Can energy spent by mechanically sucking or blowing air be compensated by lower drag? 1

[DasKleineWunder]

If a mechanical device (consuming energy) is used to blow or suck air to reduce the vehicle aerodynamic drag
Can this energy consumed by the mechanical device be overcompensated by a lesser amount of energy required to move the car at the same speed?
Thus net energy savings

Is there a thermodynamic law or something preventing this from happening?

Of course it all depends on vehicle speed.
At lower speeds when aerodynamic drag isn't important these mechanical air blowing/sucking devices should be turned off.

 

[rb1957]

"Thus net energy savings" ... no, it'll take more energy than will result from the reduced drag.


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[3DDave]

Probably not, but there are cases.

For example, a boundary layer trip can cause a transition from laminar to turbulent flow that limits laminar flow detachment. There is a large decrease in drag if laminar flow detachment is prevented. It is a classic Aero 101 wind tunnel test. Sometimes you can borrow wasted energy - P-51 radiator exit design, for example, to decrease drag.

 

[imcjoek]

The concept isn't entirely bonkers.

http://en.wikipedia.org/wiki/Northrop_X-21

But it has never proven to be very practical.

 

[rb1957]

yes, there are mechanical ways to reduce drag.

yes, there are BL suction applications (a la Blackburn Buccaneer) that improve performance.

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[berkshire]

I would suggest checking the papers of Dr. August ( Gus) Raspet at Mississippi state university Starkville
He did a lot of research on this subject.
B.E.

You are judged not by what you know, but by what you can do.

 

[KENAT]

DasKleineWunder - are you talking about

http://en.wikipedia.org/wiki/Boundary_layer_suction

While I was at uni there was a bunch of research going on into this with wing sections or engine pylon parts with lots of little holes in the skin and some kind of suction applied to suck in the sluggish boundary layer... There have been a number of NASA programs testing this and related theories too.

One major flaw is that these holes tend to get plugged in real world settings outside of the wind tunnel.

I'm not quite so certain as rb1957 that it necessarily violates total system conservation of energy in theory at least.

(I believe you'll find Buccaneer was boundary layer blowing rb1957.)

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[GregLocock]

The more general question, is it possible to add a device to a system that takes energy to run it, but will reduce the overall system energy requirement.

This doesn't violate any laws of thermodynamics, it just pushes the system efficiency towards the Carnot law efficiency limit.





Cheers

Greg Locock


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