Aerodynamics Resource Thread

Thought it might be cool to start a resource thread for understanding and applying aerodynamics. I was hoping to get a discussion rolling and pool links some of you might have saved in your bookmarks for reference. If you'd care to share them I can consolidate them and build an index on the first page. When you click on this thread in the future, info can be obtained quickly.

A good starting point for discussion might be on the relationship between the undertray and the rear of the vehicle body. What is the "ideal" angle of attack for a diffuser or does that depend on shape, speed, and location of wing? I'm sure there's more than one fluid dynamics and aerospace guru lurking around here?

Link to start this off: http://strangeholiday.com/oops/stuff...304.092016.pdf


Aerodynamics of Race Cars
Joseph Katz

Department of Aerospace Engineering, San Diego State University, San Diego, California 92182;

email: jkatz@mail.sdsu.edu

Should be a good thread!

Aerodynamics, chassis and tyres - Forum - F1technical.net

Quote:

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Originally Posted by J. Fast

What is the "ideal" angle of attack for a diffuser or does that depend on shape, speed, and location of wing?

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Depends on all of these, and it is by no means a simple problem. Diffusers do a good job of increasing overall downforce, but do so at the expense of increased drag. Furthermore, a diffuser that has not been optimally designed can have local areas of DECREASED downforce. These local areas of high static pressure can cause high adverse pressure gradiants that, if significantly high, can lead to flow separation.

not to mention it also depends on the underbody in front of the diffuser as well, and the exhaust exit location.

Is there much benefit for us to flat panel the whole underside? Prop shaft excluded.

I thought the whole point of a rear diffuser was to straighten the airflow out, thus speeding up its journey underneath the car, thus creating 'free' downforce without drag.

André

There are a lot of factors to be considered.. A common angle I've heard is 5-10 degrees.. A diffusers effectiveness can be decreased if too much air is escaping out the sides of the car as well. iirc there was an old racing series that ran some kind of flexible side skirts that almost touched the ground to prevent the air escaping out from under the car before the diffuser.

The diffuser and the wing are meant to work together, the goal is to create the most downforce with the least ammount of drag, as with everything there are compromises. Without windtunnel testing, being in the ballpark is all you can ask for.

The flexible sideskirt thing was done in Formula 1 back in the early-mid eighties I believe. As was the revolutionary fan on the Renault car that got banned after only a couple of races because it obliterated the opposition by sucking the car into the track! Nobody could keep up.

André

Yes, flat bottoming would help, as well as venturi ducts but that won't happen on our cars. T-case, prop shaft, exhaust, all that is in the way but we are pretty close as is.
Diffusers, up to any angle you want really, until it stalls. Once it stalls it's just drag. Here's a little secret, angle of the car. Flat is not what you want. Take the entire car and put it on an angle. This is what will net you huge gains in DF.

It's a shame this thread didn't go any further. Would anyone be interested in posting up their home-brewed examples and reasoning for aspects of design/effectiveness
What do you guys think about using Alumilite or Dibond sheets for splitter/diffuser making?
Our gas tank is very much in the way as far as having a diffuser goes, would it be better to have a little less angle and be able to extend the diffuser a bit more past the ending of the gas tank? Perhaps Erron can pitch in on this as I have seen his diffuser and know it starts where the gas tank ends. What was your reasoning behind this? did you consider it differently but change your mind, why?

That's good material for sure.
I basically wanted to bridge the gap between the bumper and gas tank. It starts at the front of the gas tank too.

hmm, well I can see that it is attached to the fuel tank guard, but it looks flat to me in the picture.
http://img703.imageshack.us/img703/2221/diffuser2.jpg
is there a small angle there as well?

There is. It's only a few degrees if you measure it from the car, but with the cars rake, it's about 7.

Quote:

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Originally Posted by Rakuny

It's a shame this thread didn't go any further. Would anyone be interested in posting up their home-brewed examples and reasoning for aspects of design/effectiveness
What do you guys think about using Alumilite or Dibond sheets for splitter/diffuser making?
Our gas tank is very much in the way as far as having a diffuser goes, would it be better to have a little less angle and be able to extend the diffuser a bit more past the ending of the gas tank?

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As it relates to our cars and is quite helpful here's some stills I took from my PRM racecar video subscription on U-Tube and ninja edited... You can just stare at them... stare some more... and stare some more. That's what I did. Then I went and bought some material and built some shit. I don't have all the pics loaded ATM but you'll get the point.

http://i788.photobucket.com/albums/y...initions-1.png
http://i788.photobucket.com/albums/y...ports/PRM4.png
http://i788.photobucket.com/albums/y...ports/PRM3.png

Now, to answer your question regarding utilizing Alumalite... I used Econopanel (alumalite is the same thing) for my entire under tray. I selected it because it's lightweight, rigid, durable, and a good way to go if you're in the trial and error phase of setting up your aero. It's relatively inexpensive and quite easy to manipulate also.

http://i788.photobucket.com/albums/y...s/DSCN3445.jpg
http://i788.photobucket.com/albums/y...s/DSCN3448.jpg
http://i788.photobucket.com/albums/y...114_170833.jpg


As for my undertray. I've 1 1/14" stand-offs secured to the bottom of the cab of my cars which allows me to affix econopanel to.

http://i788.photobucket.com/albums/y...114_170106.jpg

This is underpanel sheet
http://i788.photobucket.com/albums/y.../Econolite.jpg
http://i788.photobucket.com/albums/y...s/DSCN3433.jpg
http://i788.photobucket.com/albums/y...s/DSCN3432.jpg


My input on your question regarding rear diffuser angles? When I first started researching and reading about aero several years ago at 3stech I was caught up on ideal aerodynamic angles, geometry, and so forth. I came to discover you really can't be dead set on them. What we're really looking for is effective airflow management and maintaining pressure/airflow separation under the vehicle. Be critical of the entrance and exit regions at the under tray and ensure air moving in the laminar flow direction remains uninterrupted. The biggest hindrance to the effectiveness of the under tray are not the entrance and exit angles (unless they're too extreme), typically its lateral wind shear and tire scatter. Therefore, IMO, what's most important over the entrance and exit angles of the splitter and rear diffuser is the presence of end fences. The end fences provide separation and block airflow/pressures not in equilibrium, or traveling in the laminar direction of flow, from interfering with the airflow from the entrance to exit at the under the body.

Worry less about the angles at the splitter and diffusers entrance and exit (somewhere between 1 degree and 10 is great and generally wont stall) and spend more time on pressure region separation from the sides and tires. You will get the greatest net gains for downforce and drag reduction if you focus more on those two things.

The entrance to exit transition at the underbody should be smooth and free from voids and gaps. Those two things create space for low pressure to migrate to because it's the path of least resistance. They break up the laminar flow column and cause stalling.

Flat bottom, no voids, smooth transition from entrance -> to under tray -> to exit for introduction of air to underbody and onto the rear diffuser. Use end fences to block flow from outside airstream. End fences... to work effectively for tire scatter need to extend to at least where the rims meet the tires. The end fence should at minimum extend verticall into the underbody to the same finish plane of the rim/tire bead.

Hate to say it because I myself don't post some stuff because of it but... as it applys to aero, when you don't have a wind tunnel... Take something that's working and tested for someone else on your exact same car and PLAIGARIZE!