This article looks to provide an overview on ANSYS Fluent and its tools to analyze, as well as optimize, the vehicle's aerodynamic performance.
Prerequisites:
- 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
Steps in BLUE represent actions one would do inside ANSYS.
ANSYS
This is ANSYS workbench. We are using ANSYS's Fluent Module to do our analysis.
Double click on "Fluid Flow "Fluent" to load the module.
The module has five parts: geometry, mesh, setup, solution and results. Each have their respective status symbol.
Up To Date: This is what we want. It indicates the step is complete.
Unfullfilled: The previous step is incomplete and "upstream" data is missing
Attention Required: Previous step is complete, but action needs to be taken to proceed.
Update Required: Something in a previous step was changed and the step needs to be updated.
Refresh Required: For our pruposes, its the same as update required.
So looking at our current module, we have everything unfulfilled and attention is required on the geometry step. Each cell must be Up to Date before starting the next step.
Double click geometry to open the Geometry Modeller.
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 the model and win tunnel in half and only solve for either the right of left half. This reduces our calculation time by around half!
For straightforward, yet accurate, simulation purposes only the air is modeled to move around the car (as opposed to car moving thorugh air). For a fully accurate representation, the ground can be simulated to be moving (generally not worth the additional complexity and solving time).
Click `File>Import External Geometry File", then click "Generate"
Our geometry is loaded and we can proceed to build the wind tunnel to contain our "air". The car will also be cut in half in this step.
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.
This presents a challenge: if the wall cushion is too small then the "air" will interact with the wind tunnel walls and give us poor results, but if the wall cushion is too large it will take heavy computation time to solve the system.
For this tutorial, we will use three-five times the car dimensions as a cushion (ie. the car is 2m wide, the total cushion is 10m). In reality, the optimal cushion value will depend on many factors (areas of interest, competency at meshing) and can be determined through experimentation (run the same setup with varying cushion values to see how close the walls can get before the results changes drastically).
Our car is symmetrical about the YZ plane (this may vary for other models).
Select "Number of Planes" to 1 and the symmetry plane to be YZPlane. Input the cushion values as seen below.
Click "Generate"
Meshing
Setup/Solution