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How to get time average tangential velocity in swirl flow.

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lakhi:
I'm working on the swirl flow and I'm not able to get desired time average tangential velocity. It is said that in swirl flow, where tangential component is not independent and is coupled to pressure, simply time averaging the tangential velocity (as is represented in FLUENT) is not the actual procedure. All I need to have is the x-velocity "vx" , the y-velocity "vy" and the z-velocity "vz" . The mean z-velocity "vz" is same as the mean axial velocity "vaxial" . The mean tangential velocity "vθ" is to be obtained using Fluent custom field function:

vθ = vx cos θ + vy sin θ

where "vx" is the time averaged x-velocity, "vy" is the time averaged y-velocity and "θ" is the angular coordinate.

Now when I open Fluent custom field function, nowhere i can find "θ". The above expression tells that the coordinate system used is the Cartesian coordinate system because tangential velocity vθ is expressed in terms of vx and vy. But how to express this "θ". Do I have to write a UDF for this? If yes, how  :-[ . Suggestions will be appreciated.

infocfd:
You can convert the equation:
vθ = vx cos θ + vy sin θ

into Cartesian coordinate.
x = r cos θ
y = r sin θ

lakhi:
Thanx infocfd, I finally got it.

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