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 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!
Click `File>
Meshing
We will be using Solidworks to model the surface of the aerobody. Solidworks has a number of powerful surface modelling features but they are sometimes difficult to use, especially for those just starting out. The following video offers a good introduction to modelling surfaces in solidworks:
Full Sized Model
Once, we have designed the aerrbody in CAD, we will then obtain full sized models of the car body. These will likely either be made of foam or MDF. More complex curves will be made using a CNC machine while less complex geometry can be made by hand. The surface finish of the model is very important because any imperfections on the model will end up on the final body panels.
Fibreglass Mould
The next step is to create a fibreglass mould or tool of the body panel. This is done by first applying a coat of mould release to the model. Then tooling gelcoat is applied followed by several layers of fibreglass and resin. The result is a fibreglass mould into which you can do a carbon fibre layup.
Final Layup
The final carbon fibre layup is complex and there are numerous ways to do it (infusion layup, wet layup, etc.). This step will always require the use of vacuum bagging. A process where a plastic bag is wrapped around a the mould and all the air is sucked out; thus applying pressure to the carbon fibre and transferring all the detail of the mould to the final part.
There is an excellent video series on youtube that shows the entire process of making a fibreglass mould, followed by layups inside that mould: