Underwater Pressure Research Articles

Research on damage effect of underwater multipoint synchronous explosion shock waves on air-backed clamped circular plate

An underwater multipoint synchronous explosion can enhance the damage effect of an underwater explosion shock wave on a structure. This study investigates the damage effect of an underwater multipoint synchronous explosion on an air-backed clamped circular plate. First, we derive the theoretical formula for the superposition of underwater explosion shock wave pressure. A numerical simulation was conducted using the ALE (Arbitrary Lagrange-Euler) algorithm to investigate the damage effect of underwater explosion shock wave pressure on a clamped circular plate, and an experiment was conducted in a tank. By changing the conditions (explosive distance, explosive spacing, and number of explosives), the damage effect of an underwater multipoint synchronous explosion on an air-backed clamped circular plate was thoroughly investigated. The results show that the shock wave pressure of an underwater two-point synchronous explosion has a nonlinear superposition effect that is approximately 1.48–2.52 times greater than that of a single-point explosion. As the distances between the explosives and between the explosives and the plate decrease, the damage effect increases. When the total mass of the explosives is constant, the synchronous explosion of eight explosives has the greatest damage effect on the clamped circular plate.

Underwater fiber-optic salinity and pressure sensor based on surface plasmon resonance and multimode interference.

In this paper, an underwater fiber-optic sensor based on surface plasmon resonance (SPR) and multimode interference (MMI) is presented for simultaneous measurement of salinity and pressure. This sensor is based on a single-mode-multimode-single-mode-multimode-single-mode structure with a gold film deposited on the middle single-mode fiber and the fiber structure is wrapped around an elastic cylinder to constitute a sensing head. In the fiber structure, the SPR region produces a resonance dip to measure salinity, and the independent MMI region achieves narrow and salinity-insensitive interference dips to measure pressure. Performance of the sensor is predicted by calculation, and the MMI spectrum is simulated by using the finite-difference beam propagation method. By experimental tests for salinity and pressure, the sensitivities of 0.36 nm/‰ and -1.42nm/MPa are achieved, respectively, and the cross talk is also proved to be insignificant. This study provides an important application direction for SPR-MMI sensors and a prospective method for ocean detection.

Pinhole measurements of hydroacoustic pressure fluctuations beneath wall-bounded turbulent flows

The common flush-mounted-transducer measurement method for underwater surface pressure fluctuations results in transducer-size-limited spatial resolution, which may be inadequate for the fluctuations produced beneath a turbulent boundary layer (TBL). For water moving over a smooth surface at speeds of a few meters per second, the relevant length scales may be less than 100 mm with fluctuation magnitudes far less than 1% of the free-stream dynamic pressure. Unfortunately, such size and sensitivity requirements cannot simultaneously be met with common instrumentation. However, mounting microphones behind pinholes is a possible solution for wall-bounded airflows. This presentation covers the effect of pinhole mounting pressure transducers for measuring surface pressure fluctuations in wall-bounded turbulent flows in water. The experimental work is conducted in a turbulent channel flow at varying flow speeds (0.5 to 7 m/s) using pressure transducers with a 5-mm-diameter sensitive area that are flush mounted and mounted behind variable size pinhole openings. The measured pressure-fluctuation power spectra are compared to each other and to results from past direct numerical simulations, in the 10 Hz to 10 kHz frequency range, to determine the utility of pinhole mounting for underwater TBL pressure-fluctuation measurements. [Sponsored by ONR.]

