Assessment Of Turbulence Models Research Articles

Numerical investigation of heat transfer in spirally coiled corrugated pipes: Assessment of turbulence models

The Archimedean spiral coil made of a transversely corrugated pipe represents the radiant heat absorber of a parabolic dish solar concentrator. The main advantage of the considered design is the coupling of two passive methods for heat transfer enhancement: coiling the flow channel and changing the surface roughness. The aim of this numerical study is to assess the capability of RANS models of different complexity (realizable k-?, SST k-?, and RSM linear pressure-strain) to adequately represent the heat transfer phenomena in the considered complex flow geometry for wide ranges of Reynolds and Prandtl numbers. The obtained results indicate that the realizable k-? model with enhanced wall treatment is inadequate to simulate the heat transfer for all flow conditions, while both SST and RSM slightly overestimate experimental data in the turbulent region and are able to predict laminarisation at low Reynolds numbers. The SST model predictions are more accurate in the transitional and at the beginning of the turbulent region, irrespective of the curvature ratio. The RSM predictions are generally more accurate in the turbulent region. Numerically obtained circumferential distributions of local Nusselt number reveal that considered turbulence models are unable to completely anticipate the interactions between the complex flow in the basic section of the pipe and the vortex flow within the corrugations.

Assessment of Turbulence Models for Unsteady Separated Flows Past an Oscillating NACA 0015 Airfoil in Deep Stall

This paper provides 2D Computational Fluid Dynamics (CFD) investigations, using OpenFOAM package, of the unsteady separated fully turbulent flows past a NACA 0015 airfoil undergoing sinusoidal pitching motion about its quarter-chord axis in deep stall regime at a reduced frequency of 0.1, a free stream Mach number of 0.278, and at a Reynolds number, based on the airfoil chord length, , of . First, eighteen 2D steady-state computations coupled with the SST model were carried out at various angles of attack to investigate the static stall. Then, the 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations of the flow around the oscillating airfoil about its quarter-chord axis were carried out. Three eddy viscosity turbulence models, namely the Spalart-Allmaras, Launder-Sharma , and SST were considered for turbulence closure. The results are compared with the experimental data where the boundary layer has been tripped at the airfoil’s leading-edge. The findings suggest that the SST performs best among the other two models to predict the unsteady aerodynamic forces and the main flow features characteristic of the deep stall regime. The influence of moving the pitching axis downstream at mid chord was also investigated using URANS simulations coupled with the SST model. It was found that this induces higher peaks in the nose-down pitching moment and delays the stall onset. However, the qualitative behavior of the unsteady flow in post-stall remains unchanged. The details of the flow development associated with dynamic stall were discussed

Assessment of turbulence models and mesh types for the simulation of multi-body separation by coupling the CFD and rigid body dynamics

Turbulence models are considered one of the most important sources of numerical errors in the CFD studies on different aeronautical problems. The mesh type can also affect the numerical accuracy and computational efficiency. In this paper, various simulations on the store separation from an aircraft model in the transonic flow have been carried out by coupling the CFD and the rigid body dynamics on the overset mesh. The influence of S-A model, Standard k-epsilon model, SST k-omega model and inviscid model on the simulation results is examined. Different mesh types of both unstructured mesh and structured mesh as component part in overset are also considered. Grid-independence study are first carried out. The effects of different turbulence models and mesh types are then examined by comparing the trajectories, velocities and Eulerian angles. Recommendations are given to get better results for the simulation of multi-body separation problem.

Assessment of Turbulence Models over a Curved Hill Flow with Passive Scalar Transport

