Week 3 - External flow simulation over an Ahmed body.

Aim :- To generate mesh of given Body and perform velocity & pressure contours.

Ahmed Body :- The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed.

The reason for the negative pressure in the wake region:-

The negative pressure is due to the velocity difference applied at a certain Reynold number and the rear angle, which at a low Reynolds number when this difference in pressure happens the wake obtained increases the drag due to the pressure drop behind the Ahmed body, which at a high Reynolds number due to the vortices generated it produces small amounts of pressure thrusting the body.

The significance of the point of separation

The point of separation defined by the Ahmed body and the flow, the alpha angle depending on which value is at it causes change in flow direction, forces required, lift on the body, induced drag due to “tip vortex”, at a critical value flow in slant stalls and reduced lift-reducing as well drag, all depending on the value of alfa and the Reynolds number.

Procedure :-

• Outlet is located at a distance of 5 m from the right side face of Ahmed body along the X axis.
• Inlet is located at a distance of 2 m from the left side face of Ahmed body along the X axis.
• Top face of the encloser is at a distance of 1m from the top side face of the Ahmed body along the Y axis.

Boundary Conditions:

Inlet Velocity, V=25m/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.

Meshing:

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:

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 12.4mm.

SETUP :

Solver setup: I use Energy Models, density based solver, hybrid method, in voscous model i use k- epsilon

pressure contour plot :-

velocity contour:-

velocity vector plot:-

Ahmed Body Refined Setup :- for refined setup or for velocity analysis i used second enclosure, small enclosure compare to previous enclosure.

Link :- https://drive.google.com/file/d/1wqg-LFNvotyQKaVDvLn2QA5SSbbrtEUO/view?usp=share_link

Result and Conclusion:

The baseline mesh gave a poor result, it wasn't able to capture the flow around the Ahmed body and the wake structure, due to coarse mesh.

The refined mesh gave better results. The plots show the wake formation accurately and the flow around the Ahmed body was also captured pretty well.