An Assessment of Five Turbulence Models in Predicting Turbulent Separation

2004 ◽  
Author(s):  
Robert E. Spall ◽  
Warren F. Phillips ◽  
Nick Alley
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Mach Number ◽  
Turbulence Models ◽  
Streamwise Velocity ◽  
Numerical Results ◽  
Flow Conditions ◽  
Turbulent Separation ◽  
Controlled Flow ◽  
Flow Case

Four different turbulence models were employed to predict the flow over a wall-mounted Glauert-Goldschmied body. The models evaluated include: 1) two-layer k–ε, 2) shear stress transport, 3) low-Reynolds number k–ω, 4) Spalart-Allmaras, and 5) v2−f. Calculations were performed for both an uncontrolled case, and a controlled-flow case which used steady suction through a slot located at the 65% chord station. The flow conditions include a freestream Mach number of approximately 0.1, and a chord Reynolds number of just under 1 million. For each model, the numerical results over predicted the experimentally determined re-attachment length. An examination of streamwise velocity profiles at several stations downstream of the trailing edge revealed considerable variation in the predictions of the five turbulence models.

scholarly journals Numerical Modeling of Flow Over a Rectangular Broad-Crested Weir with a Sloped Upstream Face

Water ◽  
2018 ◽  
Vol 10 (11) ◽  
pp. 1663 ◽  
Author(s):  
Lei Jiang ◽  
Mingjun Diao ◽  
Haomiao Sun ◽  
Yu Ren
Keyword(s):  
Numerical Simulations ◽  
Discharge Coefficient ◽  
Separation Zone ◽  
Turbulence Models ◽  
Numerical Results ◽  
Upstream Face ◽  
Flow Conditions ◽  
Flow Features ◽  
The Mean ◽  
Absolute Percent Error

The objective of this study was to evaluate the effect of the upstream angle on flow over a trapezoidal broad-crested weir based on numerical simulations using the open-source toolbox OpenFOAM. Eight trapezoidal broad-crested weir configurations with different upstream face angles (θ = 10°, 15°, 22.5°, 30°, 45°, 60°, 75°, 90°) were investigated under free-flow conditions. The volume-of-fluid (VOF) method and two turbulence models (the standard k-ε model and the SST k-w model) were employed in the numerical simulations. The numerical results were compared with the experimental results obtained from published papers. The root mean square error (RMSE) and the mean absolute percent error (MAPE) were used to evaluate the accuracy of the numerical results. The statistical results show that RMSE and MAPE values of the standard k-ε model are 0.35–0.67% and 0.50–1.48%, respectively; the RMSE and MAPE values of the SST k-w model are 0.25–0.66% and 0.55–1.41%, respectively. Additionally, the effects of the upstream face angle on the flow features, including the discharge coefficient and the flow separation zone, were also discussed in the present study.

scholarly journals Numerical Analysis of the Aerodynamic Characteristics of NACA-4312 Airfoil

2020 ◽  
Vol 01 (02) ◽  
pp. 29-36
Author(s):  
Md Rhyhanul Islam Pranto ◽  
Mohammad Ilias Inam
Keyword(s):  
Reynolds Number ◽  
Drag Coefficient ◽  
Lift Coefficient ◽  
Turbulence Models ◽  
Aerodynamic Characteristics ◽  
Numerical Results ◽  
Ansys Fluent ◽  
Coefficient Increase ◽  
Lift And Drag ◽  
Lift And Drag Coefficient

The aim of the work is to investigate the aerodynamic characteristics such as lift coefficient, drag coefficient, pressure distribution over a surface of an airfoil of NACA-4312. A commercial software ANSYS Fluent was used for these numerical simulations to calculate the aerodynamic characteristics of 2-D NACA-4312 airfoil at different angles of attack (α) at fixed Reynolds number (Re), equal to 5×10^5 . These simulations were solved using two different turbulence models, one was the Standard k-ε model with enhanced wall treatment and other was the SST k-ω model. Numerical results demonstrate that both models can produce similar results with little deviations. It was observed that both lift and drag coefficient increase at higher angles of attack, however lift coefficient starts to reduce at α =13° which is known as stalling condition. Numerical results also show that flow separations start at rare edge when the angle of attack is higher than 13° due to the reduction of lift coefficient.

