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Airfoil leading-edge fluid-blowing control is computationally studied to improve aerodynamic efficiency. The fluid injection momentum coefficient Cμ (the ratio of injection to incoming square velocities times the slot's width to airfoil's half chord length) varies from 0.5% to 5.4%. Both static and dynamic conditions are investigated for a NACA0018 airfoil at low speed and Reynolds number of 250 k based on the airfoil's chord length. The airfoil is dynamically pitched at a reduced frequency (the pitching tangential speed to the freestream speed ratio), varying between 0.0078 and 0.2. Reynolds-averaged Navier–Stokes (RANS) and unsteady RANS (URANS) is used in the simulations as based on the Transition SST and Spalart–Allmaras models, generally achieving good agreement with experimental results in lift and drag coefficients and in the pressure coefficient distributions along the airfoil. It is found that oscillating the airfoil can delay stall, as expected, in dynamic stall (DS). Leading-edge blowing control can also significantly delay stall both in static and dynamic conditions as long as sufficient momentum is applied to the control. On the other hand, for a small Cμ such as 0.5%, the leading-edge control worsens the performance and hastens the appearance of stall in both static and dynamic conditions. The airfoil's oscillation reduces the differences between pitch-up and pitch-down aerodynamic performances. Detailed analysis of vorticity, pressure, velocity, and streamline contours is given to provide plausible explanations and insight to the flow. [ABSTRACT FROM AUTHOR]

The circular synthetic jet (SJ) for different dimensionless stroke lengths and at Reynolds number Re = 5000 was investigated in this paper. Particle image velocimetry (PIV) was used. The flow was measured at a distance of 240 mm from the orifice, and this area was divided into two fields of view (FOV). The parameter fields were created by the injunction of these two FOVs. The time-averaged velocity, turbulent kinetic energy (TKE), turbulence intensity, vorticity field, centerline, and profiles of SJ were presented and discussed. Additionally, the jet half-width of SJ was investigated. The data discontinuity at a line of the FOVs was discussed. The impact of the dimensionless stroke lengths on the parameters of SJ at Re = 5000 was discussed. [ABSTRACT FROM AUTHOR]

This study investigated the effect of a mixed obstacle layout on the deflagration mechanism of propane--air premixed gases. Most previous studies focused on a single type of obstacle, changing the shape and number of the obstacles to observe the effect on the flame deflagration characteristics. However, in real explosion accident sites, obstacles are often a mixture of different types. Little literature exists on the deflagration characteristics of hybrid barriers in semiconfined spaces. In this paper, the deflagration characteristics of propane-air premixed gas with a mixed structure of hurdles and square obstacles was studied. First, the effectiveness of numerical simulations was demonstrated by comparing experimental and large eddy simulation (LES) results for the flame dynamics with a single flat plate obstacle. Based on this, the flame behavior for different layouts of square obstacles in a mixed obstacle configuration was further simulated using the large eddy simulation method, focusing on the flame behavior, overpressure characteristics, and flow field structure in the vicinity of the obstacle. The results showed that a mixed obstacle promoted flame evolution more than a single obstacle when the square obstacle was within a critical distance from the ignition source location at the same moment in time. When the flame front crossed the first hurdle-type obstacle, the flame pattern spread in a "cat's paw" pattern to the unburned portion of the tube. In addition, the increased distance of the square obstacle from the ignition source did not allow the peak overpressure and the peak rate of overpressure rise to show a positive feedback mechanism. Finally, the strength of the vorticity in the flow field was positively correlated with the distance of the square obstacle from the ignition source. The results of study provide theoretical for the prevention of explosions. [ABSTRACT FROM AUTHOR]

Rotating cavitation water jet technology is widely used in many industrial fields such as pipeline cleaning, unblocking, cutting, etc. To more accurately analyse the flow characteristics of rotating cavitating water jets, the specific impact of the nozzle rotational velocity on the evolution of the whole flow field was systematically explored in this work. More specifically, a numerical simulation study of a three-nozzle rotating cavitation nozzle was carried out using the Large Eddy Simulation method of the WALE sub-lattice model. After determining 12MPa as the inlet pressure condition. The tangential velocity, vorticity, cavitation cloud development patterns, and wall pressure changes in the internal flow field at different rotational speeds were compared. From our analysis, it was demonstrated that as the nozzle rotates faster, the tangential velocity of the jet increases, leading to a deflection of the jet. The degree of deflection is positively related to the rotational speed. Under the action of shear, the vortex structure will gradually increase in size as the jet develops and finally breaks up and disintegrates. The period of cavitation cloud development and maximum volume fraction increases with increasing the rotational speed; When the rotational speed increases, the striking pressure of the rotating jet on the wall first increases and then decreases. The fitted jet curve can better reproduce the jet development pattern. The derived fitting formula allows the determination of the corresponding impact angle according to the magnitude of the rotational velocity. The existence of a higher nozzle rotation speed induces a greater curvature of the fitted curve, the deflection angle, and the impact angle. Our work provides a robust theoretical reference for the field application of the rotating nozzle cavitation water jet technology and can be used as technical support for industrial well-bore unblocking and pipeline descaling. [ABSTRACT FROM AUTHOR]

Turbulent spots are regions of turbulence surrounded by laminar flow that appear during the late stages of boundary layer transition. While turbulent spots are often studied in isolation, they usually occur near low-speed streaks and other disturbances during transition. This paper investigates the interaction between a turbulent spot and a subcritical low-speed streak using direct numerical simulations. The results, analyzed from streak instability and vorticity points of view, reveal mechanisms of the destabilization of the streak by the spot and provide insights into spot evolution in a realistic environment. Additional simulations involving intentional local control of portions of the streak provide further insight into the interaction mechanisms and potential transition mitigation strategies. [ABSTRACT FROM AUTHOR]

During the operation of a water-jet pump, cavitation generates noise and vibration, causes surface erosion of the hydraulic components, and reduces the performance of the pump. Suppressing the cavitation is beneficial for improving the stability of the energy system of the water-jet pump. In order to investigate the mechanism of cavitation suppression and optimize the cavitation performance of the water-jet pump, the unsteady cavitation flow was studied by numerical simulation and experiment in this paper. Using high-speed photography technology on a closed test platform, the cavitation flow structures in the water-jet pump were captured, and the physical process of cavitation evolution was revealed. Based on this, in order to obtain the cavitation flow characteristics closely related to the cavitation performance, the cavitation flow in the impeller tip clearance was studied by numerical simulation, the vorticity variation rate in the tip clearance was analyzed, the effects of different cavitation conditions on the vorticity in the tip clearance were revealed. Additionally, this paper analyzed the pressure pulsation characteristics of the tip clearance under different cavitation conditions, and emphatically analyzed the influence of the cavitation flow on the tip clearance pressure pulsation. [ABSTRACT FROM AUTHOR]

