Herman Medwin

Bio: Herman Medwin is an academic researcher from Naval Postgraduate School. The author has contributed to research in topics: Diffraction & Bubble. The author has an hindex of 15, co-authored 41 publications receiving 1694 citations.

Fundamentals of Acoustical Oceanography

Abstract: Fundamentals of Acoustical Oceanography an important reference for specialists in acoustics, oceanography, marine biology, and related fields. This book also encourages a new generation of scientists, engineers, and entrepreneurs to apply the modern methods of acoustical physics to probe the unknown sea. The book is an authoritative, modern text with examples and exercises. It contains techniques to solve the direct problems, solutions of inverse problems, and an extensive bibliography from the earliest use of sound in the sea to present references. The book provides background to measure ocean parameters and processes, find life and objects in the sea, communicate underwater, and survey the boundaries of the sea. Fundamentals of Acoustical Oceanography explains principles of underwater sound propagation, and describes how both actively probing sonars and passively listening hydrophones can reveal what the eye cannot see over vast ranges of the turbid ocean. This book demonstrates how to use acoustical remote sensing, variations in sound transmission, in situ acoustical measurements, and computer and laboratory models to identify the physical and biological parameters and processes in the sea.

Speed of sound in water: A simple equation for realistic parameters

Abstract: A simple equation is presented for the dependence of sound speed on temperature, salinity, and depth of water. The comparison with Del Grosso’s NRL II shows discrepancies of the order of tenths of m/sec for realistic values of the parameters.Subject Classification: 30.25.

Bubble sources of the Knudsen sea noise spectra

Abstract: Single coherent bubble contributions to the incoherent underwater noise of spilling breakers have been studied in an anechoic laboratory facility. The waves are generated by a plunger, they propagate 17 m along a 1.2×1.2‐m water waveguide, and ‘‘spill’’ and create bubbles at the surface of a 3×3×3‐m anechoic cube of water. Several species of bubbles have been identified. In general, they act as transient dipoles of duration from 2 to several milliseconds, with peak axial source strength of the order of tenths of pascals, at 1 m. The noise is emitted when the bubble is within hundreds of micrometers or a few millimeters of the surface. Bubbles were observed in the 2 decades of frequency from 500 to 50 000 Hz. The average of the individual bubble events yielded a spectrum that slopes at about 5 dB/oct from 1 to 20 kHz, the same as the Knudsen wind noise spectra at sea. The magnitude of the laboratory breaker noise during continual wave‐breaking events was approximately 80 dB re: 1 μ Pa2/Hz at 1 kHz, which i...

Ambient and transient bubble spectral densities in quiescent seas and under spilling breakers

Abstract: Recently published observations, using laser holography near the ocean surface, have shown that the densities of 10 to 15 micron radius bubbles can be as high as 106 (per cubic meter per micron radius increment) within 3 m of the surface of quiescent seas, thereby supporting the acoustically derived values of two decades ago. The accumulated evidence suggests that these quiescent microbubble densities off the coast of California are proportional to a−4 for radius range 10 < a < 60 microns and a−2.5 for larger bubbles. A calibrated, floating, multi-frequency, acoustical resonator has now been used to obtain the bubble spectrum for 9 radii from 30 to 240 microns, 25 cm under breaking waves in the open sea. At this depth, bubbles larger than 60 microns have a radius dependence approximately a−2.5 and smaller bubble densities vary approximately as a a−4. Our densities are in good agreement with recently published laboratory studies of bubbles, using laser scattering under wind-blown surfaces with about the same surface frictional velocity U*. Older, photographically derived densities at ocean depth 75 cm are close to the new data only over the limited range 50 to 100 microns. Past inconsistencies between acoustical results and optical results were apparently due to an inability of some optical techniques to identify small bubbles in the difficult ocean environment.

Acoustical determinations of bubble‐size spectra

Abstract: The exact integration of the equations for acoustical cross sections of bubbles is considered to show when the dependence of the bubble number on radius can be determined by scatter and attenuation experiments in a liquid containing a broad spectrum of bubble radii.

A consistent approach to definitions and symbols in fisheries acoustics

Abstract: Long-standing problems with acoustical terminology in fisheries applications such as echo-integration indicate the need for a more consistent approach. Based where possible on existing terms, a scheme of explicitly named quantities is proposed, backed by clearly stated definitions and preferred symbols. The emphasis is on scattering phenomena because the terminology in this area presents the main source of difficulty. Starting with the scattering equations for a small target, the volume, area, and line coefficients relevant to multiple, distributed targets are defined, leading to practical formulas for the important application of remote biomass estimation from echo-integration. The aim is to incorporate, as far as possible, common practice in fisheries-acoustics terminology and related fields. The developed scheme has been commended by the ICES Fisheries Acoustics Science and Technology Working Group as a constructive approach to better communication standards in fisheries-acoustics publications.

Sea Salt Aerosol Production: Mechanisms, Methods, Measurements, and Models - A Critical Review

Abstract: Sea salt aerosol (SSA) exerts a major influence over a broad reach of geophysics. It is important to the physics and chemistry of the marine atmosphere and to marine geochemistry and biogeochemistry generally. It affects visibility, remote sensing, atmospheric chemistry, and air quality. Sea salt aerosol particles interact with other atmospheric gaseous and aerosol constituents by acting as sinks for condensable gases and suppressing new particle formation, thus influencing the size distribution of these other aerosols and more broadly influencing the geochemical cycles of substances with which they interact. As the key aerosol constituent over much of Earth's surface at present, and all the more so in pre-industrial times, SSA is central to description of Earth's aerosol burden.

Interaction of Water Waves and Currents

Abstract: Publisher Summary This chapter discusses the varied physical circumstances in which interactions among water waves and currents occur. Different mathematical approaches, relevant observations, and experiments that are applicable to all or some of these physical circumstances are described. The emphasis is on waves and their interaction with preexisting currents rather than on wave-generated currents. Common simplifying assumption is that the waves are of sufficiently small amplitude for the free-surface boundary conditions to be linearized and evaluated at, or close to, the mean free surface. Most progress can be made in this subject with such a constraint, but wherever possible, finite-amplitude effects are discussed. Unlike some other common forms of wave motion, water waves involve water motion varying with direction perpendicular to the space in which they propagate. The chapter concludes on the interaction of waves generated by a ship with the flow around it.

Scale dependence of bubble creation mechanisms in breaking waves

Abstract: Breaking ocean waves entrain air bubbles that enhance air-sea gas flux, produce aerosols, generate ambient noise and scavenge biological surfactants. The size distribution of the entrained bubbles is the most important factor in controlling these processes, but little is known about bubble properties and formation mechanisms inside whitecaps. We have measured bubble size distributions inside breaking waves in the laboratory and in the open ocean, and provide a quantitative description of bubble formation mechanisms in the laboratory. We find two distinct mechanisms controlling the size distribution, depending on bubble size. For bubbles larger than about 1 mm, turbulent fragmentation determines bubble size distribution, resulting in a bubble density proportional to the bubble radius to the power of -10/3. Smaller bubbles are created by jet and drop impact on the wave face, with a -3/2 power-law scaling. The length scale separating these processes is the scale where turbulent fragmentation ceases, also known as the Hinze scale. Our results will have important implications for the study of air-sea gas transfer.