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-   -   question about governing equation in CFX using rotating/non rotating reference frame (http://www.cfd-online.com/Forums/cfx/71691-question-about-governing-equation-cfx-using-rotating-non-rotating-reference-frame.html)

rystokes January 12, 2010 07:14

question about governing equation in CFX using rotating/non rotating reference frame
 
Hi,

I'm trying to use cfx to obtain flow field of liquid which is confined in a cylindrical domain, the domain is covered by "wall", no other boundary conditions. It's stationary initially, then the wall starts to rotate around a fixed axis with an accelerating speed, when it reaches a peak value, then starts to decelerate till stationary, then once again accelerating and decelerating in the similar way. The problem seems quite simple. I used rotating reference frame and non rotating reference frame method to solve it and made comparison about the velocity field. I wrote a CEL for the control rotating speed omega. Firstly, I used rotating reference frame by assigning the control speed value omega to the "angular velocity" in the domain motion tab and zero velocity for wall velocity with no slip condition, so I can get relative velocity values from post-cfx. Another approach I used is setting up zero value for angular velocity in the domain motion tab, and assigning the control speed value omega to the "angular velocity" in the wall velocity tab. In this way, I can get absolute velocity values. It has three velocity components, radial, tangential and axial. There should be no difference for radial and axial components either using rotating or non rotating refernce frame, and absolute tangential should equal to relative tangential + omega x r .

However, I found there is large deviation between these two approaches for the velocity results that I got respectively, especially for the tangential component. When the wall is accelerating to rotate, the absolute tangential component is also keeping growing, however, the relative doesn't, and only accelerate for a shorter time. I'm quite puzzled about it. Thus my questions are what are the governing equations(Navier-stokes) when using either the rotating or non rotating reference frame respectively in CFX PLEASE?

Thank you very much in advance!


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