## Diffuser Pressure Peak

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Diff-user wrote:so was that your doubt or have i just explained something you already knew and wasted a few kilobytes of F1T storage space ? (boy i need to learn some people skills)

I already knew it, but it's a good explanation for anyone reading this thread who has no idea what anyone is talking about.
tomspotley

Joined: 7 Jul 2012

damn!!!... i JUST understood what the post was all about....
well i do kind agree with the "being stretched" theory..... the way is see it the flow is being pulled and made to turn about a (relatively) sharp curve and it just pulls that curved section back in the opposite direction..... the rest of the diffuser has a less aggressive profile and thus this effect is less visible there..... and acceleration can be both by change in magnitude or change in direction... this is the latter neh? called convective acceleration in fluid mechanics i believe
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Diff-user

Joined: 11 May 2012

It is not just stretching - a radial pressure gradient is the cause / effect (I am with hoolus on this) of the centipetal acceleration of the fluid particles which follow a curved path. Another example of this is low pressure inside vortices.
shelly

Joined: 5 May 2009

That's another thing I hadn't thought of. A pressure gradient can exist normal to the streamlines, causing a change in direction but no change in speed. However Bernoulli's doesn't tell us anything in that case (only concerned with velocity magnitude).
tomspotley

Joined: 7 Jul 2012

A perfectly normal pressure gradient would cause a small increase in velocity magnitude. You end up with your old velocity vector plus a new one perpendicular to it.
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hollus

Joined: 29 Mar 2009
Location: Copenhagen, Denmark

tomspotley wrote:That's another thing I hadn't thought of. A pressure gradient can exist normal to the streamlines, causing a change in direction but no change in speed. However Bernoulli's doesn't tell us anything in that case (only concerned with velocity magnitude).

The only pressure that really matters is that acting normal to the bodywork surfaces.
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riff_raff

Joined: 24 Dec 2004

tomspotley wrote:That's another thing I hadn't thought of. A pressure gradient can exist normal to the streamlines, causing a change in direction but no change in speed. However Bernoulli's doesn't tell us anything in that case (only concerned with velocity magnitude).

IIRC, the Bernoulli Eq. is only valid along a streamline or in an irrotational flow.

Edit: What I wrote was a little unclear. Just saying that the Bernoulli Eq. would show that the flow along the streamline is in fact slowing down, and then applying conservation of mass and assuming incompressibility would show that there's flow going in other directions. Same thing I believe that incompressible Navier-Stokes would show.
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jon-mullen

Joined: 10 Sep 2008
Location: Big Blue Nation

The diffuser works kind of like a blowgun, if you blow slightly then the air can actually go around the projectile(diffuser kink) with relatively little obstruction. If the airflow intensifies then you get a pressure gradient, and what happens to the airflow after the projectile has left the blowgun? The net airflow going through the blowgun increases starting all the way from your lungs, now imagine this, happening really fast(to the point that statistical analysis is more accurate than finite), over and over and over again, and you pretty much understand what happens at the diffuser.

It seems to me that the diffusers of the last two years have been modeled more after the frong wings, than just an out and out diffuser. Hence the gurney tabs, and more careful sculpting of not just the underside but topside of the diffuser.
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godlameroso

Joined: 16 Jan 2010
Location: Miami FL

I've just read this topic and found some answers to my questions on how diffusers work... I was confused more or less as tomspotley about the "contraddiction" of diffusers vs nozzles.

I found this post extremely helpful:
viewtopic.php?p=359737#p359737

And this one too:
viewtopic.php?p=359784#p359784

But now I have some further doubts:

1) if the car had no diffuser (as stated in the latter post) and only a flat floor, wouldn't an expansion occur all the same? It would be sudden and not gradual as in the diffuser, but wouldn't this pressure difference (between floor and environment behind the car) produce an acceleration of the underflow and thus downforce?

2) (opposite "extreme" case) If the car had the whole floor shaped as a diffuser, would it produce more downforce than a flat floor and a diffuser only at the end?

Maybe I'm still lacking some physics knowledge to understand perfectly diffusers... but a good explanation, maybe with the effect of the two "extreme" cases, would be helpful. Thanks in advance and nice topic!
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LogicPro

Joined: 1 Apr 2013
Location: Northern Italy

2) f1 now has flat floor and small diffuser due to rules, because full floor ground effect was too effective and not safe in some situation, so they banned it. And yes, full floor diffuser give lots of downforce.
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stez90

Joined: 10 Jul 2012

LogicPro wrote:2) (opposite "extreme" case) If the car had the whole floor shaped as a diffuser, would it produce more downforce than a flat floor and a diffuser only at the end?

Maybe I'm still lacking some physics knowledge to understand perfectly diffusers... but a good explanation, maybe with the effect of the two "extreme" cases, would be helpful. Thanks in advance and nice topic!

The full-floor diffuser might not develop as great a pressure depression. But the ΔP would extend over a greater area that, in turn, would produce a greater force.
olefud

Joined: 12 Mar 2011

Not sure if this is the technically correct spot for it, but it is a good video.

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Cam

Joined: 2 Mar 2012

Maybe not the smoothest presenter, but it goes a bit further to explain.
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Cam

Joined: 2 Mar 2012

LogicPro wrote:1) if the car had no diffuser (as stated in the latter post) and only a flat floor, wouldn't an expansion occur all the same? It would be sudden and not gradual as in the diffuser, but wouldn't this pressure difference (between floor and environment behind the car) produce an acceleration of the underflow and thus downforce?

It would sudden and behind the car, and the pressure difference wouldn't be acting over any body work so the downforce generated would be minimal.
Cold Fussion

Joined: 19 Dec 2010

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