A race car in a sub2d?

[boss]

Hi guys,

This a little bit off topic, but I couldn'd find a better place to put this threat. Also because all car aerodynamics forums are about F1 an NASCAR. Let's give it a try.

I study aeronautical engineering in the Netherlands and I am now part of a race car team. The cars cooling is very poor so I have a new project ;) (also for school). The main problem is the immense heat under the cars rear fiberglass body part (also because the engine needs as cool air as possible). So I think cooling holes are the most essential. I've been very busy putting the cars cross sections in sub2d. I had to make the cars front end round, otherwise the program would not recognise the shape. The main question is where to put inlets and outlets for cooling? Off course I'd need cfd progams for a good result, but I think this is as good as it gets for now. I have a few questions, to not make the car even worse!

- Do the pictures below look normal? I'm a bit confused by the high pressure zone at the cars rear end. And is it possible to look at the cars aerodynamics with a 2d program?
- Where would you put the cooling holes? Please keep in mind that the turbo is placed at the rear right of the car next to the wheel. (sorry, no picture of that).
- I found some info about naca ducts, These are great for air inthakes, but what would you use for air outlets? An inversed naca duct maybe?

Here are the pictures, please let me know what you think!

Luuk

[Andy W]

Ferrari and Renault have two different solutions - I'm not sure the Ferrari is using this to evacuate air from the engine compartment, but the Renault surely is. I like the Renault - if it works for sharks, must be good for aerodynamics? :)

[boss]

hmmm, thanks that's very interesting!

I like F1 a lot, more because of looking at the cars than watching the races. Never thought about using this solution... :rolleyes: . I really like the renaults by the way, they have a lot of lips and flaps and other parts at the car for better aerodynamics, looks so cool!

Any other brilliant ideas?

Thanks, Luuk

[BMatthews]

There's brute force and then there's the smart way. The brute force method uses large openings to force the passing air to where you want it to be. The clever and tricky way would model that car's shape in 3D and run that pressure distribution estimation software to find areas where there is a positive pressure point and put inlet holes in that area and the find negative pressure points and put the outlets there. Unfortunetly it's just not possible for the average mind to look at pictures of a car, regardless of how attractive it is, and "see" where the high and low pressure points on that care are located and how big the area of high and low pressures are to determine how big the slots or holes can be.

So you're on the right track with using the computer to find the high and low pressure areas but I also think your idea that you can't use a 2D program to examine a 3D shape is correct. The 2D program assumes an infinite width I would imagine so it's not really valid to use that software other than for some suggested general trends. One error I see right off is that you didn't include the rear wing in either of the side views. That wing will create a big effect. You also definetley cannot use the idea of the side view as shown as there is no ground effect allowed for. That alone will totally change a lot of the flow dynamics.

The high pressure zone on the back of the body as shown both is and isn't a surprise to me. Apparently the low pressure air has been pushed up and away from the body by the forward area and once clear of the disturbance shape I guess it's coming back together with a snap and you're getting a high pressure spot.

That would suggest to you that closing those openings in the back of the body and putting gills on the upper deck and upper rear fenders where the most green is located should work best. In fact if that high pressure point shown in the program holds true for the real car you may well be sucking back in the hot exhaust gasses as well as trapping the engine's crankcase heated air in there!

But then there's the lack of data for any effects from the wing to consider. That wing could well create a bad pressure point that would negate any suction on the rear of the upper deck.

But check that it's not speed and length related to where there's a wave effect going on. If your program has settings for air density and viscosity and body size check that those are all accurate so you know you're working at the proper Reynolds number so there's no scale effects at work.

If you can't get the full 3D CFD software working then what about the good ol' experimental method? set up the car with some small plastic tubes running to various spots on the car and run all the tubes to a manometer bank in the drivers compartment for some good ol' experiments. One of the sensitive combination vacuum/pressure guages that can be quickly switched from tube to tube would work well for tests. A few runs with the 6 or 8 tubes at various points on the body should soon determine where there's good suction points for the outlets and a good pressure or at least neutral point for the intakes. Or if you find a really good suction point then maybe you don't NEED any intakes. The air that sneaks in around the lower skirt would supply the outside air and you end up with more suction for the body shell and a win-win situation. A cooler engine compartment and more suction downforce.

I used to road race stock sedans back in the 1970's and was keenly interested in the whole racing sport then so I'm reasonably familiar with what you're trying to do and how important it is to make one item do two jobs wherever possible.

[boss]

Thanks bruce,

I'll try to make a wing and maybe a ground surface. One other thing is that the cars engine compartment at the rear end is not closed. If I have time and the skills I might try to figure out a diffuser. I might try the manometer, but unfortunately a report has to be ready within 3 weeks. But maybe I'll try it later. Very interesting comment about maybe not needing any inthakes. You suggest to make a air inlet at the cars bottom and suck away the air underneath the car? Your comments really help me!

