# under relaxation & Periodic Flows

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 April 17, 2010, 04:24 under relaxation & Periodic Flows #1 Member   Ardalan Join Date: Feb 2010 Location: Baku Posts: 50 Rep Power: 9 -What is the difference between under-relaxation and false time step? What are the advantages and disadvantages of each one? -For problems which have a steady state we stop calculations when time derivative is zero. What about periodic flows for example flow past a circular cylinder?

 April 17, 2010, 06:55 #2 New Member   Tanmoy Join Date: Apr 2010 Posts: 3 Rep Power: 9 under relaxaation scheme is a power scheme during Iteration.I have used it in FDM ,but widely used in FVM.good for retarded convergence.yet stable.When periodically motions are there no decay solutions with respect to time will observed.dont know about false time step..know about false diffusion!

 April 17, 2010, 11:55 #3 Senior Member     p ding Join Date: Mar 2009 Posts: 315 Rep Power: 11 Dear friends: For steady problem, when we use the unde-realxation, you can find that the discretized equation is equivalent to the unsteary problem.

 April 17, 2010, 13:13 #4 Member   ganesh Join Date: Mar 2009 Posts: 40 Rep Power: 10 Dear Ardalan, Under relaxation is generally employed when the code shows a tendency to diverge and is therefore more a stabilization tool. False time or pseudo time stepping actually is a convergence acceleration procedure, where ideas from steady--state solvers can be employed for a genuinely unsteady problem. It is possible that solvers might use both these concepts. For periodic problems, as the name suggests we stop the code when a periodic state is acheieved, in other words, the mean of integrated coefficients like Cl and Cd are invariant with time. Regards, Ganesh

 April 18, 2010, 00:43 #5 Member   L.Y LIN Join Date: Mar 2010 Posts: 47 Rep Power: 9 above masters,where could i find some books about these tow concepts?i really want know them,thanks!

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