The dynamics of two-dimensional vortex shedding from a rotating cluster of three cylinders was investigated using Computational Fluid Dynamics (CFD) and Dynamic Mode Decomposition (DMD). The cluster was formed from three circles with equal diameters in mutual contact and allowed to rotate about an axis passing through the cluster centroid. While immersed in an incompressible fluid with Reynolds number of 100, the cluster was allowed to rotate at nondimensionalised rotation rates (Ω) between 0 and 1. The rotation rates were nondimensionalised using the free-stream velocity and the cluster characteristic diameter, the latter being equal to the diameter of the circle circumscribing the cluster. CFD simulations were performed using StarCCM+. Dynamic Mode Decomposition based on the two-dimensional vorticity field was used to decompose the field into its fundamental mode-shapes. It was then possible to relate the mode-shapes to lift and drag. Transverse and longitudinal modeshapes corresponded to lift and drag, respectively. Lift--drag polars showed a more complex pattern dependent on Ω in which the flow fields could be classified into three regimes: Ω less than 0.3, greater than 0.5, and between 0.3 and 0.5. In general, the polars formed open curves in contrast to those of static cylinders, which were closed. However, some cases, such as Ω = 0.01, 0.22, and 0.28, formed closed curves. Whether a lift-drag polar was closed or open was deduced to be determined by the ratio of Strouhal numbers calculated using lift and drag time series, with closed curves forming when the ratio is an integer. [ABSTRACT FROM AUTHOR]

The flow phenomena around a rotating elliptic cylinder placed in a narrow channel is studied numerically. The walls of the channel act as a confinement that limits the flow in the transverse direction. The confinement ratio (β) ⁠, nondimensional rotation rate (α) ⁠, and the Reynolds number (Re) span across multiple values. A parametric study is done to identify the variations in drag-coefficient (CD) ⁠, lift-coefficient (CL) ⁠, and moment coefficient (CM) with changes in β, α ⁠, and Re. Near-field and far-field vorticity contours are discussed in detail. fast Fourier transform (FFT) of the time-periodic lift signals are presented to understand the shedding-frequency characteristics. Furthermore, CM values are analyzed for possible cases of autorotation. It is observed that confinement acts to delay the shedding of vortices. However, a complete suppression is not obtained even for the maximum value of β ⁠. This is likely because of the sharp flow separation at the edges of the cylinder, which tends to promote the formation of a vortex. Hovering vortices are observed for α>1 ⁠, and a special case is identified for which the hovering vortex never dissipates. [ABSTRACT FROM AUTHOR]

Secondary flow is a prominent feature of channel bends; it alters the streamwise velocity and bed shear stress distributions. Experiments were conducted to investigate the complex pattern of secondary flow in a narrow and sharp openchannel bend and the underlying mechanism of generation of multiple circulation cells. Compared with the moderate bends, the sharp bends are characteristic of multiple circulation cells from the 90° section. In addition to the curvature-induced circulation cell (S1) and turbulence-induced counterrotation circulation cell (C1) near the outer bank, another circulation cell (S2) was observed near the inner bank and was attributed to flow separation. A termby-term analysis of the vorticity equations indicates that the centrifugal term favours S1 and C1 while opposing S2. The turbulence-related term accounts for the formation of C1 and S2. The advective transport term redistributes vorticity and maintains the existence of S2. The dependence of secondary flow structure on Reynolds number and aspect ratio was also explored. With an increase in the Reynolds number from 23000 to 37000, both the strength and size of C1 are reduced by 50%, whereas the size of S2 increases by 20%, and its strength slightly decreases. With a decrease in the aspect ratio from 3.3 to 2, the strengths of S1, S2, and C1 are doubled, and the sizes of C1 and S2 increase by 90% and 20%, respectively. [ABSTRACT FROM AUTHOR]

• Broadcast spawning can be explained using Lagrangian particle-tracking algorithm. • The fertilization rate reaches the maximum at 0.1 m/s of flow velocity. • Fertilization actively occurs at the recirculation zone behind the sea urchin body. • When gametes are aggregated by vortex motion, possibilities of fertilization rise. Broadcasted fertilization for the reproduction of invertebrates is accomplished through a complicated interaction between spawned gametes and the surrounding flows. Most gametes encounter each other in the vicinity of the sea urchin body where unique flow structures develop, and so analysis of local flow characteristics allows us to better understand the effect of flow on the fertilization process. This study applied a Lagrangian framework based on computational fluid dynamics to estimate the fertilization rate of eggs in a range of flow velocities (0.025–0.2 m/s) and the fertilization rate was the highest at U = 0.1 m/s, which is an intermediate flow speed. Among the four classified sub-zones, such as the aboral surface, wake, substrate, and water column, fertilization occurred most frequently in the wake and substrate regions. The relationship between fertilization rate and flow structures was investigated using three parameters: 1) standardized Morisita index to quantify the pattern of gamete distribution, 2) length of the recirculation zone to specify the areas where the eggs are most frequently fertilized, and 3) integral scale to estimate the dimension of vortex structures downstream. The results of this study show that the fertilization rate is higher inside the recirculation zone, especially when strong vortex structures are observed because the flow provides a favorable condition for the gametes to aggregate and collide with each other. [ABSTRACT FROM AUTHOR]

The goal of this study is to interpret the cross-sectional increases of rinnenkarren systems with the use of analytical model and CFD simulation. In rinnenkarren, water accumulation from the catchment was approximated using an analytical method based on field data. The length of eddies appearing at tributary junctions was studied by CFD in model channels. The results of the analytical and numerical models were compared against morphometrical parameters of rinnenkarren surveyed in the Totes Gebirge (Austria). It is found that there is a relationship between catchment size and channel development. Along small catchments, channel development is random. However, channel development along large catchments is controlled by water concentration. Decrease in the slope angle of the catchment results in an increase in the volume of water entering the channel and development of tributary channels. When water inflow is not concentrated in a single place, several smaller tributary channels emerge. When it is concentrated, only one large-sized and long tributary develops. At the junctions of large tributaries significant vorticity was identified in the CFD models. In addition to the previous model studies, the similarity between the lengths of the simulated vorticity sections and the local field hollowings was revealed. [ABSTRACT FROM AUTHOR]

