Test on the Ahmed body

Aim:

To simulate a case of ahmed body and check for grid independence of the solution.

Objectives:

• Describe Ahmed\'s body and its importance.
• Explain the reason for the negative pressure in the wake region.
• Explain the significance of the point of separation.

Expected Results:

1. Velocity and pressure contours.

2. The drag coefficient plot for a refined case. ( For 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.

1. Ahmed body and its Importance:

Model:

The Ahmed body was described originally by S. R. Ahmed in 1984 . Three main features were seen in the wake:

1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model

2. The B recirculation region that is formed due to the separation at the base of the model.

3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.

The wake was shown to be highly dependent on slant angle. For slant angles less than 12°, the flow remains attached over the the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30° the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.

The Ahmed body was first defined and its characteristics described in the experimental work of Ahmed . Two configurations with slant angles of 25°and 35°are considered as a test case. For this configurations detailed LDA Measurements have been performed by Becker, Lienhart and Stoots [2,3] in the LSTM low-speed Wind-tunnel with a cross-section of 1.87x1.4 m2 (width x height) with a bulk velocity of 40 m/s. The test sectionof the wind-tunnel was 3/4 open (only ground plate present). The distance between the body and the plate representing the ground is 50 mm. In the experiment perform by the Ahmed [1], flow velocity was taken 60 m/s, Reynolds number was 4.29 million based on model length.

Geomentry used:

As the body is perfectly symmetric, we can run the simulation by considering the only half body. This is the best practice where you can save on the number of cells and get the results faster.

Baseline mesh basically means using the mesh generated by using default settings of ansys meshing component. so, in this case setuo we make a cuboidal shape and subtract the ahmed body from and then defining the remaining volume as fluid domain. this represents the geomentry obtained in the wind tunnel.

Dimensions Used:

Mesh:

This is the view of mesh near the ahmed body.

Named selections:

Inlet (velocity 25m/s)

Outlet

Symmetry-wall

car-wall

Results and conclusion:

Residuals:

Drag plot:

the result of pressure difference on the rear and front causes the drag on the body.From the drag plot .the drag coefficient is around 0.33

Lift plot:

Velocity contour:

Pressure contour:

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 air flow 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 seperation , leaving a vacuum or low pressure space where the air is not able to fill properly.

Velocity contour vector plot:

grid independency test :

The grid Independency test has performed up to 8 revisions from the base line mesh.

Mesh become fine as we increase the cell count and formation of wake region become clearer which helps us to get better results.

as in independency test the drag and lift coefficient remains almost same for every mesh element size..after some point it will converged so thet it cannot update the values as seen in above figure.

2. significance of the point of separation :

Boundary layer separation occurs due to adverse pressure gradient which means the flow takes place from low pressure area to high pressure area. Due to viscosity the velocity of fluid particles reduces. A point comes when the velocity of fluid particles first reduces to zero at the wall (also known as separation point) and then to negative or the direction of fluid particle changes at the wall and the areas close to the wall while the layers of fluid above them continues to flow in same direction. This leads to boundary layer separation

3. reason for the negative pressure in the wake region :

The pressure goes low to maintain the flow when there is an increase in the velocityof the flow.

It can be the result of flow seperation at the curved surface interface of the body.

The pressure becomes negative due to flow recirculation.

The pressure turns negative to counter balance the effect of shear stress.

The flow gets accelerated in those regions which results in negative pressure.