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The transition process within a Laminar Separation Bubble (LSB) that formed on a compressor blade surface was investigated using Large Eddy Simulations (LESs) at a Reynolds number of 1.5 × 105 and incidence angles of 0°, +3°, and +5°. The vortex dynamics in the separated shear layers were compared at various incidence angles and its effects on the loss generation were clarified through entropy analysis. Results showed that transition onset, which was accurately identified by the Linear Stability Theory (LST), was significantly promoted at the increased incidence angle. As such, the development of LSB was suppressed and the relative role of viscous instability played in the transition process was weakened. At the incidence angle of 0°, two-dimensional spanwise vortices detached from the blade surface and rolled up periodically, which were further stretched and eventually evolved into large-scale hairpin vortices. As time passed, the fully developed hairpin vortices broke down into small-scale eddies. Meanwhile, the flow near the wall reversely ejected into the outer separated shear layers and a sweeping process happened subsequently, forcing the separated shear layers to reattach and accelerating the generation of turbulent fluctuations. By comparison, the strength of vortex rolling-up was weakened at higher incidence angles, and the vortex pairing and breakdown of large-scale vortices were less pronounced. Therefore, the level of turbulent fluctuations that generated in the separated shear layers was reduced. Detailed entropy analysis showed that the turbulent dissipation effect related to the Reynolds shear stresses determined the largest amount of positive entropy generation, which declined to a lower level as the incidence angle increased from 0° to +5°. Correspondingly, the profile loss was reduced by 50.4%.

Laboratory experiments were performed to characterize the distinct features of hairpin-like vortical structures induced via pulsed, wall-normal jets in laminar (LBL), and turbulent boundary layers (TBLs). Hotwire anemometry was used to characterize the disturbed flow at multiple streamwise, transverse, and vertical locations. The dominant spatiotemporal structure of the induced motions was reconstructed using Taylor’s hypothesis and time-averaged, phase-locked data. Results support the concept that the meandering of very-long, low-momentum streaks observed in the logarithmic layer and lower part of wake regions of TBLs results from the upwash of hairpin vortices or packets. Comparing the induced hairpin-like vortices in the LBL and TBL reveals the modulation of the mean shear; using the second-order derivative of the mean streamwise velocity, we show a linkage between the mean shear gradient and appearance of low- and high-momentum streaks in the logarithmic layer. Streamwise velocity profiles with a pitot tube by Smith (Effect of Reynolds number on the structure of turbulent boundary layers, 1994) at ten Reynolds numbers from Reθ = 4601 to 13,189 are also inspected to evaluate such linkage in a broader range of Re. As Reynolds number increased, the vertical regions of the negative second derivative of mean streamwise velocity increased. The minimum value of mean shear gradient decreased, indicating that the mean shear stress gradient increased to form more noticeable low- and high-momentum streaks in the logarithmic layer. Besides, the Reynolds tensor’s principal stresses and principal axes angles associated with the hairpin vortex legs stretching are demonstrated by DNS data (Spalart, P. R. 1988) as Reynolds number increased.

We have studied the resonance curves of a quartz tuning fork of the fundamental frequency 32 kHz immersed in superfluid 4He and mixtures 3He–4He with 3He concentration of 5 and 15% in the temperature range of 0.35–2.5 K at saturated vapor pressure. Two types of experiments have been carried out, with a tuning fork both not covered by a bulb and coated by a bulb, i.e., in the restricted geometry. In both cases, the velocity-force dependences for the tuning fork showed a linear damping force at low peak velocities and extra drag due to the appearance of vortex lines accompanying the transition to turbulence under increasing peak velocity. These dependencies are mainly determined by the density of the normal helium component. There is a pronounced difference between superfluid 4He and mixtures of 3He in 4He, where 3He impurity particles provide a constant temperature-independent contribution to the normal component of the mixture. The extra contribution to the damping force, so-called “turbulent drag force”, decreases with concentration increase at the same peak velocity of the tuning fork that can be explained by the extension of the range of laminar flow with an increase in the concentration of 3He. We found that the drag coefficient in superfluid 4He and mixtures 3He–4He reaches a plateau at different peak velocities and different exciting forces and explained this fact by different conditions for vortex formation, depending on the different thickness of the near-wall viscous layer. The comparison between the data obtained in restricted and unrestricted geometries shows that there is an excessive dissipation of the tuning fork motion associated with the emission of the first sound wave in unrestricted geometry.

