Ahmed Body Challenge
Theory :The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools.
Ahmed body is a generic car body (a simplified car model) i.e the flow of air around Ahmed body captures the essential flow features around an automobile.
ANSYS is a general purpose FEA and CFD simulation software. It gives numerical prediction of the behavior of a system. These numerical predictions need to be validated by experimental data.
People working in vehicle aerodynamics use Ahmed body to validate their numerical model because the experimental data from wind tunnel testing is available for Ahmed body. Once the numerical model in validated, it is used to design new models of the car.
Refining the mesh
Case1:
Outer enclosure element size: 100mm
inner enclosure element size :35mm
no inflation layer was set
Total node count :71156
Total element count :287490
Meshing
Velocity vector plot:
The blue border running along the wall is basically because of the law of the wall (no slip ) condition ,which says the velocity there is zero this is bacuse the flow is subsonic.
Also an interesting phenomina is observed behind the body where the direction of vector is reversed .which signifies the eddie formation.
Pressure contour
As the velocity in front decreases we see a good amount of pressure rise in that area.
Pressure contour across the ahmed body.
Pressure across the body is can be determined from the following which can be further used for structural analysis .
Max pressure is 102833pa.
To creat charts of velocity at acertain distance beind the ahmed body line plotting was used .
Position of line
Chart of velocity at specified position.
we observe that just behind the ahmed body the velocity goes negative.
Case 2:
Outer enclosure element size: 50mm
inner enclosure element size :18mm
inflation layer was set with 4 layers and total thickness of 4mm .
Total node count :129987
Total element count :480243
Meshing
Velocity vector plot:
In this due to inflation set we get clean, uniform and a very small zero velocity border across the body .
Pressure contour
Pressure contour across the ahmed body.
Case 3:
Outer enclosure element size: 50mm
inner enclosure element size :18mm
inflation layer was set with 4 layers and total thickness of 4mm .
Total node count :129987
Total element count :480243
K-epsilon model predicts well far from the boundaries and k-omega model predicts well near wall .
also k-omega is sensitive to to free-stream conditions and suitable for complex boundary layers under adverse pressure gradient and seperation.
so keeping the mesh same we changed the model to k-omega .
Velocity vector plot:
Pressure contour:
Pressure contour across the ahmed body.
Conclusion:
- Simulation time was drastically increased as the element count increased.
- we obtain better accuricy and smoother plots as the elements increased due to the fact that less approximation was done in this process.
- Due to flow seperation behind the ahmed body we observe negative pressure because seperation of flow takes place in that area.
- Cases are arranged in ascending order of their accuricy .
Negative pressure region :
(Note:The pressure actually is positive .As we are dealing with gauge pressure (0 gauge pressure is highest in our simulation) so ansys shows results in comparison with the highest specified pressure in the simulation.)
The pressure goes negative because of the flow seperation .similar phenomina is observed with velocity as direction of flow reverses .
Actually flow seperation is boundary layer seperation and it occurs whenever the change in velocity of fluid in either magnitude or direction is too large for the fluid to adhere to the solid surface .