A note on the amplitude modulation phenomenon in non-canonical wall-bounded flows

The amplitude modulation phenomena, defined originally by Mathis et al. (J. Fluid Mech., 628, 311-337; 2009), corresponds to a unique non-linear interaction between Reynolds number (Reτ) dependent large-scale motions and Reτ-invariant inner-scale motions observed in canonical wall-bounded flows. While similar non-linear interactions can be observed in non-canonical wall-bounded flows, linking them to amplitude modulation is questionable due to the fact that each non-canonical effect is associated with distinct variations in the energies of both the large and inner scaled motions. This study investigates non-linear triadic interactions as a function of various non-canonical effects by analyzing published hot-wire datasets acquired in the large Melbourne wind tunnel. It is found that triadic interactions, across the entire turbulence scale hierarchy, become statistically significant with increasing intensity of non-canonical effects such as rough walls, pressure gradients, and spanwise or wall-normal forcing. This stands in contrast to previous observations made in canonical flows, where only the interaction between inner scales and inertia-dominated large scales was considered dynamically significant for increasing Reτ. The implications of these findings are discussed for near-wall flow prediction models in non-canonical flows, which must take into account all coexisting non-linear triadic interactions in wall-bounded flows.