Ahmed Body Challenge
OBJECTIVES:
- Setup Ahmed Body geometry in ANSYS Fluent.
- Study the variation of flow parameters.
- Post process the results and documentation
SCOPE:
- Clean up CAD and extract fluid domain.
- Prepare and verify FV model of Ahmed Body geometry.
- Understand the flow parameters variation and check for grid dependency test.
SOFTWARE USED:
- ANSYS, SpaceClaim - To prepare FV model and setup cases of Ahmed Body Geometry.
- ANSYS, Meshing - To mesh the given model.
- ANSYS, FLUNET - To setup the boundary conditions and to solve the given model.
- ANSYS, CFD Post - To perform Post Processing.
INTRODUCTION:
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 and was first defined and characterized in the experimental work of Ahmed. 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.
Fig 1. Flow over an Ahmed Body
GEOMETRY:
The geometry description is as shown below:
Fig 2. Ahmed Body 3D Geometry
Fig 3. Ahmed Body Front View
Fig 4. Ahmed Body Side View
Fig 5. Ahmed Body Rear View
The Ahmed Body Geometry in this project is studied under 4 different mesh sizes. They are:
- Case 1: Mesh Size : 413.6 mm (Base Mesh size/Default Mesh size)
- Case 2: Mesh Size : 100 mm (External Enclosure)) 15 mm (Internal Enclosure)
- Case 3: Mesh Size : 100 mm (External Enclosure) 15 mm (Internal Enclosure) With Inflation Layers (0.1mm - First Layer, 20 Layers)
The geometry given is known as the solid region/dry region. This portion of the model is not used for external fluid flow analysis. Hence, an enclosure region is to extracted from the domain for the analysis. This process is carried out in spaceclaim tool.
Fig 6. Fluid Domain for Case 1 (Base mesh size/Default mesh size)
Fig 7. Fluid Domain for Case 2 and Case 3
Once these methods are completed, then the model is taken for meshing.
MESH DETAILS:
The fluid voume once extraced is then extraced to the ANSYS Mesher to generate mesh.
CASE 1:
Fig 8. Ahmed Body Base Mesh
Fig 9. Ahmed Body Base Mesh - Sectional View
Fig 10. Ahmed Body Base Mesh - Body Mesh
- Element Size : 507.83 mm
- Nodes : 8119
- Elements : 40015
- Mesh Quality :
1. Element Quality:
- Min : 0.29
- Max : 0.99992
- Average : 0.82837
- Standard Deviation : 9.8128e-002
Fig 11. Ahmed Body Case 1 - Element Quality
2. Aspect Ratio:
- Min : 1.1648
- Max : 6.9009
- Average : 1.8744
- Standard Deviation : 0.47273
Fig 12. Ahmed Body Case 1 - Aspect Ratio
3. Skewness:
- Min : 1.2591e-004
- Max : 0.77809
- Average : 0.24255
- Standard Deviation : 0.12611
Fig 13. Ahmed Body Case 1 - Skewness
- Mesh Check (FLUENT):
Fig 14. Mesh Qualtity Check in Fluent
CASE 2:
Fig 15. Ahmed Body Case 2 Mesh
Fig 16. Ahmed Body Case 2 - Sectional View
Fig 17. Ahmed Body Case 2 - Body Mesh
- Element Size : 100 mm (External Enclosure) 15 mm (Internal Enclosure)
- Nodes : 331338
- Elements : 1866887
- Mesh Quality :
1. Element Quality:
- Min : 8.6727e-002
- Max : 0.99982
- Average : 0.82909
- Standard Deviation : 0.12887
Fig 18. Ahmed Body Case 2 - Element Quality
2. Aspect Ratio:
- Min : 1.0397
- Max : 32.613
- Average : 1.8924
- Standard Deviation : 0.67709
Fig 19. Ahmed Body Case 2 - Aspect Ratio
3. Skewness:
- Min : 1.7333e-004
- Max : 0.94262
- Average : 0.23829
- Standard Deviation : 0.16944
Fig 20. Ahmed Body Case 2 - Skewness
- Mesh Check (FLUENT):
Fig 21. Mesh Qualtity Check in Fluent
CASE 3:
Fig 22. Ahmed Body Case 3 Mesh
Fig 23. Ahmed Body Case 3 - Sectional View
Fig 24. Ahmed Body Case 3 - Body Mesh
- Element Size : 100 mm (External Enclosure) 15 mm (Internal Enclosure) With Inflation Layers (0.1mm - First Layer, 20 Layers)
- Nodes : 410692
- Elements : 1992683
- Mesh Quality :
1. Element Quality:
- Min : 3.4967e-003
- Max : 0.99986
- Average : 0.76675
- Standard Deviation : 0.23805
Fig 25. Ahmed Body Case 3 - Element Quality
2. Aspect Ratio:
- Min : 1.0397
- Max : 417.55
- Average : 5.9603
- Standard Deviation : 20.273
Fig 26. Ahmed Body Case 3 - Aspect Ratio
3. Skewness:
