Ahmed Body External Aerodynamics CFD Benchmark using TCFD® This report presents the benchmark validation of a CFD simulation of the famous external aerodynamics test case - Ahmed body, using TCFD® . Ahmed body is a generic car body (a simplified car model) i.e the flow of air around the Ahmed body captures the essential flow features around an automobile. The aim of the benchmark validation was to evaluate the TCFD®, computational fluid dynamics (CFD) software, and compare its results with the measurement data. The particular goal of this benchmark is to compare the Drag Coefficient, V elocity Profiles, Velocity field, and ® the time-averaged Wake structure of the TCFD simulation results with the real experimental measurement in the wind tunnel [1], [2]. Keywords CFD, VALIDATION, BENCHMARK, TCFD, SIMULATION, EXTERNAL, AERODYNAMICS, AHMED BODY, INCOMPRESSIBLE, RANS, AIR, STEADY-STATE, AIRFLOW, AUTOMATION Benchmark Parameters ● ● ● ● ● ● Typical flow speed: 40 m/s Reynolds Number: 4.29 M Flow model: incompressible Mesh size: 20M cells Medium: air Slant angle: 25º ● ● ● ● ● ● Reference density: 1.2 kg/m3 Dynamic viscosity: 1.8 × 10-5 Pa⋅s CPU Time: 700 core.hours Turb. Model: k-omega SST Simulation type: Virtual Tunnel Turbulence intensity: 1% Preprocessing The original CAD model of the Ahmed body was in the STEP file format. Original STEP files are usually too complex for comprehensive CFD simulations, so certain preprocessing (cleaning) CAD work is generally required. While the original CAD model for this project was simplified and cleaned using Salome open-source software, any other standard CAD system can be used instead. The principle is always the same: the surface model has to be created; all the tiny, irrelevant, and problematic model parts must be removed, and all the holes must be sealed up. This Ahmed body CAD model is reasonably simple. The surface model in the STL format is created as input for the meshing phase. This preprocessing phase of the workflow is extremely important because it determines the simulation potential and limits the CFD results. Mesh The computational mesh was created in an automated workflow using snappyHexMesh a pplication. A cartesian block mesh was used for the initial background mesh to mimic the virtual tunnel. The Ahmed body model itself is 1044mm long, 338mm high, and 389mm wide. The virtual tunnel is 5m high, 11m long, and 6m wide. Basic mesh cell size is a cube of 10mm edge. One Refinement box is included within the refinement level 5. The inflation layer of the mesh, at every wall, has 5 cells, of Expansion ratio 1.25 between cells. The mesh refinement levels can be easily changed, to obtain the coarser or finer mesh, to better handle the mesh size. In case of need, any other external mesh can be loaded directly in MSH, CGNS or OpenFOAM format. Mesh Elements points faces internal faces faces per cell hexahedra # 21227593 61836352 60982292 6.048639 19907349 Mesh Elements prisms pyramids tet wedges polyhedra cells # 42078 0 6 355738 20,305,171 TCFD® Case Setup ● ● ● ● ● ● ● ● Simulation type: Virtual tunnel Time management: steady-state Physical model: Incompressible Number of components: 1 Mesh Mesh size: 20M cells Inlet: Velocity 40 m/s Outlet: Static pressure Turbulence: RANS ● ● ● ● ● ● ● ● Turbulence model: kOmegaSST Wall treatment: Wall functions Speedlines: 1 Simulation points: 1 Fluid: Air Reference pressure: 1 atm Reference density: 1.2 [kg/m3] Dynamic viscosity: 1.8e-05 [Pa.s] Post-processing TCFD® includes a built-in post-processing module that automatically evaluates the required quantities, such as efficiency, forces, force coefficients, flow rates, and much more. All these quantities are evaluated throughout the simulation run, and all the important data is summarized in an HTML report, which can be updated anytime during the simulation for every run. Furthermore, visual postprocessing of the volume fields can be done with ParaView. Results #1 - The Drag Coefficient The Drag Coefficient is a very important total value (dimensionless quantity), which is a measure of the object's resistance to the airflow. A major contribution to the drag of a vehicle stems from the pressure drag which is a consequence of flow separation. According to measurement in the wind tunnel [1], the drag coefficient of the Ahmed body, with the slant angle 25º, has the Drag Coefficient equal to 0.285, while simulation software TCFD® predicts the Drag Coefficient to 0.2848. The difference between the simulation and measurement is so little that it below the drag coefficient image resolution. Ahmed body (slant angle 25º) Drag Coefficient Measurement result according to [1] 0.2850 CFD result according to TCFD® 0.2848 Results #2 - Velocity Profiles Another important quantity is the velocity profile on the centerline of the Ahmed body, at a certain distance (x), from the tail of the body. In this simulation study, it has been evaluated the x-velocity component in the four distances of x = 23, 103, 183, and 243 mm, and compared with the measurement results [2]. Results #3 - Velocity Field Another important goal is the flow visualization on the plain located downstream in the wake, behind the Ahmed body. In the original paper from S.R.Ahmed [1], the authors suggest examining the cross-flow velocity distribution, at three downstream positions in the wake at positions (a), (b), and (c). Shown are the velocity distributions in the half of the symmetric transverse planes at xA/l =-0.077, -0.19 and -0.479. The cross-section boundary of the bubble is shown. Region of reversed flow is cross-hatched. The formation of the side vortex is clearly visible. It is also seen that its core is fed by the separation bubble. Velocity vectors