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Non dimensional BL thickness

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Old   December 5, 2017, 15:22
Default Non dimensional BL thickness
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Hi Folks,

I am doing some simple CFD work on paper and am trying to non dimensionalise the maximum boundary layer thickness of a turbulent flow travelling over a airfoil (simplified as a flat plate). I've given it a go but I'm not sure if I'm doing it right.

I think that to non dimensionlise the BL I can simply do:

Normal BL [m] * frictional velocity [m/s] / kinematic viscosity [m^2/s]

I would then set the free stream velocity and the fluid density to equal 1 and then calculate the non-dimensional BL thickness using the equation above and I think I'd get the same dimensionless BL thickness as if I used actual flow values (for example the defaults in Fluent of rho = 1.2047 and dynamic viscosity = 1.789e-5)

Am I going about this right?

Thanks,

Fred
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Old   December 5, 2017, 15:57
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Filippo Maria Denaro
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for a turbulent BL you can estimate the non-dimensional thickness as

delta(x)/x = C Re_x^(-1/5)
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Old   December 5, 2017, 16:56
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Thanks for your reply!

Thank you for that equation. Out of interest, does the way I've done it worked? (and also making U_freestream and rho = 1)
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Old   December 6, 2017, 02:50
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In addition to changing the free stream U and rho, you will also have to change the dynamic viscosity.
The dynamic viscosity should be such that the Reynolds number is the same in dimensional as well as non-dimensional cases.
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