- Min : 1.7333e-004
- Max : 0.95831
- Average : 0.23982
- Standard Deviation : 0.16813
Fig 27. Ahmed Body Case 3 - Skewness
- Mesh Check (FLUENT):
Fig 28. Mesh Qualtity Check in Fluent
BOUNDARY CONDITIONS DEFINITION:
The following boundary conditions is obtained for solving 3 different cases:
Fig 29. Boundary Conditions Description
1. Model : Ahmed Body
2. Fluid : Air
- Density : 1.225 `(kg)/m^3`
- Specific Heat (`C_p`) : 1006.43 `J/(kg* K)`
- Thermal Condutivity : 0.0242 `W/(m*K)`
- Kinematic Viscosity : 1.7894e-05 `(kg)/(m*s)`
3. Inlet Velocity:
- Inlet : 50 m/s
4. Inlet Temperature :
- Inlet : `300 K`
5. Condition : Steady State
6. Flow : Turbulent
7. Outlet : Velocity Outlet
RESULTS :
The Following Results are obtained:
1. Pressure Contour:
- Case 1:
Fig 30. CASE 1 - Pressure Distribution around the body
Fig 31. CASE 1 - Pressure Distribution on the body
- Case 2:
Fig 32. CASE 2 - Pressure Distribution around the body
Fig 33. CASE 2 - Pressure Distribution on the body
- Case 3:
Fig 34. CASE 3 - Pressure Distribution around the body
Fig 35. CASE 3 - Pressure Distribution on the body
From the above contours, we can observe that the pressure plots looks similar to each other but with increase in mesh count, the pressure variation that is captured becomes more accurate.
2. Velocity Contour:
- Case 1:
Fig 36. CASE 1 - Velocity Distribution
- Case 2:
Fig 37. CASE 2 - Velocity Distribution
- Case 3:
Fig 38. CASE 3 - Velocity Distribution
From the above contours, with the increase in the mesh size the velocity developed around the body becomes more accurate. In CASE 3, since boundary layer is introduced the point of seperation can be observed clearly.
3. Velocity Streamlines:
- Case 1:
Fig 39. CASE 1 - Velocity Streamlines
- Case 2:
Fig 40. CASE 2 - Velocity Streamlines
- Case 3:
Fig 41. CASE 3 - Velocity Streamlines
The above figures show the flow pattern that is observed during the flow over the body.
4. Velocity Vectors:
- Case 1:
Fig 42. CASE 1 - Velocity Vectors
- Case 2:
Fig 43. CASE 2 - Velocity Vectors
- Case 3:
Fig 44. CASE 3 - Velocity Vectors
The above images show the velocity patters that is developed furing the flow. In CASE 3, we can observe the separation point is easily visible.
5. Velocity Plot:
- Case 1:
Fig 45. CASE 1 - Vertex Average of Velocity Magnitude
- Case 2:
Fig 46. CASE 2 - Vertex Average of Velocity Magnitude
- Case 3:
Fig 47. CASE 3 - Vertex Average of Velocity Magnitude
From the above plots, we observe that come at the same plot. Hence this proves that the baseline mesh provides a satisfiable results, hence a fine mesh can be avoided helping in reducing computational power and time.
6. Resudials:
- Case 1:
Fig 48. CASE 1 - Residuals
- Case 2:
Fig 49. CASE 2 - Residuals
- Case 3:
Fig 50. CASE 3 - Residuals
7. Resudials:
- Case 1:
Fig 51. CASE 1 - Coefficient of drag plot
- Case 2:
Fig 52. CASE 2 - Coefficient of drag plot
- Case 3:
Fig 53. CASE 3 - Coefficient of drag plot
8. Velocity Variations:
- Case 1:
CASE 1: Velocity profile animation
- Case 2:
CASE 2: Velocity profile animation
- Case 3:
CASE 3: Velocity profile animation
From the above set of animation we can observe that with increase in the mesh count, the velocity fluctiactions around the body becomes more significant.
WHY THE PRESSURE GOES NEGATIVE:
As from the above three cases, we can observe there are some areas where pressure is negative. There can be many reasons for such negative pressure.
- Velocity increases when it flow overs the ahmed body, so in order to maintain the flow pressure becomes negative. (Simple case of law of conservation of energy)
- Flow seperation takes place when the fluid hits the ahmed body,which leads to the formation of the air bubbles which are at very low pressure.
- Pressure could also become negative in order to over come the shear stress.
- Other reason could be due to flow recirculation.
Point of Seperation:
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