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March 20th, 2011, 03:31 AM  #1 
Newbie Joined: Mar 2011 Posts: 6 Thanks: 0  predictorcorrector scheme
Hi! I'm asking for help to understand the realization of the iterative predictorcorrector scheme (I'm not a specialist in this field) for PDE system . The description of a method attached in article. The question concerns algorithm realization under the formula (A5), resulted in article (A5) where  function in the right part of the equations, and  variables. , as I have understood, these are predicted value of all variables from, for example, Euler's method. Then it is necessary to calculate variables and to compare them with predicted, repeating algorithm before achievement of necessary accuracy. But how to calculate if in the right part there are the same , for me not clearly? In any way I don't understand the dependence $ of two variables means? How to use this formula in calculations? I will be glad to any concrete algorithms or the references to it. Wait for your suggestions. 
March 20th, 2011, 08:53 PM  #2 
Member Joined: Nov 2007 Posts: 73 Thanks: 0  Re: predictorcorrector scheme
Without more references (I can't see the attachment), I guess the corrector method is implicit. Roughly speaking, you have both sides. You can solve the equation for using any method for zeros, like Newton's method. Be happy! 
March 21st, 2011, 07:49 AM  #3 
Newbie Joined: Mar 2011 Posts: 6 Thanks: 0  Re: predictorcorrector scheme
Thank you for answer I have partial differential equations system so the using of zero methods is more complex, some help I have found in the article where the iterative PC method was used. Sorry for attachment, here it is http://zalil.ru/30711340 
March 21st, 2011, 07:59 AM  #4 
Member Joined: Nov 2007 Posts: 73 Thanks: 0  Re: predictorcorrector scheme
Yes, I know it is a PDE system. But when you have the discretized equation, this one on the form u^{new}... is not longer a PDE. That's the point in almost all numerical methods. Think for instance in an implicit RungeKutta method for ODE's.

March 21st, 2011, 08:27 AM  #5 
Newbie Joined: Mar 2011 Posts: 6 Thanks: 0  Re: predictorcorrector scheme
The discretization of ODE and using predictor corrector method is more simple. For me not clear how I must use corrector cycle: for every equation separately or for all system in one cycle. For example the second equation discretization will be does it right to use such scheme where is calculate from other equation on previous step?

March 22nd, 2011, 04:36 AM  #6 
Member Joined: Nov 2007 Posts: 73 Thanks: 0  Re: predictorcorrector scheme
No, I think your suggestion is not correct. If you use the U^{predict} instead of you're not going to obtain a correction as n increases. You have a system for the correction, and this system involves all components of the unknown. Let's see. Your unknown is . Using the predictor you obtain . Let's take . The correction procedure is an iteration in the form: More precisely: The point is in the right hand side of every equation you have ALL the unknowns. This forces to solve ALL the system at the same time. One method to compute U^{n+1} is to write: This is a problem about find a zero of a function . You can use the Newton method (or any method for zeros). Be happy! 
March 22nd, 2011, 08:44 AM  #7 
Newbie Joined: Mar 2011 Posts: 6 Thanks: 0  Re: predictorcorrector scheme
Thank you for developed answer Please explain what you mean when use . For example for equation (A4c) . What the dependence on two parametrs means? 
March 22nd, 2011, 01:52 PM  #8 
Member Joined: Nov 2007 Posts: 73 Thanks: 0  Re: predictorcorrector scheme
I am not sure about what you meant. The F dependence on two parameters just meant (in this case) that F depends on the n and the n+1 approximated solutions. 
March 22nd, 2011, 08:37 PM  #9 
Newbie Joined: Mar 2011 Posts: 6 Thanks: 0  Re: predictorcorrector scheme
I don't understand how the expression for will look exactly (n  time step or number of iteration in timespatial step ? ). If and so if we use corrector scheme does it right to write and from where we get on the first corrector iteration for example ? 
March 23rd, 2011, 05:07 AM  #10 
Member Joined: Nov 2007 Posts: 73 Thanks: 0  Re: predictorcorrector scheme
That's the point. The method it's not explicit. It's an implicit method. That means you need to solve an equation (normal equation, not differential one) to compute every approximation U^{n+1} using U^n. Check this wikipedia article: http://en.wikipedia.org/wiki/Explicit_a ... it_methods I think it can helps you. Note in the wikipedia example the implicit formula is easy to solve, but in your case probably you need to solve it numerically too (Newton method or any other method for zeros). 

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