In this paper, thermal transfer with natural convection in a tilted annular cylinder with a Cu-water nanofluid has been numerically studied. The hot interior and cold exterior elliptical surfaces of the enclosure were maintained at constant temperatures h T and c T, respectively. The governing equations were solved by the stream functionvorticity approach. The finite volume approach was utilized to discretise the controlling equations. The volume fraction range of the nanoparticles and the Rayleigh number was as follows: 0 < φ < 0.08 and 104 < Ra <106, respectively. The inclination angles were γ = 30°, 45° and 60°. Results were given as isotherm contours, streamlines, average and local Nusselt numbers. The results indicate that the thermal transfer ratio increases with an increase in the tilt angle, regardless of the nanoparticle size values. and the impact of the inclination angle on the heating transfer rate is more important the higher the Rayleigh number and the more convection there is. [ABSTRACT FROM AUTHOR]

The thermal performance of alumina-copper/water hybrid nanofluid in buoyancy-driven heat transfer of a U-shaped cavity with a heated wavy wall is investigated in detail throughout this manuscript. A three-node triangular finite element method is used to solve the system by considering the Galerkin weighted residual algorithm. A Newton–Raphson algorithm with a damping coefficient is used as the convergence criterion. Numerical and experimental comparisons of previously published results are compared with the present calculations to ensure confidence in the present modeling. The physical representation of the modeling is presented through the streamlines, isotherms, and vorticity distribution. To quantify the overall heat transfer performance, the average and local Nusselt numbers are used for various combinations of parameters. It is found that the higher the cold rib dimension, the undulations, and the amplitudes of the heated corrugated walls produce a higher heat transfer rate. The inclusion of a hybrid nanofluid may inhibit the heat transfer rate when the length of the hot wall exceeds the length of the cold wall. As low as 4 % and up to 16 % thermal performance increase of utilizing hybrid nanofluid is observed compared to pure water in a wavy U-shaped enclosure. [ABSTRACT FROM AUTHOR]

Cavitation inception in tip leakage flows remains a challenging topic in the engineering field, as the effect of tip gap width on inception is unclear. The present study is devoted to an analysis of the effect of gap width on tip leakage cavitation inception by using full-wetted numerical simulations. Numerical results show that the cavitation inception number is strongly related to the dimensionless gap width t, which is defined as the ratio of tip gap width to the maximum hydrofoil thickness, and the reason behind it is explained by the specific flow structures. The cavitation inception number of suction side (SS) sheet cavitation decreases gradually with the increase of dimensionless gap. The cavitation inception numbers of tip leakage vortex (TLV) and tip separation vortex (TSV) increase first and then decrease with the increase of the gap, reaching the maximum at t=0.2 and t=0.3 respectively. The main reason is that in the gap range of 0.2-0.3, TLV and TSV cores have the highest vorticity and the lowest pressure. [ABSTRACT FROM AUTHOR]

This paper is presented to compare various wavy leading-edge protuberances on oscillating hydrofoil performance and power efficiency. The unsteady turbulent 3D flow simulations were carried out by using the StarCCM+ software. The 3D hydrofoil with a straight at the leading-edge is a NACA0015 section with a chord of 0.24 m and an aspect ratio of 7. Four new types of hydrofoils are proposed with wavy leading-edge protuberance. The RANS equations with the realizable k-e turbulence model are used to predict the turbulent flow around the hydrofoil under different conditions. In order to validate, a comparison of the numerical results of the forces coefficients and power efficiency of non-protuberance oscillating hydrofoil are shown in good agreement with experimental data. Then, four new profiles on the leading-edge of the hydrofoils are simulated and many results of the pressure distribution, vorticity contour, streamline velocity, and power coefficient for five hydrofoil types are presented and discussed. It is concluded that the hydrofoil of Type-M can achieve constantly higher efficiency of over 46% by employing appropriate heave and pitch amplitudes. [ABSTRACT FROM AUTHOR]

The lift force of a rectangular semiwing in stationary ground effect was computed based on the spanwise circulation distribution inferred from wake crossflow measurement and was compared with the direct force balance data at Re =2.48 ×105. The lift calculated was found to be in good agreement with the force-balance measurement for ground distances h larger than 10% of the airfoil chord c. In close ground proximity (h/c = 10%), a large discrepancy however existed due to the appearance of a multiple vortex system consisting of a tip vortex, a corotating ground vortex, and a counter-rotating secondary vortex. The counter-rotating secondary vortex offset the vorticity of the tip vortex. By considering its contribution additive, the discrepancy observed between the lift forces computed and measured with the force balance in close ground proximity became marginal. The impact of ground effect on lift-induced drag was also discussed. [ABSTRACT FROM AUTHOR]

The vortex-induced vibration (VIV) performance and aerodynamic mechanism of VIV control of a split three-box girder bridge were investigated numerically by Computational Fluid Dynamics (CFD). Large-scale section model wind tunnel tests show that the integrated countermeasure combined by 30% ventilation ratio grids and new scheme for maintenance tracks can effectively restrain vertical and torsional VIVs. The reliability of numerical method is verified by comparing static three-component coefficients and Strouhal numbers obtained from CFD and wind tunnel tests. The flow structure and changes of aerodynamic forces were analyzed via delayed detached-eddy simulation (DDES). For the original section, large-scale vortices form with shedding behind the safety barrier of the upstream box and behind the maintenance tracks, as well above the downstream highway box, introducing periodic pulsating lifts and moments that result in severe VIVs. The grids can effectively reduce the size of vortices in the gaps, prevent the flow from fluctuating and reduce the pressure difference between upper and lower surfaces. The torsional VIVs at non-negative attack angles are not controlled because the grids cannot interfere with the vortices behind the maintenance tracks. The new scheme for maintenance tracks, which has grid-like effects, can eliminate the torsional VIVs at non-negative attack angles by preventing the vortices behind maintenance tracks from impacting downstream boxes. The aerodynamic force components are analyzed to find that box 1 mainly contributes the mean values and box 3 mainly contributes the fluctuating values. The integrated countermeasure can reduce the mean and RMS values of aerodynamic forces to varying degrees. The streamlines and RMS pressure contours indicate that the integrated countermeasure effectively reduced the wake height and vortices motion range, which is conducive to VIV control. This work lays a solid foundation for an in-depth understanding of the aerodynamic characteristics and optimization mechanism of split three-box girder bridges. [ABSTRACT FROM AUTHOR]

