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Old   October 14, 2008, 10:49
Default Re on a flat plate
  #1
Francis
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Hi guys,

I am wondering how to calculate the Reynolds number on a flat plate, where the flow coming from the parallel direction of the plate. How to choose the charactistic length of it?

Many thanks,

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Old   October 14, 2008, 10:54
Default Re: Re on a flat plate
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Jed
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How long is the plate? Rather, how long is the piece you are interested in?
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Old   October 14, 2008, 10:57
Default Re: Re on a flat plate
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Francis
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Are you indicating the aspect ratio matters? Like a thin aerofoil?
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Old   October 14, 2008, 11:22
Default Re: Re on a flat plate
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Jed
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The domain of interest is what matters. As you increase the size of the domain, the Reynolds number grows. So assuming the domain is (in principle) infinite, the answer to your original question is `anything you want'.
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Old   October 14, 2008, 11:28
Default Re: Re on a flat plate
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Francis
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Isn't it like a thin aerofoil? Take the chord (thickness) of the flat plate for the characteristic length?
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Old   October 14, 2008, 11:58
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Jed
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Maybe I misunderstood `plate'. I assumed you were just looking at flow near a formally infinite plane. If you have two sides, a positive thickness, and somewhere in the domain that the flow is not separated then the scales are not arbitrary.
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Old   October 14, 2008, 12:58
Default Re: Re on a flat plate
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Francis
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Hhmmm, thank you Jed. We use "flat plate" quite a lot in the UK.
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Old   October 14, 2008, 13:10
Default Re: Re on a flat plate
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Jed
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Is this your geometry?

http://www.grc.nasa.gov/WWW/wind/val...rb/fpturb.html

If so, where is the thickness of the plate? You need to decide how far to the right your domain extends. That will define the length scale.
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Old   October 14, 2008, 13:13
Default Re: Re on a flat plate
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Francis
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I get you, thank you very much.
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Old   October 14, 2008, 15:47
Default Re: Re on a flat plate
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Louis
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Note also that for semi infinite flat plate models, people often refer to Rex (=U*x/nu), where x is the distance from the plate leading edge. You will see things like skin friction coefficient plotted as a function of Rex.
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