A series of Direct Numerical Simulations (DNS) at low Reynolds number is performed to investigate the flow and the heat transfer in ducts with complex geometry, which are frequently encountered in many engineering fields, especially in aeronautics. A finite-difference approach and the immersed-boundary method are adopted to solve the governing equations in the computational domain. The flow and heat transfer phenomena in a circular pipe are studied in detail for laminar and turbulent regimes; thus the code is validated through a comparison of present DNS results with those provided by some trusted references. Straight ducts with regular polygonal cross sections represent the first type of ducts with complex geometry analyzed in this paper. The behavior of the pumping power expenditure is studied when the number of polygonal sides changes. The secondary motions are then investigated, and their effects on the mean velocity and temperature contours are studied. The shapes of vortices in secondary motions are compared with those provided by an analytical approach, which is based on the eigenfunctions of the Laplacian operator. Also the geometries pioneeringly studied by Nikuradse in 1930 are taken into account.

Time-resolved particle image velocimetry measurements were performed downstream of a swept backward-facing step. The measurements allow detailed analysis of the interactions between the unsteady instabilities and the stationary crossflow vortices. Different mechanisms are identified that lead to the modulation of the different families of unsteady instabilities that occur downstream of the step. For the low-frequency spanwise-travelling mode, the modulation occurs due to a redistribution of momentum when the instability encounters regions of large spanwise shear of the wall-normal and streamwise velocity. However, the higher-frequency streamwise-travelling instabilities undergo the familiar ‘lift-up’ mechanism when they encounter the regions of large vertical velocity due to the presence of the stationary crossflow vortices. The process leading to large velocity spikes, and ultimately to a laminar breakdown to turbulence, is identified as a constructive interaction between the different unsteady instabilities, coupled with an interaction with the stationary crossflow vortices when the phases align properly.

This paper deals with a mathematical modeling of flow stability of Newtonian and non-Newtonian fluids in the gap between two concentric cylinders, one of which rotates. A typical feature of the flow is the formation of a vortex flow, so-called Taylor vortices. Vortex structures are affected by the speed of the rotating cylinder and the physical properties of the fluids, i.e., viscosity and density. Analogy in terms of viscosity is assumed for non-Newtonian and magnetorheological fluids. Mathematical models of laminar, transient and turbulent flow with constant viscosity and viscosity as a function of the deformation gradient were formulated and numerically solved to analyze the stability of single-phase flow. To verify them, a physical experiment was performed for Newtonian fluids using visualizations of vortex structures-Taylor vortices. Based on the agreement of selected numerical and physical results, the experience was used for numerical simulations of non-Newtonian magnetorheological fluid flow. Web of Science 9 10 art. no. 1832

A high-order transition route from inertial to elasticity-dominated turbulence (EDT) in Taylor–Couette flows of polymeric solutions has been discovered via direct numerical simulations. This novel two-step transition route is realized by enhancing the extensional viscosity and hoop stresses of the polymeric solution via increasing the maximum chain extension at a fixed polymer concentration. Specifically, in the first step inertial turbulence is stabilized to a laminar flow much like the modulated wavy vortex flow. The second step destabilizes this laminar flow state to EDT, i.e. a spatially smooth and temporally random flow with a $-3.5$ scaling law of the energy spectrum reminiscent of elastic turbulence. The flow states involved are distinctly different to those observed in the reverse transition route from inertial turbulence via a relaminarization of the flow to elasto-inertial turbulence in parallel shear flows, underscoring the importance of polymer-induced hoop stresses in realizing EDT that are absent in parallel shear flows.