Compressibility and Rarefaction Effects on Pressure Drop in Rough Microchannels

2006 ◽  
Author(s):  
Giulio Croce ◽  
Paola D’Agaro ◽  
Alessandro Filippo
Keyword(s):  
Reynolds Number ◽  
Mach Number ◽  
Pressure Drop ◽  
Slip Flow ◽  
Major Effect ◽  
Flow Conditions ◽  
Fully Developed Flow ◽  
Compressibility Effect ◽  
Wide Range ◽  
Poiseuille Number

A numerical analysis of the flow field in rough microchannel is carried out with a finite volume compressible solver, including generalized Maxwell slip flow boundary conditions suitable for arbitrary geometries. Roughness geometry is modeled as a series of triangular shaped obstructions. Relative roughness from 0% to 2.65% were considered. Since for truly compressible flow we have no fully developed flow condition, the simulation is performed over the whole length of the channel. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 200 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to 0.0249. Performance charts are presented in terms of both average and local Poiseuille number as a function of local Kn, Ma and Re. In particular, it appears that roughness strongly decreases the reduction in pressure loss due to rarefaction. Thus, roughness effect is stronger at high Kn. Furthermore, compressibility effect has a major effect on pressure drop, as soon as local Mach number exceed 0.3.

scholarly journals Transfer functions for flow predictions in wall-bounded turbulence

2019 ◽  
Vol 864 ◽  
pp. 708-745 ◽  
Author(s):  
Kenzo Sasaki ◽  
Ricardo Vinuesa ◽  
André V. G. Cavalieri ◽  
Philipp Schlatter ◽  
Dan S. Henningson
Keyword(s):  
Reynolds Number ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Large Scale ◽  
Transfer Functions ◽  
Streamwise Velocity ◽  
Wall Region ◽  
Multiple Input ◽  
Wall Shear ◽  
Near Wall

Three methods are evaluated to estimate the streamwise velocity fluctuations of a zero-pressure-gradient turbulent boundary layer of momentum-thickness-based Reynolds number up to $Re_{\unicode[STIX]{x1D703}}\simeq 8200$, using as input velocity fluctuations at different wall-normal positions. A system identification approach is considered where large-eddy simulation data are used to build single and multiple-input linear and nonlinear transfer functions. Such transfer functions are then treated as convolution kernels and may be used as models for the prediction of the fluctuations. Good agreement between predicted and reference data is observed when the streamwise velocity in the near-wall region is estimated from fluctuations in the outer region. Both the unsteady behaviour of the fluctuations and the spectral content of the data are properly predicted. It is shown that approximately 45 % of the energy in the near-wall peak is linearly correlated with the outer-layer structures, for the reference case $Re_{\unicode[STIX]{x1D703}}=4430$. These identified transfer functions allow insight into the causality between the different wall-normal locations in a turbulent boundary layer along with an estimation of the tilting angle of the large-scale structures. Differences in accuracy of the methods (single- and multiple-input linear and nonlinear) are assessed by evaluating the coherence of the structures between wall-normally separated positions. It is shown that the large-scale fluctuations are coherent between the outer and inner layers, by means of an interactions which strengthens with increasing Reynolds number, whereas the finer-scale fluctuations are only coherent within the near-wall region. This enables the possibility of considering the wall-shear stress as an input measurement, which would more easily allow the implementation of these methods in experimental applications. A parametric study was also performed by evaluating the effect of the Reynolds number, wall-normal positions and input quantities considered in the model. Since the methods vary in terms of their complexity for implementation, computational expense and accuracy, the technique of choice will depend on the application under consideration. We also assessed the possibility of designing and testing the models at different Reynolds numbers, where it is shown that the prediction of the near-wall peak from wall-shear-stress measurements is practically unaffected even for a one order of magnitude change in the corresponding Reynolds number of the design and test, indicating that the interaction between the near-wall peak fluctuations and the wall is approximately Reynolds-number independent. Furthermore, given the performance of such methods in the prediction of flow features in turbulent boundary layers, they have a good potential for implementation in experiments and realistic flow control applications, where the prediction of the near-wall peak led to correlations above 0.80 when wall-shear stress was used in a multiple-input or nonlinear scheme. Errors of the order of 20 % were also observed in the determination of the near-wall spectral peak, depending on the employed method.

scholarly journals Depth-averaged velocity and bed shear stress in unsteady open channel flow over rough bed

2018 ◽  
Vol 40 ◽  
pp. 05071 ◽  
Author(s):  
Jnana Ranjan Khuntia ◽  
Kamalini Devi ◽  
Sebastien Proust ◽  
Kishanjit Kumar Khatua
Keyword(s):  
Shear Stress ◽  
Unsteady Flow ◽  
Steady Flow ◽  
Cross Sections ◽  
Open Channel ◽  
Bed Shear Stress ◽  
Flow Conditions ◽  
Lateral Velocity ◽  
Rough Beds ◽  
Flow Case

