Underwater Pressure Research Articles

Correlation of the peak pressure generated by an underwater spark discharge with energy absorption in a spark channel

In this paper, we have explained the correlation between the amplitude of the underwater shock pressure generated by a spark discharge in water and the energy absorption in a spark channel. With the well-known theory of shock wave formation by an accelerating piston, the peak pressure is considered to be generated by the energy absorption in the spark channel until the expansion speed of the spark channel reaches its maximum value. To obtain the maximum channel speed, we solve the power balance equation with experimentally measured voltage and current waveforms as a transient heating source of the spark channel. It is found that the experimentally measured peak pressure is well correlated with the energy absorption until the maximum channel expansion speed, irrespective of the operating parameters such as the capacitance and the voltage. It is also found that the energy absorption in the spark channel is significantly influenced by the spark channel resistance, indicating that an appropriate resistance model for the spark channel is necessary for an improved analysis of an underwater spark discharge system. The physics-based estimation of the peak shock pressure presented in this paper is expected to be useful for optimizing underwater spark discharge systems utilized in various application fields.

Application of polyvinylidene fluoride for pressure measurements in an underwater explosion of aluminized explosives

To study underwater explosions, a new type of an underwater pressure sensor using polyvinylidene fluoride as a sensing element is developed and well calibrated. The sensor is then applied in underwater explosion tests of RDX explosives with addition of aluminum in the fiber or powder form. The results show that the peak pressure of shock waves slightly decreases when aluminum is added to RDX, and the peak pressure of the fiber aluminum composite explosive is slightly inferior to that of the explosive containing the aluminum powder at the same measuring positions.

Acoustic power measurement of high-intensity focused ultrasound transducer using a pressure sensor

The acoustic power of high-intensity focused ultrasound (HIFU) is an important parameter that should be measured prior to each treatment to guarantee effective and safe outcomes. A new calibration technique was developed that involves estimating the pressure distribution, calculating the acoustic power using an underwater pressure blast sensor, and compensating the contribution of harmonics to the acoustic power. The output of a clinical extracorporeal HIFU system (center frequency of ~1 MHz, p+ = 2.5–57.2 MPa, p− = −1.8 to −13.9 MPa, ISPPA = 513–22,940 W/cm2, −6 dB size of 1.6 × 10 mm: lateral × axial) was measured using this approach and then compared with that obtained using a radiation force balance. Similarities were found between each method at acoustic power ranging from 18.2 W to 912 W with an electrical-to-acoustic conversion efficiency of ~42%. The proposed method has advantages of low weight, smaller size, high sensitivity, quick response, high signal-to-noise ratio (especially at low power output), robust performance, and easy operation of HIFU exposimetry measurement.

Hearing of the African lungfish (Protopterus annectens) suggests underwater pressure detection and rudimentary aerial hearing in early tetrapods.

In the transition from an aquatic to a terrestrial lifestyle, vertebrate auditory systems have undergone major changes while adapting to aerial hearing. Lungfish are the closest living relatives of tetrapods and their auditory system may therefore be a suitable model of the auditory systems of early tetrapods such as Acanthostega. Therefore, experimental studies on the hearing capabilities of lungfish may shed light on the possible hearing capabilities of early tetrapods and broaden our understanding of hearing across the water-to-land transition. Here, we tested the hypotheses that (i) lungfish are sensitive to underwater pressure using their lungs as pressure-to-particle motion transducers and (ii) lungfish can detect airborne sound. To do so, we used neurophysiological recordings to estimate the vibration and pressure sensitivity of African lungfish (Protopterus annectens) in both water and air. We show that lungfish detect underwater sound pressure via pressure-to-particle motion transduction by air volumes in their lungs. The morphology of lungfish shows no specialized connection between these air volumes and the inner ears, and so our results imply that air breathing may have enabled rudimentary pressure detection as early as the Devonian era. Additionally, we demonstrate that lungfish in spite of their atympanic middle ear can detect airborne sound through detection of sound-induced head vibrations. This strongly suggests that even vertebrates with no middle ear adaptations for aerial hearing, such as the first tetrapods, had rudimentary aerial hearing that may have led to the evolution of tympanic middle ears in recent tetrapods.