The main objective of this thesis is the numerical study of airflow and pollutant dispersion within urban street canyons for various scenarios of street geometries, wind characteristics and thermal stratification. In this context, a new two-dimensional flow solver based on the Navier-Stokes equations and the finite volume method was developed to simulate incompressible laminar or turbulent viscous flow fields. The computational code incorporates a local grid refinement technique in order to increase the numerical accuracy in areas of particular interest keeping computational effort low. Additionally, inclined to the flow direction wall boundaries are implemented on Cartesian meshes with the aim to simulate the flow field developed around complex curvilinear geometries. The algebraic system of the discretized differential equations is solved utilizing an extension of the ADI algorithm for unstructured grids, which was also developed. The overall method is extended to three dimensions in order to study the flow field within complex topography of cities. Simulations revealed that where one primary vortex is developed, high levels of pollutants are found in the leeward side of street canyons. Convection proves to be the principal mechanism of transport within canyons while the mechanism of pollutant dispersion from street canyons to the outside flow layers is turbulent diffusion and the possible unsteadiness of flow fields. In cases where buildings have inclined roofs, mechanisms of pollutant dispersion vary considerably due to the appearance of double structure vortices. When unstable stratification prevails, an increase in the intensity of the vertical movements and the dominant recirculation is observed. Thermal instability favors pollutant dispersion and increases the unsteadiness of flow fields. When intensive temperature stability occurs, the levels of turbulent kinetic energy and velocity speed are significantly reduced. The numerical investigation of wind flow and pollutant dispersion in a large area of Athens indicates that the pollutant levels depend heavily on the geometric asymmetry of the street canyons, the roofs’ geometry and the angle between the prevailing wind direction and the roads. Open space areas within building complex seem to be places where levels of pollution appear low.%%%%Ο βασικός στόχος της παρούσας διατριβής eίναι η αριθμητική μeλέτη της ροής του αέρα και της διασποράς ρύπων σe οδικές χαράδρeς για διάφορα σeνάρια γeωμeτριών των οδικών χαραδρών, πνέοντος ανέμου και θeρμοκρασιακής διαστρωμάτωσης. Σe αυτά τα πλαίσια αναπτύχθηκe νέος διδιάστατος υπολογιστικός κώδικας ο οποίος έχeι την δυνατότητα προσομοίωσης στρωτής ή τυρβώδους ασυμπίeστης συνeκτικής ροής, eπιλύοντας τις eξισώσeις Navier-Stokes μe την μέθοδο των πeπeρασμένων όγκων. Στον υπολογιστικό κώδικα eνσωματώθηκe μια τeχνική τοπικής πύκνωσης του υπολογιστικού πλέγματος μe σκοπό την αύξηση της υπολογιστικής ακρίβeιας σe πeριοχές ιδιαίτeρου eνδιαφέροντος μe μικρό υπολογιστικό κόστος. Eπιπλέον, αναπτύχθηκe μία νέα τeχνική διαχeίρισης…

High-amplitude free-stream turbulence and large surface roughness elements can excite a laminar boundary layer sufficiently enough to cause streamwise oriented vortices to form. The latter is accompanied by streaks of varying amplitudes that ‘wobble’ through inviscid secondary instabilities and, ultimately, transition to turbulence. In this paper, we formu- late a mathematical framework for the optimal control of compressible boundary layers to suppress the growth rate of the streamwise vortex system before breakdown occurs. This has a commensurate impact on the wall shear stress and heat transfer at the wall. Flow instabilities are introduced either through roughness elements equally separated in the spanwise direction or via free-stream disturbances. The compressible Navier-Stokes equations are reduced to the boundary region equations (BRE) in a high Reynolds number asymptotic framework wherein the streamwise wavelengths of the disturbances are assumed to be much larger than the spanwise and wall-normal counterparts. The method of La- grange multipliers is used to derive the adjoint compressible boundary region equations via an appropriate transformation of the original constrained optimization problem into an unconstrained form. In the present formulation, the wall transpiration velocity represents the control variable while the wall shear stress or the vortex energy represents the cost functional. Our study shows that this kind of control approach induces a significant reduc- tion in the kinetic energy and wall shear stress of the boundary layer flow. Contour plots visually demonstrate how the primary instabilities gradually flatten out as more control iterations are applied.