An incoming canonical spatially developing turbulent boundary layer (SDTBL) over a 2-D curved hill is numerically investigated via the Reynolds-averaged Navier–Stokes (RANS) equations plus two eddy-viscosity models: the K−ω SST (henceforth SST) and the Spalart–Allmaras (henceforth SA) turbulence models. A spatially evolving thermal boundary layer has also been included, assuming temperature as a passive scalar (Pr = 0.71) and a turbulent Prandtl number, Prt, of 0.90 for wall-normal turbulent heat flux modeling. The complex flow with a combined strong adverse/favorable streamline curvature-driven pressure gradient caused by concave/convex surface curvatures has been replicated from wind-tunnel experiments from the literature, and the measured velocity and pressure fields have been used for validation purposes (the thermal field was not experimentally measured). Furthermore, direct numerical simulation (DNS) databases from the literature were also employed for the incoming turbulent flow assessment. Concerning first-order statistics, the SA model demonstrated a better agreement with experiments where the turbulent boundary layer remained attached, for instance, in Cp, Cf, and Us predictions. Conversely, the SST model has shown a slightly better match with experiments over the flow separation zone (in terms of Cp and Cf) and in Us profiles just upstream of the bubble. The Reynolds analogy, based on the St/(Cf/2) ratio, holds in zero-pressure gradient (ZPG) zones; however, it is significantly deteriorated by the presence of streamline curvature-driven pressure gradient, particularly due to concave wall curvature or adverse-pressure gradient (APG). In terms of second-order statistics, the SST model has better captured the positively correlated characteristics of u′ and v′ or positive Reynolds shear stresses (<u′v′> > 0) inside the recirculating zone. Very strong APG induced outer secondary peaks in <u′v′> and turbulence production as well as an evident negative slope on the constant shear layer.

Effect of Turbulence Modeling Selection Within Helios for Small Quadrotor Aerodynamics

The high-fidelity rotorcraft simulation framework HPCMP CREATE-AV Helios was used to study the impact of turbulence modeling methods on the prediction of a small quadrotor unmanned aerial vehicle. First, the vehicle’s rotor was simulated as an isolated rotor in both hover and forward flight for turbulence model assessment. Predictions using a fully turbulent turbulence model were compared with predictions from several laminar–turbulent transition models. Both comparison with experiments and computational performance were considered. The laminar–turbulent transition models predicted flow separation near the tip for all flight conditions simulated, leading to lower torque and thrust than the fully turbulent model, which predicted attached turbulent flow. The fully turbulent model was shown to provide reasonable comparison with experiments with the lowest computational cost and was used for full vehicle simulations. The full quadrotor configuration calculations were performed in hover and forward flight. The forward flight simulations were performed with and without a detached eddy simulation method, and similar interactional aerodynamics were predicted. The Helios simulation suite is shown to be well-suited for prediction of aerodynamic performance of small-scale rotorcraft.

Turbulence model assessment and heat transfer phenomena inside a rectangular channel under forced and mixed convection

A very high-temperature gas-cooled reactor (VHTR) is an attractive option for hydrogen production due to its high operating temperature. The reactor cavity cooling system (RCCS), a passive safety system of the VHTR, surrounds the reactor vessel with rectangular riser channels and removes decay heat from it. Under emergency operation conditions such as a decrease in the flow rate of the RCCS, the flow condition inside the RCCS riser can become a mixed convection condition accompanying heat transfer deterioration. To design and operate the VHTR with reliable safety, accurate predictions for various convective heat transfer conditions in RCCS riser channels are required. In this study, by comparing the results of an air flow visualization experiment, the predictability of the turbulence models was assessed for the convective heat transfer and complex flow characteristics near the corner region of the rectangular duct confirmed from the experimental study. In this paper, the heat removal rate through the test section of the visualization experiment was quantified under various convective heat transfer conditions, and local temperature measurement data from the experiment were presented to describe the heat transfer deterioration phenomena inside a rectangular riser in turbulent mixed convection. Linear and nonlinear eddy viscosity models were evaluated by comparing the heat transfer coefficient, local temperature, local flow structure including secondary flow, and turbulence quantities obtained from the experiment. Furthermore, improvements to turbulence models for enhancing the prediction of mixed convection heat transfer in rectangular riser ducts were proposed.

