Fluid turns into solid at given T (via viscosity UDF or solidifcation model?)

jpina · April 3, 2016, 04:41

Dear all,

I'm carrying out a VOF simulation where a fluid is cooled due to its contact with the surrounding walls.

I'm using a UDF for seting the viscosity of the fluid with the Cross-WLF model. Since it's a polymer, the cooler it gets, the more viscous it turns out to be.

The fact is that the Cross model I use for the viscosity only applies when the temperature of the fluid is bigger than its melting temperature. When the polymer temperature is lower than the melting temperature, the viscosity should be infinite. I have tried to use an if statement in the UDF in order to set a viscosity of 1e20 (i.e., infinite) when the temperature is lower than the melting T. However, this leads to crashes and divergences.

I have thought, that maybe I should use the solidification and melting build-in model of ANSYS Fluent and track the mass fraction instead of the volume fraction. Would this be a good approach? When a region was converted to solid, will it remain still? That's what I am interested in.

Maybe my approach with the if statement would work with using some other solver settings? Maybe you could point me which?

Thanks a lot!

jpina · April 3, 2016, 04:48

I ansywer to one of my questions, according to ANSYS Fluent help Chapter 21 (http://www.afs.enea.it/fluent/Public.../PDF/chp21.pdf), when a fluid solidifies, its velocity is zero:

"Under Parameters, specify the value of the Mushy Zone Constant (Amush in Equation 21.2-6). Values between 104 and 107 are recommended for most computations. The higher the value of the Mushy Zone Constant, the steeper the damping curve becomes, and the faster the velocity drops to zero as the material solidifies."

April 3, 2016, 04:41	Fluid turns into solid at given T (via viscosity UDF or solidifcation model?)	#1
jpina Senior Member Jordi Pina Join Date: Mar 2015 Posts: 157 Rep Power: 11	Dear all, I'm carrying out a VOF simulation where a fluid is cooled due to its contact with the surrounding walls. I'm using a UDF for seting the viscosity of the fluid with the Cross-WLF model. Since it's a polymer, the cooler it gets, the more viscous it turns out to be. The fact is that the Cross model I use for the viscosity only applies when the temperature of the fluid is bigger than its melting temperature. When the polymer temperature is lower than the melting temperature, the viscosity should be infinite. I have tried to use an if statement in the UDF in order to set a viscosity of 1e20 (i.e., infinite) when the temperature is lower than the melting T. However, this leads to crashes and divergences. I have thought, that maybe I should use the solidification and melting build-in model of ANSYS Fluent and track the mass fraction instead of the volume fraction. Would this be a good approach? When a region was converted to solid, will it remain still? That's what I am interested in. Maybe my approach with the if statement would work with using some other solver settings? Maybe you could point me which? Thanks a lot!

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April 3, 2016, 04:48		#2
jpina Senior Member Jordi Pina Join Date: Mar 2015 Posts: 157 Rep Power: 11	I ansywer to one of my questions, according to ANSYS Fluent help Chapter 21 (http://www.afs.enea.it/fluent/Public.../PDF/chp21.pdf), when a fluid solidifies, its velocity is zero: "Under Parameters, specify the value of the Mushy Zone Constant (Amush in Equation 21.2-6). Values between 104 and 107 are recommended for most computations. The higher the value of the Mushy Zone Constant, the steeper the damping curve becomes, and the faster the velocity drops to zero as the material solidifies."