# Epsilon Convergence Trouble

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 August 26, 2007, 19:04 Epsilon Convergence Trouble #1 Carlos Guest   Posts: n/a Hello, I am simulating airflow over a vehicle using the realizable k-epsilon turbulence model on a grid of ~ 8 million cells. I am using a hybrid grid of hex and tet cells and the simulations are second order. For slow air speeds I get satisfactory convergence, however for faster speeds I have trouble with the residual levels for epsilon. All other residuals are converged ok (below 10^-5) and everything levels off after 6000 iterations. Epsilon does level off too, but there is a medium amplitude oscillation present and the average residual value is around 10^-4. I have tried relaxing the solution which helps generally but epsilon is largely insensitive. I tried lowering the under relaxation factor for epsilon from 0.8 to 0.6 but this has little or no effect. Any ideas or secret recipies for improved convergence with this turbulence model? Many thanks, Carlos.

 August 27, 2007, 01:55 Re: Epsilon Convergence Trouble #2 AAA Guest   Posts: n/a Hi I think you've nailed the problem Carlos. With 8 million cells and such low residuals after 6000 iterations, you are my man. But what velocities are we talking here?

 August 27, 2007, 07:40 Re: Epsilon Convergence Trouble #3 Carlos Guest   Posts: n/a Hi Triple A, In terms of the appearance of the residuals yes I do have a converged solution, however I'm not entirely happy with the residual level for epsilon. I have read many times that the default convergence limit is 10^-3 and this is not enough for 3D problems. As an absolute bare minimum these days I try to get everything below 10^-4. Usually with k-e models everything plays ball except for epsilon. I am using a velocity of approx 22m/s which is about 50Mph. The flow is both internal and external as I am looking at ventilation in livestock trailers. Does anybody know of any other method other than altering the relaxation of the solution to get lower residuals? Is it worth looking at Multigrid methods to try and lower things? It just seems like something which takes a long time to set up. And am I correcting in thinking that Multigrid simply SPEEDS UP convergence and doesn't actually help the convergence levels? Maybe I am just being paranoid here but I am trying to be as rigorous as possible in writing a paper for a good journal. Thanks for your time. All the best, Carlos.

 August 27, 2007, 08:20 Re: Epsilon Convergence Trouble #4 Dusan Guest   Posts: n/a it is truth, it has to be at least half of cells, i think

 August 27, 2007, 11:22 Re: Epsilon Convergence Trouble #5 AAA Guest   Posts: n/a Hi Carlos, I usually have slightly lower residuals when I perform adaption. Try performing adaption on velocity gradients and see if this helps. Regards

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