Abstract
The regeneration mechanism of streamwise vortical structures in the self-sustaining process of wall-bounded turbulence is investigated. Resolvent analysis [1] is used to identify the principal forcing mode which produces the maximum amplification of the response modes in the minimal channel for the buffer [2] and logarithmic layer [3]. The identified mode is then projected out from the nonlinear term of the Navier-Stokes equations at each time step from the direct numerical simulations (DNS) of the corresponding minimal channel. The results show that the removal of the principal forcing mode is able to significantly inhibit turbulence for the buffer and logarithmic layer while removing the subsequent modes instead of the principal one only marginally affects the flow. Analysis of the dyadic interactions in the nonlinear term shows that the contributions toward the principal forcing mode come from a limited number of wavenumber interactions. Using conditional averaging, the flow structures that are responsible for generating the principal forcing mode, and thus the nonlinear interaction to self-sustain turbulence, are identified to be spanwise rolls interacting with meandering streaks.
Highlights
The structure of near-wall turbulence has been extensively investigated over the past halfcentury
Simulations of the minimal channel for the buffer and logarithmic layer with a fixed mean streamwise velocity profile were performed to isolate the structures at a prescribed scale
The most amplified nonlinear term corresponding to the most energetic wavenumber was computed from the resolvent analysis using the mean velocity profile of the minimal channel simulations
Summary
The structure of near-wall turbulence has been extensively investigated over the past halfcentury. Important progress was made in the early 1990s using the “minimal flow unit” approach, which revealed that buffer layer streaks can self-sustain even when motions at larger scales are inhibited and that their existence, relies on an autonomous process [2]. Jimenez & Pinelli [15] further confirmed that this near-wall process is independent of the flow in the logarithmic and outer regions by showing the survival of the near-wall motions in the absence of outer turbulence. The consensus from these studies, along with many others that followed
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