Eddy-viscosity Turbulence Model Research Articles

3D simulation of gas-laden liquid flows in centrifugal pumps and the assessment of two-fluid CFD methods

An assessment of a two-fluid model assuming a continuous liquid and a dispersed gas phase for 3D computational fluid dynamics (CFD) simulations of gas/liquid flow in a centrifugal research pump is performed. A monodisperse two-fluid model, in conjunction with a statistical eddy-viscosity turbulence model, is utilized. By a comprehensive measurement database, a thorough assessment of model inaccuracies is enabled. The results on a horizontal diffuser flow reveal that the turbulence model is one main limitation of simulation accuracy for gas/liquid flows. Regarding pump flows, distinctions of single-phase and two-phase flow in a closed and semi-open impeller are figured out. Even single-phase flow simulations reveal challenging requirements on a high spatial resolution, e.g., of the rounded blade trailing edge and the tip clearance gap flow. In two-phase pump operation, gas accumulations lead to coherent gas pockets that are predicted partly at wrong locations within the blade channel. At best, a qualitative prediction of gas accumulations and the head drop towards increasing inlet gas volume fractions (IGVF) can be obtained. One main limitation of two-fluid methods for pump flow is figured out in terms of the violation of the dilute, disperse phase assumption due to locally high disperse phase loading within coherent gas accumulations. In these circumstances, bubble population models do not appear beneficial compared to a monodisperse bubble distribution. Volume-of-Fluid (VOF) methods may be utilized to capture the phase interface at large accumulated gas cavities, requiring a high spatial resolution. Thus, a hybrid model, i.e., a dispersed phase two-fluid model including polydispersity for flow regions with a dilute gas phase, should be combined with an interphase capturing model, e.g., in terms of VOF. This hybrid model, together with scale-resolving turbulence models, seems to be indispensable for a quantitative two-phase pump performance prediction.

Numerical Investigations of parabolic trough collectors using different nanofluids

This paper presents three-dimensional numerical simulations of parabolic trough collectors (PTC) based on twolow-Reynolds eddy viscosity turbulence models, namely; Launder and Sharma k-epsilon and k-omega SST models. Forthe simulations, water was used as the Heat Transfer Fluid (HTF) with four different nanoparticles; Al2O3, TiO2, CuO andCu. Different volume fractions (φ) of the nanoparticles were investigated for various Reynolds (Re) numbers with uniformheat flux. Results showed that the overall performance of the system is more sensitive to changes in the thermal properties of nanofluids than the thermal properties of the HTF. At a volume fraction of 6% and a Re number of 70,000, the Nusselt number (Nu) enhancement of nanofluids TiO2-water, Al2O3-water, CuOwater, and Cu-water were found to be 21.5%, 20.2%, 18.11%, and 15.7% respectively while the performance evaluation criteria (PEC) were 1.214, 1.2, 1.18 and 1.155 respectively.

Improved CFD prediction of flows past simplified and real-life automotive bodies using modified turbulence model closure coefficients

In spite of its shortcomings, faster turnaround time and cost-effectiveness make the Reynolds-averaged Navier–Stokes modeling approach still a popular and widely used methodology in many industrial applications, including the automotive industries in general, but the motorsports sector in particular. Existing literature suggests that all Reynolds-averaged Navier–Stokes models generally fail to predict pressure and velocity flow fields with a reasonable accuracy, especially in the vehicle wake region. Recent numerical works suggest that, when using two-equation eddy viscosity turbulence models, improved correlation between the experiment and computational fluid dynamics is not achievable through only mesh refinements or adding additional corrective terms in the turbulence transport equations, and additional efforts are necessary for better predictions. In this backdrop, the prediction improvement strategy adopted in this paper is based on the realization that the turbulence model closure coefficients are normally specified as constants and their values are determined from either a single observation or a simple functional form is assumed and that these coefficients are constructed and/or constrained to behave correctly in extremely limiting circumstances. Subsequently, this study investigated the influence of a few selected turbulence model closure coefficients on the veracity of computational fluid dynamics predictions by analyzing simulations run with turbulence model closure coefficient values that were different from the commonly used default ones. This was done by first investigating the individual effect of each model parameters on the prediction veracity, and then a combination of model closure coefficient values was formulated in order to obtain a prediction with the best experimental correlation. This procedure was applied to four different test objects which include NACA 4412 airfoil at 12° angle of attack, the 25 and 35° slant angle Ahmed body, and a full-scale sedan type passenger vehicle. The shear-stress transport [Formula: see text] was chosen as the turbulence model because of its popularity in automotive applications. It was observed that, irrespective of the test model, the computational fluid dynamics predictions are somewhat independent of some closure coefficients, while the prediction showed strong dependencies on certain model constant values, especially those in the dissipation equation. The present study demonstrated that, by using a modified set of closure coefficient values, very well correlated computational fluid dynamics predictions were possible. The findings from this study simply reiterate a 20-year-old conclusion by Stephen Pope that the closure coefficients for a model are not static and universal but instead are dynamic and must be calibrated for specific flow situations.

