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-   -   Flow behind a cylinder (http://www.cfd-online.com/Forums/fluent/32413-flow-behind-cylinder.html)

Vasily October 30, 2003 11:00

Flow behind a cylinder
 
Hi, I'm puzzled with the following problem in Fluent: Trying to get wake behind a cylinder, I found strange difference in the flow development when I use different materials. Namely, for the air, everything is fine, but when I switch into water, no wake appears even for extremely large Re. I have read how people tried to introduce different disturbances into the flow and thus to destabilize the solver. Does anyone know why is this happening? Thanks

thomas October 30, 2003 18:01

Re: Flow behind a cylinder
 
Hi, First what are you using LES or DNS ?. In case you realize all what I am gonna say do not heasitate to give-up ! A teacher taught us a story in class about a PhD student working on a 2D DNS solver and who has first obtained the same results than you: No wake even by increasing the Re. They introduced a 'Noise' in the flow condition symbolized by a small periodic moves of the flow obstacle. With that they obtain much better results showing wake and vortex break-up. So what all that means physically ? Experimentally it has been showed the transition to laminar to turbulent flow can depends on the experimental condition. For axample someone walking next to your pilote can induce enought disturbance to your system to change the Re number of your transition. It is exactly the smae thing in LES and DNS ! you are simulating perfect experimental condition (that you cannot achieve in reality). Therefore the disturbance that all authors are talking about is just the way to introduce non-ideal flow condition (to help you for example it can be someone walking next to your pilote). As I have personnally never worked on LES or DNS and just know some basic theory on it I cannot what to do and which parameter you might have forgot to set if it is the case (check the fluentuser website ! ). A classical disturbance could be to introduce a 'noise' at the inlet via a random function.

To sum-up numerical simulation does not take into account REAL PHYSICAL DISTURBANCES. They need to be introduce by periodic or unsteady boundary condition.

That is all, I just hope I brought you some further informations and I a sure some specialist will bring you better advices to set all your parameters. Ciao

PS : 'Geugnon -> equipe de L2 -> la honte ' :)

Vasily October 30, 2003 18:21

Re: Flow behind a cylinder
 
Hi, Thomas Thanks for discussion, I have two remarks:

1) Aren't we doing exactly the same with the air, modeling an 'ideal experimental conditions ' ? What is the mechanism that allow the flow to develop in the right way? notice, that in this case everything is much more realistic. Vortex shading occure at right Re. 2)I'm just wondering, what will be the shading frequency of vortices in case of periodically induced disturbances, would it be Strouhal number or more close to the frequency of the 'noise' ?

Vasily

thomas October 30, 2003 18:34

Re: Flow behind a cylinder
 
What a quick answer :) ! Concerning the results difference involved by the use of different material I do not think I am enough competent into that field to bring any answer. Just make sure you have set the same exact parameter. However concerning the periodicity of the vortex shedding I have been taught that This frequency does not depend on the disturbance you put in your system. Clearly, your system do not amplify the distrubance you put in it. I wanted to introduce that in my previous pamphlet but thought it would have been too long. Bye


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