Steady-state fluid flow analysis over Ahmed body to simulate wake formation in ANSYS FLUENT

Objective

• Describe Ahmed body.
• Perform a grid dependence test on the geometry of Ahmed body.
• Describe why pressure becomes negative in some places.

 

 

Ahmed body

 

The Ahmed body was described originally by S.R. Ahmed in 1984. Ahmed body is a simplified version of a passenger car. It is a bluff body with rounded edges and a blunt face. The upper rear end of Ahmed body is slanted, whose angle can be varied to see its effect on the fluid flow. The model is supported on small cylindrical legs instead of wheels. This oversimplified geometry of a passenger car captures some of the very important physics of the fluid flow around the geometry.

Ahmed body is able to capture some of the essential fluid flow features at the nose of the vehicle, around the vehicle and wake formation at the rear of the vehicle. In the Ahmed body experiment, three main features were seen in the wake, (1) The A recirculations 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.

 

Importance of Ahmed body

• The experiments have been conducted on the actual Ahmed body model. Hence, a large amount of experimental data on Ahmed body is available. Engineers around the world can use this experimental result to validate their numerical results to formulate new CFD codes.
• We can use Ahmed body to quickly understand the fluid flow around the simplified model of car and understand the various factors responsible for drag behind the car. This leads to a better car design.

 

 

                                                                      CASE 1

 

1. Geometry

• This is the geometry of Ahmed body. It has a blunt face and rounded edges, and it also has a slanted upper back. It is supported on the cylindrical legs
• To simulate the fluid flow around the Ahmed body we have to create a wind tunnel. We can do so by enclosure feature in SpaceClaim.

 

Enclosure

• Here we can see the enclosure for the wind tunnel, in the image we can see that it contains a second smaller enclosure. The small enclosure is created for local mesh refinement around the Ahmed body. This is done to better simulate the fluid flow around the body and capture the fluid flow with greater accuracy.

Dimensions of outer enclosure:

• Length - 8.044 m
• Width - 1.389 m
• Height - 1.338 m

Dimensions of inner enclosure:

• Length - 2.544 m
• Width - 0.909 m
• Height - 0.838 m

 

Intersection

• After creating the outer and inner enclosure we have to rectify the problem of the intersection. We can do it by using the intersection feature in Ansys. This feature will remove any intersection between the outer and inner enclosure.
• The removal of the intersection is necessary to produce a nice and seamless mesh at the boundary of the inner enclosure.

 

2. Meshing

Before starting the meshing procedure we have to create named selections for the inlet, outlet and enclosure walls. 

• Method: In this step, we will insert the meshing method as "multizone". This method will produce high-quality Hexa mesh in the outer enclosure. The element size for meshing is 100 mm.
• Body sizing: After that will insert body sizing for the inner enclosure for performing local mesh refinement. Element size: 50 mm.
• Face sizing: For the cylindrical legs we have to insert face sizing option. This will properly generate the mesh according to the small cylindrical legs. Element size: 5 mm.
• Inflation layer: To properly simulate the fluid flow around the boundary of Ahmed body and to capture the physics very near to the body we have to insert the inflation layer. A total of 5 layers are created and the maximum thickness for the inflation layers is 12.5 mm. (Max inflation layer thickness = 5*1.2^5).
• Now we can generate the mesh.

 

 

• In the above image, we can see the mesh details and the total number of elements.
• Element size for inner enclosure: 50 mm.
• Element size for outer enclosure: 100 mm.
• Total nodes: 57046.
• Total elements: 188221.

 

 

 

 

 

Mesh metric

 

3. Setup

• Steady-state analysis.
• Solver: density-based.
• Velocity formulation: absolute.
• Solution method used: implicit.
• Viscous model: k-omega.
• Fluid: Air.
• Now we have to define the boundary condition.
• Inlet velocity: 50 m/s and Temperature: 300 k. Outlet gauge pressure: 0 pascals.
• In the postprocessing menu, we will create a plane and name this plane cut-plane-z, which will divide the geometry exactly in two halves. This plane will have normal in the z-direction.
• Now we have to initialize the solution, after initializing the solution we have to define the contour plots for velocity and also create solution animation.
• Run the calculation for 500 iterations.

 

4. Solution

Residuals

 

• The above chart is for velocity data at 2 m and 3 m from the origin and behind the Ahmed body.
• To plot the velocity data at 2 m and 3 m from the origin we first have to create lines at these distance.

 

Pressure contour

• The above contour is of pressure around the Ahmed body. In the above image, we can see that the pressure is very high at the front end of the Ahmed body and at some places around the vehicle the pressure is negative.

 

Velocity contour

 

Velocity vectors

• The above image is of velocity vectors on the velocity contours. We can see that because of coarse mesh the wake formation is not proper at the rear end of Ahmed body.

 

                                                             Velocity animation

               

 

• Here we can see the velocity animation and velocity contour. In the animation, we can see that the wake formation behind the Ahmed body is not captured properly. This is happening because of the large mesh elements. We have to refine the mesh to capture the wake formation properly.

 

 

 

                                                                       CASE 2

 

1. Geometry

The geometry setup will be the same as case 1. In case 2 we have to refine the mesh in the inner enclosure around the Ahmed body.

 

2. Meshing

Before starting the meshing procedure we have to create named selections for the inlet, outlet and enclosure walls. 

