Numerical study of turbulent channel flow perturbed by spanwise topographic heterogeneity: amplitude and frequency modulation within low- and high-momentum pathways

We studied the effects of spanwise heterogeneous, topographically-driven secondary flows on inner-outer interaction in turbulent channel flow. This topography induces domain-scale, counter-rotating vortices. The vortices are anchored to the topography such that prominent upwelling and downwelling occurs above the low and high roughness, respectively. We have quantified the extent to which such secondary flows disrupt the distribution of spectral density across constituent wavelengths throughout the depth of the flow, which has direct implications for amplitude and frequency modulation. The distinct outer peak associated with large-scale motions -- the “modulators” -- is preserved within the upwelling zone, but vanishes in the downwelling zone. Within the downwelling zones, structures are steeper and shorter. Single- and two-point correlations for amplitude and frequency modulation demonstrate insensitivity to resolution across cases. We also show a pronounced crossover between the single- and two-point correlations, a product of modulation quantification based upon Parseval's theorem (i.e., spectral density, not the wavelength at which energy resides, defines the strength of modulation).