The Importance of Front Splitter Leading Edge Radius

[fe1rx]

My recent brake and oil cooler duct installation had me doing some yarn tuft flow visualization at the front of the car, so I took the opportunity to also do some flow visualization of my front splitter. In particular, I wanted to look at the effect of the leading edge fairing/cuff that I had installed with respect to increasing the air flow under the car (more flow meaning higher velocity meaning lower pressure on the underbody and more front downforce).

I had built some bracketry (I call it an air rake) that allowed me to easily tuft the air flow approaching the front of the car. For this examination I located the tufts on the vehicle centerline, and I installed a GoPro to an arm attached to the front tow hook. While tufting us more commonly applied to surfaces, with some caveats it can be revealing away from a surface also.



My basic splitter is fabricated from 3/8” HDPE with a full (3/16”) radius on the leading edge. I currently run a separate add-on leading edge fairing that increases the radius on the lower edge of the splitter to 1”.



The fairing is also fabricated from HDPE and is screwed on.



The underside transition between the splitter and the fairing is smooth.



I looked at the results for both leading edge configurations at speeds of 60, 80, 100 and 120 km/h, with little observed difference based on speed, except that at 60 km and below the weight of the yarn tufts causes them to droop and not fully reflect the path of the incoming air. At 80 km/h and above there is enough energy in the air to fully support the yarn tufts such that their curvature is reflective of the actual streamline/streamtube in which the yarn is emersed.

The following images show the flow at 120 km/h with and without the fairing.





The results are dramatically different. Without the fairing all approaching air below approximately the lowest tuft flows under the car, the rest over. With the fairing all the air below the second lowest tuft flows under the car, the rest over.

The reason for this is not immediately apparent, so I lowered the camera’s vantage point and tufted under the splitter leading edge.



In this configuration, this time at 80 km/h, it is apparent that there is much more turbulence under the splitter without the fairing, and that this turbulence effectively obstructs flow under the car.





Even though the spitter is installed with essentially zero incidence angle relative to the ground plane, it is clear that it operates at a significant angle of attack due to the downward approach of the incoming air. When the leading edge radius is small, the approaching air cannot follow the tight radius and it separates from the lower surface of the splitter.

The desirability of a large radius on the lower leading edge of a front splitter is well known and has been widely commented on, so that it has an effect was not at all surprising to me. What did surprise me is how much more air flows under the splitter with the addition of a modest radius.

[tracer bullet]

Originally Posted by fe1rx

What did surprise me is how much more air flows under the splitter with the addition of a modest radius.

Agreed! Do you think you felt any difference in downforce? I vaguely recall you setting up a camera in the wheel well once in the past to measure how much the springs compressed.

I don't know how you have time for these things but I'm glad you do and love reading about them. Thank you.

[fe1rx]

Originally Posted by tracer bullet

Agreed! Do you think you felt any difference in downforce? I vaguely recall you setting up a camera in the wheel well once in the past to measure how much the springs compressed.

I don't know how you have time for these things but I'm glad you do and love reading about them. Thank you.

You are welcome!

The camera approach was followed up by damper position sensors and a pitot-static system which allow me to measure downforce pretty well. By that time I had installed the fairings so all my "accurate" downforce measurements are with the fairings on, and I have never done any back-to-back track testing with and without the fairings.

https://www.1addicts.com/forums/show....php?t=1801907