Importance of the diffuser?

[johnny99]

Would it not be a better idea if the area 1 was also flat, exactly the same as area 2

John

i used to think that the diffuser created downforce by causing the air to expand and drop in pressure, until i read Simon McBeathe's book.




what i still can't quite get my head around is why the air increses velocity from the outside to section 2. I know the cross sectional area of the space between the air and the ground decreases => an increase in velocity which gives the low pressure.But i keep thinking that there would be a high pressure zone at section 1.

Can anyone clear things up?

[timbo]

Can anybody explain why there's pressure minimum at the transition from floor to diffuser?
It is pretty straightforward why the first minimum of pressure is at the front of the floor, as air would slow down due to friction along the floor length. But what causes pressure drop at the transition from floor to diffuser?

[ESPImperium]

Would it not be a better idea if the area 1 was also flat, exactly the same as area 2

John

i used to think that the diffuser created downforce by causing the air to expand and drop in pressure, until i read Simon McBeathe's book.




what i still can't quite get my head around is why the air increses velocity from the outside to section 2. I know the cross sectional area of the space between the air and the ground decreases => an increase in velocity which gives the low pressure.But i keep thinking that there would be a high pressure zone at section 1.

Can anyone clear things up?

Not really as youre not channeling the same volume of air under the car, so a slight funner means more air, means more pressure, means more downforce as the end result.

Thats what i have learnt looking at Williams front wing "snow-plough" and the same idea a little diffrent and a little futher back on the Brawn.

[xpensive]

@ timbo:
The reason why the lowest pressure is at the transition from flat-floor to diffuser is that the air-speed under the car is peaking at that point.

The diffuser works as a suction-device speeding up the air, why air-speed will taper off farther front.

[timbo]

@ timbo:
The reason why the lowest pressure is at the transition from flat-floor to diffuser is that the air-speed under the car is peaking at that point.

That is pretty clear, of course but, why:

The diffuser works as a suction-device speeding up the air, why air-speed will taper off farther front.

Because air slows along the diffuser length and pressure also increases.

[slimjim8201]

Made a model of the Allard J2-X (not a very good one). You can clearly see how the diffusers function under the car in terms of high and low pressure regions:

[Callum]

Very nice slimjim, I really like the Allard, how did you manage to get a picture of the underfloor? What software did you use?

[repaf1]

The diffuser works as a suction-device speeding up the air, why air-speed will taper off farther front.

I need some clarification here. Am I correct in saying... The diffuser is speeding up the air flow under the floor section, (with its suction effect) and this increases total car downforce. The diffuser section itself doesn't really create much local downforce as the airflow in the diffuser section is slowing down.

So in general the downforce the diffuser creates is total car/floor downforce, and not downforce localized at the rear of the car?

[xpensive]

It think this has been discussed and explained upstreams on this very thread.

[walter]

mulsannescorner.com has an interesting page about that car. Looking at the underbody I was interested in whats happening right behind the wheels... the pressure is lower to the outside of each wheel rather than the center where my logic would put it. can anyone explain this or should this be ignored?

[repaf1]

It think this has been discussed and explained upstreams on this very thread.

Not really, no mention of the diffusers local downforce production, only that it aids in total car Down-F. I'd like to know if the diffuser itself produces downforce locally just as the rear wing does, and if so, how does it do this, is it the decreasing pressure gradient in the diffuser that created downforce in that spot, and speeds airflow under the floor?

[timbo]

It think this has been discussed and explained upstreams on this very thread.

Not really, no mention of the diffusers local downforce production, only that it aids in total car Down-F. I'd like to know if the diffuser itself produces downforce locally just as the rear wing does, and if so, how does it do this, is it the decreasing pressure gradient in the diffuser that created downforce in that spot, and speeds airflow under the floor?

I thought along the same lines and I concluded that you have to consider air above the floor and diffuser to understand how diffuser adds to downforce.
If you have clean high-velocity air stream above diffuser that would result in lower static pressure in the air directly above diffuser, and therefore the diffuser would work as suction device. It would make diffuser create a bit of lift, but would lower the pressure under the floor.

[roger44]

Are we omitting a fundamental underbody aerodynamic in the diagram being shown in this topic?

the floor is NOT flat to the ground, it is angled nose down, in fact the front splitter is almost touching the track hence the sparks seen from the front sometimes, espceially underbraking.
The air as it moves back towards the rear expends as the gap to the ground gets wider, this air becomes less dense, hence vacumn.
To encourage the air to flow backwards faster and cause greater vacumn, the exhausts create hot air at the rear which is less dense, diffuser air is drawn into this low pressure area creating suction on the diffuser which draws the air from under the car

[xpensive]

Interesting point actually, repaf1. However, my guess is that the air-speed leaving the tip of the diffuser would be more or less the same on top as well as below, pretty much as on a wing profile. Thus making the diffuser itself rather neutral downforce-wise?

Where's that Irish aeronautical engineer when you need him anyway?

[timbo]

The air as it moves back towards the rear expends as the gap to the ground gets wider, this air becomes less dense, hence vacumn.

You are wrong. When the tube expands air slows and that increases static pressure. That is for subsonic flow.