# transistion

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 May 29, 2002, 10:24 transistion #1 cfdguy Guest   Posts: n/a can any one help. at what Re does a 2-D channel flow becomes turbulent? suppose i have a obstruction,ie., if study flow past a square cylinder, at what Re the flow becomes turbulent? can any one help? thanx in advance cfdguy

 May 30, 2002, 05:09 Re: transistion #2 Adarsh Guest   Posts: n/a depends on the free stream turbulence and other disturbance effects.. such as sound.Critical reynols no. is influenced by many factors in an experiment.Can't say precisely. Adarsh

 May 30, 2002, 07:23 Re: transistion #3 cfdguy Guest   Posts: n/a hi adarsh thank you i had done flow visualisation study of flow over a bluff body (sqr cylinder)at re=4000 in a water channel. the picture seems to show smooth flow lines. can i say that the flow is laminar. how can i decide it. bye cfdguy

 May 30, 2002, 08:39 Re: transistion #4 FVSuppe Guest   Posts: n/a Look up the book by Cebeci & Smith - 'Analysis of Turbulent Boundary Layers'. There are some correlations and formulas that may be useful to you. As far as I remember there are some formulas to predict start of transition for bluff bodies.

 May 30, 2002, 17:21 Re: transistion #5 Jongdae Kim Guest   Posts: n/a 1. What is you turbulence intensity at inflow boundary? 2. Aspect ratio ? 3. End condition ? (Do you use end plates which are parallel ?) 4. How to calculate Reynolds number ? Re=U*D/nu, where U is inflow velocity, D is cylinder section dimension, nu is kinematic viscosity. In the flow around square cylinder, unsteadiness starts from around Re=45. Around Re=150, another unstability starts. (These Re. numbers may be incorrect.). Around Re=250, drag and lift coefficients seems to be non-periodic, random. Flow of Re<240, plot of time history of drag and lift coefficients are smooth. Flow of Re>250, flow is in the transient stage. These are the experience based on my computational work (LES around bluff body). At Re=4000, Flow could still be laminar. But in my understanding, it's very difficult or impossible to maintain the flow as laminar unless you intend to keep the flow laminar.

 May 31, 2002, 05:47 Re: transistion #6 cfdguy Guest   Posts: n/a hello kim thank you for your message. here r the answers to ur queries. (1)tur.intensity is 1% (2) blockage ratio=(height of cylinder/height of the channel) is (1/14). (3)aspect ratio=(height of cylinder/depth of water) = (1/8) (3)the end plates are parallel (4)re is same as you defined.and it is 4000. awaiting your reply thank you cfdguy

 June 6, 2002, 09:17 Re: transistion #7 cfdguy Guest   Posts: n/a hi all y no more discussion on this topic. bye

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