in the cross-hatched region indicate the existence of an upper and lower region of reversed flow; the axis of the upper region is curved upwards in direction of the core of the side edge vortex. Results #4 - Wake Structure Another important goal of this study is examining the wake structure. In the original paper from S.R.Ahmed [1], the authors observed that the wake flow exhibits a triple deck system of horseshoe vortices. Strength, existence, and merging of these vortices depend upon the base slant angle. In a time-averaged sense, the regions of flow separation exhibit complex kinematic macro structures. Such structures in the wake, which is the major separated flow region of a vehicle flow field, determine the drag experienced by the body. The shear layer, coming off the slant side edge, rolls up into a longitudinal vortex, in a manner similar to that observed on the side edge of low aspect ratio wings. At the top and bottom edges of the flat vertical base, the shear layer rolls up as indicated, into two recirculatory flow regions A and B, situated one over another. As the flow over the slant surface is influenced by the vortex C coming off the side edge, the strength of vortex A is dependent upon the strength of vortex C. The flow observation described in [1] was clearly confirmed with the TCFD®simulation. Conclusion The very complex CFD analysis of the Ahmed body aerodynamics, using TCFD®, was performed successfully. It has been shown, that the TCFD® simulation provides very good agreement with the measurement data, in terms of total integral quantities (Drag Coefficient), 1D quantities (Velocity profiles), 2D effects (Velocity planes), and 3D effects (Wake structure). According to measurement in the wind tunnel [1], the drag coefficient of the Ahmed body, with the slant angle 25º, has the Drag Coefficient equal to 0.285, while simulation software TCFD® predicts the Drag Coefficient to 0.2848. The difference between the simulation and measurement is so little that it below the measurement drag coefficient image resolution [1]. The numerical difference of the Drag Coefficient of the Ahmed body between measurement and the TCFD® simulation is less than 0.1% (while the authors of the measurement estimate the measurement itself has an error about 1%, [1] ). In the CFD simulation, the velocity profiles are extremely sensitive to mesh density at the wall. Overall the velocity profiles predicted by the simulation show good agreement with the measurement. Visual characteristics (cross-flow velocity distribution at three downstream planes, and the triple-deck system of horseshoe vortices in the time-averaged wake) of the flow, show very good agreement between the measurement and TCFD® simulation. All the data are freely available. Potential questions will be answered on request at info@cfdsupport.com. [1] S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE-Paper 840300, 1984 [2] H. Lienhart, C. Stoots, S. Becker, Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model), DGLR Fach Symp. der AG STAB, Stuttgart University, 15-17 Nov. 2000 [3] H. Lienhart, S. Becker, Flow and Turbulence Structure in the Wake of a Simplified Car Model, SAE 2003 World Congress, SAE Paper 2003-01-0656, Detroit, Michigan, USA, 2003 [4] C. Hinterberger, M. García-Villalba, W. Rodi, Large Eddy Simulation of flow around the Ahmed body. In "Lecture Notes in Applied and Computational Mechanics / The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains", R. McCallen, F. Browand, J. Ross (Eds.), Springer Verlag, ISBN: 3-540-22088-7, 2004 About TCFD® CFD SUPPORT LTD. creates a new generation of CFD simulation tools. TCFD® massively increases productivity in CFD simulations. It successfully merges the benefits of both open-source and commercial code: due to its open-source nature, TCFD® is perpetual for an unlimited number of users, jobs, and cores, and it is further customizable. Due to its commercial nature, TCFD® is professionally supported, well tested, ready for the industry, robust, accurate, automated, offers a graphical user interface (GUI), documentation and much more. Since TCFD® is unlimited, it is well-suited to demanding workflows like optimization, transient or aeroacoustics. TCFD® scales CFD simulations to the available hardware resource. TCFD® is fully automated and flexible. Its beauty is, that it is the user who decides how deeply to dive into CFD or not at all. TCFD® can be used as a black box (data in - data out) or as a fully sophisticated CFD code where all the options remain open at the same time. TCFD® was originally designed for simulations of rotating machinery like Pumps, Fans, Compressors, Turbines, etc. TCFD® proved to be so effective, that it was later extended with many other applications to cover even a wider range of CFD field. TCFD® shows great performance at the external aerodynamics of various objects. TCFD® numerical solver is open-source. TCFD® is not dependent on other software but it is fully compatible with standard other software packages. TCFD® is fully automated and can run an entire workflow with a single command: from data input, the new case is written down, the mesh is created, the case is set up and simulated, the results are evaluated and the results are written down into a report It operates both in graphical interface (GUI) and in batch mode. TCFD® is from the beginning developed to fit any existing CAE workflow. TCFD® has a modular (plugin) character. Any part of it can be used within other applications. It has strong interfaces to cover a wide range of input and output data. Technical support of TCFD® is unlimited. www.cfdsupport.com | +420 212 243 883 | info@cfdsupport.com https://www.cfdsupport.com/tcfd.html
