# Changing Courant Number Changes Results ?

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August 20, 2019, 01:14
Changing Courant Number Changes Results ?
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I am new to Fluent and would appreciate guidance from experts in here.

Bear with me on the detailed description and the efforts sustained on it.

Was trying to simulate “Bladeless fan” in 3D. After trying for few simulations, couldn’t get to the default convergence level. This prompted us to move to 2D axi-symmetric model to check on the flow characteristics of the “Bladeless Aerofoil” fan profile and later use the learnings in 3D simulation. How wrong was the prognosis 😊

Details of the 2D Axi-symmetric model are given below. Crux of the challenge is that whenever Courant Number is changed, the whole results patterns change, as can be seen from the attached pictures on velocities.

Realizable k-e turbulence model, with scalable functions, Coupled solver, nodal interpolations, all equations to be solved in second order, were chosen for quick turnaround.

For the given model, if we change the courant number, to work on the convergence, the flow patterns change dramatically. Not sure as to which results are correct and which are not. And in all the cases in the pictures attached, the solutions are converged.

Changing the inlet velocity does not seem to have effect on influencing the flow patterns.

Can experts in here be kind enough to guide us as to what exactly is happening with the solution and which is the correct solution and why?

How to work on solution convergence when controlling Courant number.

By the way, in all the cases shown below, the courant number was changed at the start of the re-initialized simulation and “not” in between.

Concern also about the behavior of residuals for different Courant numbers?

Is it right / correct?

Awaiting guidance

Mesh size - ~ 80,000 nodes

Quality - Skewness - Max 0.76

Edge Sizing: 1/2000 of Chord Length

Inflation Max Thickness: 1/200 of Chord Length

Leading Face: 25 times chord length

Bottom Face: 25 times chord length

Trailing Face: 35 times chord length
Attached Images
 Courant_Behavior_Velocity_Plot_Inlet_6.PNG (90.5 KB, 25 views) Courant_Behavior__A_Residual_Plot_Inlet_6.PNG (114.2 KB, 21 views)

 August 21, 2019, 00:53 #2 New Member   R P Join Date: Aug 2019 Posts: 5 Rep Power: 3 Good Day Everyone, Any guidance on resolving the issue will go a long way in a better understanding of Bladeless Fan Flow Appreciate your help

 August 26, 2019, 04:51 #3 Senior Member   Join Date: Aug 2014 Location: Germany Posts: 292 Rep Power: 10 Between Co 50 and Co 25 the result changes look pretty drastic, so i would double check everything in terms of boundary conditions and models.. I'm not familiar with external flow simulations, it just my gut feeling that something major changed between the two simulations. Co 100 / Co 50 and Co 25 / Co 10 look like reasonable changes..

 August 26, 2019, 06:10 #4 Member     Join Date: Mar 2017 Location: TR Posts: 41 Rep Power: 6 Results seems not converged, use hybrid or fmg initialization and run all cases again, wait until residuals not change too much. Hope this will help.

August 26, 2019, 11:12
#5
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Quote:
 Originally Posted by BlnPhoenix I'm not familiar with external flow simulations, it just my gut feeling that something major changed between the two simulations. Co 100 / Co 50 and Co 25 / Co 10 look like reasonable changes..
Appreciate your guidance. All conditions remain the same. Just changed the Co number. That's it.

I was scratching my head when I saw the results. Not sure why it is happening. Still haven't been able to pin down the reasoning behind it.

Quote:
 Originally Posted by atelcikti1 Results seems not converged, use hybrid or fmg initialization and run all cases again, wait until residuals not change too much. Hope this will help.
Have used Hybrid initialization for all the case studies. Default residual values taken for study 0.001

Not sure how to take the Simulation on Bladeless fan forward.

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