Study of Pressure and Velocity characteristics of the ahmed body by performing grid dependency tests
Study of Pressure and Velocity characteristics of the ahmed body by performing grid dependency tests
Ahmed Body and its importance:
The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features.
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[1].
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.
Reference
[1] S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE-Paper 840300, 1984
Simulation
The ansys fluent simulation on ahmed body was carried out by considering three different cases of grid.
Geometry
Boundary Conditions used for all Cases
Velocity at inlet = 50 m/s
Flow Medium = Air
Slover = Density Based Solver, K- epsilon standard Model
Mesh
The ahmed body is surrounded by two enclosures. For the first enclosure, hexahedral grid is considered and for the second enclosure, tetrahedral grid is considered. The inflation layers around the ahmed body walls are also taken into consideration to precisely study the flow separation at its rear end.
CASE 1
For this case following parameters were considered
Size of Hexahedral Grid = 100 mm
Size of Tetrahedral Grid = 50 mm
No. of elements = 178314
No. of inflation layers was considered 5 with growth rate of 1.2 mm with maximum thickness 12.5 mm
Plots
a) Residual Plot
No. of iterations for Convergence : 670
CFD Post Processing Results
1. Velocity Distribution
2. Pressure Distribution
3. Vector Diagram
Case 2
For this case following parameters were considered
Size of Hexahedral Grid = 100 mm
Size of Tetrahedral Grid = 40 mm
No. of elements = 282340
No. of inflation layers was considered 6 with maximum thickness 10 mm
Plots
1. Residual Plot
No. of Iterations for Convergence = 844
CFD Post Processing Results
1. Velocity Distribution
2. Pressure Distribution
3. Vector Flow
Case 3
For this case following parameters were considered
Size of Hexahedral Grid = 100 mm
Size of Tetrahedral Grid = 35 mm
No. of elements = 404428
No. of inflation layers was considered 8 with maximum thickness 8 mm
Plots
1. Residual Plots
No. of iterations for convergence: 750
CFD Post Processing Results
1. Velocity Distribution
2. Pressure Distribution
3. Vector Flow
Conclusion
1. As the mesh is made more finer and smoother, there is better re-circulation region development in case 3 as compared to case 1 as observed from vector flow diagrams.
2. There is better velcity profile formation in case 3 and case 2 as compared to case 1 due to variation of sizes of grid.
3. There is not much significant difference in pressure profiles of cases under consideration.
Q3. In the post processed results, explain the variation in pressure as to why it goes negative - 50 points
As it can be clearly seen from the pressure profile above, the pressure at front of the ahmed body is high (red area) as plane is normal to the direction of flow air. After striking the body , the flow gets divided between the upper part and lower part of the body and flow gains some velocity at the expense of pressure. As the flow passes over the ahmed body and due to slant at the rear end of the ahmed body , flow separation takes place which results in formation of a re-circulation region at the rear end and developing a region of very low pressure (-ve pressure).