Assessment of turbulence models using DNS data of compressible plane free shear layer flow

The present paper uses the detailed flow data produced by direct numerical simulation (DNS) of a three-dimensional, spatially developing plane free shear layer to assess several commonly used turbulence models in compressible flows. The free shear layer is generated by two parallel streams separated by a splitter plate, with a naturally developing inflow condition. The DNS is conducted using a high-order discontinuous spectral element method (DSEM) for various convective Mach numbers. The DNS results are employed to provide insights into turbulence modelling. The analyses show that with the knowledge of the Reynolds velocity fluctuations and averages, the considered strong Reynolds analogy models can accurately predict temperature fluctuations and Favre velocity averages, while the extended strong Reynolds analogy models can correctly estimate the Favre velocity fluctuations and the Favre shear stress. The pressure–dilatation correlation and dilatational dissipation models overestimate the corresponding DNS results, especially with high compressibility. The pressure–strain correlation models perform excellently for most pressure–strain correlation components, while the compressibility modification model gives poor predictions. The results of an a priori test for subgrid-scale (SGS) models are also reported. The scale similarity and gradient models, which are non-eddy viscosity models, can accurately reproduce SGS stresses in terms of structure and magnitude. The dynamic Smagorinsky model, an eddy viscosity model but based on the scale similarity concept, shows acceptable correlation coefficients between the DNS and modelled SGS stresses. Finally, the Smagorinsky model, a purely dissipative model, yields low correlation coefficients and unacceptable accumulated errors.

Assessment of turbulence modeling for massively-cooled turbulent boundary layer flows with transpiration cooling

We assess Reynolds-averaged Navier–Stokes (RANS) turbulent closures for the prediction of a turbulent boundary layer with transpiration cooling via comparison with a high-fidelity direct numerical simulation database. This study considers the canonical zero-pressure gradient, flat-plate, turbulent boundary layer over a massively cooled wall, with transpiration cooling. The simulations are conducted at a low-subsonic Mach number and we study two transpiration cooling configurations with uniform and slit injection at various blowing ratios. The DNS and RANS simulation setups are nearly identical. The RANS-based turbulence models perform well in the qualitative estimation of the velocity and thermal boundary layer evolution at low-blowing ratios (F = 0.2 and 0.6%); more significant differences are noted at higher blowing ratios (F=2.0%). The RANS models, especially the Spalart–Allmaras model, over-predict turbulence production near the wall which results in faster growth in the boundary thickness; this error becomes more pronounced at higher blowing ratios. Despite the greater mixing of momentum, the thermal mixing is under-predicted compared to the DNS in the uniform blowing case but over-predicted for the slit case. These results suggest that modeling errors in the temperature distribution due to turbulent thermal flux modeling can be significant even if the velocity is correctly modeled.

Assessment of turbulence models for the prediction of Bénard-Von Kármán vortex shedding behind a truncated hydrofoil in cavitation conditions

The transient cavitating flow in the wake of a hydrofoil at zero incidence angle has been simulated using a homogeneous mixture cavitation mass transfer model combined with both Reynolds Average Navier-Stokes (RANS) and Scale Resolving Simulation (SRS) turbulence models. The hydrofoil geometry corresponds to a 2D NACA 0009 with a truncated trailing edge which has already been extensively investigated in the High-Speed Cavitation Tunnel of the EPFL. The hydrodynamic conditions of interest correspond to a free stream velocity of 20 m/s (Re = 2 · 106) without cavitation and with two different degrees of cavitation. To improve the prediction of the vortex shedding behing the hydrofoil, the γ − Rθt transitional boundary layer model has been coupled with the turbulence models. At cavitation-free regime, all the turbulence models with the exception of the SST and LES WALE ones have the ability to predict the experimentally measured vortex shedding frequency. Nevertheless, the results indicate that, neither the SST nor the DES-SST γ − Rθt , can predict the vortex shedding frequency increase which has been experimentally observed when cavitation occurs. In contrast, the numerical results provided by the SST γ − Rθt and the SSTCC γ − Rθt show the capability to predict the expected shedding frequencies for both non cavitation and cavitation conditions. Beyond all expectation, the results provided by the LES WALE seem not only to overestimate the vortex shedding frequency at cavitation free conditions but also to underestimate the frequency when the cavitation number is significantly reduced.

A Turbulence Model Assessment for Deep Part Load Conditions of a Francis Turbine

Today, hydropower plants received increased attention as they have the ability to stabilize the electrical grid. However, this is accompanied by more start-stop sequences as well as operation in off-design conditions. One big task is to develop improved runner designs that can resist the increased pressure fluctuations. In the scope of this study, a Francis turbine is simulated for a deep part load operating point with the use of the SAS model and the more recently developed SBES model with ANSYS CFX. Both turbulence models are hybrid RANS-LES models. The simulation results are compared against model test data from a measurement at high cavitation number. For the evaluation of the pressure fluctuations comparison is furthermore made against experimental data at low cavitation number. All in all, SAS and SBES model predict similar flow characteristics. Both, the velocity profiles in the PIV measurement plane and the predicted pressure fluctuations, show only minor differences. The comparison against experimental data shows that the amplitude of the broadband frequency spectrum, which results from the occurrence of vortices, is overestimated at the investigated locations on the runner blades.