Improvement in Savonius Wind Turbines Efficiency by Modification of Blade Designs—A Numerical Study

Abstract Savonius wind turbines are special class of vertical axis wind turbines (VAWTs). These are low-cost drag-driven turbines and are known to be inefficient. It is proposed in this study that a simple modification to the turbine blade design can yield a significant improvement in power efficiency. The performance of the new design is extensively studied on openfoam-v1812, a popular open source computational fluid dynamics (CFD) library. The flow equations coupled with equations of rotation of the turbine are solved on an overset mesh framework. This study also serves as a validation of recently released overset support in openfoam. The turbulence is incorporated by coupling Reynolds-averaged Navier–Stokes (RANS) with shear stress transport (SST) κ − ω eddy viscosity turbulence model. The turbulence parameters are set to produce a flow with the Reynolds number, Re = 4.8 × 105. To have better confidence in simulations, this study also presents a comparison of numerical flow over conventional Savonius turbine designs with the published data. It is observed that a majority of CFD analysis on wind turbine designs are performed for the fixed tip speed ratio on a traditional static mesh structure. But, in this CFD study, a wind-driven rotation of Savonius turbine is simulated on an overset dynamics approach. The results of the study are compared and discussed based on the predicted moment and power coefficients, pressure variation on the blades, flow velocity field, and wake analysis. The study indicates that the blade design presented here has a potential to increase the power efficiency of a Savonius wind turbine by 10–28%.

Influence of moving plate velocity on conjugate heat transfer due to the impingement of an inclined slot jet

In this paper, a dimensionless numerical study of the flow-field and heat transfer characteristics of an incompressible turbulent slot jet impinging obliquely over a moving surface of finite thickness is presented. Simulations were performed using [Formula: see text] eddy viscosity turbulence model. The temperature field was solved simultaneously in the solid and the fluid domain. For a fixed impingement distance and a fixed Reynolds number, the impingement angle ([Formula: see text]) and plate velocity ([Formula: see text]) were varied in the range of 30–75∘ and 0–0.3, respectively. In the results, the length of the potential core depends on the jet inclination, which increases with increase in jet angle. The jet angle and plate velocity have more influence on the uphill side compared to the downhill side. The location of stagnation displaces toward the uphill side as the inclination angle decreases, and the drifting of stagnation point is noted with the variation in plate velocity. The average skin-friction coefficient increases with increase in [Formula: see text] and [Formula: see text], and the influence of [Formula: see text] on the skin-friction coefficient is reduced as [Formula: see text] increases. The maximum Nusselt number ([Formula: see text]) increases with increase in [Formula: see text], and the drifting of [Formula: see text] is observed with increase in plate velocity. It is found that the average Nusselt number increases quickly with increase in plate velocity for lower angles of impingement. The distribution of local heat flux follows the same trend as the local Nusselt number.