Research on bubble jet of underwater explosion at the bottom of fixed square plate

<p indent=0mm>The water jet formed after the collapse of the bubble in the near-field underwater explosion can cause serious damage to the local structure of the ship, and the formation of the jet is asymmetric, which makes it difficult to measure its data in the experiment. At present, studies on the pressure time history curves of the bubble jet in the underwater explosion are still relatively rare. In order to study the characteristics of the jet, the underwater explosion test of <sc>2.5 g</sc> TNT explosive at the bottom of fixed square plate at different detonation distances was carried out. First, the accuracy of the test data was verified by the underwater pressure sensor. Second, the image of the whole process from the explosive initiation to the bubble jet generated by the action of the bubble and the steel plate was obtained by high-speed camera. Finally, the time-history curve of water jet pressure was measured by the wall pressure sensor in the center of the steel plate, and another group of wall pressure sensors was set <sc>15 cm</sc> away from the center of the steel plate as a control group to ensure that the measured jet data were not clutter. The test data recorded by the underwater sensor show that the error between the shock wave overpressure, the maximum bubble radius of the bubble pulse period and the empirical formula is within 10%. The results recorded by the high-speed camera and wall pressure sensor show that three jets can be observed when the burst distance is <sc>28 cm,</sc> but the pressure time history curve is not measured due to the long burst distance. When the burst distances are <sc>15,</sc><sc> 17.5</sc> and <sc>20 cm,</sc> three jet pressure time history curves are measured. However, due to the short burst distance and the presence of a large number of bubbles between the steel plate and the water surface under the action of the shock wave, it is difficult to observe the test phenomenon with high-speed cameras. The pressure time history curve shows that three jets are generated after the first bubble pulse peak, and the jet pressure can reach 1/4 of the shock wave. The jet appears 1–3 ms after the first bubble pulse peak, and the time interval of each jet is between <sc>0.9</sc> and <sc>2.4 ms.</sc> The duration of jet load is between 4–27 μs, and it is greatly reduced as the distance increases.

Similarity Law between Centrifuge Scale Test and Prototype Underwater Explosion

Shock wave and bubble pulsation caused by underwater explosion destroy the hydraulic structure. However, the realization of the underwater explosion prototype test is restricted by many factors, such as the site environment. Furthermore, the repeatability of the test scheme is not strong. The centrifuge scale test provides a new way of studying the damage degree of the structure under the action of underwater explosion. The similarity relationship refers to the bridge between the scaled model and the prototype, which cannot achieve complete similarity in practice. The centrifuge-scaled model test is performed by increasing the acceleration of a certain multiple. Meanwhile, the model reduces the corresponding ratio in the geometric layout to achieve the geometric similarity with the prototype test. Therefore, the applicability of the centrifuge scaling method in the study of the dynamic response of the structure in underwater explosion needs to be explored further. In this work, the underwater explosion scaling test numerical model for 1 g RDX (equivalent to 1.62 g TNT) charge under different centrifugal acceleration conditions is established, and the calculation results of underwater pressure and dynamic response of the steel plate are compared with the centrifuge test results. A prototype model is established to study the similarity relationship between the centrifuge scale test and the prototype model when the steel plate structure is in the stage of small deformation and linear elasticity. The application of the similarity ratio in the scale test of underwater explosion the centrifuge is discussed. The application of the centrifuge in the study of the failure response of the hydraulic structure in underwater explosion is expanded by establishing the model and comparing with the experimental results.

Experimental and Analytical Study of under Water Pressure Wave Induced by the Implosion of a Bubble Generated by Focused Laser.

In various domains of material processing, such as surface cleaning and surface treatment, cavitation phenomenon may become an alternative to traditional methods if this phenomenon is well understood. Due to experimental and mathematical difficulties in theoretical models, it is still a challenge to accurately measure the physical mechanism of the fluid/structure interactions. In this study, we verified the feasibility of using polyvinylidene fluoride (PVDF) sensors to quantitatively measure the under-water pressure wave generated by the collapse of a single cavitation bubble. The electrical signal obtained by PVDF can be converted into pressure information only by using the sensor material parameters provided by the supplier. During the conversion process, only the capacitance of the acquisition chain needs to be additionally measured. At the same time, a high-speed video recording system was used to visualize the evolution of the cavitation bubble. The Gilmore analytical model and an associated wave propagation model were used to simulate the pressure peak of the first collapse of the cavitation bubble. This theoretical pressure was compared with the experimental results. The result showed that, for bubbles with a normalized standoff distance larger than 5, the PVDF sensor had the ability to quantitatively measure the pressure wave generated by a single cavitation bubble.