In this note, a longstanding indeterminacy in the solution of self-similar planar mixing layers is resolved. It is shown that the boundary condition that offers the largest set of self-similar solutions is the specification of the vertical velocity at infinity, which also quantifies the value of the stream function and the net inflow/outflow there. This is referred to as the entrainment boundary condition. Other conditions that have been suggested to close the problem yield a subset of solutions and thus represent a loss of generality. The entrainment boundary condition determines the spatial growth rate and the spreading parameter for a given mixing layer, which were previously thought to be determined through experiment and empirical matching to a linearized version of the problem. These statements are validated using existing experimental data. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper aims to summarize publications related to new generation of tube geommetry, thermal performance, and flow characteristics in different cross-sectional manners, and the effect of crucial parameters on thermal–hydraulic performance. An alternating cross-sectional tube has an enhanced vorticity without installing vortex generator devices. Multi-longitudinal vortices are induced along the length of the tube: this makes a significant enhancement in the thermal performance of the tube. Many published papers have focused on the heat transfer and flow characteristics of the alternating cross-section tubes in various configurations in the past two decades.Although the cross-sections had different shapes, the flow behaviors and vortex generation were similar. The geometric parameters, like aspect ratio, pitch length, transition length, alternating angle, phase-shifting, and the total length, influenced the thermal–hydraulic performance of the alternating cross-section tube. The flow characteristics in an annular region differed between the alternating cross-section tube as internal and external tubes. Moreover, some researchers proposed the Nu and f correlations for the practical design of heat exchangers. Finally, future research works and improvement guidelines for alternating cross-section tubes are revealed. [ABSTRACT FROM AUTHOR]

This survey covers the mathematical theory of steady water waves with an emphasis on topics that are at the forefront of current research. These areas include: variational characterizations of traveling water waves; analytical and numerical studies of periodic waves with critical layers that may overhang; existence, nonexistence, and qualitative theory of solitary waves and fronts; traveling waves with localized vorticity or density stratification; and waves in three dimensions. [ABSTRACT FROM AUTHOR]

Stall is a complex flow phenomenon in centrifugal pumps at part load conditions. However, there is no clear description of the stall evolution process and mechanism, which is critical for stall control. Based on a highfrequency Particle Image Velocimetry (PIV) system (10k Hz) and a non-refraction experimental bench, emphasis is laid on the flow structures near the initial stall conditions. The results show that as the flow rate decreased, the flow separation occurred at the middle of the blade suction side and then evolved into a stall vortex which moved to the impeller's inlet direction and kept growing. Subsequently, it broken into two vortexes when reaching the location where the impeller cross-sectional area is the smallest. One stall vortex continued its motion toward the passage inlet direction, while the other vortex separates to the impeller outlet. As the stall vortex's size at the impeller inlet enlarged, the flow incident direction at the impeller inlet was directed to the blade suction side, which caused the stall vortex on the suction side to disappear. The stall mechanism is explained in detail using both experiments and numerical simulations. Meanwhile, the Scale Adaptive Simulation-Shear Stress Transport (SAS-SST) hybrid model is used to simulate several flow rate conditions near the stall initial stage. The findings indicate that the increasing adverse pressure gradient and the high-pressure zone move along the suction blade towards the impeller inlet as the flow rate is reduced; however, the relative velocity is constantly decreasing. When the fluid can't provide enough kinetic energy to maintain its continuous flow along the suction surface, flow separation occurs. The stall vortex, which results from flow separation, blocks the passage impeller. The increasing adverse pressure from the impeller outlet to the inlet is the main cause of flow separation; and the adverse pressure gradient is a major manifestation of the stall vortex. [ABSTRACT FROM AUTHOR]

The intake port of a CA4DD diesel engine was investigated by a computational fluid dynamics (CFD) numerical simulation to enhance the intake swirl and improve the flow characteristics in the engine. The uniform design method was applied to study the influence of guide vanes with different parameters on the intake process at various valve lifts. Nine guide vane models were established and compared to the base model using CATIA™ 3D CAD software. Numerical simulations were conduct with Xflow software based on the lattice Boltzmann method (LBM). The distributions of the streamlines, vorticity, velocity and turbulent intensity in each cylinder were simulated and analyzed. The results show that the influence of the guide vanes on the swirl ratio was greater than 37%, and the flow coefficient was less than 5% compared to the base model. Scheme 5, H7.5-L50-θ20 (guide vane height of 7.5 mm, length of 50 mm and angle of 20°), provided good performance. The flow characteristics of the optimal guide vane model were verified through a steady flow test. When the guide vane aspect ratio and angle were within the ranges of 3.5-6.9 and 12.2°-20.2°, respectively, the swirl ratio had the best effect at maximum valve lift. This study provides a theoretical basis for improving the performance of dual-intake diesel engines. [ABSTRACT FROM AUTHOR]

Large-scale turbulent flow features in liquid (water) and gas (air) phases in the film region of two-phase slug flow are identified by using proper orthogonal decomposition (POD) of the vorticity fields. Linear combination of POD vorticity modes is used for the qualitative visualization of coherent structures. The vorticity fields are computed from the instantaneous two-dimensional velocity fields measured using a combined particle image velocimetry and laser-induced fluorescence technique (PIV-LIF). Vorticity modes are calculated and compared with the curl of POD velocity mode. POD analysis revealed the presence of dominant vortical structures embedded in both liquid and the gas phases. It is also observed that the gas phase revealed more eddies than the liquid phase. The proportion of enstrophy is higher in the gas phase as first POD vorticity mode contained 7.5% of the total enstrophy, while for the liquid phase; the first mode captured 6.8%. Linear combination of vorticity modes provided effective qualitative information of the coherent structures in both phases. POD-vorticity modes when compared with POD-velocity modes revealed few similarities among the pair of identified vortical structures. Based on the results, it is concluded that POD vorticity revealed hidden flow features of both phases of slug flow, which eventually provides in-depth and comprehensive description of this complex slug flow phenomenon. [ABSTRACT FROM AUTHOR]

The flow resulting from the rotation of a series of thin plates that initially separate two gases of different densities is analyzed using direct numerical simulations (DNSs). The 90-deg plates' rotation forms a vorticity shear layer and a density interface in between the tips of two neighboring plates. Results of this study show that the shape of these layers strongly depends on the plate's tip-based Reynolds number that can be varied thanks to a parametrization of the plates' opening law. Different regimes are identified corresponding to single- or multimode initial interfaces, with or without the occurrence of starting vortices during the formation of the shear layer. The density interfaces resulting from this procedure are particularly well-suited to serve as initial conditions for the study of the Richtmyer-Meshkov (RM) instability-induced mixing. Results of this study also provide a description of vortex formation in stratified flows. [ABSTRACT FROM AUTHOR]

