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December 27, 2016, 01:23 
No Vortex shedding for flow past cylinder

#1 
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Shashank
Join Date: Sep 2014
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I am trying to simulate a 2D case for flow past cylinder. I have taken the diameter of cylinder as 1m. The case has been set up for a Re=80 at which the flow should exhibit vortex shedding. I get perfect vortex shedding results when I use the ViscousLaminar Turbulence model but any other model does not yield in vortex shedding. I have fine mesh near the wall for getting refined results. I do understand that kepsilon and komega models are not able to show the shedding as they do not solve near wall flows accurately. But I think I should get vortex shedding results by using SSTkOmega, but it is not so.


December 27, 2016, 02:13 

#2  
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Quote:
Moreover, eddy viscosity turbulence models model all turbulent scales and can give you only average info. Use laminar model and transient solver to get realistic results. 

December 27, 2016, 03:33 

#3 
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Shashank
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I am not able to understand that at Re=80 how the flow can still be laminar if the vortex shedding has already begun?
Is it that just because the axisymmetric wake is still intact at this Re range and the shedding occurs periodically at regular intervals the flow is still laminar? Sent from my Moto G (4) using CFD Online Forum mobile app 

December 27, 2016, 04:24 

#4  
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Quote:
Vortex shedding may be laminar with formation of laminar vortex street and turbulent with formation of turbulent wake. 

December 27, 2016, 12:02 

#5  
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Lucky
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Quote:
If the flow is laminar use the laminar flow model and not a turbulence model. Unsteady simulations using laminar or turbulence models can capture inviscid vortex shedding. 

December 28, 2016, 02:38 

#6 
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Katherine
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Are you using a mesh that is symmetric about the centerline? I've found this same problem when using a symmetric mesh with a uniform velocity inlet condition. The way that I solved it was to introduce a perturbation in the inlet velocity condition. Using OpenFOAM, this would be a small velocity in +y that would only last for a short time (again, using OpenFOAM this was accomplished with the "ramp" function). As an example, this would be changing the inlet velocity condition from u={u_inf, 0} for a 2D case to u={u_inf, 1} from time=0.5 seconds to time=1 seconds. After introducing this perturbation in the ydirection, you would again change the velocity inlet boundary condition to u={u_inf, 0} (I.e. you would again only have a horizontal component of velocity).
As mentioned previously, you shouldn't need a turbulence model for Re=80 since this is laminar flow. The problem might be that you have a symmetric mesh and you need a perturbation to introduce the vortex street behind your cylinder. Sent from my Nexus 5X using CFD Online Forum mobile app 

December 28, 2016, 04:22 

#7  
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Quang Le Dang
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Quote:
Firstly, please using laminar model Secondly, run transient and wait very long time. The last time I have the same problem. However if I run enough time, they will have fluctuations and vortex Sent from my LGF260S using CFD Online Forum mobile app 

December 28, 2016, 09:35 

#8 
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Shashank
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Thanks @Antanas for pointing the difference between laminar and turbulent vortex shedding w.r.t Re. I ran the simulations again this time for a higher Re (Re=350) using laminarviscous as well as other turbulence models. Even at this range laminarviscous model does show vortex shedding which can be easily differentiated as compared to that which was obtained at Re=80. Also for this range the turbulence models SSTkomega, Scale Adaptive Simulation and the Transition SST Model do predict the turbulent vortex shedding however in case of SSTkomega the values of Drag Coefficient which I obtain are really vague (around 24 whereas at Re=350 it should be around 1.3 which I do obtain using other turbulence models). The other problem is using RSM model I am unable to get the turbulent vortices, RSM not being an eddy viscosity model should be able to predict the shedding but it is not so!!


Tags 
cylinder, turbulence models., vortex street 
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