Undertray and Diffuser Question

[e36jon]

Greetings Works

Here's a pic of a straight wall diffuser with section planes evenly spaced along its length. If you calculate the cross sectional area of the diffuser throat at each section and plot them against length from the diffuser you'll get a straight line (shown).



In this second example I curved one of the sidewalls to mimic what I am seeing in your update. When you plot these areas you'll get a curve (shown). I believe this is not a good thing.



In your case, where the vertical walls are maybe pinching a bit, I think you can increase the roof height to get back to a consistent expansion ratio. ( If it's all done gradually with no sharp transitions...)

Hopefully this is clear.

Cheers,

Jon

[NOT A TA]

I've been following along because I'm working on a similar project on my track day car and considered starting a thread here about my underbody. Mine is on a production based car and like you I have no regulations to adhere to.

Is this a car you're actually building or a design exercise? Scratch built car or production based? Front or rear engine? Do you have any front/rear weights?

Everybody likes pics so here's me doing some highway speed tuft testing using another car of the same model as mine for baseline observations. I've also been using 360 3D VR video so I can blow it up on the big screen and look at individual tufts in certain areas if I want.

[e36jon]

OK, looks like I need some love on how to post an image...

[Works]

@NOT A TA - ha ha thats pretty cool

@e36jon - Im super curios as to what youve got on those pics ... how are you posting them?

[Works]

Is this a car you're actually building or a design exercise? Scratch built car or production based? Front or rear engine? Do you have any front/rear weights?

I'm building the car from scratch. Its essentially a single seater space frame. This is the second car I'm building from the ground up, and this time I wanted to try and get as much right, up front. The other car was iterations and experiments, with a lot of parts that I had found and mashed together. Case in point is the diffuser, which was a cheap fiberglass unit I got online.

[Works]

I really don't want to get off topic here, but my other car is pretty interesting (I think). Its currently gone through a complete tear down after obliterating the motor. Plans for the motor (Suzuki GSXR WP 1100, 90ish model) is to turbo it next.
The bodywork was pretty disastrous, so I've started experimenting by 3D printing a new body. The design is based off the 1989/90 Ferrari 641, which is a car Ive always loved.

This is how that car started:



This is where Im at with the bodywork currently.



I plan on 3D printing the diffuser Im trying to design in this thread.

[e36jon]

@e36jon - Im super curios as to what youve got on those pics ... how are you posting them?[/quote]

I have the images in a file on G-Drive. Here's a link: https://photos.google.com/share/AF1QipN ... hzXzRFYWZB

Trying to figure out how to get them into a photos folder on there instead.

Jon

[NOT A TA]

Jon, the pics Works and I post are hosted by Photobucket. It makes it easy to insert pics on forums. Might want to look at a site like that.

I like the old car, very cool. Is the new one going to be full body?

[Works]

Yes full body car.

OK Jon, I think I get what you're saying now ... Basically we have our volume of air compressing mid diffuser, as we effectively have an "hourglass" shape along one axis.

We need the air to move in a continuously expanding chamber, and the curves currently are breaking that.

Correct?

[e36jon]

[quote="NOT A TA"]Jon, the pics Works and I post are hosted by Photobucket. It makes it easy to insert pics on forums. Might want to look at a site like that.

Which of the link options on photobucket should I use? I copied the "IMG" link and inserted it, but still no image...

Belay that, just sorted it out...

[NOT A TA]

Yes copy and paste the IMG link into the box here as though it was text without using the toolbar here. Make sure you don't have it next to a font. I hit enter a couple times to drop the curser and create a space between text and any pics and to create a space between multiple pics.

[e36jon]

K Jon, I think I get what you're saying now ... Basically we have our volume of air compressing mid diffuser, as we effectively have an "hourglass" shape along one axis.

We need the air to move in a continuously expanding chamber, and the curves currently are breaking that.

Correct?[/quote]

Greetings Works

Yes, your paraphrase is spot on.

The strakes are still worth having, as dividing up the bulk flow into smaller cells will give a better result (Not that I understand all of the causes for that.), so if you can't change the geometry to remove the hourglass shape as seen in plan-view maybe you can play with the shape of the roof (floor?) of the diffuser such that you get back to steady linear expansion.

Cheers,

Jon

[coaster]

The guy in the trans am should put aero concerns aside and worry first about putting that car on a serious diet.

[NOT A TA]

The car shown in the pic was just used for the highway tuft testing as a baseline because I'd swapped all of the old suspension from the project car into that one so it had the same ride height, body rake, and tire sizes. The car I'm doing the aero work to has gone on a diet.

Pic below is the car I'm working on. I should really start a thread to get opinions on my splitter/undertray/diffuser design.

[Cold Fussion]

Yes full body car.

OK Jon, I think I get what you're saying now ... Basically we have our volume of air compressing mid diffuser, as we effectively have an "hourglass" shape along one axis.

We need the air to move in a continuously expanding chamber, and the curves currently are breaking that.

Correct?

Essentially the diffuser will have a maximum expansion angle before you start seeing flow separation, so to maximise the amount of expansion you'd want to run along the critical angle all the time, rather than ramping up the expansion as you move along. Furthermore with a longer diffuser your critical angle will be lower than if you have a shorter one. It's worth giving Joeseph Katz's book a read as he describes many of the fundamentals of race car aero.