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aerodynamic experiment - Babkin Racing

Babkin Racing
Date: 2007-10-30 15:49
Subject: aerodynamic experiment
Security: Public
Tags:aerodynamics, mechanical
I've been wondering, what is the flow of air over the hood? Is there a low pressure or a high pressure area? These questions come from the need to improve the cooling in my Mustang race car, which seems to be limited by the flow of air through the radiator. A friend told me that he had the cooling improved a lot by using a tall hood with air hole son the back. But then would there be a high-pressure area in front of the windshield? Then at speed things would get worse, back pressuring the space behin dthe radiator and reducing the flow. But the middle of the hood or the area right behind the radiator looks like a pissible good low-pressure area. Then drilling holes in the hood would be a cheap and easy fix.

So how do I check? I've seen a picture in a book, with a car on the proving ground being covered with tufts of rope which highlight the air flow. The rope looks too complicated but I have a cheap replacement: shredded paper! Stick it on the car with masking tape and drive the car, and the home-made replacement for a windtunnel is ready.

An issue with the race car is that it's not street-legal. It would be nice to try the method and see that it works before going to the race track. So I've started with my everyday car, Volvo-740. There are a couple of side benefits to it. First, it has a flat easy-to-see hood. Second, I have a decent idea of how the air flows over it. In the light snow there is usually no need to use the wipers, the snow just flows over the windshield, highlighting the air flow. It might not be obvious from its shape but Volvo had one one of the lowest Cd figures for its time.

I've shedded some paper, and stuck it on the car. Here is how it looked from outside and inside:

Then drive the car! Here is the sequence of the pictures:

20 mph:

30 mph:

40 mph:

50 mph:

60 mph:

And here are some videos:

25-35 mph:

25-35-50-60 mph:

I didn't go any faster than 60mph - firstm there are speed limits around here, and second driving the car and shooting the images takes too much attention. A separate camera operator, or better yet an operator in another car would be useful.

You can see that only the tufts near the nose flap, and as the speed grows, those on the sides start moving too but the tufts on the hood itself are all flat. AFAIK Volvo gets its low Cd from using its blunt nose to push the air high enough, so that then the rest of the car just tucks under without creating any additional resistance. Which seems to match the picture I see.

Looks like the method works, and the paper shreds hold together fine up to at least 60 mph.

Doesn't quite answer the question of the pressure though. I think it should be low, sine the car moving over the car has speed, and as such should have lower pressure, and suck out the air from over the hood. But needs more measurements. I've got some spare MAP sensor from Mustang sitting around, stay posted!

[ Russian translation: http://sab123.livejournal.com/102639.html ]
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Bob Normal
User: bob_the_normal
Date: 2007-10-30 22:02 (UTC)
Subject: (no subject)
First I see discussion of aerodynamics, THEN I see a Volvo...

I lol'd.

(I have love for the Volvo's anyhow, but you gotta admit it's probably one of the funniest cars to do aero testing on.)

I don't think I'd trust this test much unless you found yourself on a road where the wind just didn't exist at all. And I don't know if you're joking about the low CD, but the box design on Volvos isn't doing them any CD favors... wall in front, vaccuum in back? I suppose it really depends on how what speed you're talking about though.

Interesting though, looks fun... if I had a spare minute I'd go play around with tape and paper right now and see how it went. Keep it up!

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Babkin Racing
User: babkinracing
Date: 2007-11-02 17:26 (UTC)
Subject: (no subject)
No joke. It's true, about Volvo's low Cd. It just shows that there is a lot of difference between what looks sleek and what actually is sleek.

As for the wind, I doubt that it made much difference. How fast could it be? I don't think much faster than 10 mph. Anyway, it was not that much about the exact figures. More about seeing the trend and testing the method itself.
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