NUWC Newport Division

The hearing thresholds of two adult manatees were measured using a forced-choice two alternative paradigm and an up/down staircase psychometric method. This is the first behavioral audiogram measured for any Sirenian, as well as the first underwater infrasonic psychometric test with a marine mammal. Auditory thresholds were obtained from 0.4 to 46 kHz, and detection thresholds of possible vibrotactile origin were measured at 0.015-0.2 kHz. The U-shaped audiogram demonstrates an upper limit of functional hearing at 46 kHz with peak frequency sensitivity at 16 and 18 kHz (50 dB re: 1 microPa). The range of best hearing is 6-20 kHz (approximately 9 dB down from maximum sensitivity). Sensitivity falls 20 dB per octave below 0.8 kHz and approximately 40 dB per octave above 26 kHz. The audiogram demonstrates a wider range of hearing and greater sensitivity than was suggested from evoked potential and anatomical studies. High frequency sensitivity may be an adaptation to shallow water, where the propagation of low frequency sound is limited by physical boundary effects. Hearing abilities of manatees and other marine mammals may have also been shaped by ambient and thermal noise curves in the sea. Inadequate hearing sensitivity at low frequencies may be a contributing factor to the manatees' inability to effectively detect boat noise and avoid collisions with boats.

Direct measurements of streamwise wavenumber-frequency spectra of turbulent wall pressure fluctuations were made in an acoustically quiet water tunnel. A linear array of evenly spaced flush mounted pressure sensors was used to measure the wall pressure field at 48 streamwise locations. This array provided over 24 dB of resolution (sidelobe rejection) in the wavenumber domain, leading to an accurate estimate of the “convective ridge” and part of the subconvective and low wavenumber portions of the spectrum at discrete frequencies. Boundary layer parameters, including the mean wall shear stress, boundary layer thickness, displacement thickness, and momentum thickness, were derived from mean streamwise velocity measurements for 8100 < Rθ < 16,700. Time and length scales derived from these parameters were used to nondimensionalize the measured spectra. The effectiveness of different scalings for nondimensionalizing the low and convective wavenumber regions at discrete frequencies was evaluated.

When a rigid body collides with a liquid surface with sufficient velocity, it creates a splash curtain above the surface and entrains air behind the sphere, creating a cavity below the surface. While cavity dynamics has been studied for over a century, this work focuses on the water entry characteristics of deformable elastomeric spheres, which has not been studied. Upon free surface impact, an elastomeric sphere deforms significantly, giving rise to large-scale material oscillations within the sphere resulting in unique nested cavities. We study these phenomena experimentally with high-speed imaging and image processing techniques. The water entry behaviour of deformable spheres differs from rigid spheres because of the pronounced deformation caused at impact as well as the subsequent material vibration. Our results show that this deformation and vibration can be predicted from material properties and impact conditions. Additionally, by accounting for the sphere deformation in an effective diameter term, we recover previously reported characteristics for time to cavity pinch off and hydrodynamic force coefficients for rigid spheres. Our results also show that velocity change over the first oscillation period scales with the dimensionless ratio of material shear modulus to impact hydrodynamic pressure. Therefore, we are able to describe the water entry characteristics of deformable spheres in terms of material properties and impact conditions.

Click data from a tagged Mesoplodon densirostris was compared with broadband acoustic recordings from an 82 hydrophone wide-baseline array located in the Tongue of the Ocean, Bahamas.Two detectors, a Fast Fourier Transform (FFT) based detector and matched filter, were evaluated in white noise and with the acoustic recordings from the array for performance detecting M. densirostris clicks.The matched filter performed the best, allowing 92% of the tagged animal's clicks to be detected on at least one hydrophone.Time Difference of Arrivals (TDOAs) between the DTag and the surrounding hydrophones were computed.These TDOAs were used to compute a three-dimensional hyperbolic localization track of the tagged animal.A maximum detection range of 6500 m from the tagged animal to the recording hydrophone was observed.Offset aspect angles were determined from the DTag heading information and the bearing to the receiving hydrophone.Clicks within 30 degrees were detected at the farthest ranges, while clicks were detected at all off-set angles at closer ranges.

Scaled worlds preserve certain functional relationships of a complex task environment while paring away others. The functional relationships preserved are defined by the questions of interest to the researcher. Different scaled worlds of the same task may preserve and pare away different functional relationships. In this paper we use the example of Ned to discuss the use of scaled worlds in applied cognitive research. Ned is based on a detailed cognitive task analysis of submarine approach officers as they attempt to localize an enemy submarine hiding in deep water. For Ned we attempted to preserve the functional relationships inherent in the approach officer's information environment while paring away other aspects of his task environment. Scaled worlds attempt to maintain the realism inherent in the preserved functional relationship while being tractable for the researcher and engaging to the participant.