Improved Eddy-Viscosity Modelling of Turbulent Flow around Porous–Fluid Interface Regions

The RANS modelling of turbulence across fluid-porous interface regions within ribbed channels has been investigated by applying double (both volume and Reynolds) averaging to the Navier–Stokes equations. In this study, turbulence is represented by using the Launder and Sharma (Lett Heat Mass Transf 1:131–137, 1974) low-Reynolds-number $$k-\varepsilon $$ turbulence model, modified via proposals by either Nakayama and Kuwahara (J Fluids Eng 130:101205, 2008) or Pedras and de Lemos (Int Commun Heat Mass Transf 27:211–220, 2000), for extra source terms in turbulent transport equations to account for the porous structure. One important region of the flow, for modelling purposes, is the interface region between the porous medium and clear fluid regions. Here, corrections have been proposed to the above porous drag/source terms in the k and $$\varepsilon $$ transport equations that are designed to account for the effective increase in porosity across a thin near-interface region of the porous medium, and which bring about significant improvements in predictive accuracy. These terms are based on proposals put forward by Kuwata and Suga (Int J Heat Fluid Flow 43:35–51, 2013), for second-moment closures. Two types of porous channel flows have been considered. The first case is a fully developed turbulent porous channel flow, where the results are compared with DNS predictions obtained by Breugem et al. (J Fluid Mech 562:35–72, 2006) and experimental data produced by Suga et al. (Int J Heat Fluid Flow 31:974–984, 2010). The second case is a turbulent solid/porous rib channel flow to examine the behaviour of flow through and around the solid/porous rib, which is validated against experimental work carried out by Suga et al. (Flow Turbul Combust 91:19–40, 2013). Cases are simulated covering a range of porous properties, such as permeability and porosity. Through the comparisons with the available data, it is demonstrated that the extended model proposed here shows generally satisfactory accuracy, except for some predictive weaknesses in regions of either impingement or adverse pressure gradients, associated with the underlying eddy-viscosity turbulence model formulation.

Destabilization of a planar liquid stream by a co-flowing turbulent gas stream

In Airblast atomization, the disintegration of a liquid stream is assisted by a co-flowing high speed gas stream. The shear instability develops at the interface, forming interfacial waves that propagate and eventually break into droplets downstream. In the present study, the destabilization of a planar liquid stream by a co-flowing gas stream with different turbulence intensities is investigated through direct numerical simulation. A parametric study is conducted to investigate the effect of gas inlet turbulence intensity on the interfacial instability near the nozzle exit and the development of two-phase mixing layer downstream. The gas-liquid interface is resolved by a momentum-conserving volume-of-fluid method. A digital filter approach is used to generate temporally and spatially correlated turbulent velocity fluctuations at the gas inlet. The interfacial stability is absolute for all cases considered. The dominant frequency and the spatial growth rate corresponding to the most unstable mode are measured and compared with experiments and spatial-temporal linear stability analysis. The numerical results of the dominant frequencies agree well with the experimental data. Both the dominant frequency and the spatial growth rate increase with the gas inlet turbulence intensity, after passing a threshold value. The linear stability analysis with the turbulent eddy viscosity model captures the increasing trends of the dominant frequency and the spatial growth rate but significantly underpredicts the values. Two-phase turbulence statistics, including Reynolds stresses and turbulent kinetic energy dissipation, are also presented. It is shown that the gas inlet turbulence tends to enhance interaction between the two streams near the nozzle exit and to reduce the interaction downstream.

Application of eddy-viscosity turbulence models to problems in ship hydrodynamics

ABSTRACT With the rapid advent of computational methods in all fields of engineering, several areas have emerged as significant sources of ambiguity. Among these is the selection of a turbulence model to close the Reynolds averaged Navier-Stokes equations. In ship hydrodynamics, this has been particularly difficult to resolve due to the complex nature of the problem. The present study aims to alleviate the ambiguity inherent in the field. This is done by performing a series of tests on turbulence models and comparing the outcomes with experimental results. The results are analysed via a modified bivariate plot, which reveal a strong candidate for the optimum choice of turbulence modelling. The time per iteration also points towards the same candidate, identified as the standard k-ω model, as a good choice within the software used to perform the analysis.

