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Amplitude and frequency modulation in wall-turbulence

The enormous impact and significance of wall-turbulence in various applications ranging from transportation and energy generation systems to meteorology and oceanography cannot be understated. Therefore, there is a need to develop methods that can predict, model and control wall-turbulence in order to meet current and future challenges. Recently, a variety of exciting control and modelling strategies have been proposed. However, almost all these ideas are based on our understanding of low Reynolds-number flows. In higher Reynolds-numbers, the large-scale structures farther away from the wall interacts with the small-scales near the wall and significantly alters the perspectives gained from examining low Reynolds-number flows. In this study, we present evidence that quantifies the extent of the interaction between the large- and the small-scales in high Reynolds-number turbulent boundary layers. Results indicate that both the amplitude and the frequency of the near-wall structures are significantly altered by the larger structures in high Reynolds numbers. This has important consequences in development of control strategies aimed at controlling the near-wall structures for drag reduction.