CFD simulation of the air flow around a car model Ahmed body
Introduction:
It is necessary to maintain the balance between lift and drag force in automobiles. In order to maintain control for steering and braking, especially at high speeds, an increase in the drag force should be needed. However, increasing the drag force which leads to an increase in fuel consumption. Hence, these two forces must be carefully balanced. The two forces can be controlled by, adjusting the dimensions of the automobile body and flow parameters.
The Ahmed body is the standard body which can be used to validate the results from CFD simulations. The Ahmed body is a very simple bluff body which has its shape simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies. The generic Ahmed body reference model is used to carry out computations for studying of the aerodynamic parameters.
Fig. Dimensions of Ahmed body
`psi` - Slant angle
Mesh details:
| Case 1 | Case 2 | Case 3 | |
| Element size of Outer enclosure | 0.1 m | 0.1 m | 0.1 m |
| Element size of Inner enclosure | 0.05 m | 0.04 m | 0.033 m |
| Number of Inflation Layer | 5 | 5 | 8 |
| Growth rate | 1.2 | 1.2 | 1.2 |
| The total thickness of inflation layers | 0.0125 m | 0.0125 m | 0.0215 m |
| Element size of 4 round shaped base of Ahmed body | 0.005 m | 0.005 m | 0.005 m |
| Total number of elements | 178369 | 273572 | 450559 |
The Academic version of Ansys has an element count limit of 0.5 million. Hence, the maximum number of elements have been limited to 0.45 million.
Fig. The meshing of outer and inner enclosures
Fig. Inflation layer around the boundary of the Ahmed body
Fig. Element Quality for Case 1
Fig. Element Quality for Case 2
Fig. Element Quality for Case 3
In all 3 cases, the quality of elements is more than 5%. It can be considered as a good quality mesh.
Simulation Details:
Type of simulation: Steady-state
Type of model: K-epsilon (Turbulence model)
Dynamic viscosity of a fluid: 1.7894e-05 `(Kg)/(m.s)`
The density of a fluid: 1.125 `(Kg)/(m^3)`
Boundary condition:
Inlet velocity: 50 m/s
Outlet pressure: 1 atm
The surface of the Outer enclosure: Symmetry
The surface of Ahmed Body: wall
Solution Method:
Formulation: implicit
Flux type: Roe - FDS
The above details used for all three cases. The Velocity and Pressure profiles are calculated at the midplane of the enclosure.
Results from case 1:
1)
The above graph shows that the residuals from solving governing equations getting constant after 1100 iterations.
2)
The above profile shows velocity contour at the midplane of the enclosure. It shows near to zero velocity at the backside and frontside of the Ahmed body. It reaches maximum velocity at around the corner of the front side region.
3)
The above profile shows that velocity vector at each node. It shows the direction of flow at each element. Due to the higher number of elements in the inner enclosure, vectors are close to each other.
4)
The above 3D profile shows that the pressure profile of Ahmed body. The flow becomes almost stagnant at the front side of the body. Hence, the pressure reaches the maximum at the front side of the body. The smooth curves around the front region are creating wakes. It causes negative pressure on those regions. The slanted side on rear creates flow separation which creates the wakes. It causes negative pressure on that slanted surface.
5)
The above profile shows pressure contour at the midplane of the enclosure.
6)
The above graph shows that magnitude of velocity at 2 and 3 m away from the Ahmed body in the downstream direction.
Results from case 2:
1)
The above graph shows that the residuals from solving governing equations getting constant after 1000 iterations.
2)
The above profile shows velocity contour at the midplane of the enclosure. It shows near to zero velocity at the backside and frontside of the Ahmed body. It reaches maximum velocity at around the corner of the front side region. The profile is a bit smoother than the previous case because of more number of elements.
3)
The above profile shows that velocity vector at each node. It shows the direction of flow at each element. Due to the higher number of elements in the inner enclosure than the previous case, vectors are very close to each other.
4)
The above 3D profile shows that the pressure profile of Ahmed body. The flow becomes almost stagnant at the front side of the body. Hence, the pressure reaches the maximum at the front side of the body. The smooth curves around the front region are creating wakes. It causes negative pressure on those regions. The slanted side on rear creates flow separation which creates the wakes. It causes negative pressure on that slanted surface.
5)
The above profile shows pressure contour at the midplane of the enclosure.
6)
The above graph shows that magnitude of velocity at 2 and 3 m away from the Ahmed body in the downstream direction.
Results from case 3:
1)
The above graph shows that the residuals from solving governing equations getting constant after 2500 iterations.
2)
The above profile shows velocity contour at the midplane of the enclosure. It shows near to zero velocity at the backside and frontside of the Ahmed body. It reaches maximum velocity at around the corner of the front side region. The profile is a bit smoother than the previous case because of more number of elements.
3)
The above profile shows that velocity vector at each node. It shows the direction of flow at each element. Due to the higher number of elements in the inner enclosure than the previous case, vectors are very close to each other.
4)
The above 3D profile shows that the pressure profile of Ahmed body. The flow becomes almost stagnant at the front side of the body. Hence, the pressure reaches the maximum at the front side of the body. The smooth curves around the front region are creating wakes. It causes negative pressure on those regions. The slanted side on rear creates flow separation which creates the wakes. It causes negative pressure on that slanted surface.
5)
The above profile shows pressure contour at the midplane of the enclosure.
6)
The above graph shows that magnitude of velocity at 2 and 3 m away from the Ahmed body in the downstream direction.
Grid dependence test:
| Case 1 | Case 2 | Case 3 | |
| Maximum Velocity magnitude | 106.1 m/s | 106.3 m/s | 106.2 m/s |
| Maximum Pressure | 1490 Pa | 1505 Pa | 1548 Pa |
From the above table, it was evident that the difference in flow parameters between each case is considerably small. Hence, the solutions are independent of grid size.
Animation video of flow around Ahmed body:
Conclusions:
1) The flow becomes almost stagnant at the front side of the body. Hence, the pressure reaches the maximum at the front side of the body.
2) The smooth curves around the front region are creating wakes. It causes negative pressure on those regions.
3) The slanted side on rear creates flow separation which creates the wakes. It causes negative pressure on that slanted surface. It gives the drag force for the Ahmed body.
4) the velocity magnitude reaches near to zero at the front and backside of the Ahmed body. It reaches the maximum at smooth curves around the front region and slanted region.