Comparison of k-ε models in gaseous release and dispersion simulations using the CFD code FLACS

Several model validation studies on gas dispersion scenarios have been conducted in the past on the Reynolds averaged Navier Stokes (RANS) based eddy viscosity turbulence models. However, many of these studies are based on a limited number of validation cases involving simple geometries and conformal mesh. In the area of safety engineering, the application of RANS-based CFD for consequence analysis is a widely used methodology. Best practice on use of CFD in this context, as the document developed in the COST Action 732 (Franke et al., 2007), focus primarily on validation and verification aspects as well as simulation setup and definition of input data. Guidelines on turbulence models also exist, among which the ERCOFTAC CFD Best Practice Guidelines, and the works of Meroney et al. (2016) and Mcbride et al. (2001). However, there is no unique recommended model for dispersion simulations. The objective of the present study is to assess the three well-known RANS eddy viscosity models, namely, Standard k-ε, Re-Normalization group (RNG) k-ε and Realizable k-ε, in a representative range of gas dispersion cases by comparing models’ behavior with experimental data. The current validation cases include dense CO2 release in a cross-wind, impinging hydrogen jet, and a dense chlorine jet release in an industrial site. All the simulations were conducted using the commercial CFD code FLACS. Turbulence models were assessed based on the ability to reproduce experimental concentrations, required computational-time and numerical-stability. Overall, Standard k-ε and RNG k-ε models were found to be reasonably good in all cases. Nevertheless, Realizable k-ε model shows promise in yielding good results in cases involving complex-geometries and dense-phase gas-releases. These results may also be explained with the interplay between the Porosity/Distributed Resistance subgrid models used in FLACS and turbulence models.

Parametric study on a wall-mounted attached ventilation system for night cooling with different supply air conditions

Night ventilation is a sustainable approach to achieve comfortable built environment with relatively low energy consumption in summer. However, the cooling performance of conventional night ventilation in the hot summer climate zone is unsatisfactory due to the insignificant climatic potential. Considering that a large proportion of the heat gains is stored in the building envelope, particularly in the external wall, we propose a novel wall-mounted attached night ventilation (WANV) for night cooling. This novel system is capable of generating a downward air jet that directly flows over the internal surface of the west-external wall to achieve enhanced convective cooling performance on the west wall during the nighttime. The present paper numerically investigates the air flow patterns and temperature field distributions in the test chamber with WANV under different supply air conditions (air velocity and temperature). The shear-stress transport k–omega turbulence model, a blending two-equation eddy-viscosity turbulence model which is suitable to capture airflows with strong streamline curvature and separation, is employed in the numerical study. Comparisons between the experimental data and the simulation results are carried out, and the good agreement suggests that the numerical model is able to predict the air velocity and temperature profiles of the chamber with WANV. The simulation results show that intensive air recirculation and enhanced convection are achieved due to the high turbulent air momentum in the scenario with WANV. A similar tendency of the decay of air velocities in the near-wall region is observed regardless of the supply air velocity. Furthermore, a correlation between the air velocity and the height above the floor is proposed. The cooling performance of WANV is significantly improved by decreasing the supply air temperature. Additionally, the impact of supply air conditions on the cooling performance is quantified using the relative heat releasing effectiveness index, which suggests that the optimal supply air velocity is approximately 4 m/s in hot summer at Chongqing or locations with similar climate conditions.

Towards the distributed burning regime in turbulent premixed flames

Three-dimensional numerical simulations of canonical statistically steady, statistically planar turbulent flames have been used in an attempt to produce distributed burning in lean methane and hydrogen flames. Dilatation across the flame means that extremely large Karlovitz numbers are required; even at the extreme levels of turbulence studied (up to a Karlovitz number of 8767) distributed burning was only achieved in the hydrogen case. In this case, turbulence was found to broaden the reaction zone visually by around an order of magnitude, and thermodiffusive effects (typically present for lean hydrogen flames) were not observed. In the preheat zone, the species compositions differ considerably from those of one-dimensional flames based a number of different transport models (mixture averaged, unity Lewis number and a turbulent eddy viscosity model). The behaviour is a characteristic of turbulence dominating non-unity Lewis number species transport, and the distinct limit is again attributed to dilatation and its effect on the turbulence. Peak local reaction rates are found to be lower in the distributed case than in the lower Karlovitz cases but higher than in the laminar flame, which is attributed to effects that arise from the modified fuel-temperature distribution that results from turbulent mixing dominating low Lewis number thermodiffusive effects. Finally, approaches to achieve distributed burning at realisable conditions are discussed; factors that increase the likelihood of realising distributed burning are higher pressure, lower equivalence ratio, higher Lewis number and lower reactant temperature.

