GD Aero Additions & CFD
January 10, 2016
This is something a bit different from normal. I’ve wanted to perform some CFD on the car for quite a while now and decided to start tackling this as a fun side project. The usefulness of CFD, especially when inexperienced in it, is always questionable, but hopefully down the line I will learn more about its methods and improve my models with time. I have little to no experience with CFD so take these results with a huge pinch of salt.
I decided to start with some basic 2D analyses for ease and simplicity; a 3D model would be both extremely time consuming and ultimately futile. I utilised Ansys’ Fluent software with 2D profiles created in Creo 3.
The main purpose was to look at and compare pressure and velocity profiles when the splitter and wing are added. Due to the simplicity of the model with an inviscid fluid the absolute results will not be very accurate so I am merely comparing them to one another and looking at the percentage differences. If anything these are more like pretty pictures to look at. With that being said it is clear that the aero additions have a significant effect.
The Vehicle Profile:
This was initially traced from a blue print of a Nissan 200SX S14A. The undertray was modelled to include some of the prominent features on the car; with the profile of the floor of the car being very different in various places I tried to include things such as the front sub-frame, void behind the engine, rear sub-frame and boot profile.
There are 3 profiles: V1 (standard), V1.5 (with splitter added) and V2 (wing and splitter added).
As mentioned before this analysis was done with an inviscid fluid (ideal fluid with no viscosity) so this will add a reasonable amount of error, but it still shows the effects of the aero additions well. The air velocity was set to 100km/h. With no viscosity or surface friction the Cd is not at all accurate and is ignored. The Cl (lift) is compared between models.
In the graphical results all the scales between each version are the same.
The pressure distribution gives you a good idea of the lift being generated at different parts along the car. At V1 it can be clearly seen that there is quite a high pressure under the front of the car as air is forced down under it; this is generating quite a lot of lift. There are a few high pressure points at the windscreen and boot, which are to be expected, but the low pressure above the roof as the air is sped up over will also generate a lot of lift when you compare it to the pressure under the car.
The addition of the splitter makes a huge difference to the underbody pressure. The air hitting the front is forced up limiting the airflow under the car; not only is the pressure under the engine lower but the pressure differential between the top and bottom of the splitter will created quite a lot of downforce as well; it shows how stiff and well supported the splitter needs to be. I was surprised just how noticeable the effect of the splitter is and this should create a noticeable difference to the car.
Swapping the spoiler at the back for a wing performs as you’d expect; high pressure above and lower underneath generating downforce. In future iterations I am going to measure the lift of the wing at various places to see where it may be most effective, or if it matters at all.
With the addition of the splitter (V1.5) the lift decreased by 46%, while the wing further decrease the lift by 16%.
The streamlines shown in this picture again clearly show the effectiveness of the splitter. The air being forced up allowing for a limited amount of air to go underneath at a much greater speed. This higher pressure at the front and lower pressure under the car should also increase the effectiveness of the intercooler and radiator, creating a bigger pressure differential across them.
The results once again show the wing at the rear functioning by the higher speed beneath it. Although there is quite a lot of separation behind the wing indicating that the angle of attack is too high, but its difficult to read into the boundary layers due to the accuracy of the model.
The next steps will be to continuously improve the model and look at different aero additions. I would like to investigate the position of the wing as mentioned and also look at the effect of adding a diffusor at the rear.