Ahmed Body

OBJECTIVE:

Perform a grid dependency test on the ahmed body and compare the results.

AHMED BODY:

The generic Ahmed Body reference model has been chosen as the benchmark for carrying out computations for studying of the aerodynamic parameters.

Figure shows the geometry of a simplified model known as Ahmed Body which generates characteristics similar to a real vehicle flow field neglecting the effects of wheel, engine and projected surfaces like mirror. The Ahmed Body is made up with very simple geometry, consist of a round front part and it has a slant on its rear end, whose angle can be manipulated and corresponding drag(Cd) and lift(Cl) coefficients calculated.

NUMERICAL SIMULATION:

In order to perform grid dependency test, simulations on ahmed body was carried out in 3 different cases. i.e., By generating different meshing strategies.

1) Geometry:

The ahmed body was enclosed in two rectangular boxes, outer box to capture/record the flow structures and inner box was created dimensions based on baseline setup to accurately capture the flow separations from its rear.

2) Boundary Conditions:

Standard k-epsilon model with Inlet Velocity, V=50m/s, Density of air = 1.225 kg/m3, temperature = 300k and viscosity = 1.7894e-5 kg-s/m.
Boundary conditions are - uniform velocity at inlet, uniform pressure at outlet , symmetry at wall.

3) Meshing and Results:

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

CASE 1:

Simulation was done for total No. of Mesh counts = 1,88,831.
where, 1st box domain = 100mm size of hexahedral mesh
2nd box domain = 50mm size of tetra mesh
and 5 inflation layer around the wall of ahmed body with max thickness of 12.5mm

CASE 2:

Simulation was done for total No. of Mesh counts = 3,12,026.
where, 1st box domain = 100mm size of hexahedral mesh
2nd box domain = 40mm size of tetra mesh
and 6 inflation layer around the wall of ahmed body with max thickness of 10mm

CASE 3:

Simulation was done for total No. of Mesh counts = 4,79,856
where, 1st box domain = 100mm size of hexahedral mesh
2nd box domain = 34mm size of tetra mesh
and 8 inflation layer around the wall of ahmed body with max thickness of 8mm

CONCLUSION:

From the above results of plots and images, we can observe following points:

a) Drag Coefficient from case 1 is, Cd ~ 0.25 and formation of wake region & recirculation of velocity profile is observed slightly from the vector plot.

b) From case 2 and case 3 simulation, we can observe from the above images that drag coefficient was around, Cd ~ 0.22 and formation of recirculation region was better than the case 1 simulation.

c) We can conclude that refinement of mesh from case 2 to case 3 didn\'t found any noticeable changes in drag coefficient(Cd) as well in velocity profile.
Hence, The grif size of approx. 3,12,000 cells (case 2) provides sufficient accuracy.

PRESSURE DISTRIBUTION ON AHMED BODY:

The air becomes almost stagnant as it strikes the body which results in air exerting very high pressure on front surface of the body represented by the red area.
The airflow then gets divided between the upper and lower surface of the vehicle and as it accelerates over the surface of the ahmed body pressure drops due to increase in flow velocity.

As the air continues to flow and make its way to the rear, a notch is created by the rear slant owing to flow separation, leaving a vacuum or low pressure space (-ve pressure) where the air is not able to fill properly.

Also, the result of pressure difference on the rear and front causes the drag on the body.