Flow Over Ahmed Body
Flow Over Ahmed Body
AIM: To Perfrom a grid dependency test for flow over Ahmed Body
Introduction:
Studies regarding airflow and turbulence behaviour on a car involve the use of the ‘Ahmed body’, which was first devised by S. Ahmed in 1984. The shape of the Ahmed body is designed in such a way to mimic the typical flow field features of a car bluff body, namely “a strong three-dimensional displacement flow in front, relatively uniform flow in the middle and a large structured wake at the rear”. It has had a significant impact on research in the automotive industry, used to study and validate flow over an automotive in both wind tunnel and CFD environments. In the case of CFD its simple shape reduces required computational power and time taken to model the body and run simulations. The geometry of the body, as originally described by S. Ahmed, can be seen in figure. All dimensions are fixed apart from the rear slant angle ‘α’, which can be set at different angles, typically from 0 to 30 degrees to study the difference this makes to the rear wake. Ahmed found that depending on the rear slant angle used, the drag coefficient and the amount each area of the body which contributed to the drag coefficient changed accordingly.
Simulation Procedure:
In order to perfrom the grid dependency test on Ahmed body three tyoes of mesh cases are taken:
Case 1: Double enclosure with nominal and inflation layer mesh
Case 2: Double Enclosure with fine mesh and inflation layers
Case 3: Double enclousre with refined mesh and inflation layers
Boundary Conditions:
Density & Energy based method,K-epsilon model ;Inlet Velocity : 50m/s, Fluid : Air,Temperature:300K, uniform velocity at inlet,uniform pressure at outlet, walls &symmetry walls, enclosures
Geometry for all the cases is given below with main & sub domain enclosures
Case 1:
Mesh Details:
Nodes: 153469, Elements:298850, Element Main Domain Size:50mm, Sub-Domain Size:45mm, Inflation Layers :5
In case 1 the flow is not smooth and wake regions are present but not clear
Case 2:
Mesh Details:
Nodes: 187893, Elements:385509, Element Main Domain Size:48mm, Sub-Domain Size:40mm, Inflation Layers :8
In case 2 flow is disturbed but the wake regions are distinctive compared to case 1
Case 3:
Mesh Details:
Nodes: 218297, Elements:418277, Element Main Domain Size:40mm, Sub-Domain Size:35mm, Inflation Layers :10
In case 3 the flow is smooth and the wake regions are very much distinctive
Pressures:
The air becomes almost stagnant as it strikes the vehicle which results in air exerting very high pressure on front side of the vehicle represented by the red area.But along the body the pressure gets negative,as the flow passes the body the there is a increase in velocity and to compensate the increased velocity the pressure gets dropped and at the rear slant side recircuating region (wake regions formed) due to the high velocity hence pressure goes negative.
Conclusion:
From the grid dependency test it is clear that the flow gets smother and distinctive regions are formed when the mesh gets finer, which makes the results accurate.