Can someone explain what a diffuser does?

[knucklehead]

Who's Eric?

I'm following you accept where you say that there being a smaller space under the seats to create the Venturi effect. Our cars have flat bottoms, how is that space smaller to create the low pressure?

Cheers,
Jacob

[turbophil]

Doh, Sorry about that you have the same avatar as Eric (SF Elise)... Brain fart, I suppose

[turbophil]

Originally Posted by knucklehead

Who's Eric?

I'm following you accept where you say that there being a smaller space under the seats to create the Venturi effect. Our cars have flat bottoms, how is that space smaller to create the low pressure?

Cheers,
Jacob

By under the seats, I mean the entire flat area under the car, forward of the diffuser...

Best,

Phil

[knucklehead]

Originally Posted by turbophil

Doh, Sorry about that you have the same avatar as Eric (SF Elise)... Brain fart, I suppose

Eric confuses me too

Originally Posted by turbophil

By under the seats, I mean the entire flat area under the car, forward of the diffuser...

Got it

So to answer my own question about the extended diffuser. It's not that it creates a bigger vacuum, it's that it allows for a faster transition from fast air to slow air? Kind of like having a longer onramp for a freeway?

[turbophil]

Originally Posted by knucklehead

So to answer my own question about the extended diffuser. It's not that it creates a bigger vacuum, it's that it allows for a faster transition from fast air to slow air? Kind of like having a longer onramp for a freeway?

I think that's an excellent analogy (assuming you drive faster on the on ramp than the highway traffic) with one slight change... Rather than a *faster* transition, I'd change that wording to a *potentially more efficient* transition...

Make sense?

Best,

Phil

[knucklehead]

Yes thanks. Maybe a better analogy would be a longer OFF ramp.

[snuggles1]

Originally Posted by charliex

like tim says air dams do not create downforce, a partial vacuum is created under ther car which in turn produces more downforce, but its academic anyway since neither the elise or exige have air dams, they have splitters which do generate downforce via the leading edge.

air dams and splitters aren't the same thing.

ie an air dam is just that, a splitter is an air dam with a leading edge.

been away, sorry to bring this back up... but the book i am looking at here shows charts with the resulting downforce being created by airdams and it says this is contrary to popular belief....the testers were surprised as well. I can quote page numbers if necessary. Doesnt show a ton of downforce, but it is measurable.

Gotta catch up on a few domestic things but I'll brush up on it more later.

[apexdc]

I had said some time ago I would try to post this info on another thread about aerodynamics and finally got off my butt and scanned them.

The last article is quite diffuser specific, but all are interesting.

[apexdc]

Oops! Looks like a little too high on the resolution. I think you should be able to drag and drop the image and it should print well.

These are lower res. If they are too low res, or you can't read/print the first four pages, email me and I will send them to you direct.

[knucklehead]

great article thanks.

[turbophil]

you think that's good, you ought to read his Simon's books He actually does a good job in making them digestable... Joe Katz is another good author, but his stuff is pretty dry in comparison with a lot more math-- a bit like a textbook...

Best,

Phil

[Pestodude]

Originally Posted by apexdc

Oops! Looks like a little too high on the resolution. I think you should be able to drag and drop the image and it should print well.

These are lower res. If they are too low res, or you can't read/print the first four pages, email me and I will send them to you direct.

The article referances articles in the "last couple of issues" that examined other aerodynamic tests they did with the "pair of Lotus Exige S2's"
Do you have those articles? I'm assuming those examine over the car aerodynamics.
In this article it talks about the huge gains achieved by adding "verticle extensions to the outer diffuser fences". Has Reverie or anyone else produced a diffuser (for sale) to replicate this?

[Pestodude]

Originally Posted by Pestodude

The article referances articles in the "last couple of issues" that examined other aerodynamic tests they did with the "pair of Lotus Exige S2's"
Do you have those articles? I'm assuming those examine over the car aerodynamics.

Just saw that you posted them above in high res. Can you post in the low res/ draggable format so I can print?

[holmz]

Originally Posted by apexdc

Oops! Looks like a little too high on the resolution. I think you should be able to drag and drop the image and it should print well.

These are lower res. If they are too low res, or you can't read/print the first four pages, email me and I will send them to you direct.

Can you delete the lager images from the first post so that the page loads faster - I now large and small I have to wait for.
Or delete the small...




I still don't get the airfoil part.
When I take a clearance gauge I see the distance under the front splitter is ~130mm, then it is long flat plane/plain all the way back measuring ~140mm in the back - before the diffuser starts to curl up.

If the splitter is splitting, then all the air above it is going up, and the air below is headed under.
There is no air that is speeding up, unless I bend the splitter up at the front (which I have tried), or unless there is air spilling around the spitter in the front to get underneath the car... But If the splitter is long enough that won't be likely to happen.
There is only 130mm of air that all of sudden realizes that it is now under the car - not 200mm of air that heads into 130mm chute like fan at a soccer match.

If I have air entering a room (at 1 ft/sec) by a 1 foot opening - then it is going 1'/sec in the opening.
But if I measure the air speed later in the room, where the room is 8' wide, the speed will be lower (perhaps 1/8 of a foot/sec)
The air doesn't speed up as the volume increases - I think it decreases.

So as the air under the car is headed rearwards it is either:
1) Being expanded like an accordian and getting lower pressure, or...
2) It is slowing up which gives higher pressure...

All this is long before there is anything happens with the diffuser.

Where am I going wrong?

What happens with the diffuser is also interesting - but from a speeding up sense having the front higher than the rear would cause the air to continuously speed up from front to back - thereby maximizing downforce...
This is at odds from what is normally observed.

[TimMullen]

Originally Posted by holmz

I still don't get the airfoil part.
When I take a clearance gauge I see the distance under the front splitter is ~130mm, then it is long flat plane/plain all the way back measuring ~140mm in the back - before the diffuser starts to curl up.

Actually the specs indicate that the car is supposed to have the same height front and rear (unless they changed something from the original base and sport pack suspensions).

There is no air that is speeding up, <...>
There is only 130mm of air that all of sudden realizes that it is now under the car - not 200mm of air that heads into 130mm chute like fan at a soccer match.
<...>
So as the air under the car is headed rearwards it is either:
1) Being expanded like an accordian and getting lower pressure, or...
2) It is slowing up which gives higher pressure...

All this is long before there is anything happens with the diffuser.

Where am I going wrong?

You aren't.

All the references that I've checked from previous posts are talking about the down force created by the air foil shaped underside of race cars. Yes, in those cases the down force is created near the center of the car. But we have a flat bottom we aren't generating "air foil down force" from the flat bottom. The down force is being generated at the diffuser by the air expanding and lowering the pressure.

What happens with the diffuser is also interesting - but from a speeding up sense having the front higher than the rear would cause the air to continuously speed up from front to back - thereby maximizing downforce...
This is at odds from what is normally observed.

No, if you had the front higher (with the flat bottom), you would be compressing air under the car, generating lift, not down force.

[knucklehead]

Which is why the Rake(sp) is so important.