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diffuser basic question


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#1 pbukovca

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Posted 25 January 2012 - 16:33

Hello everybody, I have two simple questions.

(1) Why is the diffuser necessary to increase downforce ? I mean specifically how come a flat bottomed car with no diffuser has less downforce. In McBeath's book on aerodynamics he says the diffuser doesn't produce much downforce. He say though the pressure in the diffuser
is slightly lower than atmospheric pressure and therefore produces a little downforce, it is the effect of the diffuser on the mass flow rate under the cars flat bottom area the produces the downforce. He says it is diffuser allows the venturi airflow to rejoin with air behind the car
more efficiently. But why does this increase downforce compared to car without a diffuser ?

(2) Secondly, increase the diffuser angle (steeper slope) increase mass flow rate under the car (up to a point). Why does this change increase the mass flow rate ?

Many thanks to everybody and I apologize for any errors I made in my description if any. :)

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#2 pbukovca

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Posted 25 January 2012 - 19:34

Minor correction: I just read this month's issue of F1 Racing magazine in which Pat Symmonds says the diffuser can produce up to 10 % of the cars downforce itself. So it seems that significant downforce can come from diffuser itself.

#3 Kubiccia

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Posted 25 January 2012 - 19:40

AFAIK, the diffuser is to expand the constricted air underneath the car, kind of sucking it from the floor accelerating the flow under the car.

Without a diffuser, the air doesn't have an specific way out of the bottom of the car and many air "escapes" from the side. Because the diffuser are have less pressure than atmosphere, it attratcs most the the streamlined air from the front to the back of the car and less air underneath interacts with the atmospheric air in the side of the car which greatly would take away the generation of downforce under the car

About second part, regarding bigger slope, I would interpret as increasing the area of evacuation of the diffuser which "sucks" more air from the bottom. What I mean is that with bigger area in the diffuser, you'll have a bigger area of low pressure sucking the air from the underneath.

I'm not an expert but I think it's something like this based on all reading I made on the matter.

#4 MatsNorway

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Posted 25 January 2012 - 21:07

Here is a thread about a simple windtunnel program you could try and play around with. I would recommend a bit over the normal home pc tho..
http://forums.autosp...howtopic=158029

Remind me to upload some photos from below.

Edited by MatsNorway, 25 January 2012 - 21:07.