Self-sustained large-scale motions in the asymptotic suction boundary layer

TL;DR

This paper demonstrates that large-scale motions in the asymptotic suction boundary layer are self-sustained, similar to confined flows, indicating a robust, general mechanism in turbulent wall-bounded flows that involves complex streak and vortex dynamics.

Contribution

It provides evidence that large-scale motions are self-sustained in open boundary layers, extending previous findings from confined flows and highlighting their universal nature.

Findings

Large-scale motions are self-sustained in boundary layers.

Dynamics involve growth, breakdown, and regeneration of streaks and vortices.

Bursting behavior is linked to single-wall dynamics, differing from confined flow cycles.

Abstract

Large-scale motions, also known as superstructures, are dynamically relevant coherent structures in a wall-bounded turbulent flow, that span the entire domain in wall-normal direction and significantly contribute to the global energy and momentum transport. Recent investigations in channel and Couette flow, suggest that these large-scale motions are self-sustained, implying they are not driven by small-scale motions at the wall. Whether large-scale motions are self-sustained has however not yet been answered for open boundary layers, which are relevant for many applications. Here, using the asymptotic suction boundary layer flow at the friction Reynolds number $Re_\tau = 1\,168$ as a testbed, we show that large-scale motions are self-sustained also in boundary layers. Together with the previous investigations in confined flows, this observation provides strong evidence of the robust and general nature of coherent self-sustained processes in turbulent wall-bounded flows. The dynamics of the large-scale self-sustained process involving the growth, breakdown and regeneration of quasi-streamwise coherent streaks and vortices within the boundary layer shows temporal phase relations reminiscent of bursting as observed in buffer layer structures. The dynamics however differs from the quasi-periodic large-scale streak-vortex regeneration cycle observed in confined flows. Based on the similarity of the dynamics of large-scale motions in boundary layers and of small-scale buffer layer structures, we conjecture that the bursting behaviour is associated with the dynamical relevance of only one wall, while two confining walls lead to the quasi-periodic cycle.