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Author Topic: Drag coefficient fundamentals  (Read 6580 times)
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Bratfink
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« Reply #45 on: August 30, 2017, 08:45:17 AM »

This'll show you how sophisticated I am!

Whataya think about testing models in a captive hydraulic environment by measuring the overall pull of the flow?  (Huh?)

Water tunnels (or tow tanks) are an excellent way to maintain the Reynolds number in small scale without having to flow at very high speed. Essentially increase density instead of velocity to balance the dimensional equation. To save power many of the older water tunnels were gravity fed, as in the x plane of the test section was vertical.

Here's the problem; friction coefficients (viscosity) for water are much higher than air. Thus wake distortion, separation points and some other things behave differently to how they would in air. Still a valid test form though.   
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Bratfink
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« Reply #46 on: August 30, 2017, 09:08:32 AM »


Here is the youtube link. This is the first of maybe 8 or more videos this fellow made using his water tunnel to illustrate flow around several cars. An accurate model is needed. 

Yup, this is like what I was talking about. However, I wouldn't describe that as an accurate model. No cooling pack, no open grilles, no open wheel arches with rotating wheels, no underbody detail, no cut line sealing. All of which have an impact on the aerodynamics. But this is a great tool for basic shape development.

We actually do flow visualization similar to this in the wind tunnel but it requires lasers and high speed cameras. And what can't be made better by using lasers  evil     
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Bratfink
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« Reply #47 on: August 30, 2017, 09:11:18 AM »

Stan;

I thought about that, too. How about letting a model roll down an underwater ramp and timing how long it takes to travel between two marks on the ramp. It won't give you quantitive data but it should show up differences between model shapes. The bigger the model the better.

Regards, Neil  Tucson, AZ

Why not do it in air? Simple Coastdown testing. Run it up to speed, shut it off and shift to neutral see how long it takes to stop. You have to be careful to keep all the variables as constant as possible, but people do use this method.
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manta22
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« Reply #48 on: August 30, 2017, 10:18:49 AM »

I proposed doing it with a model, not a full-size vehicle. It is far easier to evaluate different model shapes in a swimming pool. Yes, I know people use a "coast-down" test but that is using a car that is already built.

Regards, Neil  Tucson, AZ
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Regards, Neil  Tucson, AZ
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« Reply #49 on: August 30, 2017, 04:18:55 PM »

I proposed doing it with a model, not a full-size vehicle. It is far easier to evaluate different model shapes in a swimming pool. Yes, I know people use a "coast-down" test but that is using a car that is already built.

Regards, Neil  Tucson, AZ

For shape development that is defiantly an option.
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Seldom Seen Slim
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« Reply #50 on: August 30, 2017, 04:24:56 PM »

Yes, it IS!
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Jon E. Wennerberg
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Stan Back
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« Reply #51 on: August 30, 2017, 04:35:51 PM »

Is that defiantly or definitely?
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jacksoni
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« Reply #52 on: August 30, 2017, 06:58:34 PM »

Is that defiantly or definitely?
Why yes! I believe it is!   cheers
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Bratfink
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« Reply #53 on: August 31, 2017, 07:25:37 AM »

Is that defiantly or definitely?
ha ha "defiantly". I didn't notice auto correct changed that.
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Seldom Seen Slim
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« Reply #54 on: August 31, 2017, 07:44:52 AM »

The "defiantly/definitely" error is a very common one I've seen countless times, so maybe it is an auto-correct problem.  This validates my feeling that auto-correct/auto-spell software isn't the end-all that'll make proofreading obsolete.

Thanks for letting us yank your chain.  Stan and I defiantly enjoy the exercise. cheers cheers evil
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Jon E. Wennerberg
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floydjer
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« Reply #55 on: September 06, 2017, 08:19:23 AM »

Gosh Jon, Ewe keep picking on the gnu  guise and their going two go aweigh................... wink
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