The forces on an object impacting the water are extreme in the early moments of water entry and can cause structural damage to biological and man-made bodies alike. These early-time forces arise largely from added mass, peaking when the submergence is much less than one body length. We experimentally investigate a means of reducing impact forces on a rigid sphere by placing the sphere inside a jet of water so that the jet strikes the quiescent water surface prior to entry of the sphere into the pool. The water jet accelerates the pool liquid and forms a cavity into which a sphere falls. Through on-board accelerometer measurements and high-speed imaging, we quantify the force reduction compared to the case of a sphere entering a quiescent pool. Finally, we find the emergence of a critical jet volume required to maximize force reduction; the critical volume is rationalized using scaling arguments informed by near-surface particle image velocimetry (PIV) data.

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. In this study, clicks recorded from Digital acoustic Tags (DTags) attached to four M. densirostris were linked to simultaneous recordings from an 82-hydrophone bottom-mounted array to derive the source level and beam pattern of the clicks, as steps towards estimating their detectability. The mean estimated on-axis apparent source level for the four whales was 201 dBrms97. The mean 3 dB beamwidth and directivity index, estimated from sequences of clicks directed towards the far-field hydrophones, were 13° and 23 dB, respectively. While searching for prey, Blainville's beaked whales scan their heads horizontally at a mean rate of 3.6°/s over an angular range of some +/-10°. Thus, while the DI indicates a narrow beam, the area of ensonification over a complete foraging dive is large given the combined effects of body and head movements associated with foraging.

Water entry has been studied for over a century, but few studies have focused on multiple droplets impacting on a liquid bath sequentially. We connect multi-droplet streams, jets and solid objects with physical-based scaling arguments that emphasize the intrinsically similar cavities. In particular, the cavities created by the initial impact of both droplet streams and jets on an initially quiescent liquid pool exhibit the same types of cavity seal as hydrophobic spheres at low Bond number, some of which were previously unseen for jets and droplet streams. Low-frequency droplet streams exhibit an additional three new cavity seal types unseen for jets or solid spheres that can be predicted with a new non-dimensional frequency. The cavity depth and cavity velocity for both droplet and jet impact are rationalized by an energy scaling analysis and the Bernoulli equation.

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. Experiments and numerical modelling reveal that the initial spherical shape evolves as elastic waves propagate through the material. We find that the skipping dynamics are governed by the wave propagation speed and by the ratio of material shear modulus to hydrodynamic pressure. With these insights, we explain why softer spheres skip more easily than stiffer ones. Our results advance understanding of fluid-elastic body interaction during water impact, which could benefit inflatable craft modelling and, more playfully, design of elastic aquatic toys.

Tension structures continue to be of increasing importance to military applications requiring minimal weight, small packaging volumes and enhanced deployment operations. Presently, design methods for inflated fabric structures are not well established. Analysis tools for their efficient design lag behind those for conventional structures, partly because woven fabrics do not behave as a continuum. Changes in fabric architecture occur with loading and lead to several sources of nonlinear response. In particular, effective constitutive relationships must be developed that institute the combined effects of biaxial tensile stresses from inflation and shear stresses from bending for use in structural models. Through analysis and experiment, this study addressed these architectural changes, such as crimp interchange, and their effects on the mechanical properties of uncoated plain-woven fabrics. This was accomplished through meso-scale finite element analyses and material tests using a recently developed experimental fixture. The fixture facilitated testing of a wide variety of fabrics (woven, braided, knitted, etc.) subjected to combined biaxial tensile and shear stresses. The meso-scale models and swatch-level test results confirmed that: (1) crimp interchange profoundly influenced the fabric elastic and shear stiffnesses, as changes in crimp heights occurred with increasing biaxial tensions, (2) the shear modulus was highly dependent upon the biaxial tensions and compaction of the tows at the crossover points and (3) the shear modulus was highly nonlinear and was not monotonic with rotation and shear force. This study also presents analytical and experimental methods to ascertain the elastic and shear moduli of woven fabrics for use in evaluating the performance of air beams.