On the accuracy of turbulence models for CFD simulations of vertical axis wind turbines

A comparative analysis of seven commonly-used eddy-viscosity turbulence models for CFD simulations of VAWTs is presented. The models include one- to four-equations, namely the Spalart-Allmaras (SA), RNG k-ε, realizable k-ε, SST k-ω, SST k-ω with an additional intermittency transition model (SSTI), k-kl-ω and transition SST (TSST) k-ω models. In addition, the inviscid modeling is included in the comparison. The evaluation is based on validation with three sets of experiments for three VAWTs with different geometrical characteristics operating in a wide range of operational conditions, from dynamic stall to optimal regime and to highly-rotational flow regime. The focus is on the turbine wake, the turbine power performance, and the blade aerodynamics. High-fidelity incompressible unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are employed. The extensive analysis reveals high sensitivity of the simulation results to the turbulence model. This is especially the case for the turbine power coefficient CP. The results show that the inviscid, SA, RNG k-ε, realizable k-ε and k-kl-ω models clearly fail in reproducing the aerodynamic performance of VAWTs. Only the SST model variants (SST k-ω, SSTI and TSST) are capable of exhibiting reasonable agreement with all the experimental data sets, where the transitional SST k-ω versions (SSTI and TSST) are recommended as the models of choice especially in the transitional flow regime.

Parametric data-based turbulence modelling for vortex dominated flows

ABSTRACTComputational fluid dynamics simulations employing eddy-viscosity turbulence models remain the baseline numerical tool in the aerospace industry, mainly due to their numerical stability and computational efficiency. However, many industrially relevant cases require a level of accuracy that is not routinely achieved by global turbulence models. The simulation of leading-edge vortices shed at low aspect ratio wings is one such class of flows that remains a challenge for turbulence modelling. A local approach is proposed in which a parametrised eddy-viscosity turbulence model is calibrated using experimental results of configurations and flow conditions similar to the one being analysed. In this paper, the Spalart–Allmaras one-equation model is enhanced with additional source terms, which are exclusively active in the vortex field. An automatic optimisation procedure with experimental data as reference is then applied. The resulting optimised model improves the eddy viscosity distribution for a limited but relevant range of configurations and flow conditions.

Computational investigation of axi-symmetric plume induced flow separation

The flow over a rocket may result in plume induced flow separation (PIFS) depending on flight conditions and nozzle-to-ambient pressure ratios. In the current study, the flow structure over a generic rocket model has been investigated to determine the conditions that would results in PIFS over the rocket body. The rocket used in the current study has a tangent-ogive nose with a length of 25.4 cm (nose length/diameter ∼ 4.0), followed by a 57.2-cm long cylindrical rocket body (overall length/diameter ∼ 13.0). A 45° conical nozzle with an exit Mach number of 2.7 (nozzle expansion ratio, ε ∼1.786) was used to simulate the plume. The total temperature and the total pressure in the combustion chamber were kept as 3589 K (6000 °F), and 5.86 MPa ab (850 psia). The flow conditions investigated including flight Mach numbers from 0.7 to 2.0, at five different altitudes; 0, 3.05, 6.10, 9.14 and 12.2 km (0, 10,000, 20,000, 30,000 and 40,000 feet). Based on the validation study using various eddy-viscosity turbulence models, the k-ε RNG turbulence model was selected for the parametric study. PIFS is not observed at Mach numbers less than 1.0 up to altitudes including 9.1 km (30,000 ft), and occurs only between Mach 1.0 and 2.0. The Mach number range where PIFS is observed increases with increasing altitude. However at 12.2 km (40,000 ft) PIFS is observed at every Mach number considered, including subsonic range.

