pdp.aero |
April 14, 2014 04:04 |
Quote:
Originally Posted by Emily1412
(Post 485895)
Thank you for your reply. But I still can't understand that if MGLEVEL=3 is faster than 0,1 and 2, what is the meaning of "not necessarily the fastest"?
And what's the result of unstability?
Hope for your reply. Thank you!
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Multigrid is one of the popular convergence acceleration method. Let me explain it in this way. Typically, when you are iterating a computational fluid dynamics based problem, you need to throw off errors from your simulation to reach the solution. Imagine a house full of trash, if you are transferring trash faster to the outside, you will find the solution faster or you will converge faster.
On the other hand, high frequency error will have been dumping faster in fine grid, and low frequency error will have been dumping faster in coarse grid. In Mutigrid you are displacing the solution between fine and coarse grid to make it faster to converge. You may use W cycle that is more accurate but it will take longer or V cycle that is less accurate but it will work faster for displacing the solution between fine and coarse grid. In the algebraic method you may using Gauss-Seidlel or ILU or other way to merge cells and coarse your initial fine grid because you need to level your grid into fine and coarse.
MGLEVEL=3 means that you have three grid levels from fine to coarse for displacing the solution and accelerating the convergence. If you are displacing the solution between too much level you are just wasting your time because the solution will not change any more. Besides, you may confront instability and hinder your solution from the convergence.
CFL_REDUCTION changes the CFL number for the coarse level in order for providing stability.
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