RedLS1GTO

The difference...

C5R


Enzo



I'm going to go out on a limb and say that both of these had extensive testing done...

jcsperson

I don't think it's all that critical. We're not dealing with jet-fighter speeds here. In the F4, our flaps were nearly 90 degrees to the wingline at full-flaps and 45 degrees at half-flaps. They created a ton of lift right up to the flap actuation limit of 250 KTS, nearly 290 mph, and air was being asked to bend substantially more than in that diffuser.

There is not a lot of air down there and the total "wing" area is small. Even if it had a perfectly smooth transition, the vacuum effect it would create would be minimal.

I think el es tu has it about right. That area in a stock C5/C6 Corvette is a jumble of trash hanging down into a big hollow spot that has the potential to be a high pressure area. The pictured diffuser serves to usher air that would normally build up there out behind the car. I think its primary function is to reduce drag at the bottom of the car. Any small amount of low pressure suction it would provide is simply a bonus.

Shaka

You want to sit in the front rows for a dime. Google some papers on diffusers or if you have a buddy who is an engineer, you can download SAE papers on the subject. Your local college may have a Formula SAE program and you get get on to their mailing list.

Order this book today.

http://www.amazon.com/New-Directions.../dp/0837601428

I don't have the time right now to respond to this thread properly.

TLGunman

We've found differences by changing the diffuser angle just a few degrees. On the Mosler MT900R we went from 11° to the traditional 7°, and it greatly improved. Same with the Viper GTS-R, an angle change increased the downforce.

If I put a flat section like that on a diffuser, Bob Riley would probably smack me upside the head

That said, I'd be interested to know how well that diffuser works, because eyeball windtunnels don't always tell the whole story.

TLGunman

Excellent book. I had it out last night, for a different discussion on wings vs. spoilers.

Everett Ogilvie

As Dave says, this is the heart of this matter. The curve on the pictured diffuser is guaranteed to cause flow separation. All the best and current literature says that working diffuser angles are between 7 and 11 degrees. Anything greater will likely cause separation and that is with the proper shape (flat) to boot. A curve where the angle keeps increasing guarantees separation. I don't even think the rear flat part matters in this case because the flow will no longer be attached at that point, i.e. all bets are off at that point.

By the way Dave, thanks for the work you guys did on the Mosler diffusers - I think they work pretty well! Especially THIS one

RedLS1GTO

You are absolutely correct with regards to an aircraft flap. They create a lot of lift at full extension. They also create a massive amount of drag. Think about what happens if you go to flaps down and constand power. Airspeed will decrease substantially.

In the extremely simplified view below, you can see the forces working on the airfoil.



The point of a diffuser on a car is to get the maximum amount of downforce without introducing drag. This diffuser has a shape similar to the 3rd down, and then also has the flat section which disrupts the flow. The disruption in laminar flow adds even more drag into the equation and creates a higher pressure area at the exit (not good).

Or... the flow separates before it makes the bend to the flat (this is my guess as to what happens) which in short means that you have a lot of wasted potential and are nowhere as efficient as you could be. As said above, all bets are off at that point.


I would love to see what it did in a wind tunnel. I am simply going by eyeball and I could be wrong, but I really don't think I am in this case. Would love to see some data either way.

Cobra4B

Reading over the basics of a diffuser here - http://en.wikipedia.org/wiki/Diffuser_(automotive)

So what happens when one isn't used? Does air flowing around the sides and over the top of the car create a barrier for the air flowing under the car making it harder for it to exit? It seems the role of the diffuser is to smooth the transition of the low pressure/high speed air under the car back to the normal atmosphere around the car. If so, is the diffuser more effective the further it sticks out from the rear of the bodywork?

Everett Ogilvie

There are all kinds of complex technical descriptions of what diffusers do and how they work (and believe me I have read them all!). Perhaps the "easiest" way to think about a diffuser is to think of it as a pump - on every pump you have a suction side and a discharge side. As the air just enters the most forward part of the diffuser, this is already the discharge side. Because it acts like a pump it accelerates the air flow just in FRONT of the forward-most portion of the diffuser, It is in this region (just forward of and just into the entry of the diffuser) that the low pressure region is created, and thus downforce. If you look at pressure coefficient pictures of race car undersides, the region of the greatest negative pressure coefficient is always just in front of the diffuser. If the flow separates from the diffuser (farther downstream) the draw into the forward part of the diffuser is lost, with resultant loss of the low pressure region.