Two experimental underwater acoustic projectors, a tonpilz array, and a cylindrical line array, were built with single crystal, lead magnesium niobate/lead titanate, a piezoelectric transduction material possessing a large electromechanical coupling factor (k33 = 0.9). The mechanical quality factor, Q(m), and the effective coupling factor, k(eff), determine the frequency band over which high power can be transmitted; k(eff) cannot be greater than the piezoelectric material value, and so a high material coupling factor is a requisite for broadband operation. Stansfield's bandwidth criteria are used to calculate the optimum Q(m) value, Q(opt) approximately 1.2 (1-k(eff)2 1/2/k(eff). The results for the tonpilz projector exhibited k(eff) = 0.730, Q(m) = 1.17 (very near optimal), and a fractional bandwidth of 0.93. For the cylindrical transducer array, k(eff) = 0.867, Q(m) = 0.91 (larger than the optimum value, 0.7), and the bandwidth was 1.16. Although the measured bandwidths were less than optimal, they were accurately predicted by the theory, despite the highly simplified nature of the Van Dyke equivalent circuit, on which the theory is based.

Bioinspiration—using insights into the function of biological systems for the development of new engineering concepts—is already a successful and rapidly growing field. However, only a small portion of the world's biodiversity has thus far been considered as a potential source for engineering inspiration. This means that vast numbers of biological systems of potentially high value to engineering have likely gone unnoticed. Even more important, insights into form and function that reside in the evolutionary relationships across the tree of life have not yet received attention by engineers. These insights could soon become accessible through recent developments in disparate areas of research; in particular, advancements in digitization of museum specimens, methods to describe and analyze complex biological shapes, quantitative prediction of biological function from form, and analysis of large digital data sets. Taken together, these emerging capabilities should make it possible to mine the world's known biodiversity as a natural resource for knowledge relevant to engineering. This transformation of bioinspiration would be very timely in the development of engineering, because it could yield exactly the kind of insights that are needed to make technology more autonomous, adaptive, and capable of operation in complex environments.

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. We tested the ability of the tag to measure vibration in excised whiskers in a flume in response to laminar flow and disturbed flow. We then trained a seal to wear the tag and follow an underwater hydrodynamic trail to measure the whisker signals available to the seal. The results showed that whiskers vibrated at frequencies of 100-300 Hz, with a dynamic response. These measurements are the first to capture the incoming signals received by the vibrissae of a live seal and show that there are prominent signals at frequencies where the seal tactogram shows good sensitivity. Tapping into the mechanoreceptive interface between the animal and the environment may help to decipher the functional basis of this extraordinary hydrodynamic detection ability.

It is known that the water entry of a body with a recessed, cupped nose can suppress the splash and air cavity typically observed for solid body entry (Mathai, Govardhan & Arakeri, Appl. Phys. Lett. , vol. 106, 2015, 064101). However, the interplay between the captive gas in the cup, the cavity and the splash is quite subtle and has not been thoroughly explored. Here we study the cavity and splash dynamics associated with the vertical water entry of cups and find a variety of regimes over a range of Weber numbers ( $We_D$ ) and dimensionless cup depths. Our parameter space spans a transition between slow-developing cavities with long closure times (low $We_D$ ) to fast-sealing cavities (high $We_D$ ). An important dynamic event is the evacuation of trapped gas from within the cup, which drives the ensuing cavity and splash behaviour. Through modelling, we predict the conditions for which the evacuating gas inflates a cavity that opens to the atmosphere versus inflating a submerged cavity that suppresses air entrainment from above the surface. We also compare our cup water entry findings to the impact phenomena observed for flat disks, which entrap gas on the front surface similar to cups. In doing so, we reveal the sensitivity of disk splash and cavity behaviour to impact angle, and show that disks share a common regime with cups, in which a thin splash quickly seals on the body. We deduce the mechanisms by which increasing cup depth delays the cavity seal time in this regime. These findings reveal that cups may in fact promote or suppress cavity growth, depending on the cup depth and impact conditions.

The design, fabrication, and acoustic calibration of a new 1-3 piezoelectric composite-based U.S. Navy standard (USRD-F82) are presented. The F82 dual array/parametric mode projector may be used as a reciprocal linear transducer, or may be used to exploit the nonlinear properties of the water to produce highly directional acoustic beams (4 to 3 deg) at relatively low frequencies (5 to 50 kHz, respectively). As a result of its wide bandwidth, a broad range of primary as well as secondary frequencies of operation is possible. In the linear mode of operation the transducer provides two separate arrays to be addressed topside for either transmit or receive applications. The two circular apertures are centered on the acoustic axis and have active diameters of 22.8 cm (9 in.) and 5.1 cm (2 in.). The smaller array aperture could be used to obtain broader acoustic beams at relatively high frequencies. Due to the absence of air-filled pressure release components, the transducer will operate over most ocean pressures and temperatures. A general description of the 1-3 piezoelectric composite-based transducer configuration and measured performance is presented.

