Automotive Diffuser Questions

[bensalasko]

Assuming the rules allow for it is there any advantage in extending the diffuser beyond the rear bumper? My initial thought was "the bigger the better" but I am now thinking that is not the case. My second question is about the diffuser exit. Almost all diffusers for sale for street cars (that are likely mostly for looks) have a rounded exit like the first picture below. Race cars however all seem to have virtually no upward curve at the rear of the diffuser, F1 being the easiest to see but I know ALMS cars are the same way. Based on that alone I am inclined not to have any upward curve in the diffuser. Any help in my design would be greatly appreciated, I have the rest of the car well sorted but have little to no aerodynamic experience and much of the subject is counter intuitive.

Rounded:

F1:

 

[btrueblood]

There's a wing in that photo?

 

[MikeHalloran]

I couldn't see the wing either.


Mike Halloran
Pembroke Pines, FL, USA

 

[bensalasko]

No wing, the picture was just of the top to show that the airflow would likely still be attached. However I did find a picture of a car with my top and wing if it helps:

http://www.aprperformance.com/images/zoom/PGQAUR/viewsize/AB-922000_0008.jpg

My intention is not to generate lift on the front but to generate as much downforce as possible around the center of the car. Based on a lot of race cars I have looked at it seems the front is usually a bit higher. I could be wrong or there could be another reason for that though.

 

[MikeHalloran]

So, we're still not looking at photos of _your_ car, the one you want help with?





Mike Halloran
Pembroke Pines, FL, USA

 

[bensalasko]

Thank you.

 

[Paul6]

Yes, your wing looks fine, although mounted a bit close to the rear deck.

Perhaps you've duplicated the raised front floor inlet as seen on recent Le Mans cars. That was a rule introduced by the organizers to REDUCE downforce and improve safety at very high speeds. If you go from a 2" high floor at the front to a 1" high floor mid chassis, you will develop a positive pressure zone under the car. That's why everyone runs the front as low as practical and then runs approx. 1° rise the rest of the way back to the diffuser entry. That way there is a small neg press under much of the floor because the increasing ground clearance helps compensate for the floor’s gradually thickening boundary layer.

Once your diffuser starts working, it will demand more volumetric flow than the tired flow coming from the front end can supply. That additional volume of flow will come in from the sides, and a good thing too. This side feed flow will have a thin, energetic boundary layer and because it is freshly attached, it will be able to cling to the diffuser roof as it turns the corner and heads up the hill. You can check this behavior by putting drops of thin tracer oil on the bottom of the floor and seeing where the air flow is carrying it.

Paul6