In this paper, an analytical investigation and 3D numerical simulation are presented for the breakup of floating non-Newtonian droplets in a non-Newtonian fluid. The considered geometry is a T-junction with unequal-width branches that can generate droplets with un-equal size. There is a very good agreement between the analytical solution and numerical simulation results obtained in this research. Various quantities such as branches flow rate ratio, branches velocity ratio, droplet’s length in each branch, the whole length of the droplet, vorticity and pressure have been investigated during the breakup process in this study. The results showed that the branches flow rate ratio and the branches velocity ratio were constant during the breakup process. It was also observed that the length of the droplet in each of the branches and the whole length of the droplet increased linearly during the breakup process. Also, the vorticity has its maximum at the breakup moment. [ABSTRACT FROM AUTHOR]

This study proposes and investigates the impact of a modification, accounting for the influence of vortices and flow properties on the liquid rupture, to improve the modeling of mass transfer rate in cavitation. The threshold phase-change pressure is calculated by the fluid-saturated pressure at rest and the added vortex pressure term. The explicit simulation of the fully turbulent, homogeneous compressible, cavitating flow around the NACA0015 hydrofoil and the hemispherical body is performed. Saito cavitation model and Wilcox k-ω turbulence model are implemented for the evaluation of the proposed modification. The pressure coefficient distribution -Cp and cavitation behavior, including the vapor formation-collapse processes and the flow mechanism, are investigated. The analysis shows that the present modification, coupled the local flow viscosity with the vorticity magnitude, making the cavitation model better sensitive to the flow condition. The modification has a weak impact on the steady sheet cavitation around a hemispherical body but is the key factor underlying the improvement in the predicted complex flow around the NACA0015 hydrofoil. In that, the predicted -Cp and cavity structure around the hydrofoil is improved in comparison with the existing numerical data by other research groups and that by the Singhal turbulent pressure fluctuation model. [ABSTRACT FROM AUTHOR]

To assess unsteady vortex interaction between rim seal purge flow and upstream stator, numerical investigations were conducted under different purge flow rates. The vortex distributions for the stator and cavity were investigated and the interaction processes near the cavity exit, in particular the vorticity change resulting from the ingress and egress, were analyzed. Results show the intensity of hub passage vortex (HPV) and hub trailing shedding vortex (HTSV) at stator exit is decreased as a consequence of enhancing blockage effects caused by the egress flow. However, when the purge flow rate increases, from stator exit to downstream of cavity exit, the reduction in the intensity of two vortices is weakened as the extrusion of egress flow thins their vortex tubes. The vortex inside the cavity is generated as the combined effect of relative rotation of cavity walls and nonuniform circumferential pressure mainly imposed by upstream stator. The ingress leads the positive axial vorticity near the stator hub to ingest into the cavity and eject into the main passage due to the blockage of purge flow. Furthermore, the interaction between the ingress of the mainstream and purge flow produces local negative axial vorticity. The egress flow carries negative axial vorticity mainly originated from the rotational cavity wall, and enters into the main flow passage near the rotating hub, then locations of HPV and HTSV move to the mid-span slightly with the extrusion of egress flow. [ABSTRACT FROM AUTHOR]

The possibility of suppressing laminar vortex shedding after a rectangular cylinder with a pair of rotating controllers has been investigated for Re=150. Drag and lift coefficients of the rectangular cylinder have been measured at different positions of the controller system. Numerical results show that the controller system suppresses the vortex shedding completely when installing at some suitable positions. It was found that the controller system affects the vorticity intensity in the separation bubbles at these suitable positions. The controller system may completely eliminate the separation bubble or may make them remain in attached to the cylinder. Besides, the effectiveness sensitivity of the controller system to its rotation rate and Reynolds number was analyzed when installing at the edge of the cylinder boundary layer where its position is denoted by (r/a=2, ϑ=40°). These analyses show that the controller system can still suppress the laminar vortex shedding completely when the rotation rate of the controller system and Reynolds number of the incoming flow change by less than 50%. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The blade vortex evaluation in Francis Turbine under deep part load conditions generates severe pressure fluctuations in the runner. The complex flow in a model turbine is numerically investigated based on a modified Partially Averaged Navier-Stokes method. The main emphasis is focused on revealing the correlation mechanism of blade vortex evolution and energy production. The results indicate that the modified PANS method shows significant advantages in hydro turbine’s simulation than the traditional RANS method. At deep part load conditions, the vorticity formed at the leading edge of the suction surface and the trailing edge of the pressure surface in the blade channels. The stretching term provides the most vorticity increments while the dilation term inhibiting part which only provides a decrement of the vorticity evolution. Based on the entropy production theory, the total entropy production distribution is consisting with the distribution of vorticity. At deep part load condition, direct dissipation and turbulent dissipation provide the most entropy, while at part load condition the proportion of these two-part decreased. [ABSTRACT FROM AUTHOR]

The evolution of single and twin oblique particle clouds in stagnant water was investigated using a series of laboratory experiments and the effects of controlling parameters such as sand mass and nozzle spacing were studied. The time variations of particle cloud properties such as frontal position, horizontal and vertical centroids, cloud width, and frontal velocity were measured using image analysis and particle image velocimetry (PIV) techniques. The entrainment coefficients were extracted from the measurements. It was found that the main vortex motion of the frontal heads altered after the collision and a new integrated frontal head was formed. The effects of release angle and particle interactions were studied by comparing the time histories of maximum centerline velocities. It was found that the centerline velocity of twin oblique particle clouds in comparison with twin vertical particle clouds increased with increasing nozzle spacing. The time history of the ratio of horizontal to vertical centroids in oblique particle clouds determined the potential location of sand particles and a practical model was developed to determine the size and location of particle clouds with time. The time histories of normalized cloud width indicated a significant change after the frontal head collision. The particle interactions due to frontal head collision in twin oblique particle clouds significantly increased the cloud width until particle clouds reached the swarm phase. The time at which twin oblique particle clouds reached the swarm phase was recorded and a linear model was proposed to link the time to reach the swarm phase with the cloud aspect ratio and nozzle spacing. [ABSTRACT FROM AUTHOR]

