# MikeBravo

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## Min and Max Wavenumber

Posted October 26, 2020 at 15:56 by MikeBravo

Min and Max Wavenumber

https://www.researchgate.net/post/Ho...aracterisation

Quote:
December 7, 2017
Lalit
the Nyquist theorem says that for a step sampling dt you can describe the smallest wavelenght 2*dt (three samples describe a sine). For a given period lenght T, the ratio T/(2*dt) gives the maximum wavenunber you can represent
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## Calculating energy/power spectra.

Posted October 10, 2020 at 13:50 by MikeBravo

Calculating Spectra:

Quote:
Originally Posted by FMDenaro
Well, I suggest to do the FFT of the three velocity
components at the chosed position. However, you need a correct windowing technique as your signal is not necessarily periodic in time. Then, consider that depending on the position far from the cylinder along the wake, you can or not recover the theoretical inertial slope. Furthermore, the theoretical slope should be the same, interchanging in space and time, requiring further assumption about ergodicity.
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## RANS Grid Sensitivity Divergence on LES Grid

Posted September 10, 2020 at 20:21 by MikeBravo

you can't have y+ changing between the grids when doing a grid refinement

Quote:
Originally Posted by sbaffini
Indeed, if y+ =4 is relative to the finest grid, it is confirmed to be a wall function problem. I can't double check now, but I'm pretty sure that the k-omega sst model in CFX uses an all y+ wall function, which means that a wall function is always active. While, in theory, such wall functions should be insensitive to the specific y+ value, they are not perfect and your case is very far from the typical wall
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## RANS Grid Sensitivity Divergence on LES Grid

Posted August 31, 2020 at 17:29 by MikeBravo

Reference on not changing y+ while doing a grid sensitivity study:

Quote:
Originally Posted by sbaffini
Indeed, if y+ =4 is relative to the finest grid, it is confirmed to be a wall function problem. I can't double check now, but I'm pretty sure that the k-omega sst model in CFX uses an all y+ wall function, which means that a wall function is always active. While, in theory, such wall functions should be insensitive to the specific y+ value, they are not perfect and your case is very far from the typical wall function
...
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## Y+ value for Large Eddy Simulation

Posted August 31, 2020 at 15:35 by MikeBravo

Explanation of Y+ as it relates to viscous sublayer and advection scheme:

Quote:
Originally Posted by cfdnewbie
yes, at least in the viscous sublayer. The size of your grid cell (or the number of points per unit length) determine the smallest scale you can catch on a given grid. From information theory, the Nyquist theorem tells us that we need at least 2 points per wavelength to represent a frequency (we need to be able to detect the sign change). However, 2 points per wavelength is just for Fourier-type approximations.
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