...
Note that the simulation will have the air move around the car (as opposed to car moving thorugh air). For a fully accurate representation, the ground can be simulated to be moving; but this is not worth the additional complexity and solving time (the accuracy improvements is marginal).
1.1) To load the model: Click "File > Import External Geometry File", then click "Generate".
| Expand | ||
|---|---|---|
| ||
|
You should be able to see the model appear in the window:
We can proceed to build the wind tunnel to contain our "air" (and cut in car in half in the same step).
1.2) Click "Tools>Enclosure"
Under details view, you can select how large the wind tunnel should be by changing values of the cushion. The cushion is the shortest distance between the wall and any part of the car. Input the desired cushion values. Note that the -Y value in this case represents the cushion distance to the ground, this value will depend on the suspensions and wheels (how far the bottom of the aerobody sits away from the ground).
...
Our car is symmetrical about the YZ plane (this may vary for other models), so a symmetry plane will be
1.3) Select "Number of Planes" to 1 and the symmetry plane to be YZPlane. Input the cushion values as seen below.
1.4) Click "Generate"
| Expand | ||
|---|---|---|
| ||
 |
...
Half the car and wind tunnel should appear. The car body needs to be subtracted from the fluid body (it'll be clear why during meshing).
1.5) Click "Tools> Boolean". Select "Operation" as "Subtract". Select the wind tunnel and target body and car as tool body. Click "Generate"
...
The geometery is ready to be meshed.
1.6) Double click "Mesh" to open Meshing
Meshing
This is the most critical step in the entire process. Building accurate meshes is a massive topic of study (many workshops and tutorials based on only meshing). This tutorial hopes to build basic intuition of proper meshing techniques specifically for vehicle aerodynamics.
...
Let us improve the mesh quality to more accurately resemble reality.
2.1) Click "Mesh" in the sidebar and expand "Sizing". Change "Advanced Size Function" to "On: Proximity and Curvature". Change "Smoothing" to "High". Change "Num Cells Across Gap" to "5-10"
...
These are the global mesh controls. These settings affect everything in our geometry. Advanced Size Function (On: Proximity and Curvature) is chosen as we want the car's curvature to be accurately represented; as the car is a thickened surface, the proximity portion prevents undesirable geometry.
2.2) To add the inflation layer (to model the boundary layer). Expand Inflation and set "Use Automatic Inflation" to "Program Controlled"
| Expand | ||
|---|---|---|
| ||
|
2.3) Click Generate Mesh.
This mesh is far better than the default settings. The bottom has enough cells to appropriately model ground effects. The mesh is denser overall to provide higher resolution and accurately represent the geometry. A boundary layer is added around the entire car.
...
To finish, the mesh components must be labelled.
Highlight 2.4) Highlight and right click the front wall > "Create Named Selection". Name the face "Inlet".
| Expand | ||
|---|---|---|
| ||
 |
2.5) Repeat the process. The wall behind the car is "outlet". The car cavity is "car". The symmetry plane "symmetry". The remaining three walls are "walls".
2.6) The mesh is complete. Go back to ANSYS workbench and double click "setup" to begin setting up boundary conditions and solution parameters.
...
Our computer sucks so keep settings at default and click "ok".
| Expand | ||
|---|---|---|
| ||
|
Click Model > Viscous (Laminar) Model. Select k-epsilon and Realizable.
| Expand | ||
|---|---|---|
| ||
|
These are different CFD models which can be used to solve the simulation. Realizable k-epsilon is the better ones for our purposes.
Edit boundary conditions for the inlet. 25m/s (90km/h) is used for this simulation.
| Expand | ||
|---|---|---|
| ||
|
ANSYS matches named selection to boundary conditions and everything else is already defined by default. A choice can be made for the road wall to be "moving" for the most accurate simulation; however, that is not a major concern for precision.
...
"Monitors". Here we can add various values to monitor. For this simulation, only the drag coeffient is relevant (we may look at lift/downforce in other simulations).
Click Create>dragCreate > Drag. Highlight car and click "OK". Ensure the force vector is pointing in same direction of the car.
| Expand | ||
|---|---|---|
| ||
|
Start the simulation: Go to "Run Calculation" and set number of iterations to around 1000. Click "Calculate" and press "Yes".
| Expand | ||
|---|---|---|
| ||
|
The simulation will begin to solve. We will wait for the solution to converge. If the solution hasn't converged by the end of the 1000 iterations, simply add more iterations.
| Expand | ||
|---|---|---|
| ||
 |
Post Processing
This stage is where we look at out results. What is possible?
...