Air Flow around Ahmed body

Air Flow around the AHMED BODY

1. Ahmed body and its description

• The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, the height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.

• As the burning of fossil fuels becomes a more pressing issue, manufacturers are introducing more fuel-efficient cars to the market. One main contributor to fuel burn is the car’s aerodynamic drag. Complexly shaped, cars are very challenging to model and it’s difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools.
• In the automobile industry, airflow overground vehicle is analyzed and the coefficient of drag is calculated using CFD. In this case, Ahmed body is considered as test body in Industry.

2.CFD Simulation:

CFD simulation of flow around Ahmed body is carried out in Ansys fluent. In order to refine the mesh, a small enclosure is created around the Ahmed body as shown in figure 1.

figure 1

The dimension of the second enclosure:

lengh=5.3m

hight=0.84m

width=0.91m

this model has meshed in Ansys meshes as shown in the figure. Ahmed body is suppressed for physics because we don\'t need the mesh of Ahmed body.

CASE 1:

In case 1, the meshing of the model is done using ansys mesher as shown in figure below.For the large enclosure hexa mesh is generated because it is a rectangular type body and for such body Hexa mesh is good. for a second enclosure, body sizing is done with element size 80 mm.For mesh element size is 190 mm.

Number of element:84010

number of nodes:27850

figure 2 mesh generation

Result:

for this model, Steady simulation is done using Ansys fluent.solution is obtained using an input parameter such as,

inlet velocity 50 m/s

output pressure =0 gauge

temperature=300 K

The K-epsilon model is selected as the turbulent model.

Cut plane z is created to see the velocity profile and pressure profile.

figure 3.Residual

The simulation is run for 800 iterations and the velocity profile is obtained as shown in figure below.

figure 4.velocity profile

for model velocity along the Y-axis is obtained and the graph is created as shown in the figure. Two curves are for two different locations in the X axis.

CASE 2:

In case 2, for small enclosure element size decreases and set as 75 mm.

No. of elments:95566

No. of nodes:30029

Inflation layer=5

max. thickness=12.94mm

figure 6.mesh generation

Result is obtained after 297 iteration as shown in figure 7.

figure 7. residual vs iteration

figure 8.velocity profile

figure 9.velocity u vs Y axis

CASE 3:

further mesh is refined as shown in figure 10.

No of element:98934

No of nodes:30887

figure 10.mesh generation

figure 11.Residual vs iteration

Result is obtained after 320 iteration and velocity profile is shown in figure.

figure 12. velocity profile

figure 13. velocity vs Y-Axis

from the grid dependency test we can say that In meshing as the number of element increases the result would be better as shown in 3 cases.

3.Reason behind the negative pressure at back side of body:

figure 14.pressure profile

Figure shows the back side of body there is negative pressure.The rear part of the vehicle plays a major role in the formation of the wake as the flow passes over the roof of the vehicle and reach to rear, there is a sudden change in geometry so flow starts to separate and wake formation occurs. This wake formation leads to a significant reduction in pressure at the rear part compare to the front part.