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- An aerobody model (solidworks, step, parasolid) which can be knitted and thickened to at least 10mm (higher is better).
- Lots of patience and something to kill time with as ANSYS loads and solves
CFD Overview
We want to calculate what air particles are doing around our car. Essentially we are looking to build a virtual wind tunnel and shoot air at our car to see how it reacts. To do so we must:
- Geometry: Load our car aerobody CAD and define our wind tunnel dimensions
- Mesh: Build an accurate mesh that represents reality
- Solution: Define parameters to solve
The Process
ANSYS
This is ANSYS workbench. ANSYS's Fluent Module is used to perform the analysis.
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Our current module has everything unfulfilled and attention is required on the geometry step. Each cell must be "Up to Date" before starting the next step. If "Update Required" ever appears, simply press "Update Project".
0.1) Double Right click on geometry to open the Geometry Modeller (not Space Claim).
Geometry
In this step we will load in the car model and build our "wind tunnel". Additionally, since the car is symmetrical, we will slice and simulate with only half the car model and wind tunnel in half. This reduces our calculation time by over 50%!
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Check the workbench to confirm our Geometry stage is "Up to Date".
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.
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Meshing Intuition: In general, all finite element techniques looks to break down a difficult problem into smaller simpler parts (each part is a "finite element"). Instead of analyzing flow on the entire tunnel at once, the problem is divided into a network of many air parcels. Each air parcel is represented as a node on the mesh, and every connection between parcels is an edge between nodes. Notice no nodes inside the car shaped cavity; this forces air flow to goes through edges around the car (which is why the car model is subtracted from the fluid model). Later we will define some nodes to have boundary conditions (ie. the wind tunnel inlet nodes have set velocity and car surface nodes are static walls). When solving begins every node will send/receive information (such as pressure, velocity, volume or momentum) to/from adjacent nodes through the edges and update itself. For example, if a high pressure parcel is connected to a low pressure parcel, it is not in equilibrium; thus, there must be some movement (mass flow or velocity change) from the high to low pressure nodes. ANSYS will solve hundreds-thousands of these equations between nodes for thousands of iterations until a steady state is acheived. (Note: This is NOT the full story, but an easy way to imagine what's happening inside the simulation) For the CFD technician, this means we want few nodes in locations where accuracy is insignificant and more nodes in important areas (orientated and positioned in a way to accurately simulate reality). As with all FEA, it is easy to get an accurate simulation (simply make the densest mesh possible and have it solve for three weeks); however, being proficient at CFD means being able to create a minimal mesh which keep computational time low and providing meaningful and accurate results. You can click "Generate Mesh" and mesh based on default properties (click Show Mesh to view the mesh). This gives us a very poor mesh.
 The current mesh is unacceptable: the top surface of the car only has a handful of nodes, the bottom of the car to the ground only has one row of cells and the mesh has no nodes to represent the boundary layer. |
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Or Highlight Surface of Car
Create Named Selection → label "car"
Use "All Faces in Chosen Name Selection" (vs Program Controlled) → select "car"
2.3) Click Generate Mesh.
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2.6) The mesh is complete. Go back to ANSYS workbench and double click "setup" to begin setting up boundary conditions and solution parameters.
Setup/Solution
3.1) Our computer sucks so keep settings at default and click "ok".
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Click Create > Drag. Highlight car and click "OK". Ensure the force vector is pointing in same direction of the car.
Solution > Report Definitions > Drag
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Post Processing
This stage is where we look at out results. What is possible?
Graphical or plots are availible for airflow, pressure, forces, etc.
Results → Reports → Forces → Direction Vector (in direction of air movement) → Print
Graphics → Pathlines → More Steps = Longer Line (500 - 2000) → Make sure to start with some path skips (i.e. 20) and lower as needed
Select Options → Draw Mesh (to see mesh) with path lines (select car)
You can view pathlines for entire car or for a specific line
Line/Rake → select endpoints for line
If you ever lose track of the model, use this button to zoom back to default.
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