Very few studies have been carried out in the past in estimating depth-averaged velocity and bed shear stress in unsteady flow over rough beds. An experiment is thus conducted to investigate the vertical and lateral velocity profiles under unsteady flow conditions in a rough open channel for various flow depths. One hydrogram is repeatedly passed through the rectangular flume with a fixed rigid grass bed. Using micro Pitot tube and Acoustic Doppler Velocimeter (ADV), the flow patterns are investigated at both lateral and longitudinal positions over different cross-sections. For two typical flow depths, the velocities in both the rising limb and falling limb are observed. Hysteresis effect between stage-discharge (h ~ Q) rating curve between rising and falling limbs is illustrated. Lateral distribution of depth-averaged velocity and bed shear stress are plotted at three different cross sections and compared with the steady flow conditions. In falling limb of an unsteady flow case, both depth-averaged velocity and bed shear stress distribution in the central region is higher than that of steady flow case. However, in the rising limb, the bed shear stress of unsteady flow is less than that of steady flow case. Further, in an unsteady flow, the magnitude of depth-averaged velocity is found to increase towards the downstream sections. Along the downstream positions, bed shear stress values increase for lower flow depths and decrease for higher flow depth cases.

Magnetohydrodynamic turbulent flow in a channel at low magnetic Reynolds number

2001 ◽  
Vol 439 ◽  
pp. 367-394 ◽  
Author(s):  
DONGHOON LEE ◽  
HAECHEON CHOI
Keyword(s):  
Magnetic Field ◽  
Reynolds Number ◽  
Shear Stress ◽  
Magnetic Fields ◽  
Reynolds Shear Stress ◽  
Streamwise Velocity ◽  
Magnetic Reynolds Number ◽  
Velocity Fluctuations ◽  
Turbulence Structures ◽  
Normal Magnetic Field

Effects of the Lorentz force on near-wall turbulence structures are investigated using the direct numerical simulation technique with the assumption of no induced magnetic field at low magnetic Reynolds number. A uniform magnetic field is applied in the streamwise (x), wall-normal (y) or spanwise (z) direction to turbulent flow in an infinitely long channel with non-conducting walls. The Lorentz force induced from the magnetic field suppresses the dynamically significant coherent structures near the wall. The skin friction decreases with increasing streamwise and spanwise magnetic fields, whereas it increases owing to the Hartmann effect when the strength of the wall-normal magnetic field exceeds a certain value. All the turbulence intensities and the Reynolds shear stress decrease with the wall-normal and spanwise magnetic fields, but the streamwise velocity fluctuations increase with the streamwise magnetic field although all other turbulence intensities decrease. It is also shown that the wall-normal magnetic field is much more effective than the streamwise and spanwise magnetic fields in reducing turbulent fluctuations and suppressing the near-wall streamwise vorticity, even though the wall-normal magnetic field interacts directly with the mean flow and results in drag increase at strong magnetic fields. In the channel with a strong streamwise magnetic field, two-dimensional streamwise velocity fluctuations u(y, z) exist, even after other components of the velocity fluctuations nearly vanish. In the cases of strong wall-normal and spanwise magnetic fields, all turbulence intensities, the Reynolds shear stress and vorticity fluctuations decrease rapidly and become zero. The turbulence structures are markedly elongated in the direction of the applied magnetic field when it is strong enough. It is shown that this elongation of the structures is associated with a rapid decrease of the Joule dissipation in time.

Compressibility and Rarefaction Effect on Heat Transfer in Rough Microchannels

2007 ◽  
Author(s):  
Giulio Croce ◽  
Paola D’Agaro
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Mach Number ◽  
Pressure Drop ◽  
Fluid Dynamic ◽  
Slip Flow ◽  
Flow Conditions ◽  
Rarefied Flow ◽  
Wide Range ◽  
Poiseuille Number

