Modeling and Simulation of flow around an Ahmed Body
Introduction
The Ahmed body is a generic car body. The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in experimental work of S.R. Ahmed in 1984.
In this project, external flow simulation over an Ahmed Body was performed and results were compared against experimental data.
Geometry
Using the following standard Ahmed Body geometry dimension, the geometry was created using CAD software and then imported in Converge as shown in Figure 1.
Figure 1 Geometry shown in Converge Studio
Virtual Wind Tunnel
The geometry was imported in Converge Studio and virtual wind tunnel was created. In the front, one Ahmed body length and in the back two body length was chosen, shown in Figure 2.
Figure 2 Virtual wind tunnel
Case Setup
Application: General flow
Materials: Predefined air mixtures (all other options were set to default values)
Simulation Parameters: Full hydrodynamic compressible transient flow with 0.3s end time and 1E-08 time step.
Boundary: Inlet with 40 m/s velocity in the x-direction, Outlet with 101325 Pascal. Left, Right, and Top sides were selected as symmetry and bottom two walls were selected as walls with slip near the inlet and no-slip after on as shown in Figure 3.
Regions: User define region with 40 m/s velocity in the x-direction.
Turbulence modeling: `k-omega SST` model was chosen since it captures the external flow with its automatic wall function.
Base Grid: dx=dy=dz = 0.1m with fixed embedding around Ahmed Body with a scale of 2 and embedding layer 3. Furthermore, a fine-mesh box was created to capture flow little accurately as shown in Figure 3.
Due to computational resources available, a super coarse simulation was conducted.
Figure 3 Boundary flag shows in Converge Studio
Figure 4 Y+ for Fine Mesh Simulation
A desired Y+ value between `30 <= Y+ <= 100` would require very fine mesh which would result in a large number of cells, which cannot be easily simulated on modern laptops.
Figure 5 Pressure Force (Drag force) in the X direction for fine mesh simulation
Figure 6 Pressure Force (Lift force) in the Y direction for fine
Figure 7 Drag and Lift coefficient calculation is shown in Mathcad
Approximately 17 Newton of Drag force and -133 Newton of Lift force was observed from the plots. The negative pressure in the y-direction and the negative coefficient of lift indicates that the Ahmed Body was generating downforce than lift force.
Figure 8.a Velocity shown near the tail of Ahmed Body shown for coarse mesh
Figure 8.b Velocity shown near the tail of Ahmed Body shown for coarse mesh
Velocity Profiles
Variation of velocity with height was compared with experimental data on the symmetry plane at different lengths into the wind tunnel. Note for the comparison, the origin for the experimental data was defined at the middle point on the tail of the Ahmed Body. Experimental data with error bar shown in the figures below.
Figure 9 Total Cell count for fine mesh simulation
Even with a coarse simulation, we were able to see the general trend in the flow behavior by comparing the velocity profiles against experimental data. Due to a technical issue, fine mesh simulation wasn\'t post-process for the result. Though from coarse mesh results trend we can firmly say that a fine mesh simulation would be approximately similar to the experimental data. As seen in figure 9, 17,000,000 cells were counted on a fine mesh case. This would not be possible to simulate on a laptop, thus a fine mesh simulation was provided. Unfortunately, due to technical reasons, it was post-process for this project.
Conclusion
In conclusion, Converge Studio can be used to simulate flow over Ahmed Body using correct case-setup settings and finer mesh and can be used to determine other flow characteristics such as aerodynamic drag and lift coefficients, and turbulance behavoir.