Molten salt (MS) and supercritical carbon dioxide (SCO2) are important heat transfer fluids (HTFs) in many energy systems as concentrated solar power. In this study, novel pillow plate heat exchanger (PPHE) with periodically staggered convex spots and concave spots is first proposed for heat transfer between MS and SCO2, and associated flow and thermal performance is numerically analyzed. Because of periodical convex/concave spots, flow pattern, temperature and heat transfer coefficient periodically vary by the interaction of fluid scouring effect and vortices. In downstream region of convex spot or upstream region of concave spot, the fluid temperature changes very quickly and the thermal boundary layer is reduced by fluid scouring effect, so associated heat transfer coefficient is relatively higher. In upstream region of convex spot or downstream region of concave spot, vortices appear and thermal boundary layer is relatively thicker, so heat transfer coefficient will be lower. In flat plate region, its heat transfer coefficient is lower than that in convex spot region and is higher than that in concave spot region, while its heat flow percentage in total system is more than 50% for large surface area ratio. The heat transfer coefficient of PPHE is remarkably influenced by its structures, it increases 28.1% with channel height rising from 2.4 mm to 4.8 mm and increases 40.4% with spot longitudinal distance dropping from 42 mm to 18 mm. In addition, flow friction factor and heat transfer correlations of HTFs in present PPHE from laminar to turbulent flow are respectively predicted with different channel height and spot longitudinal distance.

Strut serves as an effective and robust flameholder in a scramjet combustor, and the resultant wakes have a great impact on the fuel mixing and flame stabilization. Present work focuses on the non-reactive flow induced by strut without fuel injection, removing the effect of chemical reactions on turbulence. To investigate the transition process of turbulent wakes, both high-resolution experiments and numerical simulations are conducted. An advanced optical measurement technique is adopted to visualize the flow structures in different Mach conditions, and to disclose the compressible effect on the shear layer. Experimental results indicate that the scale of turbulent vortices decreases and the transition becomes difficult with the decline of Reynolds number in the cases with the same total temperature and pressure. Evidently, the viscous effect gets weakened for the supersonic flow with a high Reynolds number, giving rise to large scale vortex streets. On this basis, the intrinsic mechanism that governs the instability of shear layer is investigated by large eddy simulation. The strut-induced wakes can be divided into three parts, namely the recirculation zone, the transition zone and the downstream turbulent zone. A type of Kelvin-Helmholtz instability produced by the interactions of the reattachment shock and shear stress, should be responsible for the generation of the initial streamwise vortices during the transition process of shear layer. Thus, the flow instability rapidly grows as a form of nonlinear, magnifies, and changes the mean velocity profile of the shear layer, leading to its transition from laminar state to turbulence. Finally, the influence of turbulence on the temporal and spatial evolution of the vortices in the wakes is analyzed in deep. The upstream turbulent fluctuations facilitate the occurrence of transition process, and promote to generate larger scale spanwise vortex streets. Present work can help us understand the flow dynamics of turbulent wakes in a strut-stabilized scramjet combustor.

The accuracy of the modelling of gas–liquid flows depends strongly on a suitable modelling of the interfacial forces. Among these, drag is dominant. Most drag models reported in the literature have been derived and validated only for laminar or low-turbulent flow conditions. In this study, we numerically evaluated several drag models from the literature for high-turbulent gas–liquid flow around an obstacle in a pipe that creates a distinct vortex region. We performed Computational Fluid Dynamics (CFD) simulations and compared the void fraction and gas velocity profiles with experimental data obtained by ultrafast X-ray computed tomography. We found that all models, except Schiller&Naumann and Feng, predicted the void fraction well compared to experimental data upstream of the obstacle, i.e., for a developed two-phase pipe flow with axial symmetry. However, the void fraction downstream is greatly overestimated by all models except those that appropriately consider the turbulence effects. Based on the results, a hybrid drag model is proposed that significantly improves the prediction of the void fraction.

