Week 3 - External flow simulation over an Ahmed body.
CHALLENGE - 3: External Flow Simulation Over an Ahmed Body
AIM:
To perform the simulation of external flow over the given Ahmed Body geometry using ANSYS FLUENT by considering the given conditions in the question.
OBJECTIVE:
- To simulate and compare the results of external flow over the given Ahmed body geometry with base line meshing as well as fine meshing and to perform the grid independence test.
- To plot and study the velocity and pressure contours and of the same.
- To plot the vector plot of velocity and the plot of coefficient of drift and drag.
THEORY:
Q1. Describe Ahmed's body and its importance.
Ahmed body is a bluff model body of simple shape and basic aerodynamic properties of a car or vehicle. The major importance of such models are to study the aerodynamics behaviour of a vehicle. Many major traits of automobile industries like fundamental physics of the car geometry, lift and drag coefficients, velocity and pressure distribution around the bodu are generally solved by considering such models.
Ahmed body was first describe by S. R. Ahmed in 1984 [1].
Q2. Explain the reason for the negative pressure in the wake region.
The reason for the negative pressure in the wake region is due to recirculating flow as fast moving air past the body is not able to flow through the immediate back of the body thus it create a region where there is no pressure drop and generate a negative pressure in the wake region. The same can be obsereved in the pressure contour where a negative pressure was observed at the wake region.
Q3. Explain the significance of the point of separation.
Point of separation is the main important aspect of pressure drag at wake region. Point of separation controls the flow of air around the body by redusing the recirculating zone by allowing the air to flow through the body maximum helps to reduce this pressure drag. Thus point of separation plays an important role in vehicle dynamics.
PROCEDURE:
1. Open ANSYS Workbench 2022R2 select Fluid Flow (Fluent) from Toolbox.
2. Rename the file and left click Geometry and select New Spaceclaim Geometry.
3. Open the geometry STEP or SLDPRT of the geometry in spaceclaim.
4. As the body is perfectly symmetric, the simulation can be performed by considering the half body. For that use split body command in SpaceClaim to perform the operation.
5. Create a wind tunnel by using enclosing the geometry. For that go to Prepare > Enclosure > draw an enclosure and provide the dimension. Front face, Rear face, Top face, Bottom face and side faces should be at a distance 5m, 2m, 1m, 0m, 0.5m respectively.
6. Close SpaceClaim and double click on Mesh. Meshing window will appear. Mark Inlet, Outlet, symmetry and car-wall on geometry by right clicking each part and selecting Create Name Selection. Now select the mesh from the tree and generate mesh with an appropriate element size to get a base line meshing on the surface of geometry. The figure shown below show the geometry after meshing.
7. Close Meshing and double click on Setup, select Double Precision and Fluent window will open. Check all the values and make necessary correction if needed.
8. Select appropriate conditions mentioned below.
Solver - Density Based
Viscous - k-epsilon model
Energy - On
Material - Fluid, Air
Cell Zone Conditions
Volume - Fluid
Boundary Conditions
Inlet - Velocity Inlet, Velocity = 25m/s, Temperature = 300K
Symmetry - Symmetry
Car-Wall - Wall
Outlet - Pressure Outlet
Reference Value
Compute from - Inlet
Projected Area - 0.065741 m2
Define Coefficient of Drag and Lift
Solution Initialization - Hybrid Initialization.
9. Create a plane along the direction of flow to obtain the contours of velocity and pressure.
10. After assigning all the factors click on Run Calculation and set number of iteration and calculate.
The figure below shoews the residual plot after calculation for base line meshing case.
11. Save the contour plots of velocity and pressure, compute Coeff of drag and lift and plot the velocity vector plot in CFD post.
12. Repeat these steps for fine meshing case.
Note - 1: As the main focus is close to car geometry it is advisible to provide fine meshing near to car walls to do so create one more enclosure with front, back, top and bottom faces at 1m, 0.5m, 0.5m, 0m respectively as shown below. Delete the overlapped surface by selecting Interference from Prepare menu. From the Analysis section share topology should be turned on as Share.
Note - 2 : In meshing create Multizone Method to obtain maximum hexa meshing on the surface geometry also provide body and face sizing with appropriate element size for the geometry here 0.1m is considered for the enclosure and 0.001m for car tyre portion to obtain a proper circular shape. The figure shown below is the final meshed geometry.
RESULTS AND DISCUSSION:
PART-1 - Base Line Meshing
a. Velocity Contour
b. Pressure Contour
c. Velocity Vector
d. Coefficient of Drag plot
e. Coefficient of Lift plot
f. Coefficient of Drag and Lift Values
PART-2 - FIne Meshing
a. Velocity Contour
b. Pressure Contour
c. Velocity Vector
d. Coefficient of Drag plot
e. Coefficient of Lift plot
f. Coefficient of Drag and Lift Values
Grid Independency Test
| Cell Count | Coefficient of Drag | Coefficient of Lift | |
| Part - 1 (Baseline Meshing) | 41923 | 0.3828 | 0.2166 |
| Part - 2 (Fine Meshing) | 265704 | 0.3476 | 0.148 |
The above table respresent the grid independency test. From the table it is evident that with increase in quality and number of elements of meshing simulation results become more accurate.
CONCLUSION:
The simulation of external flow over an Ahmed body was carried out using Ansys Fluent. From the study it is concluded that the results obtained after simulation from part 1 are poor as compared to part 2. In part 2 fine and high quality meshes are used where as course meshes are used in Part -1. Thus the refined mesh allows to catch everything in the contour plot more accurately.
Animation File: https://drive.google.com/drive/folders/157-muI1RJgMl5x6kHYmKk2L4s_2UQgH7?usp=sharing