Investigation on the Effect of Underwater Acoustic Pressure on the Fundamental Mode of Hollow-Core Photonic Bandgap Fibers

Recently, microstructured optical fibers have become the subject of extensive research as they can be employed in many civilian and military applications. One of the recent areas of research is to enhance the normalized responsivity (NR) to acoustic pressure of the optical fiber hydrophones by replacing the conventional single mode fibers (SMFs) with hollow-core photonic bandgap fibers (HC-PBFs). However, this needs further investigation. In order to fully understand the feasibility of using HC-PBFs as acoustic pressure sensors and in underwater communication systems, it is important to study their modal properties in this environment. In this paper, the finite element solver (FES) COMSOL Multiphysics is used to study the effect of underwater acoustic pressure on the effective refractive indexneffof the fundamental mode and discuss its contribution to NR. Besides, we investigate, for the first time to our knowledge, the effect of underwater acoustic pressure on the effective areaAeffand the numerical aperture (NA) of the HC-PBF.

Energy balance during underwater implosion of ductile metallic cylinders.

Energy-based metrics are developed and applied to a numerical test case of implosion of an underwater pressure vessel. The energy metrics provide estimates of the initial energy in the system (potential energy), the energy released into the fluid as a pressure pulse, the energy absorbed by the imploding structure, and the energy absorbed by air trapped within the imploding structure. The primary test case considered is the implosion of an aluminum cylinder [diameter: 2.54 cm (1 in.), length: 27.46 cm (10.81 in.)] that collapses flat in a mode-2 shape with minimal fracture. The test case indicates that the structure absorbs the majority (92%) of the initial energy in the system. Consequently, the energy emitted as a pressure pulse into the fluid is a small fraction, approximately 5%, of the initial energy. The energy absorbed by the structure and the energy emitted into the fluid are calculated for additional simulations of underwater pressure vessel implosions. For all cases investigated, there is minimal fracture in the collapse, the structure absorbs more than 80% of the initial energy of the system, and the released pressure pulse carries away less than 6% of the initial energy.

In situ near-shore wave resource assessment in the Fiji Islands

Directional wave measurements were carried out at two different near-shore locations in the Fiji islands with the help of an underwater pressure transducer — a Directional Wave Recorder. The primary site which is located in the west of the main island in Fiji has a moderate energy potential of 9.81kW/m at a depth of 15m. The second site which was the focus of past wave energy measurements was also studied and the new data along with previous measurements show high energy potential at this location. This site near Kadavu Island has a near-shore energy flux of around 28.78kW/m at a depth of 18 m. The directional spread of the waves and the nature of their occurrence are presented. Additionally, the sea states during a category 2 tropical cyclone passing about 200km from the measurement location are discussed.

Finite difference computational modeling of marine impact pile driving

Computational models based on the finite difference (FD) method can be successfully used to predict underwater pressure waves generated by marine impact pile driving. FD-based models typically discretize the equations of motion for a cylindrical shell to model the vibrations of a submerged pile in the time-domain. However, because the dynamics of a driven pile are complex, realistic models must also incorporate physics of the driving hammer and surrounding acousto-elastic media into the FD formulation. This paper discusses several of the different physical phenomena involved, and shows some approaches to simulating them using the FD method. Topics include dynamics of the hammer and its coupling to the pile head, transmission of axial pile vibrations into the soil, energy dissipation at the pile wall due to friction, acousto-elastic coupling to the surrounding media, and near-field versus far-field propagation modeling. Furthermore, this paper considers the physical parameters required for predictive modeling of pile driving noise in conjunction with some practical considerations about how to determine these parameters for real-world scenarios.

