# What is the best "way" to get vortex shedding ?

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April 1, 2009, 17:48
#21
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Juho Peltola
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Quote:
 Originally Posted by harly @albcem: Which solver would you use ? - so far I was using icoFoam, because I was under the impression I don't need a turbulence model like for the 2D case Frank Bos presents. That explains also, why I used turbulence model switched off in turbFoam. - But maybe that could be the mistake? -harly
IIRC at least in some version of OpenFOAM it was:

Turbulence off:
Turbulence is not solved but the turbulent viscosity is added to the effective viscosity. Meaning that if there are non-zero k and epsilon fields present the effective viscosity is higher than the laminar viscosity.

"Laminar" turbulence model:
Returns a zero turbulent viscosity -> correct laminar viscosity.

April 2, 2009, 02:26
#22
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Alberto Passalacqua
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Quote:
 Originally Posted by harly - a switch to complete 2nd order aka backward in time and Gauss linear in space
Just a note, as Hrv suggested, to have second order on all meshes, use leastSquares for gradient evaluation. However the use of Gauss should not prevent the vortex shedding.

Quote:
 - preturbation of flow
Imho this is not necessary. As you can see in the case of the cylinder done by Frank Bos, he doesn't perturbate the flow, and the structures start to form immediately.

Quote:
 Which solver would you use ? - so far I was using icoFoam, because I was under the impression I don't need a turbulence model like for the 2D case Frank Bos presents.
Yes, icoFoam should do the job if your Re is similar to those considered by Frank Bos. Btw, what is your Re, defined as U*d/nu, where d is the cylinder/sphere diameter?

Regards,
__________________
Alberto Passalacqua

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To obtain more accurate answers, please specify the version of OpenFOAM you are using.

 April 2, 2009, 13:58 #23 Member   Daniel Harlacher Join Date: Mar 2009 Location: Davis, CA, United States Posts: 60 Rep Power: 14 So, 3D simulation is on its way - once it is finished I will post pictures (pressure, velocity plots) and then we can discuss the matter further. I use the "standard" formulation of the Reynoldsnumber: Re = U*d/nu and I am using air properties for T=20(where the heck is the degree symbol on an American keyboard?) Celcius. In the meantime I will set up a 2D case for a circular and a square cylinder in low Re(around 100 - that vortex shedding occurs) and will then try to verify the results with a reference. I will probably be able to post the first results in a couple of hours plus pictures of the meshs I used. -harly

 July 28, 2009, 06:37 #25 New Member   Bob De Clercq Join Date: Apr 2009 Location: Belgium Posts: 17 Rep Power: 14 Hi Daniel, I encounter the same problems as you with respect to the absence of any vortex shedding behind the sphere. Did you already solve the problem with the hints of your last message? At the moment, I am running a case with a perturbated inital velocity field, but I am pessimistic about the result... Many thanks. Regards, Bob

 July 29, 2009, 08:15 #26 Member   Markus Weinmann Join Date: Mar 2009 Location: Stuttgart, Germany Posts: 77 Rep Power: 14 I had similar problems when running a LES with wall-functions. Using a wall-resolved grid helped to get rid of this problem. Markus

 August 20, 2012, 12:13 #27 Member   Join Date: Jun 2011 Posts: 80 Rep Power: 11 I am in the same point as you with a cube centered in the domain by using RANS models... did you find the solution??? Antonio Last edited by maalan; September 9, 2012 at 17:38.

 April 3, 2014, 03:25 #28 Member   Join Date: Mar 2009 Location: adelaide, SA, Australia Posts: 32 Rep Power: 14 Hi Harly and All, Thanks for the posting, it's a good discussion. I'm also doing about the same simulation, flow over a square, 2d at Re= 22k. I have three grid resolution, course:30cells x 4 edges, medium:50 cells x 4 edges and fine: 80 cells x 4 edges. The course and medium have wall function and the fine without wall function. The results for coarse and medium are about the same for experiments, but for fine grid is not. Any comment why the fine grid gives the 'wrong' results? The y+ for fine grid is less than 3, so I don't use wall function. Thanks, Mali __________________ mali

 September 22, 2015, 07:33 #29 New Member   Paulina Join Date: Oct 2012 Posts: 23 Rep Power: 10 Dear harly, Did you figured out why you sometimes cannot obtain shedding? I'm doing simulation around rectangular prism with Re 1e5 and I can't get oscillations of Cd and Cl and vortex shedding using RANS. The results of LES calculations are ok. Regards, Paulina

 September 23, 2015, 03:53 #30 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 23 Paulina, can you post some of your log output? __________________ The skeleton ran out of shampoo in the shower.