With the increasing of the bypass ratio of modern aero-engines, the problem of wing-engine interference is more prominent, especially for the layout design of engine nacelle. The reverse thrust cascade is widely used in turbofan engines with high bypass ratio. In order to meet the requirements of the integrated analysis of airframe, wing, engine nacelle and hanger, the aerodynamic characteristics of aircraft/engine integration configuration with reverse cascade is numerically studied via CFD (Computational Fluid Dynamics) method. The streamline distribution, iso-surface of total temperature, vorticity distribution and total reverse thrust efficiency on different engine nacelle layouts are compared and analyzed in details. The results show that the lift coefficient of the wing decreases by 21.2% and 45.02% respectively as the engine moves forwards horizontally by 11%L and 21.2%L. The lift coefficient of the wing decreases by 2.4% and 4.82% respectively as the engine moves subsidence by -3.5%L and 3.5%L. The influential region of the reverser airflow in the radial and circumferential direction without airframe interference is significantly larger than that in the case with aircraft/engine integration. The reverser flow is susceptible to be interfered by the adjacent fuselage and wing sections, and the development of reverse thrust flow is significantly limited. Compared to the baseline nacelle location, the reverse thrust performs badly as engine nacelle moves backward and lateral horizontally. The total reverse thrust efficiency decreases gradually as the engine nacelle moves forward horizontally. [ABSTRACT FROM AUTHOR]

The objective of this work is to investigate the effect of the bionic microstructure surface on flow field structure of the slab. The motivation behind this study is to investigate the effect of the bionic microstructure parameters including the height and intersection angle of microstructure in order to improve the drag reduction characters. The numerical simulation is performed on the bionic microstructure model of the V-shaped and serrated bionic microstructures using the RNG k-ε model. The drag reduction rates of two bionic microstructures under different dimensionless sizes are ob13.79 tained. The drag reduction efficiency is up to 8.76% when the dimensionless height of microstructure h+ is at and the intersection angle 40 = for V-shaped microstructure. In addition, combined with wall temperature control of drag reduction technology, the influence of wall temperature on the drag reduction effect is also analyzed. Compared with the flow field structure of the surface boundary layer of the smooth plate, the wall microstructure divides the surface boundary layer into two parts: the bottom and the tip. The average velocity profile is moved up and the thickness of the linear bottom layer is increased. A large number of "quiet" fluids are gathered at the bottom of the surface boundary layer. In addition, the existence of wall microstructure can weaken the momentum exchange in the boundary layer and restrain the spreading vortex motion of the fluid in the near-wall region. The "secondary vortex pair" on both sides of the tip of the microstructures can effectively limit the lateral pulsation of fluid So as to achieve a good drag reduction effect. [ABSTRACT FROM AUTHOR]

The present study aims to describe flow structures and cavitation phenomenon in the submerged waterjet. A non-intrusive experimental work was performed. The waterjet was produced through a nozzle characterized by a short straight segment adjacent to the nozzle outlet. Waterjet pressures were varied from 5 to 22 MPa. The time-resolved particle image velocimetry (TR-PIV) was used to measure velocity distributions. The proper orthogonal decomposition (POD) method was employed to extract flow structures from the flowmeasurement results. Cavitation was created through increasing the waterjet pressure. A comparison of cavitation patterns at different waterjet pressures was implemented. Similarity of the distribution of average velocity is revealed as the waterjet pressure varies. The POD results indicate that two high-vorticity bands close to the nozzle, symmetrically distributed with respect to the nozzle axis, dominate the waterjet stream. Further downstream, small-scale flow structures are sparsely distributed and assume a low percentage of the total energy. Initial cavitation is featured by small-scale cavities which are formed near the high-vorticity zone. As the waterjet pressure increases, the volume fraction of cavitation increases and morphological features of cavitation change significantly as waterjet develops. At a later stage, stable cavity clouds are evidenced. A high relevance between vorticity distribution and cavitation cloud pattern is demonstrated. [ABSTRACT FROM AUTHOR]

Based on the active control theory, the synthetic jet behind the blunt body is explored by considering the control of flow around the blunt body in this paper. The Partially Averaged Navier-Stokes model was carried out for flow around circle cylinder at two subcritical Reynolds numbers (Re=1000, 3900), whose results have a good agreement with the experimental data. The results indicate that synthetic jet behind the circle cylinder has essential effects on the vortex shedding of flow around circle cylinder. The analysis of the Vorticity and Q vortex shows that the increasing velocity of synthetic jet has a strong effect on the vortex shedding of the original flow field. It is noted that the information including the coherence data and the directivity pattern with the existence of synthetic jet is different from that without synthetic jet. These results imply that the synthetic jet in the tail of the blunt body could control the flow fields around the blunt body. [ABSTRACT FROM AUTHOR]

As a specific radial flow pump, the regenerative flow pump (RFP) usually has a low efficiency. In this study, in order to explore the matching mechanism, three cases with various matching relations were investigated by the methods of theoretical calculation, computational fluids dynamics (CFD) simulation, and experiment test. The results illustrate that the theoretical prediction, numerical simulation, and experimental data are in good agreement. Furthermore, when the matching relation expressed by a ratio of the channel's and blade's radial length is equal to 1, the geometrical profiles of RFP can well guide the circulation flow into the channel at large radii and into the impeller at small radii, forming intense longitudinal vortex. The steady, strong exchange flow is characterized by the inflow and outflow regions approximately half of the isosurface. The axial vortex motion without apparent flow separation and irregular flow is observed in the impeller, a low velocity annulus exists in the medium radii of the impeller without other distinct velocity clouds, and a low velocity strip and a high velocity annulus in the channel are, respectively, performed along the blade's pressure surface and the channel's outer radii. All of this corresponds to the best pump's performance and the largest efficiency of the impeller and channel. This work promotes a systematical understanding of the matching mechanism between impeller and flow channel in the RFP and could provide some reference for the design and performance optimization for RFP. [ABSTRACT FROM AUTHOR]

The phenomenon of horn-like vortex in stator corner separation flow in an axial flow pump was first reported by Wang et al. (2020, "Investigation on the Horn-Like Vortices in Stator Corner Separation Flow in an Axial Flow Pump," ASME J. Fluids Eng., 142(7), p. 071208), and the associated external features were preliminarily presented. However, internal vortical characteristics of horn-like vortices, including the distributions of swirling strength, the deformation mechanism of vortex tube and the correlation with pressure fluctuation surge, are not revealed. In this paper, the newly developed vorticity decomposition approach is introduced, and thus more novel quantitative results are provided for the physics of horn-like vortex evolution in an axial flow pump. First, the distributions of absolute swirling strength, relative swirling strength and Liutex spectrum are presented to outline the vortical features of the horn-like vortex fields. Second, the deformation mechanism of the horn-like vortex tube is revealed. It is found that the horn-like vortex spatial evolution can be described by the deformation terms (Liutex stretching term, Liutex dilatation term, and curl term of the pseudo-Lamb vector) controlling the Liutex transport process. These terms constantly act on the horn-like vortex tube in an almost independent way, causing its continuous deformations in the transit process. Third, the quantitative correlation between horn-like vortex transit and pressure fluctuation surge is given. It is proved that periodic vortex transit can cause severe pressure fluctuation that is much larger than that induced by rotor-stator interaction. From multiple perspectives, a clearer evolution process of the horn-like vortex is outlined, which is conducive to controlling the corner separation flows and improving the stability of large-capacity and low-head pumping stations. [ABSTRACT FROM AUTHOR]

