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Changing differential equation in CFX

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Old   September 10, 2019, 18:11
Question Changing differential equation in CFX
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Sai Abhishek Siddhartha
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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
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Old   September 10, 2019, 19:17
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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
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Old   September 11, 2019, 02:18
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Quote:
Originally Posted by Opaque View Post
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!
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Old   September 11, 2019, 07:06
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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.
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Old   September 12, 2019, 09:37
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Quote:
Originally Posted by ghorrocks View Post
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.
Thanks for your reply!

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!
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Old   September 12, 2019, 19:46
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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.
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Old   September 15, 2019, 07:26
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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!
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Old   September 15, 2019, 19:59
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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.
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centrifugal force, cfx solver control, coriolis force, differential equations

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