Week 3 - External flow simulation over an Ahmed body.
Q1) What is Ahmed's body and its importance?
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's 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 is validated, it is used to design new models of the car.
Geometry:
Simulation setup:
1) Geometry Creation/ model prep in spaceclaim:
Using the enclosure tool in spaceclaim we prepare 2 enclosures around the ahmed body.
Since the model is symmetric we can split the body in half to reduce the cell count in meshing so less computational power is required.
The split tool was used to do this.
2) Meshing
Baseline mesh-
Statistics-
Solver settings
General
Viscous model-
Materials-
Boundary conditions-
Inlet-
Case-1
The highest quality mesh possible was used.
Mesh metrics-
Statistics-
Initialization method- Standard
No. of iterations- 500
Results-
Case 2-
Medium quality mesh
Initialization method- Standard
No. of iterations- 500
Results-
Case 3
Low quality mesh
Initialization method- Standard
No. of iterations- 500
Results-
Conclution-
The value of drag and lift coefficients for a refined case are -
Experimental Drag value= 0.38
Expected value of drag= 0.33
Error percentage= ((0.38-0.33)/0.33)* 100= 15.15%
Solution within 30% error is acceptable moreover if the drag value is compared across all the 3 cases the drag value keeps increasing as the mesh quality decreases or no. of elements decreases so if even a higher quality mesh was used for the simulation the percentage error could be reduced.
Mesh dependency test
Case No. No. of Elements Drag value Lift value
1 410631 0.38 0.255
2 198517 0.40 0.257
3 120488 0.42 0.255
2) Since the Ahmed body is a bluff body there is a negative pressure in the wake region with the increase in velocity, causes of this is the boundary separation and recirculation of flow along with shear stress, so to overcome these stresses the pressure turn negative in the wake region.
3)Flow separation occurs when the fluid boundary layer encounters an adverse pressure gradient. The point on the body where the boundary layer lifts off the surface is called the separation point. When the airflow separates off the surface, the pressure drag usually increases. To have better performance of an Automobile or any vehicles this separation needs to be avoided or delayed, for smooth flow over the surface of the vehicle.