I made a figure of the pressures along the x-axis of the car. The lowest pressures at the cars rear end are along the sides. so this would be the best place for making holes right. I REALLY want to try the manometer to find the exact best spot(s) for making holes or even better the chimneys as seen on F1 cars!

Thanks all, Luuk

[boss]

Just to keep you up to date:

I tried to make a surface below the car to simulate the track, this does not work, probably because the floor does not move along with the air. But hey, the program is for fast things flying higher than 8 centimeters of the ground :D . Now the good news, I made a wing on the car, tonight I'll measure the exact dimensions of the wing. But the spoiler has a BIG BIG effect on the aerodynamics. I never thougt the effect would be this big. Especially in the middle cross-section it creates a hughe low pressure zone! Yippi! But this is just a 2d simulation, so nothing is exact...

And a question about the manometer. It needs a hole in the bodywork to measure the pressure. I'm not sure if this is possible, I don't want to ruin the car. Any suggestions? I think putting the tube on top of the body will not be very relaiable.

I'll keep you guys up to date, Luuk

[BMatthews]

My own thoughts are that you're looking for fairly large area pressure effects so taping a small cross section tube to the exterior of the body work is not as big a no-no as you would imagine. I'm suggesting very small tubes in the area of perhaps 1/16 to 3/32 inch OD. You're not flowing a volume through it, just transferring a pressure effect. Sort of like voltage with no current, it only needs a hair like wire as long as there's no current.

Nice work with the addition of the wing. And note how the pressures change for the center of the airfoil with the roof present and the outer portion with the lower area section. The center is actually LIFTING the back end as shown by the odd high pressure zone under and blue bubble on top near the trailing edge. Good thing the tips are in better air so they make up for that. If your findings are valid in the real world then this car really can't be gaining much from the wing... or at least certainly not as much as the owners think it is. Spacing it up higher would help a lot but I know there's rules about that sort of thing.

Of course a 3D study would show that it's not as clean cut as this as there's lots of interaction in zones with the regions being as close as on the car.

The openings that I was suggesting were not on the lower side since your findings on that side of your "objects" won't mean anything because of the road surface. I was actually suggesting that your 2D findings would indicate that gills in the rear of the cabin would be in a low pressure zone so it would tend to suck air out from inside the body and/or the engine compartment. Doing so and assuming that there's no lower cover on the engine compartment would help to vacuum the car down onto the road.

Your initial 2D findings also suggest that the center openings in the rear of the car should be closed off and only the openings out past the cabin should be kept open. The strong blue and green for the outer shape in your last post shows that there's a low pressure zone at that point. But meanwhile in the cabin area there's a high pressure point. But here again you need to do a proper 3D flow and pressure study to determine if this trend stays intact. The 2D is great for studying shapes but it's leading you and us down the garden path since the clean effects seen in these estimations would be messed up by the oppsite and still strong effects in the first pic.

[Andy W]

There's also a good reason race teams run wind tunnels.. 24x7 in some cases!
..a

[boss]

Thanks agian for the reactions,

I am really finding this interesting, but I also know that CFD programs or accurate measurements are needed for realistic results. But of course this 2d method is not 100% rubbish. It's just not as accurate as I would like it to be, but I think it is enough for this poject. The car NEEDS cooling, and even these 2d pictures give a better solution than just drilling holes and making gaps at several places without thinking which place would be the best. I do now have a cross section of the wing so I'll put that in the computer as soon as possible. That will make the simulation more accurate. I think after putting the exact wing on, it will not be possible to make a better 2d simulation with SUB2D. I'll ask wether I am allowed to make small diameter holes in the body for the manometer. First I'll ckeck the regulations so that I'm not planning to make something wich is illegal. But I'd really like two of those chimneys on the car :rolleyes: . I'l also go and have a look at how other simular cars are cooled, like the audi R8 (le mans etc.).

keep you up to date!

Luuk

[Sparky Paul]

Luuk, you can't make two cars, one to test, one to compete with?
The test car can have the holes and the duct tape and the cardboard scab-ons that finalize the shape for the "real" car.

[boss]

well, the team has two of these car that both race. And because it is not a very big team they will not sacrefice a rear body for tests. Another problem is that there is very little time to test, but there will be more time in the winter. I think the only way to do real tests is to wait for a car to crash and than try to repair the rear body and use that one for tests. I've not yet asked wether I'm allowed to make very small holes, but I think the answer will be no. I did now by the way put the real wing in the computer. The low pressure area is way smaller and less strong now, hmmm.

Luuk