Study on Large-Scale Amplitude Modulation of Near-Wall Small-Scale Structures in Turbulent Wall-Bounded Flows

Recently, a novel approach to study the inner-outer interactions in turbulent wall-bounded flows has been presented by Mäteling et al. [8], which tackles the combined effects of superposition, bursting events, and amplitude modulation based on the time-dependent streamwise velocity fluctuations. This approach is extended in the current study to all three velocity fluctuation components. Due to the incorporation of the wall-normal velocity fluctuations, the bursting events can be study in more detail and independently of the superposition. By using a multivariate empirical mode decomposition for the scale separation, all velocity components at both wall-normal positions can be decomposed simultaneously to ensure proper scale alignment between the resulting modal representations. The results show that the superposition effect and the amplitude modulation exist in all three velocity components but are most intense for the streamwise fluctuations. A high level of coherence of the sweeps and ejections is also detected by conventional cross-correlation.