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plunge11 March 17, 2011 08:21

Unstable behaviour after long period of stablility
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I am trying to model a turbulent aerated jet entering normal to the surface of a tank of water, based on an experiment which was carried out. In the experiment, the jet is aerated by means of air bubbles flowing into the water through holes with a diameter of 0.5 mm, prior to the jet's entry into the tank.

I have modelled this jet using multiphase flow, using the "mixture" option since I assumed that the flow should be identified as "bubbly" flow. I have used "second order upwind" spatial discretization where the option was available, "QUICK" for the volume fraction, and “least squares cell based” for the pressure. For a flow in which the volume air-fraction is 0.27, the solution converges in just over 17,000 iterations, with the under-relaxation factors remaining as default. However, for a volume air-fraction of a higher value of 0.45, the calculation seemingly refuses to converge, continuing to display highly fluctuating behaviour even after many thousands of calculations - the 0.44 model was run for over 48,000 calculations without showing any sign of convergence. Because of a persistent message about a turbulent viscosity limit and some divergent behaviour in epsilon, k, and x-momentum, I reduced the under-relaxation factors from their defaults in the early stages of the calculation for 0.44. However, although this allowed the calculation to run without these messages, it did not move toward convergence.
In the case of an air content of 0.36, I first ran the calculation with the equations for slip velocity and volume fraction turned off and using the default under-relaxation factors. This ran ok so, at 400 iterations, I turned these two equations back on and reduced the under-relaxation factors to the following: pressure (0.2), density (0.5), body forces (0.5), momentum (0.4), slip velocity (0.1), volume fraction (0.3), turbulent kinetic energy (0.4), turbulent dissipation rate (0.4), turbulent viscosity (0.5). In this case, the calculation ran fine and appeared to be on its way to converging until around 5150 iterations at which point it began to display the same kind of fluctuating behaviour as the calculation for a jet air content of 0.44 displayed. I therefore now have no idea what to do to get the solution to converge.
I have attached files showing the vector results for the iteration behaviour and contours of a jet with air-fraction 0.44 at around 20,000 iterations, and the results for a jet air-fraction of 0.27.
My questions are:
1.Have I used the wrong multiphase model?
2.Is there a direct link between the changes in the under-relaxation factors and the calculation behaviour? Is there any effect on the final solution if under-relaxation factors are changed?
3.Is there another method I should use to prevent unstable and fluctuating behaviour and other than changing the under-relaxation factors?

I would really appreciate any help that anyone could give - I'm completely at a loss as to what I should do next :)

pz1337 April 6, 2011 09:15

Interested, did you find a solution, especially regarding " turbulent viscosity limit" ?


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