Ahmed Body Challenge
Title: Performing a Grid dependency test on an Ahmed body and compare the results.
- Introduction
The aim of this project is to simulate the Ahmed body for the Grid dependency, hence to see the effect of mesh sizing on the result of velocity and pressure distribution around the Ahmed Body.
Ahmed Body: It is a Generic Car body or simplified automobile body, which is being used in Automotive industries to simulate the flow characteristics around the Automobile. The fluid flow around the Ahmed body very accurately resembles the critical flow characteristics around the Automobile.
S.R. Ahmed developed the Ahmed specimen in his work “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984.
- Solving & Modelling approach
2.1 Geometry
The Geometric model of Ahmed body and Geometry with all the dimensions are as shown in figures below. For Modelling Air-flow Over an Ahmed Body, it needs to be captured in enclosed control volume also known as the Computational domain. Figures below show the Enclosed computational domain for Ahmed body as well as Boundary conditions applied for the flow simulation. To save computational time and space, here the symmetric boundary condition is used as the model is symmetric. Below figure shows the sliced model which will be used further for meshing and simulation.
2.2 Meshing
After creating the computational fluid domain for the model, the next step is to discretize the fluid domain. ANSYS meshing tool is used to generate a mesh in the model.
For more refined mesh locally it is needed to create a second enclosure around the Ahmed body by choosing appropriate dimensions for the second enclosure which captures complex nearby flow around the body and also wake region generated after the body.
The figure below shows the statistics and generated mesh for all the cases of the project.
1) Case 1 with coarse meshing.
2) Case 2 with medium size meshing.
3) Case 3 with fine meshing.
- Pre-processing and solver setting
The next step in the simulation after generating the meshing is to set-up the physical properties for the case.
- Type of solver: Density-based
- Simulation method: Steady-state simulation
- Turbulent Model: k-epsilon standard
- Material properties of fluid (Air)
Density: 1.225 kg/m3
Cp (Specific heat): 1006.43 J/kg-k
Thermal Conductivity: 0.0242 w/m-k
Viscosity: 1.7894e-05 kg/m-s
- Inlet velocity: 25 m/s
- Inlet Temperature: 300 K
- Outlet: Pressure type, 0-gauge pressure.
- Walls: wall type with no-slip
- Results
After setting up a case and providing appropriate initial and boundary conditions, numbers of simulations are performed on Ahmed body, mainly for the comparison of results from different mesh sizing.
4.1 Results of Case 1.
Residuals plot
Velocity distribution Contour Pressure distribution Contour
Velocity vector flow
Lift Coefficient Plot Drag Coefficient Plot
4.2 Results of Case 2.
Residuals plot
Velocity distribution Contour Pressure distribution Contour
Velocity vector flow
Lift Coefficient Plot Drag Coefficient Plot
4.3 Results of Case 3.
Residuals plot
Velocity distribution Contour Pressure distribution Contour
Velocity vector flow Pressure distribution on car Wall
Lift Coefficient Plot Drag Coefficient Plot
Tabel below shows the comparison of results from all 3 cases.
| No. of elements | No. of Nodes | Lift Coefficient | Drag Coefficient | |
| Coarse Mesh | 42460 | 8566 | 0.27120537 | 0.40138095 |
| Medium Mesh | 179224 | 70680 | 0.28960068 | 0.39337236 |
| Fine Mesh | 276107 | 103477 | 0.29017916 | 0.3688416 |
- Conclusion
After performing numerous simulations on Ahmed body with different meshing parameters, we can draw a few conclusions from this study of CFD simulations.
1) As we refined the meshing, the results are more accurate and clearly showing the wake region behind the Ahmed Body.
2) The negative pressure in a wake region with an increase in velocity is because the Ahmed body is a bluff body. The boundary separation and recirculating of the flow and also the effect of shear stress. To overcome this stress, the pressure turns negative for the balancing effect.
3) Flow separation occurs when the fluid boundary layer encounters an adverse pressure gradient. The point on the body where the boundary layer lifts off the surface is called the separation point. When the airflow separates off the surface, the pressure drag usually increases. To have better performance of an Automobile or any vehicles this separation needs to be avoided or delayed, for smooth flow over the surface of the vehicle.