# Default thermodynamic state in CFX

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 November 21, 2012, 05:23 Default thermodynamic state in CFX #1 New Member   Jey Join Date: Nov 2012 Location: Bangalore, India Posts: 2 Rep Power: 0 I am performing a CFD analysis for which I have not activated the "Thermodynamic State" of the material. I have specified Density, Dyn. Visc., Th. Cond., sp. Heat values varying with temperature. Can I know what state will CFX assume by default? Thanks, J

 November 21, 2012, 06:18 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,057 Rep Power: 86 If you define an incompressible simulation then the state is irrelevant - the fluid is defined by density and viscosity only. If you define a compressible simulation then you need density to be a function of pressure and/or temperature. This could be either modelling a gas or a liquid (ie water hammer). The phase of the material only really matters when you start doing multi phase simulations. If you are single phase then it does not matter.

 November 21, 2012, 10:23 #3 New Member   Jey Join Date: Nov 2012 Location: Bangalore, India Posts: 2 Rep Power: 0 Thanks Glen. I am solving a buoyancy case for an enclosed oil filled cavity. Trying to solve for natural convection with a heat generation (heat flux BC) at the middle of the cavity. There is no phase change involved in this problem. I have created User Defined functions for Viscosity, Density, Th. Cond & sp. heat as tabular values varying w.r.t. temperature. I have not used any pressure other than reference pressure during initialization. What kind of simulation will CFX assume for the above problem, whether incompressible or compressible?

 November 21, 2012, 18:28 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,057 Rep Power: 86 CFX will do a compressible simulation if you define density as a function of pressure/Temperature, AND you choose Total Energy as your thermal model. Otherwise you will be running incompressible. The simple approach for buoyancy is to use Bossinesq buoyancy which keeps the simulation incompressible. But if you want to get maximum accuracy (and I presume you do if you are setting variable properties) then you should use a compressible model.

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