Water Impact: When a Sphere Becomes Flat.

A large hydrodynamic force accompanies the vertical impact of bodies on water. While added mass phenomena govern these forces for both spherical and flat impactors, the dynamics of a trapped gas layer critically alters the flat case, reducing the peak pressure below that predicted by water hammer theory. An impactor with a spherical nose cap looks increasingly flat as the nose curvature approaches zero.

Design of A Highly Sensitive, Low-cost Underwater Force Plate to Record Substrate Reaction Forces.

The study of underwater walking presents major challenges because the small forces applied during underwater walking are difficult to measure due to the lack of a sufficiently sensitive force plate that functions underwater. Understanding the force interaction between the underwater walker and the substrate may lead to better understanding of the evolution, ecology, and biomechanics of underwater walking. The shift from aquatic to terrestrial life was a crucial transition in animal evolution where, underwater walking preceded the invasion of land and combines mechanics from terrestrial locomotion (substrate reaction forces) and aquatic swimming (buoyancy and drag).

Multicolor dye-based flow structure visualization for seal-whisker geometry characterized by computer vision.

Pinniped vibrissae possess a unique and complex three-dimensional topography, which has beneficial fluid flow characteristics such as substantial reductions in drag, lift, and vortex induced vibration. To understand and leverage these effects, the downstream vortex dynamics must be studied. Dye visualization is a traditional qualitative method of capturing these downstream effects, specifically in comparative biological investigations where complex equipment can be prohibitive.

Evidence of Atlantic midshipman (Porichthys plectrodon) vocalizations from an unmanned surface vehicle in the U.S. South Atlantica).

An unmanned surface vehicle (USV; Wave Glider) was deployed to study the coastal soundscape in shallow (less than 30 m) coastal waters off the coast of Cape Canaveral, FL, in July 2020 and January 2022. These surveys documented temporal and seasonal trends in biological sounds across a variety of habitats within an 812-km2 survey area, including sand shoals, sand-mud plains, and natural hardbottom. Among a broader diversity of identifiable and unidentifiable fish choruses recorded during the survey, a distinct and previously unidentified fish chorus was recorded; corroborating evidence suggests it and other sounds with similar spectral properties may be produced by Atlantic midshipman.

Sharks associated with a large sand shoal complex: Community insights from longline and acoustic telemetry surveys.

Offshore sand shoals are a coveted sand source for coastal restoration projects and as sites for wind energy development. Shoals often support unique fish assemblages but their habitat value to sharks is largely unknown due to the high mobility of most species in the open ocean. This study pairs multi-year longline and acoustic telemetry surveys to reveal depth-related and seasonal patterns in a shark community associated with the largest sand shoal complex in east Florida, USA.

Specular-reflection contributions to static and dynamic radiation forces on circular cylinders.

Interest in the response of highly reflecting objects in water to modulated acoustical radiation forces makes it appropriate to consider contributions to such forces from perfectly reflecting objects to provide insight into radiation forces. The acoustic illumination can have wavelengths much smaller than the object's size, and objects of interest may have complicated shapes. Here, the specular contribution to the oscillating radiation force on an infinite circular cylinder at normal incidence is considered for double-sideband-suppressed carrier-modulated acoustic illumination.

Physics-based characterization of soft marine sediments using vector sensors.

In a 2007 experiment conducted in the northern North Sea, observations of a low-frequency seismo-acoustic wave field with a linear horizontal array of vector sensors located on the seafloor revealed a strong, narrow peak around 38 Hz in the power spectra and a presence of multi-mode horizontally and vertically polarized interface waves with phase speeds between 45 and 350 m/s. Dispersion curves of the interface waves exhibit piece-wise linear dependences between the logarithm of phase speed and logarithm of frequency with distinct slopes at large and small phase speeds, which suggests a seabed with a power-law shear speed dependence in two distinct sediment layers. The power spectrum peak is interpreted as a manifestation of a seismo-acoustic resonance.

Analysis of thermal and viscous boundary layers in acoustic absorption by metallic foam.

