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Christophe August 11, 2006 11:13

Unsteady solution
Hi everybody,

I am currently doing an internship and i have a very simple problem to solve. So easy that i am not able to solve it alone...

I ll try to discribe as best i can my 3d pb: It is a disc (hollow cylinder) of fluid. I modeled only 4degrees to solve faster. The top and the bottom are 2 walls. The left and right are 2 periodics walls. The small cylinder is the inlet flow and the big cylinder is the outlet. The inlet is defined by the flow rate and a direction that i change.

I tried different angles for my inlet flow and i have different shape of flow. For angles the solution do not converge and i think that there is unsteadiness. I tried to solve with unsteady model but i have no idea to how parameter the iteration. I tried lot of different way.

I find one result where the residuals move periodicaly, and the shape of velocity seems to be correct to me. But when i keep the same parameters exept the nb of iteration per time step (from 60 to 350) the solution changes. Is this signify that my first solution was wrong? What the residuals are doing when the solution is converged for an unsteady flow? Is there a real solution because of the flow is always changing? how to get it?

The only thing i understand is that time step is very important because you can miss unstable mouvement if its too big or too small. But what is the effect of number of iteration per time step?

If my explaination is not clear, ask me, i ll do my best, as french as i am, to explain it better.

Thanks for any help.


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