Showing papers on "Flow separation published in 2021"

Physical insights into the heat and mass transfer in Casson hybrid nanofluid flow induced by a Riga plate with thermophoretic particle deposition

Abstract: In applied physics, Riga plate was one of the trademark inventions to overcome the poor conductivity of fluids. This provided an aid to avoid the boundary layer separation, reduce the friction as w...

Numerical investigation of the effect of surface roughness on flow and heat transfer characteristics of single sphere particle in supercritical water

Abstract: Supercritical water fluidized bed is a novel gasification reactor which can achieve efficient and clean utilization of coal. The rough surface of particle produced during grinding and thermochemical conversion processing will deeply affect supercritical water-particle two-phase flow and heat transfer characteristics. In this paper, fully resolved numerical simulation of supercritical water flow past single rough sphere particle with the Reynolds number ranging from 10 to 200 was carried out to investigate the effect of surface roughness. The simulation results show that as roughness increases, the separation bubbles generated in the dimple enhance the flow separation but has no significant effect on the drag coefficient. Particle surface-average Nusselt number decreases with an increase of roughness and surface enlargement coefficient due to the isolation effect at low Re and local separation bubbles in the dimple at high Re. Furthermore, the effect of surface enlargement coefficient on heat transfer efficiency factor for supercritical water near the critical point is greater than that under constant property condition and has a higher dependence on Re.

MHD mixed convection flow of a hybrid nanofluid past a permeable vertical flat plate with thermal radiation effect

Abstract: The magnetohydrodynamic (MHD) radiative flow of a hybrid alumina-copper/water nanofluid past a permeable vertical plate with mixed convection is the focal interest in this present work. Dissimilar to the traditional nanofluid model that considers only one type of nanoparticles, we consider the hybridization of two types of nanoparticles in this work which are alumina and copper. The governing flow and heat transfer equations are simplified to the ordinary differential equations (ODEs) with the adaptation of conventional similarity transformations which are then evaluated by the bvp4c solver (MATLAB) to generate the numerical solutions. The solutions are generated and illustrated in the form of graph to be easily observed. Although dual solutions are obtained in this study, only one solution is determined to be stable. By reducing the concentration volume of copper and increasing the magnetic and radiation parameters, the boundary layer separation can be hindered. With the occurrence of opposing flow due to the mixed convection parameter, the heat transfer can be enhanced when the concentration volume of copper is being reduced and when the magnetic and radiation parameters are being proliferated.

Advances in axial turbine blade profile aerodynamics

Abstract: The aerodynamic performance of axial turbines depends significantly on profile losses, secondary flow losses, and clearance gap losses of vanes and blades. In modern high-efficiency turbomachinery ...

A New Explicit Algebraic Wall Model for LES of Turbulent Flows Under Adverse Pressure Gradient

Abstract: A new explicit algebraic wall law for the Large Eddy Simulation of flows with adverse pressure gradient is proposed This new wall law, referred as adverse pressure gradient power law (APGPL), is developed starting from the power-law of Werner and Wengle (Turbulent Shear Flows, vol 8, Springer, New York, pp 155–168, 1993) in order to mimic an implicit non-equilibrium log-law based on Afzal’s law (Afzal, IUTAM Symposium on Asymptotic Methods for Turbulent Shear Flows at High Reynolds Numbers, Kluwer Academic Publishers, Bochum, pp 95–118, 1996) No iterative method is needed for the evaluation of the wall shear stress from the APGPL contrary to the majority of models available in the literature The APGPL model relies on the definition of three modes: the equilibrium power-law is used in regions of no or favourable pressure gradient, the APGPL is used in regions of adverse pressure gradient, and no wall model is used in separated flow regions This model is assessed via Large Eddy Simulations of flows involving adverse pressure gradient and boundary layer separation using the Lattice Boltzmann Method on uniform nested grids The flow around a clean and iced NACA23012 airfoil at Reynolds number $$Re = 188 \times 10^6$$ and the flow over the LAGOON landing gear at $$Re = 159 \times 10^6$$ are considered Results are found in good agreement with those obtained by the non-equilibrium log-law and experimental and numerical data available in the literature

Unsteady effects in a hypersonic compression ramp flow with laminar separation

Abstract: Direct numerical simulations (DNS) are performed to investigate a hypersonic flow over a compression ramp with a free stream Mach number of 7.7 and a free stream Reynolds number of based on the flat plate length. The DNS results are validated by comparison with experimental data and theoretical predictions. It is shown that even in the absence of external disturbances, streamwise heat flux streaks form on the ramp surface downstream of reattachment, and that they are non-uniformly distributed in the spanwise direction. The surface heat flux exhibits a low-frequency unsteadiness, which propagates in the streamwise direction. Additionally, the unsteadiness of the heat flux streaks downstream of reattachment is coupled with a pulsation of the reattachment position. By conducting a dynamic mode decomposition (DMD) analysis, several oscillatory modes, characterised by streamwise low-frequency periodicity, are revealed in the separation bubble flow. The DNS results are further explained by a global stability analysis (GSA). Particularly, the flow structure of the leading DMD modes is consistent with that of the oscillatory unstable modes identified by the GSA. It is therefore concluded that the global instabilities are responsible for the unsteadiness of the considered compression ramp flow.

