Can someone explain what a diffuser does?

[knucklehead]

Originally Posted by Pestodude

To show how effective a well engineered underbelly of a sportscar can be, look at the newer Ferrari's...no wing at all but no lift (or very little).

our cars are far from perfect aerodynamically.

The Elise has no wing and has no lift SEE POST #14. I'm either not understanding your point or not understanding your example.

[knucklehead]

Originally Posted by jhnmdahl

The point of lowest pressure is under the car where the velocity is highest, not at the diffuser. The downforce is created by the relatively low pressure of the air under the car, and the diffuser prevents the air from becoming turbulent while it decelerates, or "stalling", which would reduce the downforce created under the car.

It does work essentially the same as an airplane wing, where the lift comes substantially from the middle/thickest part of the wing, and not from the trailing edge...

John

Interesting. So how does the longer diffuser effect the point of lowest pressure? Does it move it forward? Toward the rear or not at all? Or does is create a higher velocity and therefore a larger amount of downforce?

Thanks everyone for sharing your knowledge. I'm finding this thread fascinating.

[Pestodude]

Originally Posted by knucklehead

The Elise has no wing and has no lift SEE POST #14. I'm either not understanding your point or not understanding your example.

Sorry, I misstated my meaning. No the Elise doesn't have "lift" but it doesn't have much downforce (compared to the Exige with a wing). The Ferraris have far more downforce with no wing than the Exige with a wing.

[knucklehead]

Originally Posted by Pestodude

Sorry, I misstated my meaning. No the Elise doesn't have "lift" but it doesn't have much downforce (compared to the Exige with a wing). The Ferraris have far more downforce with no wing than the Exige with a wing.

Is the Ferrari underbelly design the factor of better downforce or the longer wheelbase? Which allows a more gradual incline of the body shape. While the Elise/Exige's body shape must have a more abrupt angle?

[TimMullen]

Originally Posted by jhnmdahl

The point of lowest pressure is under the car where the velocity is highest, not at the diffuser. The downforce is created by the relatively low pressure of the air under the car, and the diffuser prevents the air from becoming turbulent while it decelerates, or "stalling", which would reduce the downforce created under the car.

It does work essentially the same as an airplane wing, where the lift comes substantially from the middle/thickest part of the wing, and not from the trailing edge...

John

Yes and No...
The diffuser gives a larger "volume" for the air to expand into. Expanding air drops in pressure. The air is expanding in the diffuser area and that's where the down force is being generated. The smooth under body is mostly for the decrease in drag. Any down force generated because of the flat under body is because of the shape of the top of the car instead.

The air expanding up to fill the area behind the car - along with the turbulent air spilling over the rear lip spoiler combine to decrease drag (and in the case of the spoiler, reduce generated lift).

Originally Posted by Pestodude

Sorry, I misstated my meaning. No the Elise doesn't have "lift" but it doesn't have much downforce (compared to the Exige with a wing). The Ferraris have far more downforce with no wing than the Exige with a wing.

I'm willing that the Ferrari has a lot more drag to go with that extra down force. But the Ferrari also has a lot more power to overcome the additional drag. There is no free lunch.

[Pestodude]

Originally Posted by knucklehead

Is the Ferrari underbelly design the factor of better downforce or the longer wheelbase? Which allows a more gradual incline of the body shape. While the Elise/Exige's body shape must have a more abrupt angle?

I'm sure those are factors along with the years of Formula 1 domination and all the engineering expertise that brought that about.

[TimMullen]

Originally Posted by Pestodude

I'm sure those are factors along with the years of Formula 1 domination and all the engineering expertise that brought that about.

Don't forget the Lotus pretty much invented the under body aerodynamics...

[knucklehead]

I'm probably going to get slammed for this one. BWTH, until the change of rules next year, I thought F1 was not allowed to use ground effects.

[NJdrool]

Originally Posted by TimMullen

Remember that the Elise, with just the diffuser, rear lip spoiler and the winglets on the front actually generates down force. On most cars, those aerodynamic aids just reduce the generated lift. The Exige makes even more with the splitter and rear wing.

