CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > Siemens > STAR-CCM+

Setting Time step for Implicit unsteady

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 26, 2020, 16:18
Default Setting Time step for Implicit unsteady
  #1
New Member
 
Join Date: Apr 2020
Posts: 10
Rep Power: 2
ranjithdj is on a distinguished road
Hi all,

I want to simulate flow over a bump using STARCCM+ for unsteady flow. In paper its given that time step is 10000. Since I am new to starccm. I don't know how to set it.

- In solver, under implicit unsteady it given that that default time step 0.001s what does it mean ?

-Stopping criteria: Maximum physical time given has 5s and Maximum inner iteration has 5.

I want convergence continuity, momentum, energy and SA as below 1e-5. How to incorporate 10000 time step or more to get convergence below 1e-5.

If I run for default functions convergence does not go below 1e-3.
ranjithdj is offline   Reply With Quote

Old   April 27, 2020, 02:19
Default
  #2
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 4,017
Rep Power: 48
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
If it's not clear what a time-step size of 0.001 s is in an unsteady simulation, then you need serious help.

By default there are no stopping criteria for residuals. And there shouldn't be, because they are not a measure of convergence. But if you still want them, you have to create stopping criteria for residuals. Right click on stopping criteria and create them from the residual monitors.

You will probably need to increase the maximum number of inner iterations to a larger number (999999 will work) to ensure the residual criteria is reached.

Set the maximum physical time based on the time-step size and number of time-steps you want to run.
LuckyTran is offline   Reply With Quote

Old   April 27, 2020, 02:35
Default
  #3
New Member
 
Join Date: Apr 2020
Posts: 10
Rep Power: 2
ranjithdj is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
If it's not clear what a time-step size of 0.001 s is in an unsteady simulation, then you need serious help.

By default there are no stopping criteria for residuals. And there shouldn't be, because they are not a measure of convergence. But if you still want them, you have to create stopping criteria for residuals. Right click on stopping criteria and create them from the residual monitors.

You will probably need to increase the maximum number of inner iterations to a larger number (999999 will work) to ensure the residual criteria is reached.

Set the maximum physical time based on the time-step size and number of time-steps you want to run.
time step is delta"t" which we discritize based on time marching in Control volume because of unsteadiness. My question is why it is default set 0.001s. If I keep time step as 1s and physical maximum physical time to 10000. It is 10000 time step. So what's the difference between keeping 0.001s and 1s?

What is the purpose of inner iteration? If I keep inner iteration 1 and simulate for 10000 time step and equations convergers to required convergence criteria (ex:1e-5).

Is that during each step inner iteration should converge to required convergence criteria?
ranjithdj is offline   Reply With Quote

Old   April 27, 2020, 09:46
Default
  #4
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 4,017
Rep Power: 48
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by ranjithdj View Post
What is the purpose of inner iteration? If I keep inner iteration 1 and simulate for 10000 time step and equations convergers to required convergence criteria (ex:1e-5).
Who even does this? You don't check your residuals only at the last time-step.... You must check them every time-step. Are you confusing unsteady simulations with a steady simulation?

The difference between 0.001s and 1.0 s is 0.999 seconds. I don't know what to say beyond that. The default number has to be something and in Star it is 0.001s. You can use whatever deltaT is appropriate for your problem. If you know what temporal discretization is then you should have an idea of the implications of running a case bigger/smaller deltaT.

We use iterative solvers here, for a variety of reasons. For each time-step of 0.001s, you have to solve many iterations before your calculation is converged for that time-step. If you set the inner iterations to 1, you will not only not converge, it is also unlikely you will ever reach your convergence criteria. Yes you need to converge every time-step.
LuckyTran is offline   Reply With Quote

Old   April 27, 2020, 14:59
Default
  #5
New Member
 
Join Date: Apr 2020
Posts: 10
Rep Power: 2
ranjithdj is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
Who even does this? You don't check your residuals only at the last time-step.... You must check them every time-step. Are you confusing unsteady simulations with a steady simulation?

The difference between 0.001s and 1.0 s is 0.999 seconds. I don't know what to say beyond that. The default number has to be something and in Star it is 0.001s. You can use whatever deltaT is appropriate for your problem. If you know what temporal discretization is then you should have an idea of the implications of running a case bigger/smaller deltaT.

We use iterative solvers here, for a variety of reasons. For each time-step of 0.001s, you have to solve many iterations before your calculation is converged for that time-step. If you set the inner iterations to 1, you will not only not converge, it is also unlikely you will ever reach your convergence criteria. Yes you need to converge every time-step.
Yes I have confusion between steady and unsteady. I used to work with steady condition. This time I am working in unsteady state. In CFD class I discretized the unsteady condition. But in practical, I didn't use it. Im referring some journal and they mentioned only 10000 time step. So I don't know how to take this time step
ranjithdj is offline   Reply With Quote

Old   June 13, 2020, 02:43
Default
  #6
New Member
 
Join Date: Dec 2017
Posts: 7
Rep Power: 4
Kukka is on a distinguished road
Quote:
Originally Posted by ranjithdj View Post
Yes I have confusion between steady and unsteady. I used to work with steady condition. This time I am working in unsteady state. In CFD class I discretized the unsteady condition. But in practical, I didn't use it. Im referring some journal and they mentioned only 10000 time step. So I don't know how to take this time step
The journal article might have meant simulation running for a total of 10000 time steps. Time step size is different. Suppose you select the time step size of 0.0001s, then you are performing calculations for (0.0001 x 10,000 = ) 1.0 s.
Kukka is offline   Reply With Quote

Reply

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
Time Step Continuity Errors simpleFoam Dorian1504 OpenFOAM Running, Solving & CFD 0 January 17, 2018 04:18
High Courant Number @ icoFoam Artex85 OpenFOAM Running, Solving & CFD 11 February 16, 2017 13:40
Micro Scale Pore, icoFoam gooya_kabir OpenFOAM Running, Solving & CFD 2 November 2, 2013 13:58
pisoFoam with k-epsilon turb blows up - Some questions Heroic OpenFOAM Running, Solving & CFD 26 December 17, 2012 03:34
IcoFoam parallel woes msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 02:58


All times are GMT -4. The time now is 02:54.