Calculate wind load using fluid impulse and momentum.


by FrankJ777
Tags: fluid, impulse, load, momentum, wind
FrankJ777
FrankJ777 is offline
#1
May2-13, 09:20 PM
P: 78
I recently took a crash course that included some topics in fluid mechanics. I learned that you can use the concept of impulse and momentum the calculate the force of fluid, for instance a jet of water on a turbine blade. The equations used are:

ƩFx = ρQ(Vx1-Vx2)
ƩFy = ρQ(Vy1-Vy2)

I realize that as a gas air is compressible, but I was wondering if I could calculate wind loading on a wall using the same concept, but maybe with a factor of compressibility factored in. If I treat air as an incompressible fluid, will I get a wind load that is reasonably close to the actual wind load?

Thanks
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SteamKing
SteamKing is offline
#2
May2-13, 09:33 PM
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P: 5,544
Compressibility can be neglected if the velocity of the gas is less than 0.3 Mach (~100 m/s)

To calculate the force on a flat surface from the impingement of an air stream, the stagnation pressure can be used. Stagnation Pressure = (1/2)rho*velocity^2
FrankJ777
FrankJ777 is offline
#3
May3-13, 02:08 PM
P: 78
Would using the stagnation pressure approach only apply where the wind speed goes to zero, like up against a large vertical building? What if the face of the structure isn't normal to the wind, but at some angle that allows it to reflect off at a different direction. Then could one, use the momentum/impulse concepts.

By the way, my interest is estimating the wind loads on large microwave antenna dishes attached to communications towers. The face of the dishes are generally round and covered with a tarp, so i think they should represent uncomplicated structures.


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