Tim thanks for the explanation. Here I always thought that the Exige produced more downforce since it is the sports model and the Elise is the touring model.

[jhnmdahl]

I stand by my original post. The vacuum effect of expanding air in the diffuser region pulling the car down is a myth, although the slight vacuum behind the car does help scavenge the relatively higher velocity air from under the car, where the downforce is created (under the body of the car, not in the diffuser region). Check out:

Diffuser (automotive - Wikipedia, the free encyclopedia)

Diffuser

Secrets of Underbody Tunnels, Rear Diffusers and Venturis | Symscape

for example.

John

[turbophil]

Originally Posted by TimMullen

Yes and No...
The diffuser gives a larger "volume" for the air to expand into. Expanding air drops in pressure. The air is expanding in the diffuser area and that's where the down force is being generated. The smooth under body is mostly for the decrease in drag. Any down force generated because of the flat under body is because of the shape of the top of the car instead..

That's not really right, Tim. Of course we've been through all this umteen times before

The peak downforce on a car like ours should be at the very beginning of the diffuser, where the transition from flat to diffuser takes place. After that, the area from the diffuser back is where the DF tapers off-- fast. That's why many diffusers start at or near the rear axle, to maximize DF over the rear tires. The more the diffuser transition is moves forward, the more forward that peak DF is... The diffuser is simply there to reintroduce the fast moving air to the slower (high pressure) air behind the car...

Best,

Phil

[knucklehead]

Originally Posted by jhnmdahl

I stand by my original post. The vacuum effect of expanding air in the diffuser region pulling the car down is a myth, although the slight vacuum behind the car does help scavenge the relatively higher velocity air from under the car, where the downforce is created (under the body of the car, not in the diffuser region). Check out:

Diffuser (automotive - Wikipedia, the free encyclopedia)

Diffuser

Secrets of Underbody Tunnels, Rear Diffusers and Venturis | Symscape

for example.

John

Where did your original post go?

[Brent Bauman]

That's not really right, Tim. Of course we've been through all this umteen times before

The peak downforce on a car like ours should be at the very beginning of the diffuser, where the transition from flat to diffuser takes place. After that, the area from the diffuser back is where the DF tapers off-- fast. That's why many diffusers start at or near the rear axle, to maximize DF over the rear tires. The more the diffuser transition is moves forward, the more forward that peak DF is... The diffuser is simply there to reintroduce the fast moving air to the slower (high pressure) air behind the car...

I'm glad you typed all that Phil. I didn't feel like typing it myself.

[TimMullen]

Originally Posted by jhnmdahl

I stand by my original post. The vacuum effect of expanding air in the diffuser region pulling the car down is a myth, although the slight vacuum behind the car does help scavenge the relatively higher velocity air from under the car, where the downforce is created (under the body of the car, not in the diffuser region). Check out:

Diffuser (automotive - Wikipedia, the free encyclopedia)

Diffuser

Secrets of Underbody Tunnels, Rear Diffusers and Venturis | Symscape

for example.

The first one doesn't link to anything.

The second one starts out with this statement (I've added the bolding):

Introduction
The diffuser is an area of bodywork at the rear of the car, although the term “Diffuser” is technically incorrect, it is the most popular term applied to this part of the car.
The aim of the diffuser is two fold, it works like a wing curving up and creating negative pressure under the rear of the car and hence downforce, the second action it fulfils and related to the first is that the low pressure it creates actively pulls air form under the car, this scavenging both produces a lower pressure area under the car and also acts to reduce the boundary layer

The last link is talking about sculpted under body - not just a flat bottom and a diffuser on the back. In fact, that's why F1 cars have to have the flat bottom - to reduce the aerodynamic down force generated by the entire car being an upside down airfoil.

I sand by my belief that the diffuser is creating the down force by expanding the air flow. It may be helping to suck the air from under the car at the same time, but that's only part of it.

