STEADY STATE ANALYSIS OF FLOW OVER AHMED BODY
STEADY STATE ANALYSIS OF FLOW OVER AHMED BODY
Objective
To perform an external flow simulation and a grid dependency test of air flowing over a Ahmed Body.
Ahmed Body
It is device or 3D model which is represention of standard passenger car. This model was to prove the CFD codes. It was an experimental model were data acquired to get the flow over a automobile. The model was imported in ANSYS DESIGN MODELER.
Case 1:
A STEADY STATE ANALYSIS of flow over the AHMED BODY is done using ANSYS FLUENT. The fluid used is air. Inlet velocity= 50 m/s.
Model
Mesh
The model was meshed using ANSYS mesher. This was a fundamental mesh setup. An enclosure was setup with dimensions given below.
+ve X= 5 m & -ve X= 2 m
+ve Y= 1 m & -ve Y= 0 m
+ve Z= 0.5 m & -ve Z= 0.5 m
A mesh metric that identifies elements according to their quality was employed and it can be seen that the elements of lowest quality are minimal. Most of the elements have very high quality in the range of 0.6-1. Hence, the mesh is of acceptable quality. The meshed model is shown in image below. Number of Elements= 92952, Number of nodes= 18091.
Convergence
Iteration= 500 were ran. The residual plot is plotted in ANSYS FLUENT shown below.
Result
Pressure plot
When the air hits the car pressure is maximum and velocity becomes zero for some time and as the air starts to move along the car the pressure keeps on decreasing and velocity keeps increasing. At some point when the velocity of air becomes maximum to balance it the pressure becomes negative.
The negative pressure of air flowing over ahmed body is because of the bluff bodies. The boundary separation and recirculation takes place when fluid hit the body and this results into shear stress. To overcome the shear the pressure becomes negative.
The wake region in which there is increase in the thrust force is caused by the vectors and the drag which extends behind the body. This makes the backflow which travels from high to low pressure and vortex are formed.
Significance of Separation point
Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient i.e. the positive pressure gradient. In adverse pressure gradient the pressure force adds up with viscous force and opposes the forward flow. This opposition is significant on fluid closest to the surface as the fluid there has less momentum due to small velocity and so when its momentum is further reduced due to opposing pressure and viscous forces the fluid near the surface is soon brought to rest. Further downstream the fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices. Separation point is the point where boundary layer separation starts.
Velcity contour on cutplanez is shown below.
Vector
In this we can see the air flow and the vortices being formed in the wake region.
Chart
Velocity plot is shown below.
Line 1- (2,0,0) (2,2,0)
Line 2- (3,0,0) (3,2,0)
Refine
To refine the area around the ahmed body a new enclosure has to be created to get more accurate result.
Case 2:
A STEADY STATE ANALYSIS of flow over the AHMED BODY is done using ANSYS FLUENT. The fluid used is air. Inlet velocity= 50 m/s.
Model
Mesh
The model was meshed using ANSYS mesher. An enclosure was setup with dimensions given below.
+ve X= 5 m & -ve X= 2 m
+ve Y= 1 m & -ve Y= 0 m
+ve Z= 0.5 m & -ve Z= 0.5 m
A local enclosure was setup around the Ahmed Body
+ve X= 1 m & -ve X= 0.5 m
+ve Y= 0.5 m & -ve Y= 0 m
+ve Z= 0.26 m & -ve Z= 0.26 m
Hexahedral mesh was setup with element size= 0.08m using multizone method. Face mesh 0.008m. body sizing of refine enclosure= 0.05m. inflation=5 total thickness=0.0125m.
A mesh metric that identifies elements according to their quality was employed and it can be seen that the elements of lowest quality are minimal. Most of the elements have very high quality in the range of 0.65-1. Hence, the mesh is of acceptable quality. The meshed model is shown in image below. Number of Elements= 187139, nodes= 68284.
Convergence
Iteration= 500 were ran. The residual plot is plotted in ANSYS FLUENT shown below.
Result
Contours of velocity magnitude is shown below.
Vector
In this we can see the air flow and the vortices being formed in the wake region.
Chart
Velocity plot is shown below.
Line 1- (3,0,0) (3,2,0)
Line 2- (4,0,0) (4,2,0)
`C_d`=0.39546353
Case 3:
A STEADY STATE ANALYSIS of flow over the AHMED BODY is done using ANSYS FLUENT. The fluid used is air. Inlet velocity= 50 m/s.
Model
Mesh
The model was meshed using ANSYS mesher. An enclosure was setup with dimensions given below.
+ve X= 5 m & -ve X= 2 m
+ve Y= 1 m & -ve Y= 0 m
+ve Z= 0.5 m & -ve Z= 0.5 m
A local enclosure was setup around the Ahmed Body
+ve X= 1 m & -ve X= 0.5 m
+ve Y= 0.5 m & -ve Y= 0 m
+ve Z= 0.26 m & -ve Z= 0.26 m
Hexahedral mesh was setup with element size= 0.06m using multizone method. Face mesh 0.008m. body sizing of refine enclosure= 0.04m. inflation=5 total thickness=0.0125m.
A mesh metric that identifies elements according to their quality was employed and it can be seen that the elements of lowest quality are minimal. Most of the elements have very high quality in the range of 0.65-1. Hence, the mesh is of acceptable quality. The meshed model is shown in image below. Number of Elements= 341446, nodes= 128632.
Convergence
Iteration= 500 were ran. The residual plot is plotted in ANSYS FLUENT shown below.
Result
Contours of velocity magnitude is shown below.
Vector
In this we can see the air flow and the vortices being formed in the wake region.
Chart
Velocity plot is shown below.
Line 1- (3,0,0) (3,2,0)
Line 2- (4,0,0) (4,2,0)
`C_d`=0.3780999
Case 4:
A STEADY STATE ANALYSIS of flow over the AHMED BODY is done using ANSYS FLUENT. The fluid used is air. Inlet velocity= 50 m/s.
Model
Mesh
The model was meshed using ANSYS mesher. An enclosure was setup with dimensions given below.
+ve X= 5 m & -ve X= 2 m
+ve Y= 1 m & -ve Y= 0 m
+ve Z= 0.5 m & -ve Z= 0.5 m
A local enclosure was setup around the Ahmed Body
+ve X= 1 m & -ve X= 0.5 m
+ve Y= 0.5 m & -ve Y= 0 m
+ve Z= 0.26 m & -ve Z= 0.26 m
Hexahedral mesh was setup with element size= 0.05m using multizone method. Face mesh 0.008m. body sizing of refine enclosure= 0.04m. inflation=5 total thickness=0.0125m.
A mesh metric that identifies elements according to their quality was employed and it can be seen that the elements of lowest quality are minimal. Most of the elements have very high quality in the range of 0.65-1. Hence, the mesh is of acceptable quality. The meshed model is shown in image below. Node= 176271 element= 386622.
Convergence
Iteration= 500 were ran. The residual plot is plotted in ANSYS FLUENT shown below.
Result
Contours of velocity magnitude is shown below.
Vector
In this we can see the air flow and the vortices being formed in the wake region.
Chart
Velocity plot is shown below.
Line 1- (3,0,0) (3,2,0)
Line 2- (4,0,0) (4,2,0)
`C_d`=0.38305976
Conclusion
The mesh refinement shows that as the mesh becomes more refine the results beome accurate. From Grid Dependency Test we can see that from velocity chart as the meshes get refine the results keep become finer and the formation of the wake region is more clear.