# Changing differential equation in CFX

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 September 10, 2019, 18:11 Changing differential equation in CFX #1 New Member   Sai Abhishek Siddhartha Join Date: Sep 2019 Posts: 5 Rep Power: 5 Is there a way to change the terms of Differential equation(Especially momentum equation)? I was hoping that i could 1) Input the values of Coriolis force and Centrifugal force, so as to examine their effect on my system 2) Turnoff the Coriolis force component, and only have centrifugal force Please help

 September 10, 2019, 19:17 #2 Senior Member   Join Date: Jun 2009 Posts: 1,609 Rep Power: 29 May I ask why would you want to do that? The equations would be incorrect; therefore, any conclusions will be polluted. Perhaps you want to compare the contributions to the momentum equation, i.e. a force balance. You can compute the sources yourself via expressions, and compare the ratio between them. Say Coriolis force/pressure force, Coriolis force/inertial term or the like In fact, you may want to read about the following dimensionless number associated with rotating physics Rossby/Kibel number Taylor number Ekman number

September 11, 2019, 02:18
#3
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Sai Abhishek Siddhartha
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Quote:
 Originally Posted by Opaque May I ask why would you want to do that? The equations would be incorrect; therefore, any conclusions will be polluted. Perhaps you want to compare the contributions to the momentum equation, i.e. a force balance. You can compute the sources yourself via expressions, and compare the ratio between them. Say Coriolis force/pressure force, Coriolis force/inertial term or the like In fact, you may want to read about the following dimensionless number associated with rotating physics Rossby/Kibel number Taylor number Ekman number
I wanted to examine the effects of coriolis force and centrifugal force on my working fluid. More specifically I wanted to see what effect coriolis force and centrifugal force have independently on the working fluid. Like, how the gas behaves if there is only coriolis force/centrifugal force. So changing/removing the terms in the DE can help me achieve that. Is there a way that we can change DE terms in CFX?

Thanks!

 September 11, 2019, 07:06 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,231 Rep Power: 135 You can add the centripetal and coriolis effects as a momentum source terms. That what you can turn them on and off, or double one and halve the other or whatever you like. Have a look in the documentation on the source terms used for the rotating domain model and write your own source terms for them. Note that compressible flow has additional changes in rotating frames of reference which might be a little harder to reproduce - again, see the CFX documentation for details. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

September 12, 2019, 09:37
#5
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Sai Abhishek Siddhartha
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Quote:
 Originally Posted by ghorrocks You can add the centripetal and coriolis effects as a momentum source terms. That what you can turn them on and off, or double one and halve the other or whatever you like. Have a look in the documentation on the source terms used for the rotating domain model and write your own source terms for them. Note that compressible flow has additional changes in rotating frames of reference which might be a little harder to reproduce - again, see the CFX documentation for details.

I have searched the documentation thoroughly, where I couldn't explicitly any reference to control Coriolis and centrifugal source terms. Additionally I have to solve the problem/equation in stationary domain. Could you kindly provide some reference in documentation/tutorials where these changes can be done?

Thanks a lot!

 September 12, 2019, 19:46 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,231 Rep Power: 135 You can't control the Coriolis and Centripetal terms. You can only turn them both on and off by activating the rotating frames of reference model. My post said if you want to control them individually you need to define a momentum source term using the same equations as used in the rotating frame of reference model. As you define this source term, you can define the inputs any way you like. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 September 15, 2019, 07:26 #7 New Member   Sai Abhishek Siddhartha Join Date: Sep 2019 Posts: 5 Rep Power: 5 Thanks for your reply. How do I change/deactivate the rotating frames of reference model? Also can you please guide me regarding where can I add momentum source terms in CFX? Is it part of buoyancy acceleration in boundary conditions or should I use some programming language? Thanks a lot!

 September 15, 2019, 19:59 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,231 Rep Power: 135 Please do the CFX tutorials and read the documentation. All your questions are very basic questions which are explained well in the documentation. The search function works well and you will find details of the source terms soon enough. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.