Rear diffuser

[Jetracer18]

Hi,
I'm new to this forum and it seemed like a great information source. I have been researching diffusers for race cars and have had little luck with finding much information other than what I have found on this board and through the F1 news regarding there diffusers. I am trying to learn more and more about them and need someone to bounce questions off of. The way I understand it and someone correct me if I am wrong that the purpose of the diffuser in a racing application is to 1) use the shape to create a lower press under the back of the car and create a larger pressure difference which leads to more rear downforce and 2) keep the boundary layer from separating and easing the flow from underneath the car gradually introducing it to the ambient atmosphere behind in such a way that the drag is dramatically reduced by reducing separation of the flow. Now ive heard this is supposed to occur over about a 7 to 13 degree slope. I also have read that there are supposed to be vanes running down the diffuser towards the back of the car gradually curving outwards towards the sides of the car and increasing in area (size) as they go towards the back of the car. What I do not understand is why the vanes ( there shape, why they curve how many,etc.)? Also where should the diffuser start? Directly under the rear axle, at the back of the frame etc.? I'm just trying to gain an understanding of the diffuser and what proper design concepts are necessary for me to create a basic diffuser to test? Thank you to anyone who can help!

Andrew

 

[Dave K]

Many times the diffuser start location is defined by the rule set, for whatever series you're racing. Sometimes diffuser shape is driven by the rules too.

Without a rule set to follow, I'd say your diffuser start location would depend on where you want the center of pressure.

The strakes, or vanes, that I've dealt with in the past, were more to straighten the flow coming into the diffuser, and keep the wheel created disturbance from choking the diffuser.

Check out a copy of Racecar Aerodynamics, by Katz.

-Dave

NX 5.0.6.3mp7

 

[Jetracer18]

Gunman,

Thanks for the response. I have gone through the rule book and there is not even a mention of a diffuser or any aero dynamic device under the car for that matter. So would the ideal situation be to try and make it start right under the rear wheels because this is where the downforce for "forward bite" can be gained? Or would you want to have it start just before the rear wheels? Also could you further explain the situation i saw where the fins started by running straight from the front to rear of the car and then curved outwards gradually towards the sides? I have purchased that book and am waiting now. Thank you again!

Andrew

 

[Dave K]

What type of car is this? Ideally you'd want a full flat bottom on the car, otherwise there could be too much underbody turbulence, and the diffuser won't work very well.

As to where to start it, again, it will depend on the car, what the chassis is like, where the suspension arms are, etc. My experience, the rule set wouldn't let me start any farther forward than the rear axle centerline.

-Dave

NX 5.0.6.3mp7

 

[Dave K]

These links may help:

http://www.mhest.com/spotlight/automobiles/articles/Race-CarAerodynamics.pdf

http://strangeholiday.com/oops/stuff/annurev.fluid.38.050304.092016.pdf

-Dave

NX 5.0.6.3mp7

 

[Jetracer18]

Gunman,

The car is very similar to looking like an indy car. Its open wheel and the sheet metal on the bottom is dead flat and runs from the nose all the way to the back of the car so the bottom is completely enclosed to answer that question. I am going to check out them links right now.

Andrew

 

[Dave K]

Like a FSAE car?

I'd look at where you want the center of pressure, in relation to the center of gravity, and adjust the diffuser length as needed.



-Dave

NX 5.0.6.3mp7

 

[Jetracer18]

It looks basically like an indy car for simplistic standards. Its full size and has a solid rear axle (i believe FSAE ans indy have independent). The biggest problem is generating rear bite so that would be one main goal to try and achieve.

 

[Dave K]

Do you use a rear wing? If so, you can use it to help 'drive' diffuser too.

-Dave

NX 5.0.6.3mp7

 

[Jetracer18]

There is a rear wing yes. Do you want me to post a link or send you a picture of a car somehow? The wing is flush with the rear deck and curves upwards starting even with the rear deck. (unlike Indy for example where the wing is "in the air")