# how to compute relative velocity from absolute?

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 July 8, 2010, 07:03 how to compute relative velocity from absolute? #1 Member   Join Date: Aug 2009 Posts: 67 Rep Power: 16 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

 July 8, 2010, 15:50 #2 Senior Member   Join Date: Jul 2009 Posts: 357 Rep Power: 18 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.

 July 9, 2010, 04:36 #3 Member   Join Date: Aug 2009 Posts: 67 Rep Power: 16 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?

 July 9, 2010, 08:42 #4 Senior Member   Join Date: Jul 2009 Posts: 357 Rep Power: 18 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.