## Air flow/pressure gradiant on flat perpendicular plane

## Air flow/pressure gradiant on flat perpendicular plane

(OP)

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

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

## RE: Air flow/pressure gradiant on flat perpendicular plane

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

## RE: Air flow/pressure gradiant on flat perpendicular plane

-Tyson

## RE: Air flow/pressure gradiant on flat perpendicular plane

(1/2)*rho*V

^{2}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

## RE: Air flow/pressure gradiant on flat perpendicular plane

Again,

Thanks for the help,

-Tyson