To perform grid dependence test on Ahmed body

1. Ahmed body and its importance:

The Ahmed body was described originally by S. R. Ahmed in 1984. Three main features were seen in the wake: The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model. 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. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry. This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model in validated, it is used to design new models of the car.

2. Ahmed body setups:

Setup A:

The Ahmed body is inserted into the geometry part. The enclosure of 7m *1m and height of 1m from ahmed body. Further a smaller enclosure is drawn around the ahmed body of dimension of length 3m, width of 0.5 m, and height from body 0.5m. Then the interference is removed between two enclosures and ahmed body is set to supress for physics.

In mesh, the name selection is created as inlet, outlet, symmetry and car-wall. The mesh is generated for the given body. Then to make finer mesh the method option is selected and by selecting the whole body and element size is reduced to 0.1m. For making mesh more finer the sizing option is selected and smaller enclosure is selected with element size of 0.05m and mesh is generated. and element size is reduced to 5 mm and inflation layer is used to make body fitted mesh using total thickness option. The no. of elements(cells) is 0.265 million.

In setup , steady method is selected and as the mach no. of air in this case is nearer to 0.3 so density based solver is used to include the compressibility effect. Further energy equation is selected and viscocity is change to K-epselon as this is turbulent flow case. The boundary conditions are set, velocity = 80 m/s. and outlet as pressure outlet. Then hybrid initialisation is done and calculation is done for 600 iterations.

The figure below shows the velocity variation and also velocity plot,

Fig. Velocity variation of air over ahmed body( line neares to body is line 1 and next to that is line 2)

Fig. Plot of distance from x axis( i.e. along Y direction) vs velocity (m/s).

Setup B: (Mesh is made more refine upto license limit)

The Ahmed body is inserted into the geometry part. The enclosure of 7m *1m and height of 1m from ahmed body. Further a smaller enclosure is drown around the ahmed body of dimension of length 3m, width of 0.5 m, and height from body 0.5m. Then the interference is removed between two enclosures and ahmed body is set to supress for physics.

In mesh, the name selection is created as inlet, outlet, symmetry and car-wall. The mesh is generated for the given body. Then to make finer mesh the method option is selected and by selecting the whole body and element size is reduced to 0.1m. For making mesh more finer the sizing option is selected and smaller enclosure is selected and mesh generated for element size of 0.0411m and for bottom legs of ahmed body the element size is reduced to 5 mm and inflation layer is used to make body fitted mesh using total thickness option which generates the no. of elements of 0.497 millions (maximum elements cells limit upto 0.5 millions).

In setup , steady method is selected and as the mach no. of air in this case is nearer to 0.3 so density based solver is used to include the compressibility effect. Further energy equation is selected and viscocity is change to K-epselon as this is turbulent flow case. The boundary conditions are set, velocity = 80 m/s. and outlet as pressure outlet. Then hybrid initialisation is done and calculation is done for 600 iterations. The following figure shows the results,

Fig. velocity variation of air over ahmed body

Fig. Plot of distance from x axis( i.e. along Y direction) vs velocity (m/s).

Setup C:(smaller encloser is enlarged)

The Ahmed body is inserted into the geometry part. The enclosure of 7m *1m and height of 1m from ahmed body. Further a smaller enclosure is drown around the ahmed body of dimension of length 4m, width of 0.5 m, and height from body 0.75m. Then the interference is removed between two enclosures and ahmed body is set to supress for physics.

In mesh, the name selection is created as inlet, outlet, symmetry and car-wall. The mesh is generated for the given body. Then to make finer mesh the method option is selected and by selecting the whole body and element size is reduced to 0.1m. For making mesh more finer the sizing option is selected and smaller enclosure is selected and mesh generated for element size of 0.05m and for bottom legs of ahmed body the element size is reduced to 5 mm and inflation layer is used to make body fitted mesh using total thickness option which generates the no. of elements of 0.396 millions (maximum elements cells limit upto 0.5 millions).

In setup , energy equation is selected and viscocity is change to K-epselon as this is turbulent flow case. The boundary conditions are set, velocity = 80 m/s. and outlet as pressure outlet. Then hybrid initialisation is done and calculation is done for 600 iterations. The following figure shows the results,

3. Pressure contour on ahmed body:

From the figures it is shown that the pressure on ahmed body at upsteam is high but pressure on body at downstream very low i.e. negative pressure. The reason behind that is the flow gets seperated on body at downstream and the flow is reversed also there is sudden fall of velocity as well as pressure and so vaccum occurs. So the negative pressure takes place at rear side of body. The velocity vector diagram shows the reversal of flow and drop of pressure.