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[FAQ]

Q) I am solving for a buoyancy-driven flow and am using the Boussinesq buoyancy model. Although the fluid heats up, the density variable does not change. I have set a reasonable value for the thermal expansion coefficient under material properties. Why is density not changing? A) The Boussinesq model for buoyancy is used when the fluid density material property is constant. The effect on buoyancy due to density change resulting from thermal expansion is inferred from temperature changes (i.e. the density variable does not change). If you wish to see the density change resulting from thermal expansion, you can either: 1) Run with the existing set-up. In CFD-Post, create a user variable to reflect the actual density based on the temperature field and thermal expansion coefficient using a CEL expression. 2) or, given that density is a function of thermal expansion coefficient and delta T, use this relationship for the density. Create a CEL expression involving a reference density, reference temperature, temperature and thermal expansion coefficient and use this the material property definition for density. The Full Buoyancy model can now be used, rather than the Boussinesq model.