ACCURATE SCALING AND LEVELLING IN UNDERWATER PHOTOGRAMMETRY WITH A PRESSURE SENSOR

Abstract. Photogrammetry needs known geometric elements to provide metric traceable measurements. These known elements can be a distance between two three-dimensional object points or two camera stations, or a combination of known coordinates and/or angles to solve the seven degrees of freedom that lead to rank deficiency of the normal-equation matrix. In this paper we present a novel approach for scaling and levelling to the local vertical direction an underwater photogrammetric survey. The developed methodology is based on a portable low-cost device designed and realized by the authors that uses depth measurements from a high resolution pressure sensor. The prototype consists of a data logger featuring a pressure sensor synchronized with a digital camera in its underwater pressure housing. The modular design, with optical communication and synchronization, provides great flexibility not requiring the camera housing to undergo any hardware modifications. The proposed methodology allows for a full 3D levelling transformation comprising two angles, a vertical translation and a scale factor and can work for surveying scenes extending horizontally, vertically or both. The paper presents the theoretical principles, an overview of the developed system together with preliminary calibration results. Tests in a lake and at sea are reported. An accuracy better than 1:5000 on the length measurement was achieved in calm water conditions.

Interaction of two approximately equal-size bubbles produced by sparks in a free field

A series of experiments are performed to study the interaction of two bubbles of approximately equal size in free-field water for different parameter combinations (i.e., the phase difference θ and the initial distance d*). First, a high-voltage discharge device that can generate multiple bubbles of different sizes in water and accurately control the time of bubble generation is introduced. For different discharge voltages, the maximum radius and the first oscillation period of a single bubble and the shock wave energy and bubble energy conversion rate are studied through high-speed photography, current monitoring, voltage probes, and an underwater free-field pressure sensor. Then the distance of the center of mass and average radius of two bubbles generated at different distances at the same time are compared with a theoretical model and good consistency is obtained when the initial distance is greater than the bubble diameter. Finally, the interaction phenomena of two bubbles generated with different initial distances and phase differences are divided into five categories, namely, coalescence, failed catapult, piercing jet, jet toward, and jet away, and the boundary conditions used to distinguish different phenomena are given. For the piercing jet, a high-speed fine jet will be generated in the bubble generated later. The jet will penetrate the wall of the bubble and propagate steadily in the water at high speed (∼79 m/s). The velocity of the jet in the bubble and in water generated with different parameter combinations is statistically analyzed, and the velocity distribution cloud diagrams are given.

Autonomous submersible multiport water sampler

Oceanography and limnology projects often require the collection of water samples for chemical analysis. Manual water sample collection is labor-intensive and often difficult, especially in remote locations or during nighttime hours. Here we describe a compact and inexpensive autonomous submersible multiport water sampler (AutoSampler) that is largely fabricated with off-the-shelf parts making it easier to build and maintain. The system can collect up to 12 discrete samples at user controllable times or intervals and is operated using open source Arduino hardware and software that can be user modified to meet deployment requirements. While the underwater pressure housing presented here is custom built from readily available materials, there are many commercially available pressure case options that can be used as a substitute. The electronic mounting plates and battery pack are designed so that they can easily be adapted to fit into other pressure case housings. Samples can be collected into bags or syringes and sample volume is set by adjusting how long the peristaltic pump is actuated. This AutoSampler allows research that would otherwise be too labor-intensive or logistically difficult to conduct, especially in remote locations.

SYSTEM THAT INTERFERES WITH THE FIRST LAW OF THERMODYNAMICS

The first law of thermodynamics, also known as Law of Conservation of Energy, states that work cannot be created or destroyed in an isolated system. Here-in presented is a system, tested and with calculations that interferes with the first law of thermodynamics. The proposed and tested system shows that it takes less work to pump air underwater than the work that is produced by the quantity of water lifted above the water level with that underwater air pressure that lifts the system, in addition to the work that accumulated in the air stored underwater. Unfortunately it is however it is not possible to utilize this work to generate a perpentual motion machine. It generates free energy, free energy is subject to irreversible loss in the course of such work.

