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Diffusers "Pumping Down" the Floor Pressure and the Relation to Model Base Pressure


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#1 pbukovca

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Posted 21 November 2012 - 22:01

Questoins

(1) Does anybody know where to get that famous 1994 paper by Gino Sovran where he describes how diffusers work ??

(2) My question is this basically. In that paper and several others by Xin Zhang from SouthHampton University they mention the fact that the diffuser "pumps down" the pressure under the floor ?

They basically said that BECAUSE (1) The diffuser exits to the base pressure of the model
and (2) this base pressure remains relatively constant as the ride height of the model is reduced

that the airflow under the model / car accelerates to a greater extent with the diffuser than the case where there is no diffuser and just a flat floor ?

Why is the fact that the diffuser exits to this base pressure so vital to the fact that the diffuser gives more downforce by accelerating the air more than a car / model with no diffuser ???

I have no clue why this is .

Many thanks to all.



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#2 NeilR

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Posted 28 November 2012 - 23:13

could it be as simple as a greater area available to base pressure in the diffuser and thus greater volume of air accelerated?

#3 desmo

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Posted 29 November 2012 - 02:22

The diffuser essentially MUST eventually exit into base i.e. ambient pressure-- that's all there is out there to exit into. I know this is a dodge but I guess I don't understand the question. I assume one would I suppose ideally want the diffuser to exit into below ambient pressure such as the lower pressure side of an airfoil before exiting to ambient but then that airfoil essentially becomes semantics aside a last diffuser element before the diffuser exits into ambient-- just not a strictly attached one. Someone correct me here if I'm mistaken but the diffuser is conceptually I believe simply an exit nozzle for undercar flow to maximize that flow's mass and velocity thus lowering the pressure under the floor dynamically per Bernoulli.

#4 Greg Locock

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Posted 29 November 2012 - 11:23

Katz chapter 6 includes plots from several papers, showing the /variation/ of drag and lift as you change the ground clearance.

SAE 980030 being the easiest to find

#5 Bloggsworth

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Posted 29 November 2012 - 14:57

Katz chapter 6 includes plots from several papers, showing the /variation/ of drag and lift as you change the ground clearance.

SAE 980030 being the easiest to find



And a snip @ $23

#6 Rasputin

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Posted 07 December 2012 - 19:59

What it's all about is creating a speed higher under than above the car, according to Bernoulli this will translate into a static pressure differential between the same.

When speed above the car will be basically the same as the travelling speed, you need to find a way to increase air speed under the car.

If you figure the air-speed coming out of the diffuser being the same as the air-speed from above the car when those flows meet, then the speed under the car will
have to be a relation between diffuser cross section area and the under the car cross-section area, so much higher, in order to produce the same volumetric flow.

Thus the bigger the diffuser cross section area is in relation to the cross section area under the car, the higher the speed under the car will be.

All within reason of course, otherwise the diffuser will "stall", when using a very popular term these days.

#7 Greg Locock

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Posted 07 December 2012 - 22:09

Closely aligned to this, I went to a seminar on the results of a recent round robin series of tests on most of the full size wind tunnels in Europe, and a couple in the States. These include a wide variety of designs, almost all were 3/4 open with one true closed loop. Some had one moving belt, some were static floor, some had 5 moving belts. They also had different methods of BL control.

Then they tested a whole bunch of production cars and a van, and made some mods to some of them. The improvement in CL values due to fitting a dam at the nose was HUGELY dependent on the exact tunnel configuration. They also ran each tunnel at different speeds, of course the 101 equation says that shouldn't affect Cl or Cd. it does.

Sadly I can't give any numbers as this is all internal to the round robin group (Eurpoean OEMS and a few stragglers) and proprietary, but I think the standout was that the estimates of Cd had a standard deviation of 0.01 at 140 kph. So using your pet tunnel, no names, gives you an advantage of 0.02 in aero bragging rights. Different companies also process the data in different ways, so the same car measured in the same windtunnel by two different regular users of that tunnel will give different numbers.

The interesting bit for me was comparing the hold down systems used, on a moving belt system the tire and springs allow the car to pitch, which affects the result vs speed, whereas the static floor cars always maintained attitude 'perfectly', but incorrectly.


#8 Powersteer

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Posted 08 December 2012 - 02:18

Why is the fact that the diffuser exits to this base pressure so vital to the fact that the diffuser gives more downforce by accelerating the air more than a car / model with no diffuser ???

Maybe without a diffuser the lower bumper's edge would just turn all that drag energy into turbulence.

:cool: