Discussion Forum

Hi,

I am trying to model a laser welding process, so I need to couple the heat transfer and the turbulent flow. For this I use the multiphysics node, the non-isothermal flow with k-e turbulent model. The main effects I consider are the latent heat, the Marangoni effect and the phase change.

In my case, I consider the whole region as liquid and solve the phase transition with "heat transfer module" by the effective heat capacity method. So I define the liquid volume fraction with a range from 0 to 1. (1 for liquid, 0 for solid). Then, I want to see the velocity profile only in molten region( fl=1) and use therefore the "porosity method". In brief, porosity=0 for solid, porosity=1 for liquid.

It can be defined in volume force term as following:

Fx = -C*((1-fl)^2)/(fl^3+b)*u
Fy = -C*((1-fl)^2)/(fl^3+b)*v+rho*g*beta*(T-Tinf)

fl : liquid volume fraction
C : sufficiently large number (ex) 1e6
b : constant for not divided by zero (ex) 1e-3
rho : density
g : gravity

This equation makes velocity zero in solid.


Actually the simulation has been performed with Comsol 4.2 with the same materials and parameters and it worked. Now I use Comsol 5.0 and the boundary conditions for the outlet has been changed. Earlier I used a BC ''no viscous stress'' and ''presusre point constraint'' for the outlet. Now with the 5.0 version I have to use the pressure BC, but the equations solved for are not the same. I tryied to set an ''outlet velocity'' and ''pressure point constraint'', but it doesn't work again. I get permamently the error message:

Division by zero.
- Function: /
Failed to evaluate variable.
- Variable: comp1.nitf1.kappaT
- Defined as: ((comp1.spf.muT*comp1.ht.Cp)/comp1.nitf1.Prt)
Failed to evaluate variable.
- Variable: comp1.ht.k_effxx
- Defined as: (comp1.ht.kxx+comp1.nitf1.kappaT)
Failed to evaluate variable.
- Variable: comp1.ht.res_T
- Defined as: ((((comp1.Txx*(-comp1.ht.k_effxx))-(comp1.ht.k_effyy*comp1.Tyy))-(comp1.Tzz*comp1.ht.k_effzz))+((comp1.ht.Cp*comp1.ht.rho)*((comp1.Tz*comp1.ht.uz)+((comp1.ht.uy*comp1.Ty)+(comp1.Tx*comp1.ht.ux)))))
Failed to evaluate temporary symbolic derivative variable.
- Variable: comp1.nitf1.streamlinenitf@VDN${test@12}
- Defined as: (((((-nojac(((comp1.spf.betaT*comp1.nitf1.rho)*comp1.w)))*comp1.nitf1.tau_c)*comp1.spf.res_p)-((comp1.nitf1.tau_m*nojac(comp1.nitf1.rho))*comp1.spf.res_w))-((nojac(((comp1.ht.alphap*(-comp1.nitf1.rho))*comp1.w))*comp1.ht.tau_e)*comp1.ht.res_T))
Failed to evaluate expression.
- Expression: d((comp1.nitf1.streamlinenitf)*(dvol),{test@12})


but if I plot nitf1.kappaT (thermal conductivity) or nitf1.Prt (turbulent Prandtl number), I can see that there is no zero value for this variables in the computational field.

Will really appriciate your help!

Best regards from Germany
Antoni