September 19th, 2015 - Miata Rear Diffuser CFD Analysis

As promised, I did more CFD test on an NA Miata to show how my rear diffuser kit would effect the air flow underneath the rear end of the car.

In case you weren't familiar with the function of a diffuser, here's a diffuser in a nut shell. It creates extra room for the air under the car to expand and creates a low pressure under the car. The low pressure area under the car creates down force and sucks the car down towards the road.

You want to create a diffuser with as much angle as possible. But if you create an angle that is too severe, the air will not follow the profile of the diffuser and will not create any downforce. It will just flow parallel with the ground and no downforce=no breaking necks at the track.

In the first image you can see a stock NA Miata with no diffuser. The air under the car flows parallel with the ground and eventually rises up after it passes the rear wake (the bubble of recirculating air) behind the car. No expansion under the car=no low pressure=no downforce is created.

In the second image you can see how the diffuser guides the air upwards; creating the low pressure area that results in downforce. Also the overall size of the rear wake has decreased. And notice how the airflow from the bottom of the car hits the top of the test area much earlier than the car without the diffuser. This shows that the airflow will remain attached to the diffuser's profile at 11 degrees. And that 11 degrees is not too steep of an angle. I have previously tested 11 degrees without the rear sub-frame belly pan, and it did not give good results. The diffuser worked better at around 5-7 degrees. But with a flatter under body including a flat rear sub-frame, the air remained attached to the diffuser at a steeper angle. So the next thing I would like to test is at what angle would the air not remain attached with a rear belly pan in place. The angle of my diffuser kit is adjustable, so this would be valuable information to know.

 Since the rear belly pan proves to make a significant increase in the function of the diffuser, it will without a doubt become an option that I will be offering to my customers in the near future.

**Please take this with a grain of salt. My version of Solidworks or any CFD software does not give a 100% accurate visualization of what happens in real life. But at the same time it still gives you a good idea of what would happen. There are still many real life variables to take into consideration like any underbody bracing, any gaps that would create turbulance, etc. Also you can see the boundaries of my test area does not fully envelope the car. With a larger test area, the lines were not dense enough to show you what the diffuser was doing. So i decided to sacrifice the airflow around the top of the car so that you can better see what's happening around the diffuser. 

You can see that the shallow ceiling pushes down the rear wake, which also reduces the amount of air expansion after the rear diffuser. It is my hypothesis that a CFD test with a larger test area will show an even larger low pressure region under the car. That is something that we will find out after more testing when we start using more sophisticated software! Lots of cool stuff to look forward to in the future!