Finite element solution for the classical problem of swirl flow in a cylinder with a rotating lid has been used to study the characteristic features of the stream-tube and identify the factors contributing to axial vortex breakdown. An increase of rotational Reynolds number has been found to result in (i) a decrease of total flow rate; (ii) an increase of flow rate through the boundary layer over the stationary walls; (iii) an increase of the throat area of the stream-tube, with the upstream axial vortex flow in some cases having a deficit in momentum flux needed to overcome the pressure and viscous forces; and (iv) an increase of distance for the axial flow to sustain deceleration in the diverging passage. Based on the analysis, it is hypothesized that "flow with particles in axial vortex motion having a deficit of momentum flux for axial flow when subjecting to a fluctuating radial force undergoes axial vortex breakdown." This explanation has been able to justify the disappearance of vortex breakdown at larger Re of laminar regime and the absence of vortex breakdown in small aspect ratio cylinders. We report novel results pertaining to total flow rate and its distribution within the vessel. The momentum flux of axial vortex, a main determinant of bubble breakdown, is found to be governed by the total flow rate, distribution of flow through the boundary layers, and the Reynolds number. The proposed hypothesis has been verified by analyzing two cases, one involving a passive and the other involving an active mechanism for regulating the axial momentum. [ABSTRACT FROM AUTHOR]

To obtain a better insight into the unsteady flow behavior in side channel pumps by a robust vortex identification method, this study presents the efficacy of the new Ω-criterion in characterizing the evolution of vortex structures in the turbulent flows under different time steps. The flow behavior and the underlying vorticity dynamics were revealed as well. Compared to Q-criterion, the new Ω-criterion identified all vortex structures irrespective of the intensity at a universal threshold of 0.52. Three different types of vortex structures (longitudinal, axial, and radial) were identified to be responsible for the turbulent flows in the side channel pumps. The beneficial longitudinal vortex promotes the momentum exchange flow between the impeller and side channel which leads to the high hydraulic head of side channel pumps. On the other hand, the unfavorable axial and radial vortex structures restricted in the impeller passage mitigate the exchange process accounting for the low efficiency of the pumps. From this study, it can be established that the evolution of the axial vortex structures is responsible for the largest vortex distribution in the impeller compared to the total vortex evolved. The impeller outer radius contributes about 60% of the unfavorable axial structures evolved. Using the new Ω-criterion, many reported anomalous findings have been explained. [ABSTRACT FROM AUTHOR]

The receptivity of the Blasius boundary layer over a semi-infinite flat plate with an elliptic leading edge and of aspect ratio five was investigated using a direct numerical solution of two-dimensional Navier-Stokes equations. The result of the computation where the slip condition is applied to the fluctuating component of velocity at the wall surface is compared with that of an ordinary computation using a nonslip condition. Another numerical experiment is performed where no vorticity fluctuation is supplied from a freestream while prerecorded values of vorticities at the wall in response to the passage of convecting fluctuations are used as the wall vorticity boundary condition. It is shown that vorticity fluctuations in the boundary layer can be classified according to their wavelengths. Waves with longer wavelengths originate from the freestream, whereas waves with shorter wavelengths close to T-S waves originate from the surface of the plate. In another numerical experiment, the slip boundary condition against the fluctuation component of vorticity is applied to the limited area of the wall surface. The aim of the study is to determine the part of the elliptic leading edge or flat plate that induces vorticity fluctuations, thereby resulting in the creation of T-S waves. The numerical results show that the contribution of vorticity fluctuations originating from the juncture is the most crucial, whereas the vorticities supplied in the elliptic leading-edge surface negatively affect the amplitude of vorticity fluctuations inside the boundary layer. And, the stagnation section did not show positive contribution. [ABSTRACT FROM AUTHOR]

A large eddy simulation study was conducted to investigate tire turbulent dynamic structure of a fluid flow in two staggered tube configurations, one is composed of all circular cylinders and the second is composed of circular and square cylinders. The present LES, based on the wall-adapting local eddy viscosity model, has been conducted for Rcn= 12,858, which match available experiments. The appropriate grid has been selected using the grid convergence index method so that the wall-normal coordinate y+ value is relevant for walls. Streamlines, turbulence kinetic energy contours, instantaneous vorticity contours computed indicate that wake patterns are more chaotic. In addition, flow coherent eddies within both configurations are identified via the Q-criterion. Based on the obtained findings, it can be stated that the model considered, in addition to being physically sound, demonstrated to be suitable for simulating the turbulent flow over circular and mixed staggered tube bundles with higher resolution. [ABSTRACT FROM AUTHOR]

This paper presents experimental and numerical investigations on the modification of local spanwise skinfriction over triangular riblets under the total drag reduction condition. Specifically, the mean and fluctuating vortical flow fields were measured using 2-components X-wires and computed using LES, respectively. Besides, the relationship between local skin-friction along the riblet spanwise and associated vortex evolution was also built using the vortex dynamic method. Based on these results, it was found that, compared with the smooth case, the impaired and enhanced vortex strength, and resultant viscous diffusion/energy dissipation, determined the reduction and augment of the viscous drag force over the local spanwise riblet groove, i.e., decreasing and increasing cases of local drag, respectively. Furthermore, the mean normal diffusion fluxes of normal and spanwise vorticities contributed more to the viscous drag under these two cases. Correspondingly, the relevant flow physics related to these phenomena was discussed in detail. [ABSTRACT FROM AUTHOR]

