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Old   July 23, 2003, 10:52
Default variable density water
Atit Koonsrisuk
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I would like to model the flow of water with heat addition. The most important mechanism that drive the flow is buoyancy force. I chose 'Water at RTP' as the fluid model. The output seems reasonable. However the density of water is changed very slightly. Because of buoyancy effect, I expect that the density should be changed significantly. I read from one resource that "if we specify a general fluid and specify density as function of temperature, the thermal expansivity will be ignored." I found in 'Fluids Editor' that we can specify the density of 'Water at RTP' to be the function of temperature. In 'FLuid Editors', I modify the 'Water at RTP' property. In 'Density Dependency' of 'Properties' form, I just tick in the checkbox that say 'Density depends on Temperature'. and in 'Density', I specify 998 kg/m3. After set that, I can't set the 'Buoyancy properties.' Then I set 'Buoyancy Model' in 'Fluid Models' form, and click 'Apply' at 'Fluid Domain' form, I get the error message: ----------------- The fluid 'Water at RTP Modified' for the domain 'Water' has insufficient properties for the models selected. The following problems were found. The thermal expansivity has not been set. Please change the fluid so that it has these properties, or change the domain models. ------------------ What do I do to specify 'Buoyancy Model' and then get the changing of density in 'result' file. Thank you very much.

Atit Koonsrisuk
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Old   July 24, 2003, 01:57
Default Re: variable density water
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If you run a constant density fluid, which you are, then you need to set the thermal expansivity for the flow solver to run the Boussinesque (sp?) bouyancy model. Setting the tick box for "Density Depends on Temperature" will have no effect in this case.

Once you set density as a function of temperature, using a CEL expression for example, then you can select the tick box. In this case you no longer need to set the thermal expansivity and the flow solver will use the full buoyancy model instead.


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Old   July 24, 2003, 03:07
Default Re: variable density water
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Hi, I think to have a significant change of the density you must have a large difference of temperature. After all, in my opinion, as you are using water, you can't have important variation of density because water is not very compressible and so not really expansible. Then you cannot have a large variation of mass, for two fluids at different temperature, for a same volume. But it's always a question of the interval of temperature you are using.
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