CFD Online Logo CFD Online URL
Home > Forums > General Forums > Main CFD Forum

Relation between physical and computational time

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

LinkBack Thread Tools Search this Thread Display Modes
Old   August 8, 2005, 08:21
Default Relation between physical and computational time
Posts: n/a
hi, Can anyone comment in any relation between computational time step (e.g., used in Fluent in the form of CFL etc) and real life time step.Can we relate physical and computational time in any sense? In fact, I am simulating a flow through a cylinder with moving piston in it. If i simulate the flow with a time step of 1e-4 with 10000 iterations...Can i say that this will be the state of real life flow in 1 second?

Regards, S Khan
  Reply With Quote

Old   August 8, 2005, 08:54
Default Re: Relation between physical and computational ti
Posts: n/a
One condition that must be met for the computational (or simulated) time to be equivalent to the physical time is that the boundary conditions you apply must match the physical time dependence of the system you're simulating. If you wish to predict the piston motion, you'll need a number of time steps.

The language is important. An iteration is a pass through the equations for one time step and is directed at obtaining a correct solution for that one time step. The process is repeated for as many time steps as needed. Incidently, with this definition, the often-used direct steady state solution can be interpreted as one infinite time step!

I believe that Fluent applies a number of iterations within each time step (10000 seems like a lot). You would first select a time step size so that the physical transient you wish to simulate is well resolved in time. Then you apply the number of iterations required for EACH of those time steps to get convergence for that time step.
  Reply With Quote

Old   August 8, 2005, 09:17
Default Re: Relation between physical and computational ti
Vinod Dhiman
Posts: n/a
Yes Jim is right. Iteration and time step are related the way Jim has explained. There can be as many iterations for a single time step as are required to converge the solution for that time step, and then you increase the time. That's the way numerical simulation works. Computational time step and physical time steps are very same if your initial and boundary conditions are physically correct.

Vinod Dhiman
  Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
HELP! time step too small? meangreen Main CFD Forum 7 June 1, 2018 15:40
time step and vortex shedding frequency kevin Main CFD Forum 16 November 19, 2009 06:00
Help: Set Up model in CFD with time Ranie Go Main CFD Forum 5 October 7, 2003 05:12
Solution in Physical or Computational ??? Mohammad Kermani Main CFD Forum 3 April 2, 2001 02:29
hints on time steps arturo FLUENT 14 October 31, 2000 02:03

All times are GMT -4. The time now is 21:33.