CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

how implicit is implicit?

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 13, 2004, 04:55
Default how implicit is implicit?
  #1
Joe cool
Guest
 
Posts: n/a
Hi, just a general question:

Suppose I have a time-dependent partial differential equation and would like to discretise (temporally) it into an implicit form. Does that mean that the time step size that I choose will not affected by the grid size that I choose to discretise my spatial derivatives?

Or will I also have to consider the method that I choose to solve my implicit form? Suppose, instead of solving simultaneously all the unknowns under one large matrix, I choose an iterative method, like a multiple predictor-corrector, will my time step size be more restricted then(as like in an explicit form of temporal discretisation)?
  Reply With Quote

Old   July 13, 2004, 05:03
Default Re: how implicit is implicit?
  #2
Junseok Kim
Guest
 
Posts: n/a
Even if your disretization is implicit, if the solver is iterative, then you have time step size restriction. You can estimate that with local mode analysis.

Junseok
  Reply With Quote

Old   July 13, 2004, 20:05
Default Re: how implicit is implicit?
  #3
Cool Joe.
Guest
 
Posts: n/a
How can this 'local mode analysis' be done?
  Reply With Quote

Old   July 13, 2004, 20:15
Default Re: how implicit is implicit?
  #4
Junseok Kim
Guest
 
Posts: n/a
You can check most numerical analysis text books.

Junseok
  Reply With Quote

Old   July 14, 2004, 13:18
Default Re: how implicit is implicit?
  #5
amol palekar
Guest
 
Posts: n/a
Hi, I guess the highest speed of local disturbance is critical. I think the delta_x/delta_t of the computational domain must be smaller than the physical speed of disturbances. So for the same grid size, time step has to be smaller for a shock wave of say Mach 3 then the Mach 1.5 wave. You can verify this with any one dimensional moving shock problem. -amol
  Reply With Quote

Old   July 15, 2004, 21:54
Default Re: how implicit is implicit?
  #6
Joe cool
Guest
 
Posts: n/a
Ok, great, thanks all!

I'm trying to sort this out.. so I should say that even though the scheme is implicit, it really has to depend on how the scheme is solved. But I suppose the time size restriction has to be looser than that of an explicit scheme, right?

Can anyone verify with me if a Crank Nicolson 2nd order time discretization is a semi-implicit, semi-explicit scheme?

And, amol, I'm doing an incompressible flow of low to medium Re. So for this local speed, I should be using the maximum |(U,V)| within the domain, right? I suppose this is basically the CFL criterion.
  Reply With Quote

Old   July 16, 2004, 02:02
Default Re: how implicit is implicit?
  #7
Junseok Kim
Guest
 
Posts: n/a
Usually we say Crank Nichoson is semi-implicit.

  Reply With Quote

Old   July 16, 2004, 12:10
Default Re: how implicit is implicit?
  #8
amol
Guest
 
Posts: n/a
yeah i think you are right Joe. -amol
  Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help. implicit discritization dearboy Main CFD Forum 0 November 25, 2010 22:46
Can anyone explain the difference between these implicit concepts? bearcat Main CFD Forum 0 February 20, 2010 19:39
ADI implicit doesn't converge George Papadakis Main CFD Forum 2 July 8, 2009 14:27
implicit vs explicit pXYZ Main CFD Forum 2 April 21, 2006 10:48
UDF : Chemical Reaction in Catalytic Converter adhimac FLUENT 1 March 27, 2001 15:04


All times are GMT -4. The time now is 22:31.