ANALYZING EXTERNAL FLOW OVER AN AHMED BODY

EXTERNAL FLOW OVER AN AHMED BODY

• ABSTRACT: In this report flow over an Ahmed Body has been studied using the different meshing techniques and the results are analyzed.

• WHAT IS AHMED BODY?

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. The simple geometrical shape has a length of 1.044 meters, a height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.

• The geometry used for the simulation:

CASE 1 - FINE MESH

• MESHING

• RESIDUALS

• VELOCITY CONTOUR PLOT

• VELOCITY CUT PLANE

• VELOCITY GRAPH

CASE 2 - FINER MESH

• MESHING

• RESIDUALS

• VELOCITY CONTOUR PLOT

• VELOCITY CUT PLANE

• VELOCITY GRAPH

CASE 3 - FINEST MESH

• MESHING

• RESIDUALS

• VELOCITY CONTOUR PLOT

• VELOCITY CUT PLANE

• VELOCITY GRAPH

• MESH COMPARISON OF ALL 3 CASES

(* The finest mesh has been tried to reach the maximum limit as per the academic version of the software depending upon all the conditions. )

• GRID DEPENDENCY TEST

The Grid dependency Test can be defined as the test which is performed to find the optimum number of elements in a mesh. When the mesh has reached the optimum number of elements, then refining the mesh won\'t participate in the result analysis. Here in this case, when we compare the velocity graphs of FINER MESH and FINEST MESH, we observe that the end results are almost the same in both cases.

• REASON FOR NEGATIVE PRESSURE IN CFD POST

The reason for the negative pressure at the back of the Ahmed Body is the recirculating flow in that region, which is accompanied by the flow separation and turbulence, this is what we call as wake region and the velocity near the back of the body reduces and the boundary layer thickens. Continuous retardation of flow brings the shear stress at this surface to zero. Now from this point onwards the shear stress becomes negative and the flow reverses and a region of recirculating flow develops.

RESULT: It has been found that once the number of elements in the mesh has reached the optimal value then refining the mesh more doesn\'t cause many changes in the values of obtained results.