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Old   May 29, 2002, 10:24
Default transistion
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cfdguy
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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
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Old   May 30, 2002, 05:09
Default Re: transistion
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Adarsh
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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
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Old   May 30, 2002, 07:23
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cfdguy
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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
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Old   May 30, 2002, 08:39
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FVSuppe
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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.
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Old   May 30, 2002, 17:21
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Jongdae Kim
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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.
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Old   May 31, 2002, 05:47
Default Re: transistion
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cfdguy
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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

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Old   June 6, 2002, 09:17
Default Re: transistion
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cfdguy
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hi all y no more discussion on this topic. bye
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