Assessment of Turbulence Models for Low Reynolds Number Flows and Their Computational Costs, Part 1: Staggered Tube Bank

Abstract This study investigates the accuracy of computational fluid dynamics (CFD) models to predict heat transfer in turbulent separated flows at low Reynolds numbers. This article will focus on flow in a staggered tube bank, while its companion articular will focus on a square prism (cylinder) in cross flow. Experimental data for both local heat transfer and velocity profiles are available for these cases and have been used extensively in the literature to evaluate various CFD methods. Six unsteady models were used and the results show that the unsteady shear stress transport (SST) model provided good overall accuracy relative to the mean Nusselt number for both cases. However, the SST model failed to accurately predict local variations. The partially averaged Navier–Stokes (PANS) variant of the SST model did show a marked improvement over the baseline SST model. The dynamic Smagorinsky large eddy simulation (LES) showed a much-improved fidelity to the local Nusselt number but unpredicted the actual values. The computational cost for the LES model was significant and it was found that the computationally expensive models with higher degrees of resolved turbulence did not necessarily return better results. Finally, the pressure drop results for the six models were scaled to predict the mean Nusselt number with the generalized Leveque method and were found to be very accurate. This method should prove useful to predict heat transfer performance with computationally less expensive cold flow results.

Assessment of Turbulence Models for Low Reynolds Number Flows and Their Computational Costs, Part 2: Square Prism in Cross Flow

AbstractThe accuracy of six turbulent flow modeling techniques in an unsteady solution is evaluated against experimental data for a square prism in cross flow. The selected models, shear stress transport (SST), SST-SAS, Reynolds stress model (RSM), partially averaged Navier–Stokes (PANS)-SST, detached eddy simulation (DES), and large eddy simulations (LES), models are the same as those presented in Part 1 of this study, which focused on flow in a staggered tube bank. For this geometry, the SST model proved to be effective at capturing the averaged Nusselt values per side of the square with relatively low computational costs. The SST model, however, showed poorer fidelity to the local Nusselt number profile compared to the experimental data. The LES approach provided a more accurate representation of the local Nusselt number but the computational cost was significantly higher. The PANS modification to the SST model did provide a noticeable improvement in accuracy at a reasonable cost while the SAS modification did not see the same improvement. These conclusions are generally consistent with those found for the staggered tube bank in Part 1 of this study. This study can be used as a guide for the industrial user to select a turbulence model for a similar problem with a low Reynolds number and significant flow separation.

Assessment of turbulence models for low turbulent natural convection heat transfer in rectangular enclosed cavity using OpenFOAM

This research aimed to study two-equation turbulence models, for the simulation of low turbulent natural convection heat transfer, which happened in a rectangular enclosure. These models include k–ɛ, k–ω, RNG k–ɛ, k–ω SST and Realizable k–ɛ which are two-equation turbulence models in OpenFOAM. The simulation was carried out to compare against experiment in literatures which gave a Rayleigh number (Ra) of 1.58×109. The 3D cell structure by the 50×50×100 cells for the rectangular enclosure dimensions of 0.75 m (width)×0.75 m (height)×1.50 m (length) was modeled. It gave the large y+ of 30 but was still in the initial sub-layer. It was found that the k–ω model provided the best prediction of turbulent natural convection heat transfer. The standard k and ω wall functions were carried out to simulate well with the k–ω model. It was good modeling technique which the fine cells were not necessary to define nearly enclosure walls.

