Question about Implicit Unsteady Solver and Maximum Inner Iterations

Awesomo · June 28, 2016, 05:55

Greetings,

I am working on a Simulation of a Injection Nozzle Flow with Cavitation. Right now I've got:

Stagnation Inlet Boundary and an Outlet Pressure Boundary
Eulerian Multiphase (Fuel and Gas, for now both constant density) with VOF-VOF Phase Interaction and Cavitation
Solver: Implicit unsteady Solver

I've a question how i can identify the right time steps and maximum inner Iterations? Right now the residuals are swinging highly.

Another question: Is it possible to simulate a cavitating flow with the steady Solver? Because there i get convergence and the results are reasonable.

Thanks!

taillanm · June 28, 2016, 06:01

The time step can be deduced from your cfl number, reference velocity and minimum cell size i think; as for the inner iterations you should monitor a parameter and plot it as a function of iterations to see if it converges; if you give e.g 20 inner iterations but it converges after 10, decrease it to 10

Awesomo · June 28, 2016, 06:11

If I understand you right, i can get identify the maximum inner iterations by an monitor plot. Is this possible with the residuals? Or can you give me an example how to.
Right now I'm testing it with max inner iterat. = 50 and the residuals aren't converging in the timesteps (see below attachment).

Thanks.

marmot · June 28, 2016, 07:05

Yes you can simulate cavitation assuming the flow is steady,

sometimes you start with a steady solver and if you see vapor produced and remains constant and if the vapor is expanding/collapsing than you know to switch to the unsteady solver, and look for frequency or something else,

Awesomo · June 28, 2016, 08:57

Another question: Is a third phase called "Air" needed in a Cavitation modelling?
Thanks!

June 28, 2016, 05:55	Question about Implicit Unsteady Solver and Maximum Inner Iterations	#1
Awesomo New Member d Join Date: Jun 2016 Posts: 14 Rep Power: 9	Greetings, I am working on a Simulation of a Injection Nozzle Flow with Cavitation. Right now I've got: Stagnation Inlet Boundary and an Outlet Pressure Boundary Eulerian Multiphase (Fuel and Gas, for now both constant density) with VOF-VOF Phase Interaction and Cavitation Solver: Implicit unsteady Solver I've a question how i can identify the right time steps and maximum inner Iterations? Right now the residuals are swinging highly. Another question: Is it possible to simulate a cavitating flow with the steady Solver? Because there i get convergence and the results are reasonable. Thanks!

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June 28, 2016, 06:01		#2
taillanm Member Join Date: Nov 2015 Posts: 57 Rep Power: 10	The time step can be deduced from your cfl number, reference velocity and minimum cell size i think; as for the inner iterations you should monitor a parameter and plot it as a function of iterations to see if it converges; if you give e.g 20 inner iterations but it converges after 10, decrease it to 10

June 28, 2016, 07:05		#4
marmot Senior Member kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 14	Yes you can simulate cavitation assuming the flow is steady, sometimes you start with a steady solver and if you see vapor produced and remains constant and if the vapor is expanding/collapsing than you know to switch to the unsteady solver, and look for frequency or something else,

June 28, 2016, 08:57		#5
Awesomo New Member d Join Date: Jun 2016 Posts: 14 Rep Power: 9	Another question: Is a third phase called "Air" needed in a Cavitation modelling? Thanks!