Partial cavitation of high-speed axisymmetric bodies is modeled using a steady potential-flow boundary-element technique. The effects of several key parameters defining the vehicle geometry are examined for configurations consisting of a disk cavitator followed by a conical section and ending in a cylindrical body. A single cavity is assumed to detach at the edge of the disk. A variety of conditions have been studied, including cavity closure on either the conical or cylindrical portions of the vehicle, variations in the cone angle, and variations in the radius of the cylindrical section. The results for the partially cavitating case are also compared with those for the supercavitating case.

A reversible elastic instability was observed in PZN-0.06PT high coupling single crystals when subjected to uniaxial compressions similar to those used in sound projectors. The strain magnitude at the onset of the instability supported a free energy prediction of a ferroelectric rhombohedral (FR)-ferroelectric orthorhombic (FO) phase transition. The thermal response of the normalized phase transition strain is in a good agreement with model calculation. A dc bias field drastically enhanced the crystal stability under compression. dc bias and compressive stress levels that are required for their stable operation in sound projectors have been deduced and will be presented.

Most existing sonar transducer technologies that are capable of producing broadband, high power acoustic signals required for future Navy needs, are only conceptual in design, or very early into their development stage. A high power broadband 16 element array of Hybrid Magnetostrictive Piezoelectric Tonpilz Transducer (MPT) projectors is currently being developed at Naval Undersea Warfare Center (NUWC), Newport, R.I. for these applications. The Hybrid Transducer combines the high strain magnetostrictive material, Terfenol-D, with that of Lead Zieconate Titanate piezoelectric ceramic, to create a double resonant, high power broadband device. This design provides a 1 kHz increase in bandwidth below the low end of the frequency band of a conventional piezoelectric Tonpilz transducers of the same size and weight. The theory of operation, fabrication technique and test results will be discussed for a single element and then compared with a conventional Tonpilz sonar transducer of same size and weight. Modeling and measurement data for the 16-element array will also be discussed.

The Naval Undersea Warfare Center has been funded by the Office of Naval Research Environmental Requirements Advanced Technology (ERAT) program to create a multi-disciplinary program to conduct Marine Mammal Monitoring on Navy Undersea Ranges (M/sup 3/R). The objective is to supplement extensive undersea range and signal processing hardware with state of the art passive marine mammal detection, localization and tracking algorithms. This system will provide the opportunity to determine the number of acoustically active species within an instrumented, open ocean area and to evaluate baseline behavior. The initial system will be demonstrated at the Atlantic Undersea Test and Evaluation Center (AUTEC), Andros Island, Bahamas. Many of the existing assets at AUTEC will be leveraged, including the data collected by the ongoing marine mammal monitoring program. Undersea range coverage at AUTEC includes a steep island slope feature that may serve as a feeding habitat for several cetaceans including the sperm whale (Physter macrocephalus), short-finned pilot whale (Globicephala macrorhynchus), minke whale (Balaenoptera acutorostrata), and several species of dolphins. The development of a database of marine mammal recordings at AUTEC is currently underway using five-minute recordings taken once every half hour. The M/sup 3/R program is increasing the rate of acoustic data collection at AUTEC and evaluating the ability to passively track the location of various species based on features of the transient acoustic marine mammal calls. Acoustic characteristics of species recorded at the AUTEC as well as developmental marine mammal monitoring algorithms are presented.

Abstract Bayesian mark‐recapture estimates of survival, abundance, and trend are reported for Cuvier's beaked whales ( Ziphius cavirostris ) using a Navy training range off southern California. The deep‐diving beaked whale family is exceptionally vulnerable to mid‐frequency active sonar (MFAS), which has been implicated in mass strandings and altered foraging behavior. Extremely low sighting probabilities impede studies of population‐level impacts of MFAS on beaked whales. The San Nicolas Basin hosts a Navy training range subject to frequent MFAS use and attracts high densities of Z. cavirostris . An 11‐year (2007–2018) photo‐identification program leveraged automated acoustic detection and location capabilities on the range's 1,800‐km 2 hydrophone array to enhance capture probability. Estimated population parameters for Z. cavirostris using the range included mean (90% credibility intervals) apparent annual survival of 0.950 (0.899–0.986), annual number of individuals as 121 (71–219), and annual rate of change of −0.8% (−5.6%–4.1%). Simulations show the probability of detecting abundance changes is currently low, but can be greatly improved through continued monitoring and increased effort. Complementary data collection on habitat use and demographic rates in San Nicolas and surrounding basins is also essential to relating direct effects of MFAS use to changes in vital rates and broader population outcomes.