Underwater blast wave pressure sensor based on polymer film fiber Fabry-Perot cavity.

This paper describes the theoretical and experimental aspects of an optical underwater shock wave sensor based on a polymer film optical fiber Fabry-Perot cavity manufactured by vacuum deposition technology. The transduction mechanism of the sensor involves a normally incident acoustic stress wave that changes the thickness of the polymer film, thereby giving rise to a phase shift. This transient interferometric phase is interrogated by a three-phase-step algorithm. Theoretically, the sensor-acoustic-field interaction principle is analyzed, and the phase modulation sensitivity based on the theory of waves in the layered media is calculated. Experimentally, a static calibration test and a dynamic calibration test are conducted using a piston-type pressure calibration machine and a focusing-type electromagnetic shock wave. Results indicate that the repeatability, hysteresis, nonlinearity, and the overall measurement accuracy of the sensor within the full pressure range of 55MPa are 1.82%, 0.86%, 1.81%, and 4.49%, respectively. The dynamic response time is less than 0.767μs. Finally, three aspects that need further study for practical use are pointed out.

An investigation into environment dependent nanomechanical properties of shallow water shrimp (Pandalus platyceros) exoskeleton

The present investigation focuses on understanding the influence of change from wet to dry environment on nanomechanical properties of shallow water shrimp exoskeleton. Scanning Electron Microscopy (SEM) based measurements suggest that the shrimp exoskeleton has Bouligand structure, a key characteristic of the crustaceans. As expected, wet samples are found to be softer than dry samples. Reduced modulus values of dry samples are found to be 24.90±1.14GPa as compared to the corresponding values of 3.79±0.69GPa in the case of wet samples. Hardness values are found to be 0.86±0.06GPa in the case of dry samples as compared to the corresponding values of 0.17±0.02GPa in the case of wet samples. In order to simulate the influence of underwater pressure on the exoskeleton strength, constant load creep experiments as a function of wet and dry environments are performed. The switch in deformation mechanism as a function of environment is explained based on the role played by water molecules in assisting interface slip and increased ductility of matrix material in wet environment in comparison to the dry environment.

An experimental study of steep solitary wave reflection at a vertical wall

Until now very few experimental investigations have been conducted to study the reflection of steep solitary waves at a vertical wall whereas many theoretical analyses and numerical simulations have been developed in the past. The use of experimental techniques to capture the waveform and associated phenomena during the short-time head-on collision of two solitary waves (or the reflection of a solitary wave by a vertical wall) is not an easy task. Solitary waves with amplitude a/h≤0.556 are experimentally generated by a piston type wavemaker. We have used a high speed camera and our experimental results were compared with previous studies, including both theoretical investigations and numerical simulations.Experimental values of attachment and detachment times, wall residence time, maximum run-up time and phase shift due to reflection at the wall as a function of solitary wave amplitude are new results. In addition, we found that previous theoretical results underestimate wave run-up characteristics (maximal run-up amplitude, attachment and detachment times, wall residence time), except the third-order result of Su and Mirie (1980) who calculated maximal run-up which is in good agreement with experiments. Within the range of solitary wave amplitude considered experimentally, present measurements are in excellent agreement with numerical results of Cooker et al. (1997) and Chambarel et al. (2009).For very steep solitary waves we found numerically the occurrence of a Rayleigh–Taylor instability on the top of the jet due to the head-on collision. A theoretical proof of the existence of this instability is given. Furthermore, the occurrence of this instability is corroborated by numerical computations of the vertical gradient of underwater pressure during the collision of the solitary waves.

