Adding extra terms in governing equations

Alek · January 30, 2014, 11:21

Dear friends,
Is it possible in CFX to add one or two extra terms in to governing equations? for example adding pressure gradient term to energy equation and CFX solves the modified energy equation containing pressure gradient term.

Sincerely

ghorrocks · January 30, 2014, 18:02

You cannot modify the underlying equations. Your options are:
* If you can write it as a source term then use a source term
* Find an existing CFX physical model which is close and use that.

Alek · January 30, 2014, 23:26

Thank you Glenn,
In your idea can we present the pressure gradient as a source term ?

Sincerely

ghorrocks · January 31, 2014, 00:05

Yes, you certainly can use the pressure gradient in a source term. You can get the gradient of the pressure field with p.grad, and I think to get the components it is p.grad_x.

Opaque · January 31, 2014, 17:51

Dear Alek,

I am bit puzzled by your question. What heat transfer model are you using Thermal Energy, or Total Energy ?

If you are using the Thermal Energy model, I wonder what are you trying to achieve? What type of material are you using ? incompressible ? I would suggest to contact ANSYS support and check if there is already a mechanism to activate such term.

In the case of the Total Energy model, CFX already include the pressure gradient term; therefore, you may be double accounting such effect.

Still puzzled

Alek · January 31, 2014, 23:26

Dear opaque
In fact I want to add pressure gradient term to species transport equation. My problem is compressible.

Sincerely

Opaque · February 10, 2014, 13:21

Dear Alek,

CFX nor FLUENT include the pressure gradient term in the species diffusion flux. FLUENT includes the thermal diffusion gradient though.

Keep in mind these additional terms are not trivial volumetric sources, but fluxes about the control volumes and must be discretized accordingly.

You may need to contact ANSYS (CFX / FLUENT) for support on how to implement such terms.

January 30, 2014, 11:21	Adding extra terms in governing equations	#1
Alek New Member Smith Join Date: Jan 2014 Posts: 9 Rep Power: 12	Dear friends, Is it possible in CFX to add one or two extra terms in to governing equations? for example adding pressure gradient term to energy equation and CFX solves the modified energy equation containing pressure gradient term. Sincerely

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January 30, 2014, 18:02		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You cannot modify the underlying equations. Your options are: * If you can write it as a source term then use a source term * Find an existing CFX physical model which is close and use that.

January 30, 2014, 23:26		#3
Alek New Member Smith Join Date: Jan 2014 Posts: 9 Rep Power: 12	Thank you Glenn, In your idea can we present the pressure gradient as a source term ? Sincerely

January 31, 2014, 00:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Yes, you certainly can use the pressure gradient in a source term. You can get the gradient of the pressure field with p.grad, and I think to get the components it is p.grad_x.

January 31, 2014, 17:51		#5
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	Dear Alek, I am bit puzzled by your question. What heat transfer model are you using Thermal Energy, or Total Energy ? If you are using the Thermal Energy model, I wonder what are you trying to achieve? What type of material are you using ? incompressible ? I would suggest to contact ANSYS support and check if there is already a mechanism to activate such term. In the case of the Total Energy model, CFX already include the pressure gradient term; therefore, you may be double accounting such effect. Still puzzled

January 31, 2014, 23:26		#6
Alek New Member Smith Join Date: Jan 2014 Posts: 9 Rep Power: 12	Dear opaque In fact I want to add pressure gradient term to species transport equation. My problem is compressible. Sincerely

February 10, 2014, 13:21		#7
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	Dear Alek, CFX nor FLUENT include the pressure gradient term in the species diffusion flux. FLUENT includes the thermal diffusion gradient though. Keep in mind these additional terms are not trivial volumetric sources, but fluxes about the control volumes and must be discretized accordingly. You may need to contact ANSYS (CFX / FLUENT) for support on how to implement such terms.