Broadband Acoustical Characterization of Bubbles in Stratified Environments
Abstract
This proposal aims to understand the generation mechanisms, distribution and size, and physical processes driving bubble populations, in stratified environments such as estuarine tidal fronts. Bubble plumes are a common feature of downwelling fronts, yet the densit,ies, size, and spatial distributions of bubbles in these fronts are not nearly as well-studied as those under breaking waves. Likewi,se, the impact of near surface estuarine stratification on wave-generated bubble penetration depth and spatial distribution is also,not well understood. This study proposes to develop broadband approaches to acoustically infer bubble size distributions using both,measurements of volume backscatter (20-410 kHz) as well as an instrument developed to measure frequency-dependent absorption (3-110,kHz) due to bubbles, from which a bubble size distribution can be calculated. Broadband spectral classification and echo statistical, approaches will be applied to the volume backscattering data. A holographic imaging technique will also be developed to provide an,optical ground truth to the acoustic measurements. Digital inline holography, consisting of a collimated laser source and a camera,,produces 2-D diffraction patterns of objects in the beam, which can be back-propagated to reconstruct a very high-resolution 3-D ima,ge of the sampling volume. Holography therefore offers a means of obtaining bubble size distributions with radii greater than ~80 m, over a range of bubble densities. However, it is unclear what the impact of suspended sediment and turbulence will be on the qualit,y of the holographic data, which will remain an active area of inquiry. This study proposes to apply all the mentioned techniques in, the Connecticut River estuary to gather spatial and temporal measurements of bubble sizes in tidal fronts. The focus will be the eb,b tide front, characterized by a strong convergence zone associated to the plume front, where the freshwater plume meets the ambient, coastal ocean, increased surface roughness and strong downward velocities. Entrained bubbles dominate the scattering at the plume f,ront. Existing backscatter measurements of bubbles collected at all three sites of the USRS program (James River, Connecticut River,, and Mobile Bay) will also be used in this analysis, as well as new measurements in the Connecticut River which integrate the broadb,and backscatter, holographic imaging, and attenuation measurements.Approved for public release.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 14, 2022
- Source ID
- N000142212048
Entities
People
- Andone Lavery
Organizations
- Office of Naval Research
- United States Navy
- Woods Hole Oceanographic Institution