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Old   July 17, 2004, 06:07
Default gravitational environment
  #1
gjy0775
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Hi everyone:

I am confused at the 'gravity' in CFX. I have read a lot of threads about gravity in this forum.I know that we don't need to take the gravity into account for non-buoyancy problem.But we have lots of non-buoyancy problem which need to think about gravity.In ANSYS or Fluent to deal with the gravity influence is very easy.You just go to the environment setting and specify gravitational acceleration. Could anybody tell me whether CFX have the same function and then how to consider it?

Thanks in advanced! gjy
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Old   July 18, 2004, 12:24
Default Re: gravitational environment
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Jeff
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Unless there are density differences (i.e. buoyancy) in your flow domain due to compressibility or multiple phases, gravity will have absolutely NO affect on your solution (other than adding the same source term everywhere in the domain adding needless calculations).

The only single phase, incompressible problem that may require gravity is something akin to a bucket draining problem with pressure boundaries. In this case you can simply add a body force rho*g to the entire domain.

Jeff
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Old   July 18, 2004, 23:19
Default Re: gravitational environment
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gjy0775
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Hi Jeff:

Thanks for your returns. One more question about my model.My model is a vertical oriented pipe,in which the water flow from bottom to upside.Should I add the body force to the flow domain?

gjy
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Old   July 19, 2004, 19:24
Default Re: gravitational environment
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Glenn Horrocks
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Hi,

You do not need to add the body force. Just specify a gravity vector and use a fluid model which has some density variations or other mechanism to drive convection.

Glenn
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Old   July 19, 2004, 21:26
Default Re: gravitational environment
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Jeff
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Yes, you can specify buoyancy and put in a gravity vector, but leave density constant (I think). The force that is applied is g*(rho - rho_ref) so you need to set rho_ref to zero.

You need to be very careful about how you specify your boundary conditions, however. A problem set up this way will generate a hydrostatic head in the gravity direction. Pressure boundaries need to account for this head as well.

Jeff
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Old   July 20, 2004, 12:20
Default Re: gravitational environment
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gjy0775
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Thanks
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Old   August 16, 2004, 04:18
Default Re: gravitational environment
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Abraham
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Can u tell me how to specify gravity vector in CFX 5.7?
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