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   April 21, 2006, 05:59
Default implicit vs explicit
  #1
pXYZ
Guest
 
Posts: n/a
Hi,

I have written a little program to solve an ordinary dfferential equation. The user can decide whether an explicit or implicit integration method is used. Looking on the results, I cannot see any advantage of the implicit scheme.

What do you think about it?

The mathematical bakground was:

IMPLICIT: f(t+Dt) = f(t) + Dt* df/dt|_(t+Dt)

EXPLICIT: f(t+Dt) = f(t) + Dt* df/dt|_(t)
  Reply With Quote

Old   April 21, 2006, 07:15
Default Re: implicit vs explicit
  #2
Tom
Guest
 
Posts: n/a
Try using f(t+Dt) = f(t) + 0.5*Dt*( df/dt|_(t+Dt) + df/dt|_(t) )

and vary the timestep comparing the results.

For an ode you'd should really look at explicit Runge-Kutta methods rather than your forward Euler step.
  Reply With Quote

Old   April 21, 2006, 09:48
Default Re: implicit vs explicit
  #3
Steve
Guest
 
Posts: n/a
One way of looking at it is that if f(t) is mass say and therefore df/dt is mass flow, what happens if Dt*df/dt(t) is negative and higher in magnitude than f(t)?

In the explicit scheme you'll get negative mass (bad!). In the implicit scheme the iterated estimate of df/dt(t+Dt) will prevent the negative mass.

Explicit schemes assume that the grients at (t) are constant through the step. Implicit ones don't. Explicit schemes require small steps because of this.
  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
From Explicit to Implicit formulation emanuele FLUENT 2 October 4, 2006 14:17
Euler (explicit or implicit) anybody Main CFD Forum 2 May 8, 2006 02:12


All times are GMT -4. The time now is 02:29.