Trebuchet03's JettKamm (2000 VW Jetta)

[MetroMPG]

Originally Posted by Christ

I'm pretty sure you're better off blocking the lowest grilles over the higher ones, especially if the lower ones are perpendicular to the ground... they would present a higher open area for air to flow into.

All else being equal (size of the openings), some people say it's better to block upper grille openings if they're likely above the stagnation point (where airflow "splits" between flowing up & over the car vs. underneath).

The thought is that it's better to permit that upward flowing air to continue over the relatively smooth body of the car than to redirect it through the aerodynamically dirty engine compartment & exit underneath the car.

Might as well use air that was already going to end up there (below the stagnation point - probably lower openings) to take care of cooling.

[Christ]

Good point.

[Deezler]

From an airflow to the radiator standpoint, the lower opening does everything. Blocking the top made little difference on warm up time or operating temps. On the Jetta you can block both anyway, and just leave the small slot in the very lower valence open, its enough airflow for anything but hot summer days.

Treb, any new simulations run or data compiled from previous runs?
I'm interested in seeing a model of your Kamm shell.
However for visibility and functionality reasons, I still like the novel spoiler design myself. But I need some data before I start building! What are the plan for future runs? I know you are swamped with finals, etc. right now.

The pace of which this thread originally moved has me jonesin' for more data/results!

[Christ]

Trebuchet, I have seen the image on the FP of this website that displays your CFD runs with a potential kamm design, and I just couldn't help but realize how much it looked like you were just going to put a CR-X hatch on the rear of your car.. lol.

[trebuchet03]

I hope everyone had a good holiday My CFD will be on break until Wed. I've got a final exam tomorrow in one of my classes (nothing heavy duty though - I just need to prepare).

And, I got a new box! One of the black Friday deals.... I should have lots of room to breathe now (I'll know in a couple of days).

Just popping in to say this project isn't dead, I'm just slammed with higher priority tasks

[cmroseberry]

I have only skimmed over the discussion, but I do have a few comments that hopefully are not redundant. Back in the late '80's, CFD codes had a huge accuracy problem in regions of flow separations due to difficulties with the turbulence modeling. I have been told that the situation is better now. It is actually much easier to model an aircraft with CFD than a car because the flow generally stays attached for almost the entire length of a well streamlined airplane (unless it is at a high angle of attack). The flowfield around a car is generally characterized by large flow separations particularly on the trailing end. As for optimizing the rear deck angle, you should find a copy of Aerodynamics of Road Vehicles by Wolf-Heinrich Hucho. There is usually a drag spike at an angle of about 30 degrees down from the horizontal. Also bear in mind that a car is often not aligned with the flow direction due to cross winds (Beta angle). The drag coefficients that Ford reports are "wind-averaged" values from testing performed over a range of yaw angles. It also appears that you have not included a moving ground plane in your simulation - this has a significant effect on the aerodynamic forces. One way to simulate this effect is to attach a mirror image of the vehicle about the horizontal plane (where the tires contact the pavement). Moving ground plane effects are another reason it can be more difficult to simulate a car flowfield as opposed to an airplane flowfield.

[tasdrouille]

Hi cmroseberry and welcome to EM!

Could you post something in the Introductions forum, I would be interested to read where you're coming from in the aero field.

I agree that a rolling ground with rolling wheels would best, but mirroring is better then nothing and probably a lot simpler to implement in the software. Trebuchet did a couple of runs with mirror images.

[aerohead]

Originally Posted by mfor4x4

Hey I'm new to these forums, I really like some of the ideas too.

I am curious if anyone here is familiar with the Formula 1 cars that were banned for being too quick? They had radiator fan motors mounted on the underside of the cars to actually suck the car to the track. Why not use this similar principle to improve rear aero on your car but have a tube coming out of the center of the back of your car pushing air out. Has anyone tried to model this or tried it? Would something like this help to smooth out the aero on the back of the car?

I think it was Ski-doo or Bombardier snowmobile engines which were used to vacuum the cars onto the track.I had Paul von Valkenburg's (SP?) Race Car Design book but it was loaned and never returned (drat!) I believe,at the end of his book,he showed a Corvette with rear suction,and it did reduce the wake and drag,however,as BicycleBob has already mentioned,absorbed more energy than it saved,for a net loss in efficiency.People all over the place are still working to make this work,but the jury is still out on this one.

[cmroseberry]

I attached a picture I have from my Hucho book regarding the subject of blower-derived downforce. This book, Aerodynamics of Road Vehicles, may be purchased from the Society of Automotive Engineers site: sae.org for $100. 85% of what I know about car aerodynamics is in this book. Go to "shop the store" then "book search" and then do an author search under "Hucho". Alternative Cars in the 21st Century by Robert Riley is also a great title for ecomodders. Of course if your local or college library can get these, that will be much cheaper.

[cmroseberry]

The picture did not attach - sorry. The car in the picture is the Chaparral 2J from 1979. The book also mentions the Brabham Formula I car in 1978.