As for the higher velocity air under the car that has been mentioned, actually it's not higher than any other air flow around the car. In fact, it would be slower. The air going over and around the car has to "pick up speed" to go over/around the body. The air flow under the car is at most - traveling the speed of the car (actually it's not moving at all, the car is). Then you can start talking about the boundary layer effects, and the "compression" of the air between the ground and the car, etc.

[turbophil]

Tim, I've got about 6 different books I'd be happy to lend you that may help to get you past your road block on this diffuser issue. Some of them were quite expensive. I've been through them a dozen times, so I can live without them for a while... I've got both Joe Katz and Simon McBeath variants- pick your poison

Best,

Phil

[Tracktimeplz]

Speaking of Lift and downforce

YouTube - RX-7 Crashes At High Speed

Classic lesson in physics

[RichS]

It kinda sounded like a plane taking off too...

[ZJChaser]

do the NACA ducts on the undertray hurt the performance of the undertray/diffuser or do the help it do its job?

[knucklehead]

Phil,
I'm a little confused. What part of what Tim is saying are you disagreeing with? That the diffuser creates a downforce because of vacuum? Or where the highest point of downforce is in relation to the diffuser?

Also had a question from the link that jhnmdahl provided
Secrets of Underbody Tunnels, Rear Diffusers and Venturis | Symscape

"By installing an inverted wing close to the diffuser exit it is possible to create a low-pressure area, which essentially sucks the air from the diffuser. The diffuser and wing combination permits a higher air-mass-flow rate through the diffuser, thus resulting in higher downforce. Sharp edges on the vertical tunnel walls generate vortices from entrained air and help confine the air through the diffuser and reduce the chance it will separate."

I'm having a problem envisioning what an inverted wing close to the diffuser exit looks like. At least in reference to the diffuser on the Lotus. Is it a wing connected to the end of the diffuser?

Also are they saying that a 90° corner would be more effective than the curved transitions from vertical to horizontal that our diffusers have?

[turbophil]

Eric-

THe part I disagree with it his bit about the expanding air creating the downforce. Now we may all be saying the same ting in a different language, and of course we all know that happens a lot on forums

Nevertheless, we know when the air is fastest is where the pressure is lowest (and vise versa). We're creating a venturi, right. Simply put: Big space on the front (of the car), small in the center (under our seats), and big again in the back (the diffuser and behind the car)... like a carburator. The low pressure is in the smallest area (and fastest moving air). The lowest pressure (most DF) under the car is actually where the air turns to enter the diffuser due to the boundry layer at that point. Think of the air have to "go wide to hit the apex". It speeds up to do so, thus crating a spike in *low* pressure just at that point... Just like the front of the venturi, the rear of the system is where the slower (thus higher pressure) air is. The diffuser's job is essentially to "help" the air to continue a laminar path (laminar being the fastest path) under the car and to introduce it to the slow air behind the car. The introdcution that the diffuser makes is like a lead rope for the fast air. Without the introduction, the fast air hits a brick wall of sorts behind the car of higher pressure air. Tim did touch on this point, and then he gets off track...

There's all sorts of theory behind diffusers. Some contraversial and some not. The above is widely accepted. There are certainly good arguments to be made about diffuser angles and such. Where the diffuser should start on the car dictates the angle it take to exit. That's a double edge sword since the diffuser starting point impacts the point of peak DF.

A couple other things we know... Some is just reiterating what Tim and others said... Deeper side strakes on the diffuser help keep dirty (aka high pressure) air out of the stream and help to promote laminar flow. Sharp angles on the diffuser strakes create vorticies which lower drag and stimulate velocity (notice that's much differnt than the very rounded corners on our diffusers). Side skirts help to keep high pressure air out of our low pressure stream of air under the car (thank you Mr. Chapman for figuring that out). Strakes that run down the underside of the car do similar...

The other reason I give TIm a hard time is becuase he's an advocate for the Lotus Engineers and it's just so fun to