Flow measurement and parameter optimization of right-angled flow passage in hydraulic manifold block

This study was conducted to investigate the flow field characteristics of right-angled flow passage with various cavities in the typical hydraulic manifold block. A low-speed visualization test rig was developed and the flow field of the right-angled flow passage with different cavity structures was measured using 2D-PIV technique. Numerical model was established to simulate the three-dimensional flow field. Seven eddy viscosity turbulence models were investigated in predicting the flow field by comparing against the particle image relocimetry (PIV) measurement results. By defining the weight error function K, the S-A model was selected as the appropriate turbulence model. Then, a three-factor, three-level response surface numerical test was conducted to investigate the influence of flow passage connection type, cavity diameter and cavity length-diameter ratio on pressure loss. The results show that the Box-Benhnken Design (BBD) model can predict the total pressure loss accurately. The optimal factor level appeared in flow passage connection type II, 14.64 mm diameter and 67.53% cavity length-diameter ratio. The total pressure loss decreased by 11.15% relative to the worst factor level, and total pressure loss can be reduced by 64.75% when using an arc transition right-angled flow passage, which indicates a new direction for the optimization design of flow passage in hydraulic manifold blocks.

Large eddy simulations of forced heat convection in a pin-fin array with a priori examination of an eddy-viscosity turbulence model

Reynolds-averaged Navier–Stokes (RANS) modeling remains the predominant technique for engineering simulations of turbulent flow and heat transfer. Most simulations employ the linear-eddy viscosity assumption to model the Reynolds stresses, which is known to produce inaccurate results in complex turbulent flows. The objective of this study is to provide new insight on the inadequacy of LEV models in complex engineering turbulence by performing an a priori examination of a typical LEV model, the k−ω shear-stress transport (SST) model, for flow predictions in a pin-fin array. First, LES results obtained from simulations with three levels of mesh resolution are presented and validated against published experimental data. Second, an a priori examination of the SST model is performed, considering discrepancies between the shape and orientation of the Reynolds stress tensors obtained from the SST model and the LES. The results illustrate that the LES predicted Reynolds stresses have significantly different characteristics in regions of developing versus fully developed turbulence, and in the bulk-flow versus near-wall regions. These differences are not reflected in the Reynolds stresses predicted by the SST model, which predicts very similar Reynolds stress shapes and orientations in these different flow regimes. The analysis provides some insight on correcting the shape of the Reynolds stress tensor in particular; this information will be used in future work to examine practical methods to quantify turbulence model form uncertainties.

Numerical studies on dynamic stall characteristics of a wind turbine airfoil

The present work delivers an evaluation of the S809 airfoil under static and dynamic conditions employing a computational fluid dynamics (CFD) approach. Pitching motions with three different mean angles of attack, 8°, 14° and 20°, with a reduced frequency of 0.079 and amplitude of 10° were imposed at a quarter chord position. Two widely used eddy-viscosity turbulence models, Spalart-Allmaras (SA) and Menter shear-stress-transport (SST), were tested to capture the static and dynamic behavior of the flow. A high order flux discretization, weighted essentially non-oscillatory (WENO) scheme was used in these computations to better resolve the detaching vortices. The results were validated against experimental data and an empirical correlation according to the Leishman-Beddoes model with reasonable agreement. The SA model gave the best prediction in the post-stall regime at high angles of attack. The pressure evolution and the flow field surrounding the airfoil in the onset of dynamic stall were presented and evaluated.

Epistemic uncertainty quantification for Reynolds-averaged Navier-Stokes modeling of separated flows over streamlined surfaces

It is well known that linear eddy-viscosity turbulence models can introduce uncertainty in predictions for complex flow features such as separation and reattachment. The goal of this paper is to advance our understanding of a physics-based approach to quantify this turbulence model form uncertainty in Reynolds-averaged Navier-Stokes simulations of separated flows over streamlined surfaces. The methodology is based on perturbing the modeled Reynolds stresses in the momentum equations; perturbations are defined in terms of a decomposition of the Reynolds stress tensor, i.e., based on the tensor magnitude and the eigenvalues and eigenvectors of the normalized anisotropy tensor. We demonstrate that the accuracy of the predicted Reynolds stress magnitude is strongly influenced by the turbulence production term and subsequently explore the anisotropy tensor eigenvalue and eigenvector perturbations that maximize or minimize turbulence production; these could be expected to provide bounds on the prediction of separation and reattachment locations. The method uses two user-defined parameters to identify the spatial extent of the perturbed region and the magnitude of the eigenvalue perturbations. Results for the flow over a periodic wavy wall and over a three-dimensional hill indicate that the perturbations that increase turbulence production decrease the extent of the separation region, while perturbations that decrease production increase the region of separated flow. The predicted bounds can successfully encompass the reference data, provided the extent of the perturbed region and the eigenvalue perturbation magnitudes are sufficiently large. Importantly, we observed a monotonic behavior of the magnitude of the predicted bounds as a function of the two user-defined parameters.

