Learning Forum

[snr1]

I am running a transient simulation of gas absorption in liquid with a mixture model. Initially, I ran case without invoking the species mass transfer, where the has run without any error. Then, I activated the species mass transfer, then immediately the case is diverging. how to resolve this and what are the reasons for this?.The error is shown here:nReversed flow on 4240 faces of pressure-outlet 5.Stabilizing pressure coupled to enhance linear solver robustness.Stabilizing pressure coupled using GMRES to enhance linear solver robustness.Experiencing convergence difficulties - temporarily relaxing and trying again...nnExperiencing convergence difficulties - temporarily relaxing and trying again...nnExperiencing convergence difficulties - temporarily relaxing and trying again...nnExperiencing convergence difficulties - temporarily relaxing and trying again...nStabilizing pressure coupled to enhance linear solver robustness.nStabilizing pressure coupled using GMRES to enhance linear solver robustness.nnExperiencing convergence difficulties - temporarily relaxing and trying again...nnDivergence detected in AMG solver: pressure coupled Stabilizing liq-species-0 to enhance linear solver robustness.nStabilizing liq-species-0 using GMRES to enhance linear solver robustness.nnDivergence detected in AMG solver: liq-species-0 Stabilizing gas-species-0 to enhance linear solver robustness.nStabilizing gas-species-0 using GMRES to enhance linear solver robustness.nnDivergence detected in AMG solver: gas-species-0 Stabilizing temperature to enhance linear solver robustness.nStabilizing temperature using GMRES to enhance linear solver robustness.nnDivergence detected in AMG solver: temperature Stabilizing vof-1 to enhance linear solver robustness.nStabilizing vof-1 using GMRES to enhance linear solver robustness.nnDivergence detected in AMG solver: vof-1nDivergence detected in AMG solver: pressure couplednDivergence detected in AMG solver: liq-species-0nDivergence detected in AMG solver: gas-species-0nDivergence detected in AMG solver: temperaturenDivergence detected in AMG solver: vof-1nDivergence detected in AMG solver: pressure couplednDivergence detected in AMG solver: liq-species-0nDivergence detected in AMG solver: gas-species-0nDivergence detected in AMG solver: temperaturenDivergence detected in AMG solver: vof-1nDivergence detected in AMG solver: pressure couplednDivergence detected in AMG solver: liq-species-0nDivergence detected in AMG solver: gas-species-0nDivergence detected in AMG solver: temperaturenDivergence detected in AMG solver: vof-1nError at host: floating point exceptionnError at Node 0: floating point exceptionnError at Node 1: floating point exceptionnError at Node 2: floating point exceptionnError at Node 3: floating point exceptioError: floating point exceptionnError Object: #f

[YasserSelima]

decrease the relaxation factors and decrease the time step by an order of magnitude. Also do two or three time steps with large number of iterations. 200 or so.n

[snr1]

Here are the relaxation factors. Which factor do I need to decrease?nI decreased the momentum and pressure values to 0.5nnThe time step I am using currently is 1e-4 which was calculated based on CFL number 0.1.nI changed the time step size up to 10e-8, but it's still diverging the same as previous.nnThe number of iterations for a time step is considered as 20.n

[YasserSelima]

I would try decreasing all of them. nfor the pressure outlet, can you make it opening?n

[snr1]

nI decreased all of them but resulted in the samennfor the pressure outlet, can you make it opening?........ I didn't understand this? can you explain it clearly?n

[YasserSelima]

if this is open to the atmosphere, you can change the boundary condition to opening instead of outlet pressure. This could solve some of the back flow issue. But it won't help converging. n

[YasserSelima]

if you are using implicit time step, make it explicit. This should help reaching preliminary solution.n

[snr1]

It is not open to the atmosphere in reality.nnI tried with implicit form of mixture model, but there is no change in outputn

[Rob]

Drop the courant number to around 25-50, 200 is way too optimistic for anything other than external airflow calculations. Then look at the mass transfer mechanism and rates that you're using. The back flow means you're pulling material in from outside (may be OK if it doesn't alter the result) but this could be a symptom of the problem. The rest of the messages mean the solver is struggling. n