This paper presents experimental and numerical investigations on the modification of local spanwise skin-friction over triangular riblets under the total drag reduction condition. Specifically, the mean and fluctuating vortical flow fields were measured using 2-components X-wires and computed using LES, respectively. Besides, the relationship between local skin-friction along the riblet spanwise and associated vortex evolution was also built using the vortex dynamic method. Based on these results, it was found that, compared with the smooth case, the impaired and enhanced vortex strength, and resultant viscous diffusion/energy dissipation, determined the reduction and augment of the viscous drag force over the local spanwise riblet groove, i.e., decreasing and increasing cases of local drag, respectively. Furthermore, the mean normal diffusion fluxes of normal and spanwise vorticities contributed more to the viscous drag under these two cases. Correspondingly, the relevant flow physics related to these phenomena was discussed in detail. [ABSTRACT FROM AUTHOR]

Background: Endoscopic medial maxillectomy (EMM) is a workhorse for multiple sinonasal conditions. To reduce its burden on the sinonasal physiology, several modified EMM (M-EMM) have been proposed. Objective: In order to provide a theoretical basis for EMM and its modifications, this study introduces a computational fluid dynamics (CFD) model, based on a time-resolved direct numerical simulation, describing EMM and assessing the role of the M-EMM in preserving the overall fluid dynamics of the sinonasal cavities. Methods: A normal sinonasal CT scan was converted into a geometrical model and used as a reference; 2 anatomies were then created by virtual surgery, mimicking EMM and M-EMM, with the latter sparing the anterior portion of inferior turbinate and medial maxillary sinus wall. The airflow was simulated in the models via the OpenFOAM CFD software and compared in terms of flow rate, mean and fluctuating velocity, vorticity, and turbulent structures. Results: The analysis shows that EMM induces a massive flow rate increase in the operated side, which becomes less obvious in the M-EMM model. In contrast to M-EMM, EMM induces higher velocity fields that reach the maxillary sinus. Velocity and vorticity fluctuations are negligible in the baseline model, but become increasingly evident and widespread in the M-EMM and EMM models. Conclusions: A significant disruption of the nasal fluid dynamics is observed in EMM, while M-EMM minimizes variations and reduces interference with nasal air conditioning. Our analysis provides insights into the pathophysiology of radical sinus surgery and provides a theoretical basis for the ability of M-EMM to reduce the temporary surgery-related changes on both healthy and operated sides. [ABSTRACT FROM AUTHOR]

In this paper, a numerical study of a swirling flame generated through the interaction between a central fire and its surrounding fires is performed. A swirling flame can be configured by installing a secondary's sources surrounding the central source, organized in an asymmetrical manner in order to ensure circumferential entrainment of the central flame by the supply puffs air. An analysis is performed to study this kind of flow. This analysis highlights the different zones that characterize the vertical propagation of a swirling flame; a first zone, close to the fire source, characterized by an acceleration of the flow and an increase of the temperature. A second zone marked by the passage of the temperature by a maximum while changing variation with a net decrease of the flow acceleration and a third zone where the thermal and dynamic fields change and gradually decrease. Moreover, this study shows an axisymmetric flow behavior with two different aspects of its global structure. A central region characterized by a block motion (solid core) where the flow is rotational, characterized by a concentration of vorticity, and surrounded by the rest of the space where the flow is irrotational. Moving vertically away from the active source, results show an attenuation of the axial vortex intensity which is accompanied by a disappearance of movement by block. The centerline evolution of the axial and azimuthal momentum flux enables also to highlight these different aspects of the global flow structure. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the effect of a directionally porous wing tip on the tip vortex using particle image velocimetry (PIV) on a half wing model with NACA 653218 as its airfoil section. Four different configurations of the directionally porous wing tip are tested. The vortex generated by the wing tips are examined at four different measuring planes downstream perpendicular to the flow axis. The flow field over the porous wing tip surface along the streamwise direction is obtained as well to understand the effects of the porosity on the flow which in the end affects the vortex downstream. Furthermore, the aerodynamic performance of all different configurations is compared to study their effects on the aerodynamic coefficients of the wing. The results show a high reduction in vorticity, up to 90%; tangential velocity reduction up to 67% and a significant reduction in vortex circulation in the near-far field. Effect on the lift to drag ratio is up to 20 %. [ABSTRACT FROM AUTHOR]

The ground effect on the wingtip vortex generated by a rectangular semiwing equipped with tip-mounted regular and reverse half delta wings was investigated experimentally. The passive tip vortex control always led to a reduced lift-induced drag as the ground was approached. In close ground proximity, the presence of the corotating ground vortex (GV) added vorticity to the tip vortex while the counter-rotating secondary vortex (SV) negated its vorticity level. The interaction of the GV and SV with the tip vortex and their impact on the lift-induced drag were discussed. Physical mechanisms responsible for the change in the vortex flow properties in ground effect were also provided. [ABSTRACT FROM AUTHOR]

Pump performance characteristics of pump turbines in transient processes are significantly different from those in steady processes. In the present paper, transient processes of a flow rate that increased and decreased in the pump mode of a model pump turbine were simulated through unsteady simulations using the shear stress transport (SST) k-ω turbulence model. The numerical results reveal that there is a larger hysteresis loop in the performance characteristics of the increasing and decreasing directions of the flow rate compared with those of steady results. Detailed discussions are carried out to determine the generation mechanism of obvious hysteresis characteristics using the methods of entropy production and continuous wavelet analysis. Analyses show that the states of the backflow at the draft tube outlet and the vortices in the impeller and guide/stay vanes are promoted or suppressed owing to the acceleration and deceleration of the fluid. This contributes to the difference in pump performance characteristics of the pump turbine. [ABSTRACT FROM AUTHOR]

Minimising the aerodynamic drag of commercial vehicles is important economically and ecologically. This work demonstrates the effective use of lobed-mixing geometries, traditionally used to enhance flow mixing, as a viable, passive flow control method for reducing base pressure drag of boat-tailed ground vehicles. Experiments were performed on a 1/24th-scale Heavy Goods Vehicle representative model at a Reynolds number of 2.3 x 105 with force and hot-wire anemometry measurements used to quantify drag and wake characteristics. Tests on a baseline (no boat-tail), an unaltered boat-tail, and lobed-mixing configurations with varying pitch and height were compared. Overall, the baseline and unaltered boat-tail exhibited good correlation to previous results. This provided confidence in the methodology adopted. Results using lobed mixers showed up to a 10.2% drag reduction with the added vorticity produced acting to fundamentally shift the nature of the wake. This is manifested principally through the generation of counter-rotating vortical structures which enhance crosswise flow entrainment into the base wake. This action is observed to limit flow entrainment towards the ground leading to a higher wake and a characteristic ‘waist’. Enhanced mixing is also demonstrated. Overall, results suggest the suitability of lobed mixers as an effective means for drag reduction of boat-tailed ground vehicles. [ABSTRACT FROM AUTHOR]