Role of corner flow separation in unsteady dynamics of hypersonic flow over a double wedge geometry

Abstract: This study investigates the origin and sustenance of self-induced oscillations of shock structures in a hypersonic flow over a double wedge configuration. Previously, various researchers have considered the double wedge flow configuration for inviscid flow with variations of different inflows as well as geometric parameters such as inflow Mach number ( M ∞), wedge angles, and wedge lengths. Few recently published articles reveal an unsteady flow physics involved with the hypersonic viscous flow for double wedge configuration with large second wedge angles. However, the reason for such self-sustained flow oscillations is not completely clear. The present work seeks out to investigate the origin of such oscillations in a low enthalpy hypersonic flow with different aft-wedge angles and wedge length ratios. In the current study, viscous flow over a double wedge at M ∞ = 7 and fore-wedge angle of 30° is considered. An improved version of rhoCentralFoam solver in OpenFOAM is used to investigate the double wedge flow over different grid resolutions in the separation region and shock–shock interaction region. This study corroborates the observation from the previous literature with an improvement in the range of parameters which results in a self-sustained periodic oscillation. The present study also suggests that the unsteadiness becomes possible when the incidence shock is in the proximity of the aft-wedge expansion corner as a consequence of different wedge length ratios ( L 1 / L 2) or aft-wedge angles (θ2). Flow can still be steady at a large aft-wedge angle if the incidence shock is far ahead of the aft-wedge expansion corner.

Aeroacoustic Investigation of a Propeller Operating at Low Reynolds Numbers

Abstract: This paper presents an experimental investigation of a propeller operating at low Reynolds numbers and provides insights into the role of aerodynamic flow features on both propeller performances an...

Transitional pulsatile flows with stenosis in a two-dimensional channel

Abstract: Although blood flows are mostly laminar, transition to turbulence and flow separations are observed at curved vessels, bifurcations, or constrictions. It is known that wall-shear stress plays an important role in the development of atherosclerosis as well as in arteriovenous grafts. In order to help understand the behavior of flow separation and transition to turbulence in post-stenotic blood flows, an experimental study of transitional pulsatile flow with stenosis was carried out using time-resolved particle image velocimetry and a microelectromechanical systems wall-shear stress sensor at the mean Reynolds number of 1750 with the Womersley number of 6.15. At the start of the pulsatile cycle, a strong shear layer develops from the tip of the stenosis, increasing the flow separation region. The flow at the throat of the stenosis is always laminar due to acceleration, which quickly becomes turbulent through a shear-layer instability under a strong adverse pressure gradient. At the same time, a recirculation region appears over the wall opposite to the stenosis, moving downstream in sync with the movement of the reattachment point. These flow behaviors observed in a two-dimensional channel flow are very similar to the results obtained previously in a pipe flow. We also found that the behavior in a pulsating channel flow during the acceleration phase of both 25% and 50% stenosis cases is similar to that of the steady flow, including the location and size of post-stenotic flow separation regions. This is because the peak Reynolds number of the pulsatile flow is similar to that of the steady flow that is investigated. The transition to turbulence is more dominant for the 50% stenosis as compared to the 25% stenosis, as the wavelet spectra show a greater broadening of turbulence energy. With an increase in stenosis to 75%, the accelerating flow is directed toward the opposite wall, creating a wall jet. The shear layer from the stenosis bifurcates as a result of this, one moving with the flow separation region toward the upper wall and the other with the wall jet toward the bottom wall. Low wall-shear stress fluctuations are found at two post-stenotic locations in the channel flow – one immediately downstream of the stenosis over the top wall (stenosis side) inside the flow separation region, and the other in the recirculation region on the bottom wall (opposite side of the stenosis).

Effects of roughness on a turbulent boundary layer in hypersonic flow

Abstract: Particle image velocimetry experiments were performed to study the effects of surface roughness on a hypersonic ( $$ M \approx 7.3 $$ ), flat-plate turbulent boundary layer. Diamond mesh and square bars of different heights were used to form the roughness. No significant compressibility effects were apparent in the flow response in that the behavior of the mean velocity and streamwise turbulence profiles was in general accord with similar experiments in incompressible flows. The effects of the roughness extended to about one roughness height above the roughness itself and Townsend’s hypothesis were confirmed. Outside of this region, the streamwise lengthscale and the inclination of the spatial correlation contours also showed good agreement with observations on smooth-wall and incompressible flow experiments.

