Influence of FFT Points on Turbulence Characteristics
The accuracy of the Mann turbulence model depends on the number of FFT points. One of the reason is that higher FFT points will have finer resolutions to enable an improved modelling at high frequency domain, but also relates to the assumption made when writing out and processing the grid, see the description of the Mann uniform shear turbulence model and also in the determination of the grid number for the Mann uniform shear turbulence model.
Figure 1 and Figure 2 display the spectra and coherence calculated for the Mann model using three different values of FFT points (32, 64 and 128), respectively. The results demonstrate that the accuracy of the Mann model generation is affected by the number of FFT points used in the calculations. For the spectra, it is clearly noticeable that increasing the FFT points helps the turbulence model to capture the higher frequency components. This allows the turbulent flow field to include a more correct representation of the small turbulent eddies. For the coherence, it can also be seen that the accuracy is also improved when a higher number of the FFT points is adopted. Note that the coherence displayed in Figure 2 is evaluated at a separation distance \(r\) equal to the distance between the adjacent grid points. As demonstrated in the determination of the grid number for the Mann uniform shear turbulence model, the output turbulent grid does not always coincide with the turbulent box itself. Therefore, the higher the FFT points being used, the smaller the mismatch between the output grid with the turbulence box grid. If the coherence is evaluated at a farther distance (e.g., 3 grid steps), the number of FFT points was observed to have only little impacts (not shown in this article for the sake of conciseness).
Last updated 17-07-2024