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-   -   how to compute relative velocity from absolute? (https://www.cfd-online.com/Forums/main/77952-how-compute-relative-velocity-absolute.html)

 spk July 8, 2010 07:03

how to compute relative velocity from absolute?

Hi guys,

I simulate a centrifugal pump with MRF using Fuent.
MRF calculates only the absolute velocity.
I try to compute the corresponding relative velocity for the rotating zone using Ensight writing this python file:

# Select all of the parts in EnSight to calculate this new variable on
ensight.part.select_all()
# Break out the Coordinate components
ensight.variables.evaluate("cordX = Coordinates[X]")
ensight.variables.evaluate("cordY = Coordinates[Y]")
ensight.variables.evaluate("cordZ = Coordinates[Z]")
# Calculate U = circumferential velocity (N = 1350 rpm)
ensight.variables.evaluate("U = (2*3.1415927*1350/60)*SQRT(cordX^2+cordY^2)")
# Calculate component rotational velocity
ensight.variables.evaluate("theta = ATAN(cordX/cordY)")
ensight.variables.evaluate("U_X = ABS(U) * COS(theta)")
ensight.variables.evaluate("U_Y = ABS(U) * SIN(theta)")
# Now, activate (load) the absolute velocity
ensight.variables.activate("velocity")
# Calculate components of relative velocity
ensight.variables.evaluate("W_X = velocity[X] - U_X")
ensight.variables.evaluate("W_Y = velocity[Y] - U_Y")
# Calculate the magnitude of the relative velocity
ensight.variables.evaluate("W = SQRT(W_X^2 + W_Y^2)")

But it calculates only the arithmetic value of relative velocity,
no the direction of it.

Is there anyone who can help me?
Is there other way to solve my problem?

Thanks a lot,
spk

 agd July 8, 2010 15:50

This may be a stupid question, so pardon me if it is - if you have the components of the relative velocity (Wx,Wy), (and assuming you know the basis vectors of the relative coordinate system) then don't you have the information you need? Or is the code not working to return Wx and Wy? Or is the issue that you aren't sure about the direction of the basis vectors? It's not clear exactly what you are having a problem with.

 spk July 9, 2010 04:36

Hi agd,

The problem is that fluent calculates only the absolute velocity and there is no way to see the relative velocity.
So, i use a postprocessing program (Ensight). I run the above python file and it computes the relative velocity (W, Wx, Wy) but the direction of vectors of relative velocity is the same with the corresponding of absolute velocity! I think that this code does not compute the direction of the relative velocity, but only the arithmetic value. Is there another way to see the relative velocity?

 agd July 9, 2010 08:42

When you say that the direction is the same are you referring to the basis vectors of the coordinate system, or are you saying that the relative velocity vectors are just uniformly scaled down from the absolute velocity vectors? In the first case, you would have the relative velocity vector as WxI + WyJ. In the second case, you would probably have an error in your script.

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