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Temperature dependent thermal expansion coefficient

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Old   May 16, 2019, 12:22
Default Temperature dependent thermal expansion coefficient
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Emil Helgren
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I am modelling natural convection of water in the temperature range (0 C -> 21 C) using the Boussinesq approximation, and the thermal expansion coefficient of water is changing around 4 degree Celsius, but i'm only able to input a constant value.

I tried creating a define property UDF and loaded that, but i was only able to choose the UDF on the other material properties (where piecewise linear and other options are also available).

Is there any way i can have a non-constant thermal expansion coefficient when using boussinesq density?
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Old   May 16, 2019, 12:39
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No. Even if your UDF worked I wouldn't do it because you'll break other things.

Just use a different equation of state for density (not Boussinesq).
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Old   May 17, 2019, 04:50
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Thanks for the reply!
Do you maybe have a specific method you would recommend? I am modelling phase change as well by the way, using the solidification/melting module.

I've tried using a piecewise linear density instead of boussinesq in 2D, and the solution didn't converge on any of the timesteps even at 50 iterations pr. step(!) as long as there was still ice present in the simulation. After all the ice was melted, it congerveged after 2-5 iterations each step, so i assume the problem is at least related to the phase change solving. I would really like to get a nice converging solution before taking the time to do a 3D simulation

I assume ANSYS just uses fully compressible navier-stokes when the density is defined piecewise linear? (can you confirm this?)
Do you think the problem is the enthalpy-porosity method used by the module having a hard time or is it something else? (maybe there is a better way of simulating this phase change?)
Any kind of advice is much appreciated!

I would love to post the details of my setup if needed
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Old   May 17, 2019, 13:31
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Piecewise density should work in place of a temperature dependent boussinesq.

Yes, Fluent always uses a fully compressible navier-stokes even when you use constant density.

Quote:
Originally Posted by emilhelgren View Post

I've tried using a piecewise linear density instead of boussinesq in 2D, and the solution didn't converge on any of the timesteps even at 50 iterations pr. step(!)
it converged with Boussinesq or did you not try it? That would be a hint as to what is stalling convergence.
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Old   May 20, 2019, 05:32
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it converged with Boussinesq or did you not try it? That would be a hint as to what is stalling convergence.
Yes with Boussinesq it converged at about the 10th iteration each step, and when the ice was completely melted it only used 2 iterations.

Interestingly, it looks like the the problem is with continuity (i hope i succeded in attaching an image of my residuals). I would think that a residual of 1 is pretty bad :S.

I read somewhere that if your solution doesn't converge, it doesn't necessarily mean you can't trust your results, but you certainly can't trust the time - as in, the flow development and interaction is right, but how fast things are happening is probably not true, would you agree with that, or is that too general a statement?

I don't really know what to change next, do you have any ideas?
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Old   May 20, 2019, 10:12
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A residual of 1 for continuity means the flow solution is not changing. That is, your velocity field is not changing. This can happen when there is no flow.


You should see some residual reduction vs iteration within each time-step and it looks like your energy residual is just constant. You've got some wonky setting in your case.
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