August 9, 2022 at 2:40 pmMontaSubscriber
I am simulating the two phase liquid-gas (bubbly) flow in a double pipe HE. The 2Phase flow streams inside the inner pipe and is cooled by water. I activated the multiphase mode and defined my phases. In the inlet of the pipe is a mass flow rate for each phase and volume fraction for gas phase (oxygen) defined. In addition a bubble diameter of 0.2 mm is given. The gas bubbles are considered dispersed in the continuous liquid phase. A calculation using a mixture model was carried out. It converged. In the next step I wanted to move to the eulerian-eulerian model. This time the simulation is diverging. Modeling the interphase drag force with tomiyama model did not converge. So I tried another model (Schiller-Naumann) but the convergence is only achieved at the first time step (which is 2E-4s). Before starting any iteration at the second time step a floating point exception error occures. I checked the ansys manual and found that it is recommended to start with mixture model and then switch to eulerian. This did not solve the issue.
I would appreciate any suggestions or recommendations on this issue.
August 9, 2022 at 6:33 pmPrashanthAnsys Employee
Can you check if the same case works if you initialize and run using Eulerian from the start (using generic models like Schiller-Naumann etc). Also See if reducing time step helps. If it helps, try turning on the custom models (for your case as recommended) one by one and see how it behaves.
August 10, 2022 at 5:57 amMontaSubscriber
Thanks for replying! I tried already initializing and running using Eulerian from the start and did not help that’s why I tried starting with mixture model. Time step was 2E-3s and was reduced to 2E-4s.
August 10, 2022 at 7:44 amPrashanthAnsys Employee
Try switching off surface tension and other taxing models to see if it improves.
August 10, 2022 at 7:55 amPrashanthAnsys Employee
Also check if the phase and turb BCs are set properly ? cross check using contours after initializing. Also, how is the mesh quality ? is the size small enough ? I see that the gas phase diameter is 0.2 mm.
August 12, 2022 at 6:27 amMontaSubscriber
I set the time step to 10E-6 s, which is very very small. It delivers now a CFL < 250. Convergence lasts for about 200 time steps and then diverges :/ Is there any tipps regarding meshing (1mm element size) and bubble diameter. Another thing, is it actually common to run eulerian-eulerian with small time steps otherwise it diverges?
August 12, 2022 at 7:08 amPrashanthAnsys Employee
Can you show something to look at. An image of the mesh and the residual behaviour just before divergence would be nice.
August 12, 2022 at 7:14 am
August 12, 2022 at 9:10 amPrashanthAnsys Employee
Are you using Eulerian multifluid VOF model with explicit scheme ?
August 12, 2022 at 10:40 amMontaSubscriber
No Multifluid VOF just explicit sheme.
August 12, 2022 at 7:00 pmPrashanthAnsys Employee
I was wondering how the convergence behaviour was so good when you ran with mixture model. Did you consider slip velocity when you ran with mixture model ?
August 18, 2022 at 11:45 amMontaSubscriber
slip velocity was considered right!
August 18, 2022 at 3:35 pmPrashanthAnsys Employee
I see the the two phase flow is flowing at the core. You can try one case without inflation layers at the core and check if that helps with the sudden rise in the courant number. Basically simplify the mesh and model so that you can isolate the problem.
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