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Old 07-08-2008, 04:43 PM   #1 (permalink)
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Question Active redirection of airflow...

I remember seeing some articles about some trucks experimenting with using compressed/redirected air to decrease the amount of drag. Has anyone here thought about using some compact fans to reduce/redirect high/low pressure areas?

I'm about to experiment with some high speed 92mm computer fans to play around with this idea...especially in the engine compartment.

I know most aero mods are passive ones that use fixed structures to redirect/shape the airflow around the vehicle. Like air dams, body pans, and vortex generators.

Active aero mods could "assist" in reducing areas of drag...as you could slow down or speed up the fan to generate the amount of assist based on speed...

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Old 07-08-2008, 06:17 PM   #2 (permalink)
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active flow management

Ludwig Prandtl was experimenting with active fluid dynamics beginning in the 1920s.His work is shown in Hoerner's work.NASA plays with it,and I believe there was a very recent post about research at Georgia Tech University I think.So far,the energy input for the "pump" exceeds the energy saving due to pumping.Alex Tremulis believed such things would be common place by now,with average Cds of 0.12 for the typical passenger car.If you can crack this nut,your a trillionaire!
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Old 07-08-2008, 10:29 PM   #3 (permalink)
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Hi Bam...,

The flow that was actively modified was around the rear of the truck. Look around on here, there is a long thread about this, including a link to the truck article.

According to the truck article, the airflow has to be pushed from the top, sides and bottom all at the same time to get a reduction in drag. Pushing from the top alone, causes lift and extra drag. Pushing from one sides will create a vectored thrust and more drag. Pushing air out from under the car with a fan would cause down force, and more drag.

The fans may not be designed for the speed of the airflow over a car. The blades may need to have more angle on them to be effective at highway speeds.

I am not sure how engine compartment fans could be used to reduce drag in an active fashion. Other than to allow a stagnent vent, which then would have air sucked through when the engine got too hot. But the engine fan would do this already. Pushing air out of the grill seems somewhat complicated to a simple grill block.
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Old 07-08-2008, 11:37 PM   #4 (permalink)
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Quote:
Originally Posted by BamZipPow View Post
I know most aero mods are passive ones that use fixed structures to redirect/shape the airflow around the vehicle.
Funny you should mention that, I was thinking about something similar today.

What if a large part of the surface of a vehicle was moving with zero velocity with respect to the air? Normally of course the surface moves with the same velocity as the vehicle, but it does not have to. For instance, consider this thought experiment: on a big rectangular truck mount very light rollers on frictionless bearings perpendicular to the flow of air along the top and both of the long sides. The truck gets up to a constant speed and the rollers spin up until the outer surface of each roller is stationary with respect to the air. (Or the rollers could be spun up by tiny motors, the point is the the speed of the vehicle's surface changes.) The shape of the vehicle is (close) to unchanged, but the boundary condition has changed. What would this do to the drag?
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Old 07-09-2008, 06:35 AM   #5 (permalink)
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Hi pas...,

The rollers would help pull the air around the end of the vehicle. Your not the first one to realise this. Do a google on "Magnus Effect". There have been proposals of lighter-than-air vehicles, where the Magnus Effect is used to provide controlled lift.
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Old 07-09-2008, 08:55 AM   #6 (permalink)
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Quote:
Originally Posted by aerohead View Post
... Georgia Tech University...
Ahem... as a GT grad, I have to inform you that the name is The Georgia Institute of Technology.

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