Karman vortex - steady state solution with RANS ?

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 September 27, 2018, 13:46 Karman vortex - steady state solution with RANS ? #1 Member   Join Date: Apr 2016 Posts: 90 Rep Power: 9 Hi, I am wondering if I can get a steady simulation of a karman vortex simulation? So in reality this phenomenon is highly unsteady. But in my CFD I am not interested to see the vortex. I am interested in getting a steady state solution. Is this possible with RANS? If I look at this video: https://www.youtube.com/watch?v=57-URbKeWME I think I will always see some vortex and vortex detachment, even in a RANS? So if I am interested in an steady solution I should calculate maybe with URANS and take the mean of my velocities over time? Thank you for your help Cellzone

September 27, 2018, 15:16
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Filippo Maria Denaro
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Quote:
 Originally Posted by CellZone Hi, I am wondering if I can get a steady simulation of a karman vortex simulation? So in reality this phenomenon is highly unsteady. But in my CFD I am not interested to see the vortex. I am interested in getting a steady state solution. Is this possible with RANS? If I look at this video: https://www.youtube.com/watch?v=57-URbKeWME I think I will always see some vortex and vortex detachment, even in a RANS? So if I am interested in an steady solution I should calculate maybe with URANS and take the mean of my velocities over time? Thank you for your help Cellzone

Yes, the RANS formulation is based on a statistically averaging that makes the solution steady.
In a similar way, you could consider the example of the laminar vortex shedding with alternating vortices. If you perform explicitly the time average of the transient solution, you will get a steady solution with two symmetric vortices.

 September 28, 2018, 01:18 #3 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 743 Rep Power: 14 RANS eliminates (real) turbulence, but ist dies not eliminate vortices at all. You may use for your task. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

September 28, 2018, 03:01
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 Originally Posted by piu58 RANS eliminates (real) turbulence, but ist dies not eliminate vortices at all. You may use for your task.

That depends on the solver - if the steady RANS equations are used or not.

 September 28, 2018, 03:11 #5 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,596 Rep Power: 70 RANS means one solves always the statistically steady averaged variable, URANS introduces a time variation of the mean variable. However that does not mean that the two formulations are similar in the modelling.