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

Implicit vs. Explicit

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   December 14, 1999, 08:41
Default Implicit vs. Explicit
  #1
Praveen C.
Guest
 
Posts: n/a
I would like to know why are impicit schemes in general more stable than explicit schemes.

Thanks, Praveen
  Reply With Quote

Old   December 14, 1999, 10:35
Default Re: Implicit vs. Explicit
  #2
John C. Chien
Guest
 
Posts: n/a
(1). In general, explicit methods compute the next time step solution directly based on the known current time information. It is therefore necessary to keep the time step within the limit of physics in order to obtain accurate sequential solution in time. (2). If the solution is linear in time, and the rate of change in time is linear or constant, then the time step can be made larger and still have accurate solution. (3). But, if the time rate of change is non-linear, that is, it is a curve in solution vs time space, the direct computation of the solution will always have errors in it. This numerical integration error can be magnified or accumulated in the path of time integration. (4). And at some point in time, it can prevent the further time integration because of the poor quality in the integrated solutions. (5). Actually, when the time rate of change is not constant or linear, the explicity model of integration is not accurate. The solution depends on the time continuously, and not just at the current time step. (6). The implicit methods make use of this property, by evaluating the solutions at two or more time space. In this way, one can improve the accuracy of the solution, in principle. There are many examples in the integration of an ordinary differential equation by using multi-steps. (7). Naturally, the difficulty with the implicit methods is in finding the solution itself. The solutions at non-current time space will be unknown. It is more difficult to solve. (8). This improved solution accuracy associated with the implicit methods (methods involved with more than the current time space information into the future), in principle, will be more accurate. And thus stays bounded within limits. A common word is the solution is more stable, it does not accumulate errors and becomes un-bounded, or diverge. (9). In general, if a method make use of the current time space information and the possible future solutions properties in the formulation, then the improved solution accuracy will keep the solution bounded. (thus more stable) (10). The iterative method is similar to the time integration method, if one looks at the iteration number as if it is the time step variable.
  Reply With Quote

Reply

Thread Tools
Display Modes

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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Implicit versus Explicit Deepak Main CFD Forum 16 November 29, 2014 21:54
Implicit or explicit ? Rickard.Solsjo OpenFOAM 0 May 31, 2011 04:02
ddt explicit or implicit with courant number!! mrshb4 OpenFOAM 0 December 13, 2010 12:13
Euler (explicit or implicit) anybody Main CFD Forum 2 May 8, 2006 02:12
implicit vs explicit pXYZ Main CFD Forum 2 April 21, 2006 09:48


All times are GMT -4. The time now is 09:24.