Velox Motorsports – FR-S / BRZ Rear Diffuser Development
by Eric Hazen
The people at Velox Motorsports are our kind of gearheads- engineers who believe aftermarket components should be well-engineered, functional, and shouldn't just look good enough for mass production. With the popularity of the FR-S and BRZ chassis, there has been no shortage of so-called “aerodynamic” products, that are mostly cosmetic and don't have any engineering behind them.
The lack of in-depth engineering within the aero parts industry triggered Velox Motorsports to design their own rear diffuser. They knew that they could improve on what was on the market currently. They wanted to maximize performance, leverage aesthetics, add to their already existing front splitter, and offer additional components not offered to the market previously.
Most readers know what a rear diffuser is- the cool looking thing on the back of a car. Some of you know that it is there to increase downforce. Fewer readers might know how it works. Basically, without getting too technical, a car by necessity ends up being shaped like an airplane wing. This is because it is flat on the bottom and has a curved top side. This is needed in order to have a passenger compartment in the middle somewhere.
Through something called Bernoulli's law, in short, faster moving air has lower pressure than slower moving air. The air flowing along the longer path over the roof of the car travels faster than the air going straight across the bottom, thus the body of a car tends to make lift naturally.
What the diffuser does is make the air flow under the car have a longer path, in addition to serving to spoil the lift. When the air hits the upward curving surface of the diffuser, it accelerates and the pressure drops, reducing lift, or even creating downforce.
With an extensive background in CFD (Computational Fluid Dynamics) and aerodynamics, this project was a perfect fit for the Velox Motorsports team. Looking at how to do the splitter, Velox decided to develop it around some pre-defined goals- the bullet points of them are listed below.
- Decrease lift (produce downforce)
- Reduce drag
- Aesthetically integrate it into the vehicle’s exterior to make it look as if it could be an OEM addition
- Work well with the Velox race front splitter
- Completely cover up the suspension components directly in front of the rear diffuser to further improve performance.
The design of the Velox Diffuser was optimized through CFD analysis. What is CFD analysis? CFD is a computer-aided design tool to help visualize and predict how air will flow and react to a body in motion.
Engineers use CFD in automotive design to predict and reduce drag, and to minimize lift by increasing downforce. CFD is a faster and more affordable way to design aerodynamic parts- compared to using a wind tunnel or physical testing.
Using CFD, Velox took multiple parameters of the diffuser, varied the design of them, and then analyzed the results- both numerically and graphically using CFD.
CFD is very computationally intense, and to be able to run the program on a PC, some compromises had to be made to reduce the number of calculations the computer had to run.
Each analysis of individual design iterations was done using a half-car analysis, since there wasn't any yaw testing performed. This is done using symmetry down the center line of the car. Significantly less computational processing power is necessary when using a half-car model and was the reason behind this analysis approach.
The meshing, or collection of the car's surface data, was done using first a surface mesh and then a volume mesh. To find a suitable mesh size for the analysis, a grid convergence study was done. A volume mesh count of 10 million cells was considered within the room of accuracy using mesh sensitivity analysis.
The CFD post-processing offers both visualization of the flow and numerical data. Using both of these, Velox made educated guesses as what changes were the most beneficial to optimize the diffusers shape to reach the design goals. Pressure plots allowed the Velox engineers to visualize the location of high and low pressure areas very easily.