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September 7, 2022, 18:57 |
Length scales in CFD
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#1 |
New Member
William
Join Date: Sep 2022
Posts: 8
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I am relearning the Navier Stokes equations by deriving them again from transport equations. I noted that the fundamental assumption Navier Stokes equations make is that the fluid of interest is a continuum. I understand that for a fluid to be continuum the length scale of interest needs to be much larger than the mean free path of the fluid. It is relatively easy to estimate the mean free path of a fluid, but I am not sure how to find out the length scale of interest in a CFD problem. I guess the question I can come up with now is, what defines the smallest length scale of a typical CFD problem? Are they related to the size of the eddies and things? Thanks!
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September 7, 2022, 19:12 |
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#2 | |
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Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
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September 7, 2022, 20:10 |
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#3 |
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William
Join Date: Sep 2022
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I see, thanks for pointing me in the right direction!
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September 7, 2022, 20:22 |
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#4 |
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Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
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A very quick way to estimate Kolmogorov length scale is to figure out your boundary layer thickness and go two orders of magnitude down. Viscous effects dominate the 10% of the boundary layer and 10% of that is your order of magnitude estimate for the viscous length scale.
You'll find out soon enough but the mean free path for air at ambient pressure is nanometer scale. Practical flow devices will have dimensions of milimeters or more. You need to be at crazy vacuum pressures or have a very small device for these scales to be anywhere close to each other. |
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Tags |
cfd, length scale, navier stokes |
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