September 23, 2015, 06:07
#31
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Paulina
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I'm a little bit out of topic, my calculations were made in Fluent.

I've tried to recreate simulation from an article (http://www.sciencedirect.com/science...43974X06001593) were they used same grid for LES and RANS, and enhanced wall funcion for Kato Launder k-epsilon.
Using above mentioned configuration my Cd and Cl were completely flat (as if it was laminar flow).

Yesterday I run simulation for standard wall function for a grid with higher y+. I've got oscillations of Cl, but after 2s it went flat.

I still don't know what is missing in the calculations (grid? set up?).
Attached Images
 Cl.jpg (28.5 KB, 14 views)

 September 23, 2015, 06:35 #32 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 23 How long did you let it running after the decay of the initial oscillations? It sometimes took a really long time in Fluent until the actual von Karman street started. __________________ The skeleton ran out of shampoo in the shower.

 September 23, 2015, 10:27 #33 New Member   Paulina Join Date: Oct 2012 Posts: 23 Rep Power: 10 The longest time was about 4 seconds. I assumed that nothing new will happen later. The period of one oscillation (from the article and my LES simulation) is about 0,13s. While using same grid as for LES initial oscillations were quite small and ended even before 1s. Do you think that 10s would be enough for vortex shedding to occur? I'll run simulations few more hours and let you know about the results.

 September 23, 2015, 10:49 #34 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 23 How long is your time step and did you switch all the numerical settings to 2nd order? __________________ The skeleton ran out of shampoo in the shower.

September 23, 2015, 11:20
#35
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Paulina
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Yes, all numerical settings are in 2nd order (And Fluent 15 run in double precision). BC are velocity inlet and outflow.

The time step is 0.002s or 0.001s. I used even 0.0005s but haven't seen difference in the results.
As a convergence criteria I set up epslion equal to 1e-7, so now it is 5 iterations per 1 time step.

You can see settings and how Cd and Cl look after few more seconds of calculations.
Attached Images
 Cl.JPG (23.8 KB, 7 views) Cd.JPG (20.3 KB, 5 views) settings.JPG (37.3 KB, 7 views)

 September 24, 2015, 09:16 #36 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 23 I can not guarantee that, but I once had this behavior and after a long time of waiting the vortex street started. If you have time, give it a try __________________ The skeleton ran out of shampoo in the shower.

 September 25, 2015, 05:54 #37 New Member   Paulina Join Date: Oct 2012 Posts: 23 Rep Power: 10 10 seconds passed and nothing happened. The flow field looks like after steady flow calculations. I think this is a matter of bad lack, or rather bad mesh. I'm out of ideas what to change to get right results.

 September 28, 2015, 04:40 #38 New Member   Paulina Join Date: Oct 2012 Posts: 23 Rep Power: 10 I've tried few more things but the situation is still the same. Initial oscillations are gone after few seconds and nothing else happens. If you would like to try and check here you can find my mesh and inlet profiles: https://www.dropbox.com/sh/5hui9t2vk...toMth-SJa?dl=0

December 15, 2016, 20:50
#39
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subhankar
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Hi all,
Did you solve your problems? I was actually solving flow past acylinder with D/H=0.3 and Re=100. I am getting correct strouhal number but getting very low drag co-efficient. Can you tell me why? I am really stuck at this for relly long time. I am using openFoam 3.0.1 and ubuntu 16.04. i have attached my case.

regards
Subhankar
Attached Files
 check.zip (9.0 KB, 5 views)

 December 15, 2016, 21:31 #40 Member   Join Date: Mar 2009 Location: adelaide, SA, Australia Posts: 32 Rep Power: 14 Subhankar, Have u try higher order scheme? __________________ mali

 Tags laminar, sphere, vortex shedding