Here is essentially the same explanation in more technical terms from one of the better SAE papers (see reference below, hopefully I properly credited the reference so there are no copyright issues);

"Diffuser Pumping – A subsonic diffuser is an internal-flow
device whose cross-sectional area increases in the flow
direction, producing a decrease in fluid velocity from inlet
to outlet, with a corresponding increase in static pressure.
This pressure rise can be used to increase the flow
rate through a system, as was known even in Roman
times.
When a diffuser delivers flow to a fixed exit pressure - the
base pressure of an automobile as an example - its pressure
recovery appears as a depression in pressure at the
diffuser inlet. This concept is illustrated in Figure 2 for a
flat-bottomed and a diffuser-equipped model. There is
downforce on the flat-bottomed model, but the presence
of the diffuser in the bottom drawing reduces the underbody
pressures further. That configuration will have the
greater underbody flow rate, lower underbody pressure
and the greater downforce. The diffuser can be considered
to have 'pumped-down' the underbody, inducing a
component of downward force on the vehicle."

The Aerodynamic Performance of Automotive
Underbody Diffusers (1998)
Kevin R. Cooper, T. Bertenyi , G. Dutil and J. Syms
National Research Council of Canada
G. Sovran
GM Research, Retired

Cooper and Syms have another paper, Selecting Automotive Diffusers to
Maximise Underbody Downforce which is very technical but excellent.

F4Gary

jcs, when and where did you fly the F4? Sounds like hard wings.

Cobra4B

So what's happening to the airflow when no diffuser is present? What specifically does the diffuser change?

In layman's terms it appears to ease the transition from under-car flow to the air behind the car. By easing this transition you can flow more air under the car thus creating more low pressure thus increasing the downforce.

Leave lever-arm length out of it for now. I completely get that part of it.

Everett Ogilvie

I added a quote from a SAE paper into my post above. If there is little or no flow separation within the diffuser length, it does smoothly transition the flow under the car to the conditions external to the car, but that does not tell you why it makes downforce, it only sort of tells you a little about low drag. Again, a diffuser "pumps down" part of the region under the car which is the source of the downforce. As the quote from the paper above states, a flat bottom car without a diffuser also makes downforce (with even with only a tiny bit of body rake), but the diffuser greatly increases the low pressure region.

If the underside of your car is not smooth the full length, and if you don't have a diffuser, you have little or no downforce and you have tons of drag due to the protruding parts and pieces, and myriad pathways for air to follow into the underbody and create high pressure regions (drag).

Cobra4B

I get it now... the "expansion chamber" is created when looking from the horizontal and the "fins" are just there to maintain laminar airflow.

AU N EGL

No points addition to PTA ?

jcsperson

I was a RIO in VF-151 on the USS Midway in the '80s. We flew the S model with the leading edge maneuvering slats. Those deployed either under g in ACM or acro, and when the flaps were actuated.

The B and N were hard wing jets. The J had smaller slats that reminded me of those on the E. The big slats reduced our carrier approach speed by 5-6 kts IIRC.

I had 10-12 hours in the E with the 495th TFS in Korea. It was interesting how everything was similar but different.

geerookie

First, laminar flow on a road going vehicle is a myth! The surfaces on a typical car are not smooth enough to maintain laminar flow and even if they were, as soon as it drove around the track it would be dirty enough to create a turbulent boundary layer that would cause the flow to detach.
Airflow on a race car is about reducing turbulence, maintaining proper flow direction and reducing the size of the boundary layer.

The strakes on a diffuser could serve a couple of purposes but may not be necessary depending on the situation. If they are built properly they will have no negative effect even if they are not needed but if they are built wrong and are not needed they will greatly reduce the venturi effect needed to increase the low pressure area under the car.
The outside strakes are there to prevent the turbulent air "falling" off the sides and rear fenders of the car from disrupting the airflow in the venturi.
The strakes in the middle are to maintain proper direction of airflow, in other words, to reduce/prevent turbulence.

moespeeds

I could make that thing for you, out of aluminum or CF, I don't give a damn if it works or not. What would you pay?

F4Gary

I flew F4E's in Holland in the 70's. 32 TFS. Soft wing with slats and no full flaps (or boundry layer control).

Cleared Prime and Synch.

geerookie

Now that's the Jersey spirit!
I mean American Entrepreneurial Spirit

el es tu

http://forums.corvetteforum.com/auto...iffuser-2.html



http://aero.calpolysae.org/techarticles.htm

http://ir.library.oregonstate.edu/xm...nKarlE2010.pdf

http://www-f1.ijs.si/~rudi/sola/RaceCarAerodynamics.pdf

http://strangeholiday.com/oops/stuff...304.092016.pdf

http://eprints.soton.ac.uk/42969/1/GetPDFServlet.pdf

http://www2.mech.kth.se/courses/5C12...raghi_2010.pdf