# Re on a flat plate

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 October 14, 2008, 10:49 Re on a flat plate #1 Francis Guest   Posts: n/a 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,

 October 14, 2008, 10:54 Re: Re on a flat plate #2 Jed Guest   Posts: n/a How long is the plate? Rather, how long is the piece you are interested in?

 October 14, 2008, 10:57 Re: Re on a flat plate #3 Francis Guest   Posts: n/a Are you indicating the aspect ratio matters? Like a thin aerofoil?

 October 14, 2008, 11:22 Re: Re on a flat plate #4 Jed Guest   Posts: n/a 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'.

 October 14, 2008, 11:28 Re: Re on a flat plate #5 Francis Guest   Posts: n/a Isn't it like a thin aerofoil? Take the chord (thickness) of the flat plate for the characteristic length?

 October 14, 2008, 11:58 Re: Re on a flat plate #6 Jed Guest   Posts: n/a 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.

 October 14, 2008, 12:58 Re: Re on a flat plate #7 Francis Guest   Posts: n/a Hhmmm, thank you Jed. We use "flat plate" quite a lot in the UK.

 October 14, 2008, 13:10 Re: Re on a flat plate #8 Jed Guest   Posts: n/a 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.

 October 14, 2008, 13:13 Re: Re on a flat plate #9 Francis Guest   Posts: n/a I get you, thank you very much.

 October 14, 2008, 15:47 Re: Re on a flat plate #10 Louis Guest   Posts: n/a 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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