On the role of turbulent large-scale streaks in generating sediment ridges

TL;DR

This study uses particle-resolved direct numerical simulations to investigate how turbulent large-scale streaks influence the formation of sediment ridges in open channel flows, revealing a top-down interaction and the role of turbulence organization.

Contribution

It demonstrates the connection between large-scale turbulence streaks and sediment ridge formation, highlighting a top-down causality and the statistical footprint of organized streaks.

Findings

Sediment ridges form in regions of weaker erosion under low-speed streaks.

The arrangement of ridges adapts to outer flow changes with a delay of several bulk time units.

Secondary currents are linked to organized large-scale streaks, not just sediment ridges.

Abstract

The role of turbulent large-scale streaks in forming subaqueous sediment ridges on an initially flat sediment bed is investigated with the aid of particle-resolved direct numerical simulations of open channel flow at bulk Reynolds numbers up to 9500. The regular arrangement of quasi-streamwise ridges and troughs at a characteristic spanwise spacing between 1 and 1.5 times the mean fluid height is found to be a consequence of the preferential spanwise organization of turbulence in large-scale streamwise velocity streaks. Sediment ridges predominantly appear in regions of weaker erosion below large-scale low-speed streaks and vice versa for troughs. The interaction between the dynamics of the large-scale streaks in the bulk flow and the evolution of sediment ridges on the sediment bed is best described as a `top-down' process, as the arrangement of the sediment bedforms is seen to adapt to changes in the outer flow with a time delay of several bulk time units. The observed `top-down' interaction between the outer flow and the bed agrees fairly well with the conceptual model on causality in canonical channel flows proposed by Jimenez (J. Fluid Mech., vol. 842, 2018, P1, section 5.6). Mean secondary currents of Prandtl's second kind of comparable intensity and lateral spacing are found over developed sediment ridges and in single-phase smooth-wall channels alike in averages over O(10) bulk time units. This indicates that the secondary flow commonly observed together with sediment ridges is the statistical footprint of the regularly organized large-scale streaks.