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April 26, 2020, 16:18 |
Setting Time step for Implicit unsteady
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#1 |
New Member
Join Date: Apr 2020
Posts: 11
Rep Power: 6 |
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. |
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April 27, 2020, 02:19 |
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#2 |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,674
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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. |
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April 27, 2020, 02:35 |
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#3 | |
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Quote:
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? |
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April 27, 2020, 09:46 |
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#4 | |
Senior Member
Lucky
Join Date: Apr 2011
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Quote:
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. |
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April 27, 2020, 14:59 |
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#5 | |
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June 13, 2020, 02:43 |
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#6 | |
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