Week 3 - External flow simulation over an Ahmed body.

Q1. Describe Ahmed's body and its importance.

Ahmed S., (1984) performed a series of wind-tunnel experiments in order to examine the wake structure around typical automobile geometries. The study focused on the time averaged structure obtained from visualizations of flow in the wake region for smooth quarter scale automobile models. Experiments were also performed with a bluff-body, “generic” vehicle geometry where pressure measurements, wake surveys and force measurements for different angles of base-slant were presented. Results indicated that almost 85% of total aerodynamic resistance is contributed by pressure drag and most of this drag is generated at the rear end. The impact of the aerodynamic drags on the power consumption, and consequently on the emissions of CO2 and gas for purpose of greenhouse, is then considerable.

Expected simulation results:

1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case. (For a velocity of 25m/sec, the drag coefficient should be around 0.33).
3. The vector plot clearly showing the wake region.
4. Perform the grid independency test and provide the values of drag and lift with each case.

Geometry and Design:

Meshing:

Case A

Outer enclosure: 100mm (elemental size)

Inner enclosure: 50mm (elemental size)

Face sizing for Ahmed body: 5mm (elemental size)

Inflation for ahmed body: 5layer with max thickness 12.5mm

Setup:

Graphs:

Case B;

Outer enclosure: 90mm (elemental size)

Inner enclosure: 40mm (elemental size)

Face sizing for Ahmed body: 5mm (elemental size)

Inflation for ahmed body: 5layer with max thickness 12.5mm

Case C;

Outer enclosure: 85mm (elemental size)

Inner enclosure: 32mm (elemental size)

Face sizing for Ahmed body: 5mm (elemental size)

Inflation for ahmed body: 5layer with max thickness 12.5mm

Q2. Explain the reason for the negative pressure in the wake region.

The reason behind negative pressure in the wake region is due to flow separation, re-circulating flow. It's a phenomenon that generally happens as the air moving past the body isn't able to pass through the immediate backside of the body creating a region, where there is no movement of air and thereby creating emptiness.

This emptiness created is a low-pressure region towards which the fluid column exactly behind the wake region which of atmospheric pressure tries to fill the space thus dragging the fluid column with it thus creating the region of vorticity.

This phenomenon is particularly used to great extent in F1 where the car which is chasing down another car on the main straight, just tucks in to use the tow (pull created by the –ve pressure region). The advantage of a tow can be pretty significant giving almost 10-20kmph.

Q3. Explain the significance of the point of separation

Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. The pressure gradient along the x-direction and the velocity gradient are 0. The flow no longer follows the body and shear stress turns negative.