The interaction of a National Advisory Committee for Aeronautics 0012 wing-tip vortex with a grid-generated turbulent flow was experimentally investigated. The experiments were conducted in the near- and mid-wake regions at three free-stream turbulence (FST) levels, viz., 0.5%, 3%, and 6%, at a Reynolds number, based on the wing chord length, cw, of 2×105. Stereoscopic particle image velocimetry measurements were carried out at four downstream positions: x/cw = 1.25, 3.25, 6.25, and 7.75. Streamwise vorticity contours showed that the wing-tip vortex decayed with increased FST and downstream distance. Turbulent surroundings were found to affect the meandering amplitude of the vortex, which increased with the dispersed scatter pattern of the vortex center motion, resulting in a meandering-induced turbulence. Meandering-corrected turbulent kinetic energy revealed the existence of a laminar core at the center of the vortex surrounded by low turbulence levels outside the core. This was attributed to the stabilizing Coriolis effects of the strong rotational motion inside the vortex core, which tend to re-laminarize the turbulent fluid crossing the periphery of the vortex. Snapshot proper orthogonal decomposition analysis on the coherent component of the velocity field revealed two dominant modes forming a helical dipole, consistent with the helical displacement of a Kelvin wave with an azimuthal wavenumber |m=1|. An analysis of the terms balancing the rate of decay of the mean enstrophy revealed that increasing FST increases the stretching of the mean enstrophy within the vortex core while it reduces both its transport and convection terms. Nonetheless, the latter contributions were larger in all cases studied acting as the main mechanism for mean enstrophy decay.

In this paper, the turbulent flow in rod bundles with spacer grid and mixing vanes in nuclear reactor core was studied using spectral element method with large eddy simulation (LES) and unsteady RANS (URANS) method. Combining with the radial periodical boundary conditions, the turbulent mixing phenomena in central subchannels were investigated, especially focusing on the flow field after the spacer grid and mixing vanes. Other than setting up long bare rod bundle to establish the fully developed turbulent flow, only a 2-cm-long bare rod bundle was extruded upstream the spacer grid and mixing vanes, while the fully developed laminar velocity profile was applied on the inlet surface for velocity boundary settings. The results indicate that both of the models could predict time-averaged velocity at 0.5Dh, 1Dh, 4Dh downstream mixing vanes properly, while the URANS model underestimate the root-mean-square (RMS) value of fluctuating velocity due to the inlet treatment effect. Furthermore, turbulent kinetic energy contour with transverse velocity vector was combined to investigate the behaviors of vortices formed by split-type mixing vanes, revealing the behavior of vortices for preliminary dynamic and mechanic analysis in design process of fuel assembly.

Improving the cooling performance of internal channels is important to extend the lifespan of hot components exposed to heat in many types of machinery. This study was performed to examine the effects of placing a delta-winglet-pair (DWP) upstream of a single dimple in an internal cooling channel. The local heat/mass transfer coefficients (H/MTCs) were measured in the vicinity of the dimple by the naphthalene sublimation method (NSM). The cooling performances under laminar, transitional, and turbulent flow conditions were compared at different values of the DWP incidence angle ( β ). The DWP enhanced local H/MTCs within the dimple cavity due to intensified secondary vortices. With increasing the DWP incidence angle, the distance between the symmetrical vortices increased resulting in a different level of H/MTC enhancement. The optimum DWP incidence angle ranged between 30 ∘ β 45 ∘ depending on the flow condition. The resulting area-averaged H/MTCs clearly indicated enhancement from the DWP. Under the laminar flow conditions, the H/MTCs were enhanced by 87%–125% from installing the DWP. The absolute H/MTCs were highest under the transitional flow conditions. Based on the present investigation, the installation of the DWP in the dimpled channel is highly recommended for both laminar and transitional flow conditions.

A condensate in two-dimensional turbulence confined to a finite domain was predicted by Kraichnan. The most common spatial form of the condensate is a coherent vortex of radius comparable with the domain size, which is statistically steady over times much longer than its turnover time. The vortices were successively studied during the last four decades, and their time-averaged properties were recently and extensively considered in theory and measured in experiments. Here, we consider weak perturbations of the coherent flow in the vortices. Our interest lies in slow perturbations, which implies that they are homogeneous along the streamlines of the coherent flow. We show that such kind of perturbations can be considered as waves of condensate propagating in the radial direction with some dispersion law. In the present work, the dispersion law and the propagation length of the waves are found as a function of the radial position inside a vortex flow. Cases of condensate saturated due to bottom friction and of viscous condensate are different. In the first case, there are waves with low damping. In the second case, all waves are of a similar kind as the shear waves in an unsteady laminar boundary layer.