Ahmed body challenge

Ahmed Body challenge

Objective:- To perform a grid independency test on the Ahmed body and compare the results.

Ahmed body and its importance:

Ahmed body is the benchmark model for aerodynamic simulation of the simplified car. it was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. With the help of ahmed body simulation we can study the various aerodynamic parameters like drag coefficient, lift coefficient, velocity, pressure profiles on the ahmed body and in wake region.

Geometry of Ahmed body and computational domain.

Dimensions of ahmed body:-

L =1.044 m

w = 0.389 m

h= 0.288 m

angle of slant=20°

length along slant = 0.222m

Dimensions of enclosure:-

length of domain upstream = 2.088m

length of domain downstream = 5.22m

height of domain = 1.4m

width of domain = 1.87m

Numerical modelling:-

Simulation has been carried out for three different mesh configurations and results have been compared. Pressure based solver has been used and coupled scheme is applied for the pressure velocity coupling. The flow is turbulent hence realizable K - ε model with enhanced wall treatment has been used to solve the RANS equation. The drag coefficent and lift coeffcient values have been monitored to ensure that the results are converged.

Meshing - three different mesh have been generated to carry out the grid independence test.

1) Coarse mesh - no. of elements = 409330

2) Medium mesh- no. of elements = 1053624

3) Fine mesh - no. of elements = 2425638

Results:-

grid independence test results are as follows:

mesh	no. of elements	Cd	Cl
coarse	409330	0.31103816	0.38472899
medium	1053624	0.31542284	0.38752478
fine	2425638	0.342397	0.414432

Different contour plots and profiles have been shown for the fine mesh case.

Velocity contour:

Pressure contour :

Residuals plot :

Velocity vector :

Velocity vector in the wake region :

streamlines in the wake region :

Y plus on the ahmed body

velocity profiles at different lengths from the ahmed body :

1) At x = -163 mm 2) At x = 200 mm

3) At x = 1 m

Discussions :-

from the grid independence test we can observe that the finer mesh gives the value of drag coefficient as 0.342 which is more accurate result as compared to other meshes. with the streamlines we get a clear view of the major vortices forming in the wake region. these lines show that there are two small recirculation zones at the back and one recirculation zone on the side of the ahmed body.

negative pressure in the wake region:-

The reason for the negative pressure in the wake region is the formation of two recirculation zones just at the back of ahmed body. the recirculation zones are opposite in direction and causes the flow to lose energy.