Flow over Ahmed Car Body Simulation Using Ansys Fluent
Ahmed Body Challenge
Aim:
To perform steady-state simulations on flow over Ahmed Car Body geometry studying the flow characteristics of the surrounding air using Ansys Fluent.
Procedures:
- Generating the CAD Model of a channel with the Ahmed body using Ansys SpaceClaim.
- Preparation of enclosure for optimizing mesh generation around critical areas.
- Mesh Processing with grid dependency test.
- Setting up the boundary conditions & to solve the case.
- Post Processing output files.
Ahmed Body:
Ahmed body represents a basic Car model where the essential air flow features of the air around the Ahmed body is captured which is used by the people working in Vehicle's aerodynamics to validate their numerical model with the experimental wind tunnel data available for Ahmed body. When the data collected from the numerical model matches with the experimental model, the prediction of flow around the New Model design would be precise.
1:- Pre-Processing:
1.1 CAD Preparation:
This is the Ahmed body model with which we solve the flow properties around the body.
1.2 Enclosure Preparation:
We have the Ahmed body model but to run CFD simulation a closed environment is required for the solver to extract the volume data within defined Boundry condition. So an enclosure is generated with the bounding box size 8m * 1.3m * 1.3m. The inlet will be at 2mts away from the center of Ahmed body.
In order to effectively capture the flow around the Ahmed body, another enclosure is generated with the bounding box size 2.5m * 1m * 1m. The inlet side face will be 0.5mts away from the Ahmed body. Make sure the physical topology is shared between these two enclosures.
2:- Solver Processing:
For the outer enclosure hexahedral meshing type to be used. This will reduce the computational time for the solver. Then inflation meshing to be generated at the cells near Ahmed body's wall which will be set up to 6 layers.
In this case, the number of lines connected along the circumference of the Ahmed Body's stand wall will be less which results in inaccurate simulation over the stand wall regions. To resolve this we can implement the face sizing method. By Face sizing method the number of Faces connecting along with the stand circumference increases making the mesh finer at the stand region. The meshing shape is set to triangles for the inside enclosure. The below image shows the final mesh generated with the face sizing.
2.1.- Setting up Solver:
These cases are solved using a density-based solver as the fluid medium is a compressible air with an inlet velocity of 50m/s.
In setting up physical models, we use standard K-Epsilon under the Viscous model section.
In order to capture the flow at the rear side of the Ahmed body monitor lines to be added at two positions. The below pictures indicates the position of these monitor lines (Line1 & Line2).
The above image shows us the visual appearance of the two monitor lines placed at the rear side of the Ahmed body to capture the fluid flow crossing those lines.
2.2 Grid Dependency Test:
The fluid flow around the Ahmed body is all needed, so proper base mesh sizes to be implemented at the inside enclosure. For this, the grid dependency test should be done.
2.2.1 Case-1:
Meshing:
In this case, the base element sizing was set to 150mm. The below image shows us the total number of elements generated with this mesh size is 114129 Nos.
Post-processing:
Residual Plot:
The above figure shows us the velocity, energy & continuity curve with respect to the number of iterations. Note that the solution is not converged even after 1000 number of iteration. The curves were also generated with unstable patterns.
Flow Over Monitor Lines:
The above plot shows us the velocity of the air crossing the monitor lines (line1 & line2) with respect to the case bounding box's height.
Velocity Contour:
The above figure shows us the velocity contour at the 1000th iteration. The maximum velocity found here is 75m/s.
2.2.2 Case-2:
Meshing:
In this case, the base element sizing was set to 100mm. The below image shows us the total number of elements generated with this mesh size is 188415 Nos.
Post-processing:
Residual Plot:
The above figure shows us the velocity, energy & continuity curve with respect to the number of iterations. Note that the velocity & energy equation is converged at the 600th number of iteration as the curves were generated with stable patterns.
Flow Over Monitor Lines:
The above plot shows us the velocity of the air crossing the monitor lines (line1 & line2) with respect to the case bounding box's height.
Velocity Contour:
The above figure shows us the velocity contour at the 1000th iteration. The maximum velocity found here is 73.40m/s.
2.2.3 Case-3:
Meshing:
In this case, the base element sizing was set to 50mm. The below image shows us the total number of elements generated with this mesh size is 294139 Nos.
Post-processing:
Residual Plot:
The above figure shows us the velocity, energy & continuity curve with respect to the number of iterations. Note that the velocity & energy equation is converged at the 650th number of iteration as the curves were generated with stable patterns.
Flow Over Monitor Lines:
The above plot shows us the velocity of the air crossing the monitor lines (line1 & line2) with respect to the case bounding box's height.
Velocity Contour:
The above figure shows us the velocity contour at the 1000th iteration. The maximum velocity found here is 73.40m/s.
2.3 Grid Dependency Result:
After performing the 3 case setups with respective base grid sizes the case-2 & 3 we can see that there were only minor changes in the post-processing results showing us that the case had converged. The finer the mesh the greater the possibility of accurate flow prediction. Here Case 3 has the finest mesh of all. So can we take data from case 3 as that would match with the experimental data taken for Ahmed body.
2.4 Pressure Distribution:
The above figure shows us the pressure acting over the Ahmed body's exterior faces. The maximum pressure found to be 1515Pa & minimum to be -5935Pa. We can see the pressure acting at the rear end of Ahmed body is in negative 5935Pa. This is due to the air which is in high pressure due to collision with the Ahmed body front face makes it way along top, bottom & side faces in X-axis direction creating vacuum region at the rear end of the Ahmed body. This could increase the vehicle's drag coefficient. So the designers try to design the body in such a way to minimize the drag coefficient as far as possible.