Flow Over Ahmed Body(ANSYS FLUENT)
OBJECTIVE-Simulate the flow over a ahmed body. Perform a grid dependency test and compare the results.
Ahmed 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 body allows us to capture characteristic features that are relevant to bodies in the automobile industry.
People working in vehicle aerodynamics use Ahmed body to validate their numerical model because the experimental data from wind tunnel testing is available for Ahmed body. Once the numerical model in validated, it is used to design new models of the car.
Figure of Ahmed body:
Simulation cases:
Case-1:Outer enclosure-Element size -100 mm(hexa mesh)
Inner enclosure-Element size -50 mm(tetra mesh)
Case-2:Outer enclosure-Element size -90 mm(hexa mesh)
Inner enclosure-Element size -40 mm(tetra mesh)
Case-3:Outer enclosure-Element size -80 mm(hexa mesh)
Inner enclosure-Element size -35 mm(tetra mesh)
Steps involved in the simulation
- Geometry
- Mesh
- Setup
- Solution
- Results
Case-1:Outer enclosure-Element size -100 mm(hexa mesh)
Inner enclosure-Element size -50 mm(tetra mesh)
Geometry-The 3-D model of ahmed body was imported to spaceclaim.To simulate flow over the body an enclosure was created surrounding ahmed body.For refined mesh near surface of the body an inner enclosure was created.This can be seen in the below figure.
Mesh-
No of nodes- 55242
No of elements in Mesh-177868
Inflation layer were generated to capture detailed physics near wall surface
No of inflation layers-5
Growth rate-1.2
Setup-
Solver Type-Density Based Solver
Steady state
Model-Viscous-k-epsilon turbulent model
Boundary conditions- Inlet-Velocity inlet=50 m/s, Temperature=300K
Outlet=pressure outlet=0 pa
Symmetry condition-for side walls and top wall of outer enclosure.
Wall-For bottom wall
Material properties- Air:Density=1.225 kg/m3
Method- SIMPLE
Initialization- Hybrid initialization
Solution-
The solution seems to converged at around 1200 iterations.This can be seen From the below graph that the residuals have become constant after 1200 iterations.
Residual Plot:
Results-
Velocity Contour
Pressure Contour on the body
Velocity vector at recirculation region
Case-2:Outer enclosure-Element size -90 mm(hexa mesh)
Inner enclosure-Element size -40 mm(tetra mesh)
Mesh-
No of nodes- 80986
No of elements in Mesh-286703
Inflation layer were generated to capture detailed physics near wall surface
No of inflation layers-5
Growth rate-1.2
Setup-
Solver Type-Density Based Solver
Steady state
Model-Viscous-k-epsilon turbulent model
Boundary conditions- Inlet-Velocity inlet=50 m/s, Temperature=300K
Outlet=pressure outlet=0 pa
Symmetry condition-for side walls and top wall of outer enclosure.
Wall-For bottom wall
Material properties- Air:Density=1.225 kg/m3
Method- SIMPLE
Initialization- Hybrid initialization
Solution-
The solution seems converged at around 2000 iterations.This can be seen From the below graph that the residuals have become constant after 2000 iterations.
Residual plot:
Results-
Velocity Contour
Pressure Contour on the body
Velocity vector at recirculation region
Case-3:Outer enclosure-Element size -80 mm(hexa mesh)
Inner enclosure-Element size -35 mm(tetra mesh)
Mesh-
No of nodes- 108301
No of elements in Mesh-400708
Inflation layer were generated to capture detailed physics near wall surface
No of inflation layers-5
Growth rate-1.2
Setup-
Solver Type-Density Based Solver
Steady state
Model-Viscous-k-epsilon turbulent model
Boundary conditions- Inlet-Velocity inlet=50 m/s, Temperature=300K
Outlet=pressure outlet=0 pa
Symmetry condition-for side walls and top wall of outer enclosure.
Wall-For bottom wall
Material properties- Air:Density=1.225 kg/m3
Method- SIMPLE
Initialization- Hybrid initialization
Solution-
The solution seems to converged at around 2500 iterations.This can be seen From the below graph that the residuals have become constant after 2500 iterations.
Residual Plot:
Results-
Velocity Contour
Pressure Contour on the body
Velocity vector at recirculation region
Grid Dependency test:
Case 1
Case 2
Case 3
We can see that from the above charts that the values of velocity are almost constant for all three cases.So it can be concluded that solution is independent of grid size and we have reached the state of grid independency from the the first case itself.So if we make the grid more finer it we have negligible changes in solution.
The grid dependency can be better visualised from the above graph.The velocity vs y axis graph shows negligible difference in the plot.
Why pressure goes negative on ahmed body?
1. The air strikes the ahmed body front at a higher velocity due to which the kinetic energy is converted to pressure enerny.So the pressure is highest at the front.
2. As the air passes over ahmed body the velocity is accelerated.So to maintain the flow continuity the pressure becomes lower and goes negative.However the negaive pressure is gauge pressure.
3. Pressure may go negative due adverse pressure gradeint due to flow separation.
Conclusion:
By making the mesh more finer the wake region was seen clearly.