High pressure drop and high length to hydraulic diameter ratios yield significant compressibility effects in microchannel flows, which compete with rarefaction phenomena at the smaller scale. In such regimes, flow field and temperature field are no longer decoupled. In presence of significant heat transfer, and combined with the effect of viscous dissipation, this yields to a quite complex thermo-fluid dynamic problem. A finite volume compressible solver, including generalized Maxwell slip flow and temperature jump boundary conditions suitable for arbitrary geometries, is adopted. Roughness geometry is modeled as a series of triangular shaped obstructions, and relative roughness from 0% to 2.65% were considered. The chosen geometry allows for direct comparison with pressure drop computations carried out, in a previous paper, under adiabatic conditions. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 300 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to around 0.05, and the competing effects of rarefaction, compressibility and roughness are investigated in detail. Compressibility is found to be the most dominant effect at high Mach number, yielding even inversion of heat flux, while roughness has a strong effect in the case of rarefied flow. Furthermore, the mutual interaction between heat transfer and pressure drop is highlighted, comparing Poiseuille number values for both cooled and heated flows with previous adiabatic computations.

scholarly journals Reynolds-number dependence of turbulent skin-friction drag reduction induced by spanwise forcing

2016 ◽  
Vol 802 ◽  
pp. 553-582 ◽  
Author(s):  
Davide Gatti ◽  
Maurizio Quadrio
Keyword(s):  
Reynolds Number ◽  
Drag Reduction ◽  
Skin Friction ◽  
Travelling Waves ◽  
Turbulent Channel Flow ◽  
Streamwise Velocity ◽  
Friction Drag ◽  
Reduction Techniques ◽  
Controlled Flow ◽  
Friction Drag Reduction

This paper examines how increasing the value of the Reynolds number $Re$ affects the ability of spanwise-forcing techniques to yield turbulent skin-friction drag reduction. The considered forcing is based on the streamwise-travelling waves of spanwise-wall velocity (Quadrio et al., J. Fluid Mech., vol. 627, 2009, pp. 161–178). The study builds upon an extensive drag-reduction database created via direct numerical simulation of a turbulent channel flow for two fivefold separated values of $Re$, namely $Re_{\unicode[STIX]{x1D70F}}=200$ and $Re_{\unicode[STIX]{x1D70F}}=1000$. The sheer size of the database, which for the first time systematically addresses the amplitude of the forcing, allows a comprehensive view of the drag-reducing characteristics of the travelling waves, and enables a detailed description of the changes occurring when $Re$ increases. The effect of using a viscous scaling based on the friction velocity of either the non-controlled flow or the drag-reduced flow is described. In analogy with other wall-based drag-reduction techniques, like riblets for example, the performance of the travelling waves is well described by a vertical shift of the logarithmic portion of the mean streamwise velocity profile. Except when $Re$ is very low, this shift remains constant with $Re$, at odds with the percentage reduction of the friction coefficient, which is known to present a mild, logarithmic decline. Our new data agree with the available literature, which is however mostly based on low-$Re$ information and hence predicts a quick drop of maximum drag reduction with $Re$. The present study supports a more optimistic scenario, where for an airplane at flight Reynolds numbers a drag reduction of nearly 30 % would still be possible thanks to the travelling waves.

scholarly journals Enhanced discrete phase model for multiphase flow simulation of blood flow with high shear stress

2021 ◽  
Vol 104 (1) ◽  
pp. 003685042110080
Author(s):  
Zheqin Yu ◽  
Jianping Tan ◽  
Shuai Wang
Keyword(s):  
Blood Flow ◽  
Shear Stress ◽  
Multiphase Flow ◽  
Experimental Verification ◽  
Flow Simulation ◽  
Turbulence Models ◽  
Phase Model ◽  
Force Model ◽  
Discrete Phase Model ◽  
Discrete Phase

Shear stress is often present in the blood flow within blood-contacting devices, which is the leading cause of hemolysis. However, the simulation method for blood flow with shear stress is still not perfect, especially the multiphase flow model and experimental verification. In this regard, this study proposes an enhanced discrete phase model for multiphase flow simulation of blood flow with shear stress. This simulation is based on the discrete phase model (DPM). According to the multiphase flow characteristics of blood, a virtual mass force model and a pressure gradient influence model are added to the calculation of cell particle motion. In the experimental verification, nozzle models were designed to simulate the flow with shear stress, varying the degree of shear stress through different nozzle sizes. The microscopic flow was measured by the Particle Image Velocimetry (PIV) experimental method. The comparison of the turbulence models and the verification of the simulation accuracy were carried out based on the experimental results. The result demonstrates that the simulation effect of the SST k- ω model is better than other standard turbulence models. Accuracy analysis proves that the simulation results are accurate and can capture the movement of cell-level particles in the flow with shear stress. The results of the research are conducive to obtaining accurate and comprehensive analysis results in the equipment development phase.

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