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May 10, 2018, 16:52 |
Courant number
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#1 |
Member
Omid Shekari
Join Date: Jun 2016
Posts: 43
Rep Power: 9 |
Do I get fake convergence by lowering current number (just like lowering relaxation factors)??
What is exactly courant number in a simple and apprehensible explaination? |
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May 10, 2018, 20:31 |
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#2 |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,677
Rep Power: 66 |
The Courant number is how far information/disturbances travel in one time-step. When the Courant number is >>1, your disturbance has the potential to skip through the mesh, which leads to many interesting problems (including numerical instability). Having a smaller Courant number actually improves the accuracy of your simulation because you are better able to track the evolution of these disturbances.
URF's are a different monster and better to interpret in a steady-state example. URF's affect only the convergence behavior and do not improve your accuracy. They are artificial/numerical bandaids to prevent a simulation from becoming unstable. |
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May 11, 2018, 05:01 |
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#3 |
Member
Omid Shekari
Join Date: Jun 2016
Posts: 43
Rep Power: 9 |
My simulation is also steady state,I’m just using “coupled” scheme and courant appears in “solution control”.
So how could it be interpreted in a steady state case? |
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May 11, 2018, 16:07 |
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#4 |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,677
Rep Power: 66 |
The Courant number in the COUPLED P-V scheme is actually a URF but has the interpretation of a Courant number multiplier. Check out this post #14 which has some equations to convert from this Courant number to URF's. A Courant number of Infinity corresponds to a URF of 1. Depending on the version of Fluent you use, the default Courant number is 200 or 100 which are close to a URF of 0.99. In this steady state example, lowering the Courant number does get you fake convergence just as lowering a URF would.
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May 13, 2018, 14:48 |
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#5 |
Member
Omid Shekari
Join Date: Jun 2016
Posts: 43
Rep Power: 9 |
Thanks so much
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