Numerical Prediction of the Boundary-Layer Flow Over the Bolund Hill: Assessment of Turbulence Models and Advection Schemes

The WindStation software package is applied to simulate the wind field over the Bolund hill. The standard, ReNormalization Group (RNG), realizable, and limited-length k−e turbulence models are tested, along with the quadratic upstream interpolation for convective kinetics (QUICK) and hybrid advection schemes. Comparisons with experimental results are made for the speed-up and turbulence kinetic energy (TKE) increase. The adopted mesh is Cartesian uniform in the horizontal plane and terrain following with a variable expansion factor in the vertical direction. The QUICK advection scheme leads to better agreement with the experimental data than the hybrid scheme, in particular for the TKE increase. The RNG and standard k−e turbulence models show the best performance for speed-up and for TKE increase, respectively.

Assessment of turbulence models for single phase CFD computations of a liquid-liquid hydrocyclone using OpenFOAM

ABSTRACT Hydrocyclones are widely used in industry and CFD has been used to compute them. Reynolds stress turbulence models (RSM), which are computationally costly and oftentimes hard to converge, are often recommended in these computations. The present work has selected a liquid-liquid separation hydrocyclone for which single-phase experimental tangential and axial velocity profiles are available. CFD has been employed to test simpler turbulence models than the RSM and results have been compared with experimental data. The turbulence models assessed in the present work were: standard k-ε, standard k-ε with a curvature correction term, RNG k-ε, realizable k-ε, k-ω, SST, a two-time-scale linear eddy viscosity model, nonlinear quadratic and cubic k-ε eddy viscosity models and the Gibson and Launder and LRR Reynolds stress models. Computations have been carried out with OpenFOAM 2.2.2. Results using the Gibson and Launder turbulence model have been compared to some obtained with Ansys Fluent and these were in agreement. Results have shown that all turbulence models, apart from the RSM, returned basically the same tangential velocity profiles as the standard model. All turbulence models have failed in predicting axial velocity. Assessment of the Reynolds stresses has indicated that the internal flow field in hydrocyclones might be shear dominant and that the Reynolds shear stress component is the most relevant to correctly predict tangential velocity. Geometric proportions of hydrocyclones may affect significantly the intensity of rotational and streamline curvature effects. Two-equation eddy-viscosity models are likely to be able to attend such condition, since appropriate levels of eddy viscosity are predicted at free and forced vortexes regions, however further investigation is still needed.

Assessment of turbulence models for the boundary layer transition flow simulation around a hydrofoil

Boundary layer transition is a typical phenomenon in flows around hydrofoils and has a large influence on the hydrodynamic characteristics such as lift & drag, and vortex shedding. The purpose of this paper is to assess the predictive capability of different turbulence models in computational fluid dynamics (CFD) on boundary layer transition along a hydrofoil. Two typical sub-grid scale (SGS) stress models (LES WALE and LES Smagorinsky), as well as DDES γ-Reθt, DDES, SSTCC γ-Reθt, SST γ-Reθt, and SST k−ω are selected. The boundary layer and wake flow calculated by different models are analyzed in detail and compared with the experimental data. The results show that large eddy simulation is more accurate than other models in terms of boundary layer velocity distribution, vortex shedding and wake structure prediction at the same grid conditions. The transition model considering curvature correction influences the predictive capability in the near-wall region compared with SST γ-Reθt. The flow fields calculated by different curvature correction coefficients are quite different. In this paper, the prediction in the front of the transition end point (0.85L) has achieved good results as scaling coefficient is equal to 75 or 100. The transition model considering curvature modification improves the prediction of boundary layer transition by decreasing the turbulence intensity and increasing the pressure gradient of the aft part of the curvature hydrofoil leading. It is found that the LES model is the first choice in the case of enough computing resources. However, for engineering problems, it is highly efficient to adopt SSTCC γ-Reθt model by adjusting the curvature correction coefficient.

Assessment of Turbulence Models on a Backward Facing Step Flow Using OpenFOAM®

Studies on turbulent inlet flows have a wide variety of applications, Backward Facing Step (BFS) facilitates this study. The phenomena of flow separation occur due to an abrupt change in geometry, causing the creation of a re-circulation zone and a flow reattachment point. The present numerical study focuses on the assessment of different turbulence models. The flow velocity is in the turbulent region characterized by the Re=7000. Steady State Reynolds Averaged Navier Stokes equations are solved along with various turbulence models using OpenFOAM® which is an open-source Computational Continuum Mechanics toolbox for conducting numerical simulations. The simulated data is processed using Paraview and the assessment of the turbulence models is performed via quantitative (reattachment length) and qualitative (Line Integral Convolution visualizations) methods.

