Thomas R. Osborn

Turbulence Characteristics and Dissipation Estimates in the Coastal Ocean Bottom Boundary Layer from PIV Data

Abstract Turbulence characteristics in the coastal ocean bottom boundary layer are measured using a submersible Particle Image Velocimetry (PIV) system with a sample area of 20 × 20 cm2. Measurements are performed in the New York Bight at elevations ranging from 10 cm to about 1.4 m above the seafloor. Recorded data for each elevation consists of 130 s of image pairs recorded at 1 Hz. After processing, the data at each elevation consist of 130 instantaneous spatial velocity distributions within the sample area. The vertical distribution of mean velocity indicates the presence of large-scale shear even at the highest measurement station. The flow also undergoes variations at timescales longer than the present data series. Spatial spectra of the energy and dissipation are calculated from individual vector maps. The data extend well beyond the peak in the dissipation spectrum and demonstrate that the turbulence is clearly anisotropic even in the dissipation range. The vector maps are also patched together to...

Vertical Profiling of Velocity Microstructure

Abstract A free-fall oceanographic instrument capable of producing direct estimates of the local rate of energy dissipation has been developed. The velocity sensor in an adaptation of the two-component airfoil probe to the oceanic environment. Spectra of the velocity data show that the instrument achieves complete spatial resolution of the vertical current shear and there is no apparent contamination of the signal from vibrations or body motions in the frequency range contributing to the variance of the shear. The energy dissipation is proportional to the variance of the vertical shear. Thus, it is possible to estimate the energy dissipation without comparing the velocity spectra to any “universal curve.”

Turbulent Dissipation Over the Continental Slope Off Vancouver Island

Abstract Thirteen profiles of the rate of viscous dissipation of turbulent kinetic energy &epsi¯ were made over the continental slope off Vancouver Island between 12 and 14 May 1980 in conjunction with CTD and moored current-meter observations. Systematic variability was observed in the vertical but not in the horizontal direction. Above 200 m depth numerous salt-stabilized temperature inversions were seen and dissipation rates were significantly larger than below 200 m. Dissipation rates below 200 m are the lowest ever reported and coincide with a low level of energetics revealed by the current meter moorings. Comparison with the Garrett-Munk internal wave spectrum indicates an e-folding decay time of internal wave energy of ∼50 days at depths below 200 m.

Measurements of bubble plumes and turbulence from a submarine

Abstract An experiment using turbulence probes and an array of side‐scan and vertically pointing pencil beam sonars mounted on the U.S. submarine Dolphin was carried out to measure turbulence in near‐surface regions of acoustic scattering, in particular, those caused by subsurface bubbles produced by breaking wind waves. The dataset collected during winds of 5–9 m s−1 reveals the banded patterns of bubbles associated with Langmuir circulation, even though no surface manifestations were visible. A forward‐pointing side‐scan sonar determined the “age” of bubble clouds after their generation by breaking waves. There is enhanced turbulent dissipation in the bubble clouds, and the dissipation rate close to the surface exceeds that predicted using conventional calculations based on the law of the wall and buoyancy flux. The correspondence between bubbles and turbulence implies a horizontally patchy turbulent structure near the surface. Below the base of the bubble clouds the distance between turbulent patches i...

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 f...

Measurements of Turbulence in the Upper-Ocean Mixing Layer Using Autosub

Abstract The rate of dissipation of turbulent kinetic energy has been measured with airfoil probes mounted on an autonomous vehicle, Autosub, on constant-depth legs at 2–10 m below the surface in winds up to 14 m s−1. The observations are mostly in an area limited by fetch to 26 km where the pycnocline depth is about 20 m. At the operational depths of 1.55–15.9 times the significant wave height Hs, and in steady winds of about 11.6 m s−1 when the wave age is 11.7–17.2, dissipation is found to be lognormally distributed with a law-of-the-wall variation with depth and friction velocity. Breaking waves, leaving clouds of bubbles in the water, are detected ahead of the Autosub by a forward-pointing sidescan sonar, and the dissipation is measured when the clouds are subsequently reached. Bands of bubbles resulting from the presence of Langmuir circulation are identified by a semiobjective method that seeks continuity of band structure recognized by both forward- and sideways-pointing sidescan sonars. The times...