Free bulging of thin-walled cylinders closed by two heavy plates

This study focuses on the dieless free bulging of thin-walled stainless cylinders closed by two heavy plates. Using the plastic mechanics of shells of revolution and by making several basic assumptions, a family of analytical formulae are derived to evaluate the shell bulging properties such as surface area, internal capacity, thickness distribution, equivalent strain, and bulging pressure. The analytical and numerical data were found to be in good agreement with each other; the results were validated through a free bulging experiments carried out on three nominally identical thin-walled cylinders, which will provide a reference for the manufacture of underwater thin-walled pressure structures.

Design and computational analysis of photonic crystal sensor to detect acoustic signals for underwater applications using finite difference time domain algorithm

In this paper we have designed the photonic crystal sensor for the detection of underwater pressure using finite difference time domain method. By using the photonic crystal with rods in air configuration, this photonic sensor is designed. Designed sensor is investigated for different pressure values and different values of wavelength are obtained. Two structures are designed, but design 2 photonic sensing structure is more preferred than design 1 because of its high sensitivity values. The highest sensitivity obtained is 369.29 nm/RIU i.e. for design 2. Photonic band gap is identified for each sensing structure and is used for the design of this pressure sensor.

The Influence of Various Structure Surface Boundary Conditions on Pressure Characteristics of Underwater Explosion

The shock wave of the underwater explosion can cause severe damage to the ship structure. The propagation characteristics of shock waves near the structure surface are complex, involving lots of complex phenomena such as reflection, transmission, diffraction, and cavitation. However, different structure surface boundaries have a significant effect on the propagation characteristics of pressure. This paper focuses on investigating the behavior of shock wave propagation and cavitation from underwater explosions near various structure surfaces. A coupled Runge–Kutta discontinuous Galerkin (RKDG) and finite element method (FEM) is utilized to solve the problem of the complex waves of fluids and structure dynamic response, considering the fluid compressibility. The level set (LS) method and the ghost fluid (GF) method are combined to capture the moving interface and deal with the stability of the coupling between the shock wave and structure surface. Besides, a cut-off cavitation model is introduced to the RKDG method. The validation of the numerical calculation model is discussed by comparing it with the known solution to verify the numerical solutions. Then, crucial kinds of structure surface boundary conditions include shallow-water single layer elasticity plate, double-layer crevasse elasticity plate, single layer curved elasticity plate, and double-layer curved elasticity plates are analyzed and discussed. The results and analysis can provide references for underwater explosion pressure characteristics, the impacting response of different boundary structures, and designing structures.

Modelling of underwater noise emissions by ships in Klaipėda Strait, Lithuania

One of the United Nations Sustainable Development Goals regarding “conservation and sustainable use of the oceans, seas, and marine resources” emphasizes the urgency of eliminating harmful effects on the sea and its biota, where the role of anthropogenic activities is crucial. The global trend of merchant shipping is increasing, thus enlarging underwater noise levels. As a result, greater noise can harm aquatic animals in their habitats. In the Baltic Sea, the underwater sound pressure levels are now being evaluated utilizing noise measurement, modelling, and mapping. In areas such as narrow ship passages, namely lagoons, channels, or straits, the ambient underwater noise modelling becomes very complex, even though these EU inland waters are regarded by legislation as part of the marine basin. For instance, the Klaipėda Channel (Klaipėda Strait), connecting the Baltic Sea and the Curonian Lagoon, is regarded by the national Lithuanian legislation as part of marine waters, where the environmental status should be evaluated according to the EU Maritime Strategy Framework Directive. In this narrow channel, an alternative to the modelling of ambient sound pressure levels can be applied to understand the long-term trends of vessel-sourced noise emissions. In this paper, an example of application of ship noise emission modelling for a narrow Klaipėda Harbour area is presented, along with the results obtained throughout 2015–2017. The modelled noise levels in the harbour area reached the median levels of 112.5 dB in 2015 and 102.6 dB re 1 µPa2 in 2017. The maximum emitted instantaneous sound pressure levels by ships reached 173.7 dB in 2015 and 179.4 dB re 1 µPa2 in 2017 in the area of interest.

