Hi,
It seems that due to the additional turbulence introduced by the spectral synthesizer the resulting turbulent length scales are too small to be resolved in LES mode and therefore the shielding function is triggering the RANS mode across the pipe. I would recommend to decrease the turbulence level at the inlet to see if this behavior disappears. Also, I would extend the inlet further upstream to minimize the effect of the inlet BC on the region of interest. On the other hand, it seems that the shear layers generated with the Spectral Synthesizer enable are more physical (as long as the inlet velocity is high enough). Did you ensure that the sampling was performed once the solution was statistically steady?
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