Assessment of RANS models in predicting mixing flow induced by split-type vanes in rod bundle

Turbulence model assessment is carried out based on particle image velocimetry (PIV) measurement data in a 5 × 5 rod bundle with split mixing vane for the nonlinear eddy viscosity models (NLEVMs) and the Reynolds stress transport (RST) model. The NLEVMs are applied with the two-layer wall treatment. The RST model is closed with the two-layer linear pressure strain model. The validation shows that the RST model is able to predict the mean flow vortices in the subchannel. The quadratic NLEVM predicts the similar, but smaller, vortices, while the cubic NLEVM fails to show the vortices. Comparing with the cross-flow velocities, prediction of axial velocity is relatively unsatisfactory. In general, the lateral normal Reynolds stresses are overpredicted by the models, while the axial normal Reynolds stress is underpredicted. In the near-wake region the lateral normal Reynolds stress can be reasonably predicted with the models, but the axial normal Reynolds stress cannot be predicted in a good accuracy. With the cubic term, the NLEVM predicts smaller normal Reynolds stresses, and especially small value of lateral component in the subchannel center. The NLEVMs which depend on local parameters, show relatively good prediction of the Reynolds stress in the subchannel center where the Reynolds stress is less influenced by the convection and diffusion effect. Invariant analysis of Reynolds stress anisotropy obtained with the models are carried out based on the Lumley triangle. Comparison of invariant analysis with large eddy simulation results of (Busco et al., 2019) indicates that the RST model can predict the far-wake turbulence anisotropy. The cubic NLEVM predicts non-realizable Reynolds stress close to the mixing vanes, while the quadratic model predicts inadequate turbulence anisotropy. All the models cannot predict the disk-like turbulence near the mixing vanes, which can be main reason of inaccurate prediction in the near-wake region.

Assessment of turbulence models for bubbly flows: Toward a five-equation turbulence model

The paper presents an assessment of turbulence models in dispersed bubbly flows based on an analysis of their application to uniform and uniformly sheared homogeneous turbulence. This analysis leads to the development of a five-equation model where the total turbulent fluctuations in bubbly flow is submitted to a double decomposition: First, the turbulent fluctuations are split into Shear-Induced-Turbulence (SIT) and Bubble-Induced-Turbulence (BIT) components. Secondly, The BIT contribution is split into two components corresponding to non-dissipative pseudo-turbulent part and dissipative turbulent part produced by local shear in bubble’s wake. Consequently, the total turbulent energy is split into three contributions: two dissipative quantities kt and ks produced respectively by gradient of mean velocity in the liquid phase and by local shear in bubble’s wake both are described by their modeled transport equations as well as their dissipation rates εt and εs; and a non-dissipative component kp attributed to fluctuations of potential flow around bubbles described by its modeled transport equation. This model was implemented in CFD code and validated in homogeneous bubbly turbulence situations. The numerical results show a satisfactory agreement with experimental data of turbulence.

Flow and heat transfer investigation of a circular jet issuing on different types of surfaces

In the present study, flow and heat transfer characteristics of an impinging jet issuing from a circular cross-sectional pipe on a rectangular plate, convex and concave hemispheres were investigated numerically for various Reynolds numbers and jet-to-plate distances. Steady and three-dimensional Reynolds-Averaged Navier-Stokes equations were solved iteratively utilizing finite volume method. Turbulence model assessment study reveals that the transitional Shear Stress Transport k-ω turbulence model can be used for such a problem. Regardless of the geometry of the impinging surfaces, two different flow regions were detected around y/D = 3. The first region starts from the stagnation point (y/D = 0) to the edge of the rectangular plate and hemispheres (y/D = 3). In this region, the maximum heat transfer is obtained for the jet impinging on the rectangular plate. However, in the second region that covers the wall jet zone the impinging jet on the convex hemisphere provides the maximum while the concave causes the minimum heat transfer. The most efficient jet-to-plate distance is found as H/D = 2 in the first region while heat transfer does not change with distance in the wall jet zone. It was shown that Nusselt number increases with Reynolds number when the jet impinges on the concave and convex hemispheres as observed for the impinging jet on a flat plate.