On the Structure of Turbulence in the Bottom Boundary Layer of the Coastal Ocean

Abstract

Six sets of particle image velocimetry (PIV) data from the bottom boundary layer of the coastal ocean are examined. The data represent periods when the mean currents are higher, of the same order, and much weaker than the wave-induced motions. The Reynolds numbers based on the Taylor microscale (Reλ) are 300–440 for the high, 68–83 for the moderate, and 14–37 for the weak mean currents. The moderate–weak turbulence levels are typical of the calm weather conditions at the LEO-15 site because of the low velocities and limited range of length scales. The energy spectra display substantial anisotropy at moderate to high wavenumbers and have large bumps at the transition from the inertial to the dissipation range. These bumps have been observed in previous laboratory and atmospheric studies and have been attributed to a bottleneck effect. Spatial bandpass-filtered vorticity distributions demonstrate that this anisotropy is associated with formation of small-scale, horizontal vortical layers. Methods for estimating the dissipation rates are compared, including direct estimates based on all of the gradients available from 2D data, estimates based on gradients of one velocity component, and those obtained from curve fitting to the energy spectrum. The estimates based on vertical gradients of horizontal velocity are higher and show better agreement with the direct results than do those based on horizontal gradients of vertical velocity. Because of the anisotropy and low turbulence levels, a −5/3 line-fit to the energy spectrum leads to mixed results and is especially inadequate at moderate to weak turbulence levels. The 2D velocity and vorticity distributions reveal that the flow in the boundary layer at moderate speeds consists of periods of “gusts” dominated by large vortical structures separated by periods of more quiescent flows. The frequency of these gusts increases with Reλ, and they disappear when the currents are weak. Conditional sampling of the data based on vorticity magnitude shows that the anisotropy at small scales persists regardless of vorticity and that most of the variability associated with the gusts occurs at the low-wave-number ends of the spectra. The dissipation rates, being associated with small-scale structures, do not vary substantially with vorticity magnitude. In stark contrast, almost all the contributions to the Reynolds shear stresses, estimated using structure functions, are made by the high- and intermediate-vorticity-magnitude events. During low vorticity periods the shear stresses are essentially zero. Thus, in times with weak mean flow but with wave orbital motion, the Reynolds stresses are very low. Conditional sampling based on phase in the wave orbital cycle does not show any significant trends.

A submersible three‐dimensional particle tracking velocimetry system for flow visualization in the coastal ocean

“…Similar structures are visible in about 20% of this 10-min data series. These observations are directly in line with those of Nimmo Smith et al (2005) and will now allow the three-dimensional nature of the vortical structures to be explored.…”

“…Therefore, the opportunities to examine spatial gradients in velocity or gain insight into the spatial structure of turbulence are limited and require assumptions involving Taylor's hypothesis. Over recent years, particle image velocimetry (PIV) has been applied successfully to the study of turbulence in the coastal ocean (Bertuccioli et al 1999;Doron et al 2001;Nimmo Smith et al 2002;Nimmo Smith et al 2004). PIV provides instantaneous two-dimensional velocity distributions within a two-dimensional sample plane.…”

“…The ocean PIV observations show that near-bed tidal flows are dominated by coherent structures made up of groups of vortices (Nimmo Smith et al 2005). These structures occur intermittently but play the key role in the transfer of energy from the mean flow to turbulence.…”

“…These stress-containing eddies appear to be similar to the hairpin-like structures often observed in laboratory flows (Adrian et al 2000) and numerical simulations (Zhou et al 1999), but the two-dimensional nature of the PIV measurements means that they are not able to provide the information on three-dimensional velocity structure that would confirm this. The cross-stream scale and form of these vortical structures is important when considering, for example, lateral dispersion, and may account to some extent for the large anisotropy in the turbulence observed within these flows (Nimmo Smith et al 2005).…”

Unsteady dynamics and organized structures from DNS over an idealized building canopy

“…Given the generality of this mechanism, it is therefore reasonable to expect hairpin vortex packets to be present in a wide variety of shear flows, including the present. Indeed there has been some experimental evidence for their presence over natural rough surfaces such as in the atmospheric boundary layer (Hommema and Adrian, 2002) and in the bottom boundary layer of coastal oceans (Smith et al, 2005).…”