Direct measurement of hairpin‐like vortices in the bottom boundary layer of the coastal ocean

Abstract

Laboratory measurements and numerical modeling at low Reynolds numbers (Reθ<7700) indicate the energy‐containing turbulence of boundary layer flows comprises coherent packets of hairpin vortices. Here direct measurements in the bottom boundary layer of the coastal ocean at higher Reynolds numbers (Reθ = 266,150) show tidal flows also contain packets of large vortices separated by periods of more quiescent conditions. The 1452 vortices recorded within a 20 min period are typically aligned along stream (∼8.0° from the mean flow direction) and inclined to the horizontal (∼27.0° from the seabed), with a mean period of occurrence of 4.3 s. These results lend three‐dimensional, in situ support to an interpretation of the coastal ocean bottom boundary layer as comprising coherent packets of hairpin vortices. This demonstrates a direct linkage from low Reynolds number experiments to higher Reynolds number flows, permitting fine‐scale details of particle transport and pollutant dispersion to be inferred from lower Reynolds number data.

Scaling oxygen microprofiles at the sediment interface of deep stratified waters

“…Deoxygenation in deep waters along the bottom boundary layer (BBL) is especially prone to the synergistic effects of climate change, including increasing average water temperature, strength of water column stratification, and accelerated nutrient cycling [ Yoh et al, ; Gudasz et al, 2010; Donnadieu et al, ]. Turbulence above the sediment‐water interface (SWI) amplifies DO dynamics, changing the sediment oxygen uptake (SOU) which depends on the supply of the DO fluxes from the BBL to the sediments [ McGinnis et al, ; Scalo et al, ; Steele et al, ]. SOU is important for the DO budget in stratified waters, as much of the organic matter decomposition occurs close below the SWI.…”

Turbulence and coherent structure characterisation in a tidally energetic channel

“…However, in order to circumvent this assumption one would need to collect both temporal and spatial data, which was not possible here, and could prove cost prohibitive for the tidal energy industry. Several previous studies [28,15,71] suggests that coherent structures are advected at speeds of 80 -117% of the free stream velocity thus, although not perfect, scaling the frequencies and periods with the mean flow speed is not such a reach.…”

“…Moreover, many of the significant peaks in the figure are seen to advance in the x-axis time window with movement upwards in the water column, with such events moving at ∼ 0.48 m s −1 which is 109% of the mean streamwise velocity. Adrian and Marusic [28], Nimmo Smith et al [15], Steele et al [71] find speeds ranging from 80 -117% of the mean streamwise velocity. The findings suggest that these features are likely to be redistributing momentum throughout the water column.…”