Ahmed Body


  • Vehicle Aerodynamics

Models active in this Benchmark case

  • Incompressible flow
  • Detached Eddy
  • SST Formulation
  • Double Precision


To compare the performance of Envenio’s EXN/AERO manycore CFD solver against a leading commercial CFD tools on a cost-normalized basis.

From a field of candidates, Envenio selected an engineer to run a CFD simulation using two commercial software packages. These included Fluent and EXN/Aero. While the engineer was required to be knowledgeable in Fluent, experience with EXN/Aero was not required and the engineer was provided training from Envenio in this regard.

Apart from knowledge of CFD software packages, the tester was required to:

  • have experience with unsteady turbulent flow simulation including URANS and LES/DES approaches,
  • be comfortable navigating & managing data within a linux environment,
  • have access to a linux machine for post-processing results.

The engineer was required to run a ~30 million element finite volume simulation using each solver. For the Fluent solution, the tester created a cut-cell mesh in the Ansys meshing utility that included progressive mesh refinement near the body using hanging nodes. For EXN/Aero, a fully Cartesian CGNS mesh file (with structred data type) was created in Pointwise by Envenio engineers and provided to the tester.

Upon completion of the simulations, the engineer prepared a short report that described the simulation results in terms of performance and accuracy. Specific metrics are listed in the ‘outcomes’ section.

Model Description – Ahmed Body

The Ahmed body described originally by Ahmed et al. in 1984 [1] is shown Fig. 1.  The configuration with a slant angle of has been studied extensively both experimentally and computationally. A review of recent experimental drag data over a range of Reynolds numbers is provided by Bello-Millan et al. (2016). The reference shows a wide range of experimental data, , which depends critically on the sharpness of the model edges and Reynolds number. A comprehensive computational study using both LES and DES approaches by Serre et al. (2011) reports lift and drag results for several subgrid and turbulence models including that employed by EXN/Aero. Meille et al. (2011) provide RSM RANS simulation and experimental results.

Figure 1: Ahmed body general dimensions

Mesh Description


Shape                                                    Rectangular box

Length                                                    9.2 m

Width                                                     2.0 m

Height                                                    1.4 m

Position of model:

Upwind face relative to inlet:                    2.1 m

CL offset relative to domain CL                0.0 m

Bottom face above ground plane             0.05 m

Mesh Resolution in the vicinity of the Ahmed Body:

CV Thickness Top                                  2.3e-5 m

CV Thickness Side                                 3.7e-5 m

CV Thickness Bottom                             1.3e-3 m

CV Thickness Wall                                  3.7e-5 m

CV Thickness Slant                                 1.5e-5 m


Streamwise Length Top                          0.02 m (max)

Streamwise Length Side                         0.02 m (max)

Streamwise Length Bottom                     0.01 m – 0.015 m

Streamwise Length Wall                          0.01 m – 0.015 m

Streamwise Length Slant                         0.01 m – 0.015 m


Horizontal Aspect Ratio (all)                     ~10 (max)

Vertical Aspect Ratio (all)                       ~1000 (max)

Simulation Completion Criteria

The simulation should be run for at least one ‘wash through’ of initial conditions in order to obtain accurate lift and drag numbers. The number of iterations N is calculated using the equation below:

where d is distance from inlet to outlet along the mean velocity vector,  is the mean velocity magnitude and  is the time step duration. Nominally, N will be the same for the Fluent, StarCCM and EXN/Aero simulations.

For performance measurement purposes, the engineer should monitor the ‘CPU time’ column in the CGNS file. When it has reached a statistically steady value in time, the engineer should run for 50 iterations and then stop.

Simulation Setup

Solver Control

Time Step                     0.00005 s (note EXN/Aero can actually run at 1e-4 sec)

EXN GPU Allocation     2

EXN CPU Allocation      4


X-axis orientation          Positive downstream

Y-axis orientation          Positive to the left of the body, looking downstream

Z-axis orientation           Positive upward, normal to ground plane


Ansys / StarCCM

CPU Allocation             At the discretion of the engineer

Boundary Conditions

Velocity [x,y,z]              [40 , 0, 0]

Kinetic Energy               0.0001

K Dissipation                0.0003

Wall model                   Smooth wall

Outlet                           Constant Pressure

Domain Settings

Initial Velocity               [40 , 0, 0]

Initial Kinetic Energy      0.0001

Initial K Dissipation        0.0003


Turbulence Model         Detached Eddy, SST formulation


Flow type                      Incompressible


Near-body Precision      Double-precision. Use single precision for outer blocks if possible


Constant Density           1.203 kg/m3

Constant Viscosity        1.05 x 10-5

Other notes

Wash through time        0.23 seconds (4600 time steps)

Mesh Topology             Hexahedral, written in CGNS format

Calculation Methodology



Simulation Results

The table below shows a comparison of the present simulation results to published numerical and experimental values. The Reynolds number is defined as  , the drag coefficient is  and the lift coefficient is . L is the model length and A is the frontal area. The EXN/Aero results and the Serre et al. (2011) DES results most closely match the recent experiments of Bello-Millan et al. (2016) where they fall between the sharp edge model and smoothed model results. The Fluent results using both LES and RANS match the earlier measurements of Ahmed et al. (1984) and those of Meile et al. (2011). A characteristic of higher drag coefficients in either experiment or simulation is separated flow at the edge along the rear roof line.


Simulation Timing and External Factors

Cost-Normalized Specific Performance Figures

Reporting Item

Locally Hosted

Fluent EXN/Aero
# CPUs 4 4
# GPUs 0 2
Cost $5,000.00 $25,000.00
Capital & Operating / hour $0.19 $0.95
Annual License $30,000 $30,000
License / hour $3.42 $3.42
Compute Cost / hour $3.61 $4.38
Compute Cost $16,654 $1,437
Timesteps 22,030 22,050
MSM (timestep / $) 1.33 15.34
Annual Timestep Max (MSM) 168,577 588,322


Reporting Item

Cloud Hosted

Fluent EXN/Aero
# CPUs 4 16
# GPUs 0 2
Cost $3.50 $1.05
Hosting Fee $14.00 $2.75
Annual License $0.00 $30,000
License / hour $0.00 $3.42
Compute Cost / hour $14.00 $6.17
Compute Cost $64,152 $2,028
Timesteps 22,030 22,050
MSM (timestep / $) 0.34 10.87
Annual Timestep Max (MSM) 168,577 588,322




  1. R. Ahmed, G. Ramm, Faltin, G. Some Salient Features of the Time-Averaged Ground Vehicle Wake, Automobile Aerodynamics: Wakes, Wind Effect, Vehicle Development, SAE SP569, 1984.
  2. Bello-Millan, F.J., Tl Makela, L. Parras, C. del Pino, C.Ferrera, Experimental study on Ahmed’s body drag coefficient for different yaw angles, J. Wind Eng. Ind. Aerodynamics, Vol. 157, pp. 140-144, 2016.
  3. Serre, E. M. Minguez, Pasquetti, R., Guilmineau, E., Deng, G. B., Kornhass, M., Schafer, M., Frohlich, J., Hinterberger, C. , Rodi, R., On simulating the turbulent flow around the Ahmed body: A French-German Collaborative evaluation of LES and DES, Computers & Fluids, v78 n9, pp. 10-23, 2011.
  4. Meile, W., Brenn, G., Rappenhagen, A., Lehner, B., Fuchs, A. Experiments and numerical simuations on the aerodynamics of the Ahmed body, CFD Letters, Vol. 3, No. 1, pp. 32-39, 2011.