Role of corner flow separation in unsteady dynamics of hypersonic flow over a double wedge geometry

Abstract: This study investigates the origin and sustenance of self induced oscillations of shock structures in a hypersonic flow over a double wedge configuration. Previously, various researchers have considered the double wedge flow configuration for inviscid flow with variations of different inflows as well as geometric parameters such as inflow Mach number , wedge angles, and wedge lengths. Few recently published articles reveal an unsteady flow physics involved with the hypersonic viscous flow for double wedge configuration with large second wedge angles. However, the reason for such self sustained flow oscillations is not completely clear. The present work seeks out to investigate the origin of such oscillations in a low enthalpy hypersonic flow with different aft wedge angles and wedge length this http URL the current study, viscous flow over a double wedge at Mach 7 and fore wedge angle of 30° is considered. An improved version of rhoCentralFoam solver in OpenFOAM is used to investigate the double wedge flow over different grid resolutions in the separation region and shock shock interaction region. This study corroborates the observation from the previous literature with an improvement in the range of parameters which results in a self sustained periodic oscillation. The present study also suggests that the unsteadiness becomes possible when the incidence shock is in the proximity of the aft wedge expansion corner as a consequence of different wedge length ratios(L1/L2) or aft wedge angles ({\theta}2). Flow can still be steady at a large aft wedge angle if the incidence shock is far ahead of the aft wedge expansion corner.

Study on a horizontal axial flow pump during runaway process with bidirectional operating conditions.

Abstract: The ultra-low head pump stations often have bidirectional demand of water delivery, so there is a risk of runaway accident occurring in both conditions. To analyze the difference of the runaway process under forward runaway condition (FRC) and backward runaway condition (BRC), the whole flow system of a horizontal axial flow pump is considered. The Shear-Stress Transport (SST) k-ω model is adopted and the volume of fluid (VOF) model is applied to simulate the water surface in the reservoirs. Meanwhile, the torque balance equation is introduced to obtain the real time rotational speed, then the bidirectional runaway process of the pump with the same head is simulated. In addition, the vortex transport equation and swirl number are proposed to reveal the flow characteristics during the runaway process. The results show that the runaway process can be divided into five stages: the drop, braking, rising, convergence and runaway stages, according to the changing law of torque curve. In the rising stage, the pressure difference on the blade surface continues to increase, which contributes to the abnormal torque increase. In this stage, the flow hits the pressure surface (PS) at a faster speed enlarging the pressure on PS, and the flow separation takes place on the suction surface (SS) weakening the pressure on SS. During the convergence and runaway stage, the pulsation amplitude of torque and axial force under FRC is obviously larger than those under BRC. This is because the rotation frequency of the vortex rope is the same as main pressure fluctuation frequency in impeller under FRC, which enhances the pulsation amplitude. Whereas the vortices are broken due to the inhibitive effect from guide vanes under BRC.

Effects of flow separation and shape factor of nanoparticles in heat transfer fluid for convection thorough phase change material (PCM) installed cylinder for energy technology applications

Abstract: In this study, flow separation effects on the performance of PCM embedded thermo-fluid system is numerically analyzed by using the finite element method. In the heat transfer fluid, shape effects of nanoparticles are considered. Spherical, blade, brick and cylindrical shaped alumina nanoparticles are used in water. The numerical work is performed for different values of Reynolds number (between 100 and 300), expansion ratio of the channel (0.35 and 1), nanoparticle volume fraction (between 0 and 2%) and different shape of particles. The PCM material is paraffin wax with spherical shaped capsules of 20 mm in diameter. There is significant impact of channel expansion and flow separation zone on the performance of the system. When the area expansion is introduced in the straight channel, the configuration with the highest Reynolds number resulted in performance degradation due to flow recirculation extended in the PCM region. The charging time for a straight channel is reduced by about 84% when comparison is made with the channel having an expansion ratio of 0.35. The shape factor of the alumina nanoparticles significantly affects the flow recirculations and thermal exchange between the PCM and heat transfer fluid. Among various particles, cylindrical shaped one performs best while 23.8% reduction in charging time is obtained at the highest solid volume fraction when comparison is made with pure water as heat transfer fluid.

Occurrence of global instability in hypersonic compression corner flow

Abstract: Hypersonic flow over a two-dimensional compression corner with a Mach number of 7.7 and unit Reynolds number of 4.2 × 106 m−1 is numerically investigated. Special emphasis is given to the onset of global instability with respect to three-dimensional perturbations. Global stability analysis is performed for various ramp angles and wall temperatures. It is found that the shock-induced separated flow system becomes unstable when the ramp angle is beyond a certain value. The critical ramp angle increases slightly with the wall temperature, although the length of the separation region is significantly enlarged. The global instability is shown to be closely linked with the occurrence of secondary separation beneath the primary separation bubble. A criterion is established based on a scaled ramp angle defined in the triple-deck theory to predict the global stability boundary, which depends on the free-stream conditions and geometries only.