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Francis October 14, 2008 10:49

Re on a flat plate
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,

Jed October 14, 2008 10:54

Re: Re on a flat plate
How long is the plate? Rather, how long is the piece you are interested in?

Francis October 14, 2008 10:57

Re: Re on a flat plate
Are you indicating the aspect ratio matters? Like a thin aerofoil?

Jed October 14, 2008 11:22

Re: Re on a flat plate
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'.

Francis October 14, 2008 11:28

Re: Re on a flat plate
Isn't it like a thin aerofoil? Take the chord (thickness) of the flat plate for the characteristic length?

Jed October 14, 2008 11:58

Re: Re on a flat plate
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.

Francis October 14, 2008 12:58

Re: Re on a flat plate
Hhmmm, thank you Jed. We use "flat plate" quite a lot in the UK. :)

Jed October 14, 2008 13:10

Re: Re on a flat plate
Is this your geometry?

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.

Francis October 14, 2008 13:13

Re: Re on a flat plate
I get you, thank you very much.

Louis October 14, 2008 15:47

Re: Re on a flat plate
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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