Week 3 - External flow simulation over an Ahmed body.
Flow analysis over ahmed body
INTRODUCTION
The Ahmed body is a geometric shape first proposed by Ahmed and Ramm in 1984. The shape provides a model to study geometric effects on the wakes of ground vehicles.The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry such as drag from there we can relate it to fuel consumption i.e. by reducing drag we can reduce fuel consumption.
MESHING
First we enclose the ahmed body and make a base line mesh for problem.
Then go to solver and set inlet velocity 25 m/s and outlet at atmospheric pressure. And run the simulation. The results are
Then we will fine the mesh using another enclosure near the wall's of ahmed body and the refined mesh is
Then set the solver with same boundary conditions and run the simulation following results will be obtained
Then again refine the mesh and run the simulation.
DISCUSSION
The following table shows the comparison for grid independence test and we can see that as our mesh is refined we approached the theoritical Cd value. With further refinement we can obtain the closure value and also because of finer mesh we can capture wake behind the ahmed body. We have added the inflation layer near the wall of ahmed body by considering y+ value.
| Outer body mesh size | Inner body mesh size | Total mesh | Coefficient of drag | Coefficient of lift | |
| Base case | - | - | 79483 |
0.36213247 |
0.19703476 |
| First refinement | 0.1 m | 0.05m | 166629 |
0.36295148 |
0.24642652 |
| Second refinement | 0.08 m | 0.035 m | 415947 |
0.3135752 |
0.21132757 |
| Theoritical | - | - | - | 0.33 | - |
We can see from the pressure contour plot at some places pressure is negative , the possible reason could be
- Flow recirculation
- To compesate increase in velocity.
- Overcome shear stress
- Flow separation (creation of air bubble )
The separation point is the point where the air stops sticking to an object that is moving through the air. When an object is moving through the air, the air molecules push back on it and resist the object's movement ,this resistance is called pressure drag. Also from that point developement of boundary layer starts.