unsteady scalar diffusion stopping before steady state
Posted: Fri Jun 15, 2018 11:32 pm
Hello, I was making some test because I want to perform some calculations of mixing in small channels and I found the following result:
Imagine a unidimensional diffusion problem say a structured mesh on x direction of 10 mm, with 1000 elements. On one end I set a dirichlet condition for my salar, and at the other end I set homogeneus Neuman. Velocity field is frozen at 0.
I set the diffusion coefficient at 5e(-10)*rho (with rho=998 kg/m3).
If I set the boundary condition with a scalar value of 1 (a concetration of 1/kg) the calculation stops way before the concentration spreads along x.
If I set the boundary condition with a scalar value of 1e3 (a concetration of 1e3/kg) the calculation reaches steady state OK.
If I set the boundary condition with a scalar value of 1e6 (a concetration of 1e6/kg) the calculation reaches steady state OK.
If I plot the concentration profile divided by the scalar value imposed (1e3 and 1e6 respectively, both profiles are superimposed at all time steps. The profile for a scalar value of 1, is also superimposed with the other two but only for the first time steps and then it frozes while the other two continue diffusing properly.
I guess there is probably a code variable that tells the code that steady state has been reached and it stops calculating but I don't know which is that variable, which is its default value etc.
Could you please help me with this issue?
Thanks
Imagine a unidimensional diffusion problem say a structured mesh on x direction of 10 mm, with 1000 elements. On one end I set a dirichlet condition for my salar, and at the other end I set homogeneus Neuman. Velocity field is frozen at 0.
I set the diffusion coefficient at 5e(-10)*rho (with rho=998 kg/m3).
If I set the boundary condition with a scalar value of 1 (a concetration of 1/kg) the calculation stops way before the concentration spreads along x.
If I set the boundary condition with a scalar value of 1e3 (a concetration of 1e3/kg) the calculation reaches steady state OK.
If I set the boundary condition with a scalar value of 1e6 (a concetration of 1e6/kg) the calculation reaches steady state OK.
If I plot the concentration profile divided by the scalar value imposed (1e3 and 1e6 respectively, both profiles are superimposed at all time steps. The profile for a scalar value of 1, is also superimposed with the other two but only for the first time steps and then it frozes while the other two continue diffusing properly.
I guess there is probably a code variable that tells the code that steady state has been reached and it stops calculating but I don't know which is that variable, which is its default value etc.
Could you please help me with this issue?
Thanks