Ahmed Body Challenge

Q1. Ans: The Ahmed body was described originally by S. R. Ahmed in 1984. The Ahmed body is a generic car body (a simplified vehicle model). Although it has a simple shape, the Ahmed body allows us to capture characteristic features that are relevant to organisations in the automobile industry. This model is also used to describe the turbulent flow field around a car-like geometry.

Q2. Ans: The pressure gradient is an is one of the factors that influence a flow immensely. It is easy to see that the shear stress caused by viscosity has a retarding effect upon the circulation. This effect can, however, been overcome if there is a negative pressure gradient offered to the flow. A negative pressure gradient was termed a Favourable pressure gradient. Such an angle enables the flow. A positive pressure gradient has the opposite effect and is termed the Adverse Pressure Gradient. Fluid might find it difficult to negotiate an adverse pressure gradient. Sometimes, we say the fluid has to climb the pressure hill. Depending on the flow conditions, the recirculating flow terminates, and the flow may become reattached to the body. A separation bubble was formed. There are a variety of factors that could influence this reattachment. The pressure gradient may be now favourable due to body geometry and other reasons. The other factor is that the flow initially laminar may undergo transition within the bubble and may become turbulent. Turbulent flow has more energy and momentum than laminar flow.

Q3. Ans: It has been observed that the flow is reversed at the vicinity of the wall under certain conditions. The phenomenon is termed as separation of the boundary layer. Separation takes place due to excessive momentum loss near the wall in a boundary layer trying to move downstream against increasing pressure, i.e., , which is called an adverse pressure gradient.

NUMERICAL SIMULATION:

Ahmed Body:

Boundary conditions:

K-epsilon Realizable model with Inlet Velocity, V=25m/s, Density of air = 1.225 kg/m3, temperature = 300k and viscosity = 1.7894e-5 kg-s/m were the inputs for this analysis.
Boundary conditions are - uniform velocity at the inlet, consistent pressure at the outlet, symmetry at a wall.

We have taken one model with one enclosure, and three different cases with different element sizes were used to compare.

Meshing: The Ahmed body is surrounded by a hexahedral grid to record flow structures, and the tetrahedral grid surrounds the inner enclosure, and inflation layers are created along the walls of Ahmed body to capture the flow separation from its rear accurately.

Solver: Density-Based solver

The material used: Air

Inflation :

Layers : 5 , thickness = 12 mm

BASELINE CASE: Ahmed body model with one enclosure and coarse mesh

Mesh :

We have used default size for this mesh.

Element size: 413.6 mm(default)

Nodes : 16598

Elements: 84094

Results:

Velocity profile:

Pressure contour:

VECTORS:

MESH REFINEMENT is done to have better results.

Now, we took two enclosures and did the analysis on the Ahmed body.

CASE 1: REFINED MESH NO 1

Element size : 100 mm

Inner enclosure element size: 50 mm

Nodes: 96983

Elements: 324652

MESH :

We also have shown that the solution has been converged.

RESIDUAL PLOT:

LIFT COEFF:

DRAG COEFF:

VELOCITY CONTOUR:

PRESSURE CONTOUR:

PRESSURE ON THE CAR BODY:

VECTOR:

CASE 2: REFINED MESH NO 2

Element Size : 75 mm

Inner enclosure element size: 35 mm

Nodes: 112644

Elements: 401589

MESH:

RESIDUAL PLOT:

The solution is converged.

LIFT COEFF:

DRAG COEFF:

VELOCITY CONTOUR:

PRESSURE CONTOUR:

PRESSURE ON THE CAR BODY:

VECTOR:

CASE 3: REFINED MESH NO 3

Element Size: 50 mm

Inner Enclosure element size: 35 mm

Nodes: 195317

elements: 506452

MESH:

RESIDUAL PLOT:

The solution was converged.

LIFT COEFF:

DRAG COEFF:

VELOCITY CONTOUR:

PRESSURE CONTOUR:

PRESSURE ON THE CAR BODY:

VECTOR:

CONCLUSION:

1. Comparing the baseline mesh and above three refining mesh, we conclude that increasing the elements and the nodes, the results should be accurate and from the velocity contour it clearly shows that the wake region (i.e.) slant area of the body which have low velocity and the range of the speed is more accurate.
2. In vector plot, it shows the recirculating area, the wake region, which may cause the vortex shedding and boundary separation occurs.
3. From the above result, we also conclude the thrust and drag force of the Ahmed body. Because of the slant region more circulating it increase the thrust force of the Ahmed body.
4. Increase the refining and increasing the element; the result should be more accurate. Above pressure diagram, shows the negative pressure. When the enclosed area has a lower pressure than the area around is called negative pressure.