A method for estimating acoustic absorption in foams is presented using a combination of micro-computed tomography, finite element analysis, and boundary layer loss theory. In the method, the foam is assumed to be rigid framed and the viscous and thermal boundary layers at the fluid and frame interface are assumed to be small compared to foam dimensions. The boundary layer losses are approximated using an infinite planar model.

Vector sensor cross-spectral density for surface noise in a stratified ocean-Formulas for arbitrary sensor geometries.

The cross-spectral density between pressure and particle velocity channels of two vector sensors with arbitrary orientation and separation is developed for a noise field consisting of directional sources distributed on a plane. Equations for arbitrary sensor orientations are necessary because cross-spectral density is an averaged product of two sensors' outputs that cannot be projected onto another direction after it is calculated. This prevents the use of existing methods, which calculate cross-spectral density for orthogonal particle velocity sensors, for sensors with different orientations.

Reciprocity relationships in vector acoustics and their application to vector field calculations.

The reciprocity equation commonly stated in underwater acoustics relates pressure fields and monopole sources. It is often used to predict the pressure measured by a hydrophone for multiple source locations by placing a source at the hydrophone location and calculating the field everywhere for that source. A similar equation that governs the orthogonal components of the particle velocity field is needed to enable this computational method to be used for acoustic vector sensors.

Seal Whiskers Vibrate Over Broad Frequencies During Hydrodynamic Tracking.

Although it is known that seals can use their whiskers (vibrissae) to extract relevant information from complex underwater flow fields, the underlying functioning of the system and the signals received by the sensors are poorly understood. Here we show that the vibrations of seal whiskers may provide information about hydrodynamic events and enable the sophisticated wake-tracking abilities of these animals. We developed a miniature accelerometer tag to study seal whisker movement in situ.

Pressure signature and evaluation of hammer pulses during underwater implosion in confining environments.

The fluid structure interaction phenomenon occurring in confined implosions is investigated using high-speed three-dimensional digital image correlation (DIC) experiments. Aluminum tubular specimens are placed inside a confining cylindrical structure that is partially open to a pressurized environment. These specimens are hydrostatically loaded until they naturally implode.

Elastic spheres can walk on water.

Incited by public fascination and engineering application, water-skipping of rigid stones and spheres has received considerable study. While these objects can be coaxed to ricochet, elastic spheres demonstrate superior water-skipping ability, but little is known about the effect of large material compliance on water impact physics. Here we show that upon water impact, very compliant spheres naturally assume a disk-like geometry and dynamic orientation that are favourable for water-skipping.

Effective beam pattern of the Blainville's beaked whale (Mesoplodon densirostris) and implications for passive acoustic monitoring.

The presence of beaked whales in mass-strandings coincident with navy maneuvers has prompted the development of methods to detect these cryptic animals. Blainville's beaked whales, Mesoplodon densirostris, produce distinctive echolocation clicks during long foraging dives making passive acoustic detection a possibility. However, performance of passive acoustic monitoring depends upon the source level, beam pattern, and clicking behavior of the whales.

Mechanical and electromechanical properties of PMNT single crystals for naval sonar transducers.

PMNT single crystals in the relaxor-ferroelectric lead magnesium niobate (PMN)-lead titanate (PT) system provide significant advantage for underwater sonar transducers. Compared to lead zirconate titanate (PZT) ceramics, the large electromechanical coupling factor provides significant increases in transducer bandwidth. The superior strain energy density generates higher source level across the band, and the lower Young's modulus allows considerably smaller transducers.

Low-frequency echo-reduction and insertion-loss measurements from small passive-material samples under ocean environmental temperatures and hydrostatic pressures.

System L is a horizontal tube designed for acoustical testing of underwater materials and devices, and is part of the Low Frequency Facility of the Naval Undersea Warfare Center in Newport, Rhode Island. The tube contains a fill fluid that is composed of a propylene glycol/water mixture. This system is capable of achieving test temperatures in the range of -3 to 40 deg Centigrade, and hydrostatic test pressures in the range 40 to 68,950 kPa.

The design, fabrication, and measured acoustic performance of a 1-3 piezoelectric composite Navy calibration standard transducer.

The design, fabrication, and acoustic calibration of a new 1-3 piezoelectric composite-based U.S. Navy standard (USRD-F82) are presented.