• Method: In this step, we will insert the meshing method as "multizone". This method will produce high-quality Hexa mesh in the outer enclosure. The element size for meshing is 100 mm.
• Body sizing: After that will insert body sizing for the inner enclosure for performing local mesh refinement. Element size: 30 mm.
• Face sizing: For the cylindrical legs we have to insert face sizing option. This will properly generate the mesh according to the small cylindrical legs. Element size: 5 mm.
• Inflation layer: To properly simulate the fluid flow around the boundary of Ahmed body and to capture the physics very near to the body we have to insert the inflation layer. A total of 5 layers are created and the maximum thickness for the inflation layers is 12.5 mm. (Max inflation layer thickness = 5*1.2^5).
• Now we can generate the mesh.

 

 

• In the above image, we can see the mesh details and the total number of elements.
• Element size for inner enclosure: 30 mm.
• Element size for outer enclosure: 100 mm.
• Total nodes: 122736.
• Total elements: 556957.

 

 

 

 

Mesh metric

 

3. Setup

• Steady-state analysis.
• Solver: density-based.
• Velocity formulation: absolute.
• Solution method used: implicit.
• Viscous model: k-omega.
• Fluid: Air.
• Now we have to define the boundary condition.
• Inlet velocity: 50 m/s and Temperature: 300 k. Outlet gauge pressure: 0 pascals.
• In the postprocessing menu, we will create a plane and name this plane cut-plane-z, which will divide the geometry exactly in two halves. This plane will have normal in the z-direction.
• Now we have to initialize the solution, after initializing the solution we have to define the contour plots for velocity and also create solution animation.
• Run the calculation for 500 iterations.

 

4. Solution

Residuals

 

• The above chart is for velocity data at 2 m and 3 m from the origin and behind the Ahmed body.
• To plot the velocity data at 2 m and 3 m from the origin we first have to create lines at these distance.

 

Pressure contour

• The above contour is of pressure around the Ahmed body. In the above image, we can see that the pressure is very high at the front end of the Ahmed body and at some places around the vehicle the pressure is negative.

 

Velocity contour

 

Velocity vectors

• After mesh refinement in case 2, we can see the wake formation is captured. But we have to refine our mesh further to capture the wake formation more properly.

 

                                                           Velocity animation

               

 

• Here we can see the velocity animation and velocity contour. In the animation, we can see that the wake formation behind the Ahmed body is now captured properly as compared to the first case. But we can further refine the mesh to get the best results.

 

 

 

                                                                     CASE 3

 

1. Geometry

The geometry setup will be the same as case 1. In case 3 we have to further refine the mesh in the inner enclosure around the Ahmed body to check mesh dependency of our solution.

 

2. Meshing

Before starting the meshing procedure we have to create named selections for the inlet, outlet and enclosure walls. 

• Method: In this step, we will insert the meshing method as "multizone". This method will produce high-quality Hexa mesh in the outer enclosure. The element size for meshing is 100 mm.
• Body sizing: After that will insert body sizing for the inner enclosure for performing local mesh refinement. Element size: 25 mm.
• Face sizing: For the cylindrical legs we have to insert face sizing option. This will properly generate the mesh according to the small cylindrical legs. Element size: 5 mm.
• Inflation layer: To properly simulate the fluid flow around the boundary of Ahmed body and to capture the physics very near to the body we have to insert the inflation layer. A total of 5 layers are created and the maximum thickness for the inflation layers is 12.5 mm. (Max inflation layer thickness = 5*1.2^5).
• Now we can generate the mesh.

 

 

• In the above image, we can see the mesh details and the total number of elements.
• Element size for inner enclosure: 25 mm.
• Element size for outer enclosure: 100 mm.
• Total nodes: 182774.
• Total elements: 896394.

 

 

 

 

Mesh metric

 

3. Setup

• Steady-state analysis.
• Solver: density-based.
• Velocity formulation: absolute.
• Solution method used: implicit.
• Viscous model: k-omega.
• Fluid: Air.
• Now we have to define the boundary condition.
• Inlet velocity: 50 m/s and Temperature: 300 k. Outlet gauge pressure: 0 pascals.
• In the postprocessing menu, we will create a plane and name this plane cut-plane-z, which will divide the geometry exactly in two halves. This plane will have normal in the z-direction.
• Now we have to initialize the solution, after initializing the solution we have to define the contour plots for velocity and also create solution animation.
• Run the calculation for 500 iterations.

 

4. Solution

Residuals

 

 

 

• The above chart is for velocity data at 2 m and 3 m from the origin and behind the Ahmed body.
• To plot the velocity data at 2 m and 3 m from the origin we first have to create lines at these distance.

 

Pressure contour

 

• The above contour is of pressure around the Ahmed body. In the above image, we can see that the pressure is very high at the front end of the Ahmed body and at some places around the vehicle the pressure is negative.

 

Velocity contour

 

Velocity vectors

 

• In case 3, we can clearly see the recirculation of air by looking at the velocity vectors. The mesh refinement in case 3 has captured the wake formation with great accuracy than the previous two cases.

 

                                                             Velocity animation

               

 

• Here we can see the velocity animation and above that, the velocity contour. In the animation, we can see that the wake formation behind the Ahmed body is now captured with greate accuracy, compared to the second case.

 

 

Reason for negative pressure

• The moment at which the air hits the front face of Ahmed body, it becomes almost stationary, this sudden obstruction in the fluid flow causes very high pressure at the front face of Ahmed body. This can be seen as the red color area in pressure contour.
• This high-pressure air passes along the curved surface of Ahmed body and gains very high velocity as it moves towards the rear end. This gain in velocity causes a pressure drop and pressure become negative in certain areas.
• The recirculation of air at the rear end of the vehicle causes the wake formation. The recirculation happens because of the pressure difference between top and bottom end of the Ahmed body.

 

Conclusion

Mesh refinement was performed on Ahmed body fluid flow analysis. As we refined the mesh from case 1 to case 3, we can clearly observe the proper wake formation behind the Ahmed body.