Ahmed Body

Ahmed Body is a simplified representative model of a car. It has been used to study the flow patterns around a car. Extensive studies and experiments have been conducted that describe the flow through an actual ahmed body in a wind tunnel. Thus the experimental data from the studies can be used to validate the accuracy of a CFD package. The dimensions of the ahmed body are therefore standardized.

Q2.

Geometry:

1. A second smaller enclosure was created whose dimensions measure 0.9 m from the front and rear end of the Ahmed body, 0.25 m from the top surface of the Ahmed body and 0.15 m on the sides of the body.
2. This enclosure volume was subtracted from the bigger enclosure volume by using clear interference command.
3. Then the topology of the two enclosures were shared to ensure continuity in mesh.

Meshing:

Body sizing feature (with ‘capture curvature’ and ‘capture proximity’ = Yes) has been used to refine the mesh in the smaller enclosure volume.

Inflation layer is also created to capture the boundary layer flow characteristics.

Inflation layer- inflation option smooth transition, number of layers 10, boundary scoping method named selection and car wall in named selection.

The 3 cases essentially differ in their mesh as the boundary conditions and setup is identical for all the cases:

	Body sizing	General mesh	Number of Elements	Average quality of elements
	Element size (m)	Element size (m)
Case 1	0.1	0.1	326566	0.67084
Case 2	0.05	0.1	352652	0.68704
Case 3	0.03	0.4136	420078	0.679

Results:

Case 1: Coarse Mesh

Case 2: Fine Mesh

Case 3: Very Fine Mesh

	Maximum Value of Velocity on cutplanez	Minimum Value of Pressure on car_wall
Case 1	76.8143 [m s^-1]	-5611.29 [Pa]
Case 2	77.0375 [m s^-1]	-5687.64 [Pa]
Case 3	77.9639 [m s^-1]	-5580.61 [Pa]

Discussion on results: The velocity contours of all the 3 cases don’t vary much. The vortices at the rear of the body are identical for all the 3 cases. Thus, the solution can be said to pass the grid dependence test.

Q3.

Lowest pressure

Detachment of Boundary layer due to adverse pressure gradient

Since the mass flow rate across each section in the X- direction has to be the same, the air flow speeds up as it encounters the ahmed body. Ahmed body can be seen as reason for reduction in the flow area through x-direction in the tunnel of air flowing. The increase in kinetic energy thus corresponds an equivalent reduction in pressure head.

Or the viscous forces / shear stresses in the boundary layer in the vicinity of the body cause resistance to the flow. In order to overcome this resistance, energy needs to be expended which is the reduction in the stagnation pressure or pressure head.