Turbulence Measurements with a Submarine

Abstract Measurements of small-scale velocity and temperature fluctuations have been made from the research submarine Dolphin in the open ocean off San Diego, California. The important contribution of the submarine is that it collects horizontal profiles. The submarine can depth-cycle to obtain a quasi-vertical profile of the fluctuations along a horizontal path. The noise level depends on the configuration of the instrumentation and the operating conditions of the vessel. Expressed in terms of energy dissipation, it is approximately 10−7 W m−3, comparable to that of free-fall vertical profilers. Much of the small-scale velocity and temperature data are similar to those collected with free-fall vertical profilers. A major difference is that the horizontal transects are aligned with the temperature gradient of salt fingers, which are not well detected by vertical profilers. Fingers were seen beneath the saline upper layer at values of Rρ between 2 and 4. Off San Diego, the velocity signal from the fingers ...

A Submersible Particle Image Velocimetry System for Turbulence Measurements in the Bottom Boundary Layer

Abstract This paper introduces an oceanic particle image velocimetry (PIV) system that has been under development at The Johns Hopkins University over the past three years. PIV maps two components of the instantaneous velocity distribution within a selected sample area by recording the motion of microscopic tracer particles illuminated by a pulsed laser sheet. The primary purpose of this instrument, in its current configuration, is to directly measure the Reynolds stresses, velocity profile, vorticity, and turbulent spectra in the bottom boundary layer of the coastal ocean. Due to the basic principles of PIV, the validity of these measurements is independent of any of the usual assumptions about the nature of the flow, such as the existence of a log layer in the velocity profile, a constant stress layer, or an inertial subrange in the turbulence spectra. In addition, at scales up to those of a single image, the analysis does not require the use of the Taylor hypothesis. The primary focus of this paper is ...

Bubble Clouds and Langmuir Circulation: Observations and Models

Abstract Concurrent measurements of the rate of dissipation of turbulent kinetic energy and the void fraction and size distribution of near-surface bubbles are described. Relatively high dissipation rates and void fractions are found in bubble bands produced by Langmuir circulation. The mean dissipation rates observed in the bands are close to those at which the dynamics of algae is significantly affected. The data are used to test basic assumptions underpinning models of subsurface bubbles and associated air–sea gas transfer. A simple model is used to examine the qualitative effect of Langmuir circulation on the vertical diffusion of bubbles and the representation of Langmuir circulation in models of gas transfer. The circulation is particularly effective in vertical bubble transfer when bubbles are injected by breaking waves to depths at which they are carried downward by the circulation against their tendency to rise. The estimated value of the ratio r of the eddy diffusivity of particles (resembling b...

Distribution of Energy Spectra, Reynolds Stresses, Turbulence Production, and Dissipation in a Tidally Driven Bottom Boundary Layer

Abstract Seven sets of 2D particle image velocimetry data obtained in the bottom boundary layer of the coastal ocean along the South Carolina and Georgia coast [at the South Atlantic Bight Synoptic Offshore Observational Network (SABSOON) site] are examined, covering the accelerating and decelerating phases of a single tidal cycle at several heights above the seabed. Additional datasets from a previous deployment are also included in the analysis. The mean velocity profiles are logarithmic, and the vertical distribution of Reynolds stresses normalized by the square of the free stream velocity collapse well for data obtained at the same elevation but at different phases of the tidal cycle. The magnitudes of 〈u′u′〉, 〈w′w′〉, and −〈u′w′〉 decrease with height above bottom in the 25–160-cm elevation range and are consistent with the magnitudes and trends observed in laboratory turbulent boundary layers. If a constant stress layer exists, it is located below 25-cm elevation. Two methods for estimating dissipatio...