Transient residual plot in Multiphase flow
 

Dear all,

i have some problems to interpret my residual plot for a laminar transient simulation with the VOF Model. I have read that the initalization for multiphase flows has a big influence for the solution. Is this right? Can somebody give me some advice how to interpret right the transient residual plot?
Thanks a lot!

Greetings,
cucko

Hi Benjamin,
I think you have converged solutions for each time step: each residual goes up at the beginning of each time step, then rapidly decreases to convergence you set, or it becomes "flat" and oscillates a bit.

Continuity residual is near 10^-3: you should verify if your solution is converged by monitoring some other variables (and by checking if mass balance is ok): if they don't change at the end of each time step you should have a converged solution.

Daniele

Hey Daniele,

thanks for your quick reply. I know that this "up and down" are iterations for each time step till a abort criterion. But i m doubtful because if u set the timestep size smaller your solution will congerve at first view. So the time-step size is a strong criterion. My Question is, how can i be sure that my simulation is physically correct? Set some monitoring points and view if they are changing for each time step? If they arent changing i should have a steady state problem or? It is hard for me to imagine congerving solution for transient cases.
Thank you!

Greetings,

Ben

Trust the results is up to you: however you can judge convergence by monitoring some variables (velocity, pressure, etc.) somewhere in your domain.
Solution is converged if those variables don't change at the end of each time step, before iterating to the next one: this can be a criterion to judge the convergence in the timestep.
Obviously if the values of these variables don't change from one timestep to the next you have a steady state solution.
Correct timestep size should be selected to make the solution to converge in 20-30 iterations, in each timestep.

Daniele