# Constant properties water with variable density as a function of temperature

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 October 30, 2014, 17:10 Constant properties water with variable density as a function of temperature #1 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15 Hi, In the material properties for water is it possible to use constant properties for everything except density? I want density to be a function of temperature and pressure. I've read that the Redlich Kwong real gas model may be able to do this. However if I turn it on, is it as difficult to converge with as full IAPWS (where every property is a function of pressure and temperature)? Thank you

 October 30, 2014, 19:30 #2 New Member   Ali Madayen Join Date: Apr 2014 Posts: 22 Rep Power: 12 Hi. Sorry but I can't get the problem right. Isn't it possible by a simple expression? or there is something that makes it more complicated?

 October 31, 2014, 04:16 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,786 Rep Power: 143 Yes, you can use constant properties except for density. What density equation of state do you want to apply?

 October 31, 2014, 04:47 #4 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15 I have an Eulerian-Eulerian free surface setup. I am modelling water liquid and water vapour, separated by a free surface. I would like to apply variable density for the water liquid equation of state. So far I am modelling variable density in the water vapour phase as an ideal gas assumption. I've just stumbled across a fourth order polynomial expression of density as a function of temperature for the water liquid phase, which I may implement. This is the equation: Density [kg/m^3] = (-2.98043*10^-8*T^4)+(4.93093*10^-5*T^3)+(-3.25853*10^-2*T^2)+(9.18937*T)+82.2757 Obviously to use the above expression you would need to call upon a dimensionless temperature (just divided T by 1 [K]) to give the correct units of density [kg/m^3]. Bar this approach, are there any others which would give me variable density for water liquid, but constant properties for everything else?

 November 1, 2014, 04:43 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,786 Rep Power: 143 Rather than generate a dimensionless temperature use the correct units in your function. For instance: Density = (-2.98043*10^-8 [kg m^-3 K^-4]*T^4)+(4.93093*10^-5[kg m^-3 K^-3]*T^3)+(-3.25853*10^-2[kg m^-3 K^-2]*T^2)+(9.18937[kg m^-3 K]*T)+82.2757 [kg/m^3] If you wanted variable density but constant everything else and you know this equation is the density versus temperature function then I would just use this, rather than trying to use the more sophisticated models.