pyramidal protrusions are numerically analysed for the first time. Simulations of laminar single-phase fluid flow and heat transfer characteristics are developed using a finite-volume approach under steady-state condition. Pure water is selected as the coolant and its thermo-physical properties are modelled using a set of temperature-dependent functions. Different configurations of the channel, including a plain channel and a channel with nature-inspired protruded surfaces, are studied here for Reynolds numbers ranging from 135 to 1430. The effects of the protrusion shape, size and arrangement on the hydrothermal performance of a flat-plate channel are studied in details. The temperature of the upper and lower surfaces of the channel is kept constant during the simulations. It is observed that utilizing these configurations can boost the heat transfer up to 277.9% and amplify the pressure loss up to 179.4% with a respect to the plain channel. It is found that the overall efficiency of the channels with pyramidal protrusions is improved by 12.0-169.4% compared to the plain channel for the conditions studied here. Furthermore, the thermodynamic performance of the channel is investigated in terms of entropy generation and it is found that equipping the channels with pyramidal protrusions leads to lower irreversibility in the system., (OLD) MSE-5

The dependence of curvature-induced secondary flow on the curvature ratio H/R (H is the average flow depth and R is the centreline radius of curvature), the Froude number Fr, and the dimensionless roughness coefficient Cf was systematically investigated in a series of 25 experiments in a laboratory flume. The investigated H/R values were (±0.005) 0.050, 0.073, 0.112 and 0.151, the Froude numbers were (±0.03) 0.10, 0.23, 0.32, 0.40 and 0.50, and roughness heights of the sediment bed were 0.002 m and 0.067 m. With width-to-depth ratios of 4.9 to 17.1, these experiments are more representative of small and medium-size rivers than the largest rivers on earth. Commonly used parameterizations for secondary flow predict the normalized magnitude to grow linearly with H/R, and to decrease with increasing bed roughness. This behaviour was only observed in mildly curved flows. The normalized magnitude was found to grow less than linearly with H/R in moderately curved flows, and to reach a maximum value in strongly curved flow. This confirms observations by Blanckaert (2009), who called this phenomenon the saturation of the secondary flow. He attributed the less than linear growth to nonlinear interactions between the primary and the secondary flows. For high values of H/R, secondary flow was found to be stronger over the very rough gravel bed than over the smoother sand bed. This is explained by the nonlinear interactions between the primary and secondary flows that are less pronounced over the rougher bed. The experiments did not reveal any dependence of the secondary flow on Fr. Predictions of the magnitude of the secondary flow with the nonlinear model of Blanckaert and de Vriend (2003, 2010) agreed very well with the experimental data over the entire range of investigated experimental conditions. Nonlinear effects are always important when the parameter Cf-0.25H/R is larger than an indicative value of 0.1, and negligible when this parameter is smaller than 0.05. These valu