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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.

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

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

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.