Experimental and numerical research on the underwater sound radiation of floating structures with covering layers

This paper presents experimental and numerical investigation into the underwater sound radiation characteristics of a free-floating stiffened metal box covered with three different kinds of covering layers and subjected to mechanical excitation. One box is bare while the other three are, respectively, covered with solid covering layers, chiral covering layers, and chiral covering layers filled with expanded polystyrene (EPS) foams. The equivalent elastic modulus of chiral covering layer is obtained by the homogenization theory. The finite element method and boundary element method are used to calculate the underwater sound pressure. The measured and numerical results are illustrated and the sound insulation mechanisms of three covering layers are discussed. The measured results agree with the numerical results well. The covering layers can obviously reduce the underwater sound radiation of floating structures. Compared with the solid covering layer, the chiral covering layer is less effective in suppressing the sound radiation in the low-frequency range but more effective in the medium- and high-frequency range. The chiral covering layer filled with EPS foams shows the best performance, which is more effective in suppressing the sound radiation both in the low-frequency range and in the medium-frequency range. The EPS foams have a high contribution to the added damping of the chiral covering layer.

Offshore pile driving noise—Prediction through comprehensive model development

Offshore wind energy is one of the most potent among renewables and thus the worldwide number of offshore wind turbines increases rapidly. The foundations of the wind turbines are typically fastened to the seabed by impact pile driving, which comes along with a significant amount of waterborne noise. To protect the marine biosphere, the use of noise mitigation systems, like bubble curtains or cofferdams, may become necessary. In this context, the model-based prediction of underwater sound pressure levels as well as the design and optimization of effective sound mitigation measures by using numerical models is one of today’s challenges. The current work presents a modeling approach that consists of a near field finite element model and a far field propagation model. Furthermore, it has been found necessary to generate a benchmark to allow for a qualitative and quantitative comparison between the manifold modeling approaches that are currently developed at various institutes and companies.

Properties influencing the transmission loss and mitigation of the underwater sound from marine pile driving

The transmission loss of underwater sound generated by impact or vibratory pile driving, combined with the potential mitigation of this sound, ultimately determine the regulatory zone-of-impact on marine life. For impact pile driving the pile source assumes properties of a “phased array” as shown in Reinhall and Dahl [J. Acoust. Soc. Am. 130, 1209–1216 (2011)], where sound radiation arises from the state of radial expansion which travels down the pile at a speed which is supersonic relative to the speed of sound in water. The acoustic field that develops is shown to be the dominant contributor to peak underwater sound pressure. An important range scale can be identified that is useful for both guiding measurements, and for transmission loss modeling. For vibratory pile driving, measurements made at close range (within about a one water depth) show considerable low-frequency content that can be below the cut-off frequency of the waveguide. Some key properties of impact and vibratory pile driving will be di...

Behavioral responses of a harbor porpoise (Phocoena phocoena) to playbacks of broadband pile driving sounds

The high under-water sound pressure levels (SPLs) produced during pile driving to build offshore wind turbines may affect harbor porpoises. To estimate the discomfort threshold of pile driving sounds, a porpoise in a quiet pool was exposed to playbacks (46 strikes/min) at five SPLs (6 dB steps: 130–154 dB re 1 μPa). The spectrum of the impulsive sound resembled the spectrum of pile driving sound at tens of kilometers from the pile driving location in shallow water such as that found in the North Sea. The animal's behavior during test and baseline periods was compared. At and above a received broadband SPL of 136 dB re 1 μPa [zero-peak sound pressure level: 151 dB re 1 μPa; t90: 126 ms; sound exposure level of a single strike (SELss): 127 dB re 1 μPa2 s] the porpoise's respiration rate increased in response to the pile driving sounds. At higher levels, he also jumped out of the water more often. Wild porpoises are expected to move tens of kilometers away from offshore pile driving locations; response distances will vary with context, the sounds' source level, parameters influencing sound propagation, and background noise levels.

