DES Turbulence 030808

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Heat transfer predictions using advanced two-equation turbulence models ANSYS CFX Validation Report Development and application of a zonal DES turbulence model for CFX-5 CFX-VAL17/0703 Development and application of a zonal DES model for CFX-5 © CFX Ltd. CFX-VAL17/0703 Development and application of a zonal DES turbulence model for CFX-5 F. R. Menter, M. Kuntz, ANSYS CFX Abstract This report describes the implementation of a zonal DES model in CFX-5, including the fo
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  Heat transfer predictions using advanced two-equation turbulence models   ANSYS CFX Validation Report Development and applicationof a zonal DES turbulencemodel for CFX-5 CFX-VAL17/0703  Development and application of a zonal DES model for CFX-5  CFX Ltd. CFX-VAL17/0703 Development and application of a zonal DES turbulence model for CFX-5   F. R. Menter, M. Kuntz, ANSYS CFX Abstract This report describes the implementation of a zonal DES model in CFX-5, including theformulation, numerical treatment and a detailed discussion of the results obtained when theDES model is applied to three testcases: flow over a cylinder, a cube in channel flow, andthe Ahmed car body. Table of Contents 1 Introduction.......................................................................................................................12 Turbulence models............................................................................................................32.1 SST model................................................................................................................3 2.2 SST-DES formulation Strelets et al.........................................................................42.3 SST-DES formulation CFX.....................................................................................4 2.4 Numerical treatment of DES model.........................................................................63 Testcases...........................................................................................................................83.1 Circular cylinder.......................................................................................................8 3.2 Cube in channel flow..............................................................................................103.3 The Ahmed car body..............................................................................................154 Grid induced separation - revisited.................................................................................295 Conclusions.....................................................................................................................306 Notes...............................................................................................................................316.1 Acknowledgment...................................................................................................31 6.2 References..............................................................................................................31 Appendix 1: SST-DES-CFX model set-up in CFX-5........................................................32  Development and application of a zonal DES model for CFX-5CFX-VAL17/0703  CFX Ltd. 1 1 Introduction Turbulence model development for aerodynamic applications has for many yearsconcentrated on improving the capabilities of CFD methods for separation prediction. Theindustrial motivation came from the overwhelming importance of flow separation and stallfor aerodynamic flows around wings and aircraft configurations. The accurate prediction of the stall characteristics was and is one of the main reasons for the development and use of CFD methods based on the Reynolds Averaged Navier-Stokes (RANS) equations.Validation studies of turbulence models in the 1980s have clearly shown that mostturbulence models were not capable of predicting the development of turbulent boundarylayers under adverse pressure gradient conditions. Based on that observation, new modelswere developed specifically to meet this challenge, resulting in a series of models capableof capturing boundary layer separation in good agreement with experimental data (Johnsonand King, 1984, Menter, 1993, Spalart and Allmaras, 1994).From experience with the SST turbulence model (Menter, 1993), the present authorswould argue that the capability of the model with respect to the prediction of the onset of separation is within the accuracy of the available experimental data and that no systematicdeviation between the simulations and the data is observed. Based on the experimentalevidence (which is admittedly limited for three-dimensional flows), there is currently littleneed for model improvements for that kind of flows. Of course, this does not imply thataerodynamic flows can be predicted within experimental uncertainty levels, as these flowsinvolve other effects, which pose additional challenges to the turbulence model. The mainareas of concern are the behavior of the flow downstream of the separation line, includingthe flow recovery after re-attachment (Johnson et al., 1994), the proper simulation of vortexflows and questions related to laminar-turbulent transition. In particular, the flowdevelopment downstream