Assessment of Turbulence Models in a Hypersonic Cold-Wall Turbulent Boundary Layer

In this study, the ability of standard one- or two-equation turbulence models to predict mean and turbulence profiles, the Reynolds stress, and the turbulent heat flux in hypersonic cold-wall boundary-layer applications is investigated. The turbulence models under investigation include the one-equation model of Spalart–Allmaras, the baseline k - ω model by Menter, as well as the shear-stress transport k - ω model by Menter. Reynolds-Averaged Navier-Stokes (RANS) simulations with the different turbulence models are conducted for a flat-plate, zero-pressure-gradient turbulent boundary layer with a nominal free-stream Mach number of 8 and wall-to-recovery temperature ratio of 0.48 , and the RANS results are compared with those of direct numerical simulations (DNS) under similar conditions. The study shows that the selected eddy-viscosity turbulence models, in combination with a constant Prandtl number model for turbulent heat flux, give good predictions of the skin friction, wall heat flux, and boundary-layer mean profiles. The Boussinesq assumption leads to essentially correct predictions of the Reynolds shear stress, but gives wrong predictions of the Reynolds normal stresses. The constant Prandtl number model gives an adequate prediction of the normal turbulent heat flux, while it fails to predict transverse turbulent heat fluxes. The discrepancy in model predictions among the three eddy-viscosity models under investigation is small.

Turbulence modeling effects on the aerodynamic characterizations of a NASCAR Generation 6 racecar subject to yaw and pitch changes

The characterization of a racecar’s aerodynamic behavior at various yaw and pitch configurations has always been an integral part of its on-track performance evaluation in terms of lap time predictions. Although computational fluid dynamics has emerged as the ubiquitous tool in motorsports industry, a clarity is still lacking about the prediction veracity dependence on the choice of turbulence models, which is central to the prediction variability and unreliability for the Reynolds Averaged Navier–Stokes simulations, which is by far the most widely used computational fluid dynamics methodology in this industry. Subsequently, this paper presents a comprehensive assessment of three commonly used eddy viscosity turbulence models, namely, the realizable [Formula: see text] (RKE), Abe–Kondoh–Nagano [Formula: see text], and shear stress transport [Formula: see text], in predicting the aerodynamic characteristics of a full-scale NASCAR Monster Energy Cup racecar under various yaw and pitch configurations, which was never been explored before. The simulations are conducted using the steady Reynolds Averaged Navier–Stokes approach with unstructured trimmer cells. The tested yaw and pitch configurations were chosen in consultation with the race teams such that they reflect true representations of the racecar orientations during cornering, braking, and accelerating scenarios. The study reiterated that the prediction discrepancies between the turbulence models are mainly due to the differences in the predictions of flow recirculation and separation, caused by the individual model’s effectiveness in capturing the evolution of adverse pressure gradient flows, and predicting the onset of separation and subsequent reattachment (if there be any). This paper showed that the prediction discrepancies are linked to the computation of the turbulent eddy viscosity in the separated flow region, and using flow-visualizations identified the areas on the car body which are critical to this analysis. In terms of racecar aerodynamic performance parameter predictions, it can be reasonably argued that, excluding the prediction of the %Front prediction, shear stress transport is the best choice between the three tested models for stock-car type racecar Reynolds Averaged Navier–Stokes computational fluid dynamics simulations as it is the only model that predicted directionally correct changes of all aerodynamic parameters as the racecar is either yawed from the 0° to 3° or pitched from a high splitter-ground clearance to a low one. Furthermore, the magnitude of the shear stress transport predicted delta force coefficients also agreed reasonably well with test results.