Long Period Grating-based optical fibre sensor for the underwater detection of acoustic waves

In this work, a Long Period Grating (LPG)-based acoustic sensor was designed and evaluated for underwater acoustic pressure measurement. The paper presents the sensing principle and characterization of the device, prior to discussing the results of a series of experiments carried out. The optical sensor was based on a LPG clamped, placed underwater and exposed to acoustic waves generated by an underwater loudspeaker. Its characteristics were measured by varying either the amplitude, the frequency of the acoustic waves, or the distance between the sensor and the acoustic source and the depth of the sensor in the water. The LPG-based sensor was subsequently fully characterized and the results obtained are cross-compared with data obtained from acoustic sensing measurements in air.

Experimental and Numerical Study on the Afterburning Effect of TNT

In order to investigate the afterburning effect of TNT in an open space, a double-layer container (DLC) which can be filled with different gases and enhance the afterburning effect of underoxidized explosives was designed. The charges were located in the inner container, and the outer container was filled with different gases (air, oxygen or nitrogen). The experiments were conducted under water. After initiation, the DLC cracks and provides gas for the detonation products. Underwater static pressure transducer was the main diagnostic. It is shown that pressure and impulse histories for explosions in oxygen and air are greater than those recorded for explosions in nitrogen. Moreover, the afterburning energy was calculated. Results show that the afterburning energy increases with the increase of the amount of oxygen, but cannot reach the theoretically maximum value even though there is excessive oxygen. Finally, two-dimensional numerical simulations were performed by the explicit finite element program ANSYS AUTODYN. The Miller energy release model was used to describe the afterburning process. Results demonstrate that computed pressure histories agreed with measured pressure histories well in terms of initial peak pressure, waveforms and total impulse.

The novel measurement method of liquid level and density in airtight container

This paper describes a novel method of liquid level and density measurement with application in airtight container such as oil storage tank. In order to prove the method, a multifunctional pressure-type liquidometer (MPTL) was designed. The MPTL comprises two pressure sensors for capturing the underwater pressure accurately, by which the MPTL could calculate the density of the liquid and back-calculate the level of the liquid. A digital temperature sensor was implanted in the MPTL to collect the temperature of the liquid. Series of experiments show a favorable linearity of 0.2% and a high accuracy of 0.27%. Besides, the simple fabrication, low cost and unconstrained conditions guarantee its popularity in the petrochemical industry fields. Overall, the findings of this study confirm the feasibility of the novel liquid level measure method and offer an economical scheme for mass producing.

Polymer MEMS pressure sensor arrays for fish-like underwater sensing applications

Blind cave fish are capable of sensing flows and movements of nearby objects even in dark and murky water conditions with the help of arrays of pressure-gradient sensors present on their bodies called lateral-lines. To emulate this functionality of lateral-lines for autonomous underwater vehicles, an array of polymer MEMS pressure sensors have been developed that can transduce underwater pressure variations generated by moving objects. Individual sensors are fabricated using liquid crystal polymer as a membrane material, which gives high robustness and reliability, and higher sensitivity compared with its silicon counterparts. The individual sensor in the array achieves a sensitivity of 90 mV/(m/s) with a high resolution of 25 mm/s. The underwater object detection capability of the array is demonstrated through proof-of-concept experiments. The array is capable of determining the velocity and distinguishing various distances of an underwater stimulus with high accuracy and repeatability.

A Flexible Underwater Pressure Sensor Array Using a Conductive Elastomer Strain Gauge

This paper presents a flexible pressure sensor array which is demonstrated to transduce underwater pressure variations produced by moving objects and surface waves. The sensors exhibit a 0.0014 fractional resistance change per 100 Pa, achieving a high 1.5-Pa pressure resolution. The measurement has a repeatability of 22% of the peak amplitude of the pressure waveform, due to creep. Additionally, sensor operation while bent to a 0.5-m radius of curvature is demonstrated. Each sensor consists of a strain-concentrating polydimethylsiloxane (PDMS) diaphragm and a resistive strain gauge made of a conductive carbon-black-PDMS composite. A 1-D array of four sensors with a 15-mm center-to-center spacing is fabricated, and the dynamic response of the sensors is characterized and modeled.