of separation is of major importance from an aerodynamicstandpoint and can have a significant effect on the characteristics of aerodynamic bodies.This is particularly true for ground vehicles, as they generally exhibit significant regions of separated flows, even at design conditions.From a modeling standpoint, it has been observed for a long time that RANS turbulencemodels underpredict the level of the turbulent stresses in the detached shear layer emanatingfrom the separation line (Johnson et al., 1994). This in turn seems to be one of the mainreasons for the incorrect flow recovery predicted by the models downstream of reattachment. The issue is sometimes masked by the tendency of models to under-predictthe onset and therefore the strength of the separation, which in turn results in an acceptableagreement in the recovery region. However, the improvement is only the result of acancellation of errors, as one cannot trade separation prediction capabilities againstimproved velocity profiles in the recovery region. The delayed recovery of the boundarylayer downstream of the reattachment line can lead to a premature separation under asecond adverse pressure gradient. This is a scenario which can, for example, be found onwings with a double shock structure as argued by Johnson et al. (1994). If the boundarylayer recovery after the first shock is not reproduced properly, the predicted boundary layer  behavior at the second shock is not reliable, even if the turbulence model is in principle ableto predict the separation onset for undisturbed flows. A second and even more disturbinguncertainty resulting from the incorrect prediction of the detached shear-layer concerns theflow topology downstream of the separation line. Current turbulence models cannot reliablyanswer the question of whether the flow is forming a closed separation bubble, or a fullystalled flow regime. This question is of major importance for the prediction of theaerodynamic characteristics of automobiles, which almost always exhibit regions of separated flow. The topology of these regions has a strong influence on the drag and more pronouncedly on the lift of the car.In an attempt to improve the predictive capabilities of turbulence models in highlyseparated regions, Spalart (1997) proposed a hybrid approach, which combines features of   Development and application of a zonal DES model for CFX-52  CFX Ltd. CFX-VAL17/07032 classical RANS formulations with elements of Large Eddy Simulations (LES) methods. Theconcept has been termed Detached Eddy Simulation (DES) and is based on the idea of covering the boundary layer by a RANS model and of switching the model to a LES modein detached regions. Ideally, DES would predict the separation line from the underlyingRANS model, but capture the unsteady dynamics of the separated shear layer by resolutionof the developing turbulent structures. Compared to classical LES methods, DES savesorders of magnitude of computing power for high Reynolds number flows, due to themoderate costs of the RANS model in the boundary layer region, but still offers some of theadvantages of an LES method in separated regions.There are two main concerns with the current DES formulation. The first is how quicklythe unsteady turbulent structures develop after the model has switched from the RANS tothe LES mode. This is of significance for the prediction of separated shear layers, as adelayed onset of resolved turbulent structures would aggravate the underprediction of theturbulent stresses due to a reduction of the unresolved turbulence level by the DESformulation.The second concern is with the switching mechanism employed by the current DESmethods. In order to prevent the activation of the DES limiter in attached boundary layers,it is typically required to ensure a lower limit on the local surface grid resolution. If thiscondition is violated, the integrity of the RANS model is severely compromised resulting inmost cases in grid induced separation. This issue will be addressed in the section onturbulence model formulation. For an alternative of a hybrid turbulence model without anexplicit grid dependency see Menter et al. (2003).In a first step, the flow around a circular cylinder and the flow over a cube mounted in achannel will be computed. These tests serve the evaluation of both the numerical and themodel performance of the current SST-DES implementation.In a second step, the capabilities and limitations of advanced aerodynamic RANS andDES turbulence models for automotive applications will be evaluated and discussed.Alternatives to the current RANS/DES switch will be discussed. The models will be appliedto the flow around a simplified generic car shape, known as the Ahmed car body (Ahmedand Ramm, 1984, Linehart et al. 2000). The RANS simulations have been presented at theERCOFTAC workshop on Refined Turbulence Modelling (Durand et al, 2002) in acomparison study of different CFD methods for specific testcases. A detailed report of theRANS simulations presented at the ERCOFTAC workshop is available from the authorsupon request.All simulations have been computed with the commercial CFD method CFX-5 of ANSYS. The current release version CFX-5.6 includes the SST-DES-CFX model asdescribed in the present report but in β -mode only.
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