Air flow/pressure gradiant on flat perpendicular plane

Question

Air flow/pressure gradiant on flat perpendicular plane

tys0n99 (Industrial)

(OP)

18 Sep 03 10:16

I am looking for any information/references for the air flow or air pressure gradiant on a flat plane perpendicular to the movement of the air.  I am not looking for anything really in-depth accounting for features on the wall, just for a plain flat wall.  Any help you could give would be greatly appreciated as I have been unable to find anything on the web or books that we have to reference to.  The wall(a/c unit) is set flat on a roof curb (and not considering minimal recess in the curb), I am assuming its a flat plane for this research.  Also, I believe, although I could be wrong, that this is something similar to stagnation flow in fluid dynamics, but I have no experience with that other than a co-worker told me to look towards it for something similar.

It would actually look something like this:
      |____|
____|___|__

with the recesses being very little compared to the unit itself.  I believe they are in the neighborhood of 1-2" compared to the 8.5' height of the unit.

So, if anyone can either point me to, or post examples of gradiants, what that is actually called, and/or formulas for the gradiant and pressures I would really appreciate it.

Thanks for any help,

-Tyson

Sparweb (Aerospace) · Answer 1 · 2003-09-19 17:54:12

I've got a book on order called Fluid Dynamic Drag, by Hoerner, and it's considered an indispensable source of drag data on all sorts of shapes and sizes of things. It should have drag data on this kind of shape and configuration. If you want a copy, I can post the ordering info (not available in stores).

Are you looking for a drag force to design your mounting of this a/c unit, or for something more subtle that requires a pressure distribution?

STF

tys0n99 (Industrial) · Answer 2 · 2003-09-22 08:47:04

Its for a combination of things, but mostly for mounting. They already know the basic requirements for mounting it to the roof, but they're wanting to know the more specific things, such as where the pressure is greatest, where needs most bracing, etc.

-Tyson

Sparweb (Aerospace) · Answer 3 · 2003-09-22 18:18:58

Stagnation points move with angle of attack.  In the case of a cubic box, when the wind is perfectly perpendicular  to a face, the stag point will be dead center.  To calculate that pressure use:

(1/2)*rho*V² where rho is the density of air.

The pressure will drop off as you move outward to the sides on the windward face, but to design a mounting, applying stagnation pressure over the whole face does not necessarily overestimate the force enough for some margin of safety.  Hoerner's book gives a drag coefficient of 1.05, so you actually get MORE drag force than a stag pressure X area of face.  This may be due to all the turbulence at the back.  Technically there's also a negative stag point on the back of any body, but turbulence confuses the matter.

Here's a hint: very rarely can C_D get higher than 2, because a forward face with a positive stag point, and a back face with a negative stag point equal two areas offering the same resisting pressure.

That said, Hoerner DOES quote some drag figures that are higher than 2.0, so watch out.

Pages 4-3 and 4-4 should help you out the most.

Once the flow becomes oblique to the box, all bets are off.  You might find there's a side force at some assymetrical angles.

STF

tys0n99 (Industrial) · Answer 4 · 2003-09-24 09:27:17

That did help my understanding alot. And I believe I have the majority of my information figured out. The only thing I have left to figure out is the pressure at a given point on the unit given wind speed and location. But I think I know how to do that, just havn't implemented my idea yet.

Again,
Thanks for the help,

-Tyson

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Air flow/pressure gradiant on flat perpendicular plane