Array Shading to Maximize Deflection Coefficient for Broadband Signal Detection With Conventional Beamforming

This paper develops and applies a numerical optimization procedure to compute broadband noise-adaptive weights for delay and sum beamforming that are conditioned to maximize the deflection coefficient at the output of a square law detector for a given set of underwater pressure measurements. The resulting optimal weights mitigate the effects of noise and interferers and maximize signal detection. Comparison of the optimal weights with minimum variance distortionless response weights show that the presented algorithm provides higher attenuation of interferers. We also use the noise-adaptive algorithm to find the optimal sparse array geometry for a given number of sensors and aperture. Comparison of the resulting optimal array with coprime, nested, and semi-coprime arrays shows that the proposed sparse array suppresses interferers more than the other sparse arrays.

Numerical investigation of marine propeller underwater radiated noise using acoustic analogy Part 2: The influence of eddy viscosity turbulence models

The present study focuses on the impact of eddy viscosity turbulence models on the benchmark INSEAN E779A marine propeller hydroacoustic performance under non-cavitating and open water conditions. In the numerical calculations, Realisable k-epsilon (k-ε), k-ω Shear Stress Transport (k-ω SST) and Spalart-Allmaras turbulence models, which are widely used in hydrodynamic fields, are selected. Hydroacoustic performance of the model propeller is predicted with the porous FW-H formulation coupled with Reynolds-averaged Navier Stokes (RANS) solver. This study aims to show the effects of different turbulence models on marine propeller hydroacoustic performance at high and low blade loading conditions both in the near and far-fields. The numerical results show that the underwater radiated noise (URN) levels, which are predicted by using different eddy viscosity turbulence models together with the porous FW-H formulation, are found to be similar at low blade loading conditions. The reason behind this similarity is due to the analogous wake structure and hydrodynamic field. However, when the propeller loading is high, the propeller's wake loses its stability; hence, the coherent vortex structures break-up and evolve into the far-field of the propeller's slipstream. The instability process of the propeller's wake is predicted in a different manner by eddy viscosity turbulence models, and these differences cause dissimilar prediction of the URN in the far-field. Consequently, the underwater pressure field is considerably affected by the instability of the vortex structures (as a non-linear noise source) for far-field noise estimations. As a result, vortex instability in the propeller's slipstream might be the main noise source of the URN for far-field noise estimations under non-cavitating and high blade loading conditions.

Land-based noise pollution impairs reef fish behavior: A case study with a Brazilian carnival

Anthropogenic sounds have spread across the biosphere, with threats from individual welfare to ecosystem health. Sounds are important to animals in both terrestrial and aquatic environments and detrimental effects have been shown across a wide range of taxa. Despite the ever-growing transformation of coastal lands by human activities, pollutant effects of sounds that propagate from land into coastal water (across realms) have been largely overlooked. We here show that the Brazilian carnival of Salvador, annually taking place along a coastal boulevard, elevated underwater sound pressure levels by more than 30 dB re 1 μPa2. We used remotely operated cameras to measure individual abundance and feeding activity of the Brazilian damsels (Stegastes fuscus) and we measured its flight-initiation distance to a model predator. Brazilian damsels did not abandon their reef territories, but their feeding activity and fleeing distance were significantly reduced under elevated noise levels, compared to the ambient control conditions at the same site and at a spatial control site. Apparently, carnival sounds emitted from land affected underwater behavior in our reef fish species. The behavioral effects may be critically important for individual fitness, and the detrimental impact of land-based noise pollution in coastal waters may also apply to other fish and invertebrate species. Humanity is claiming coastlines at a faster rate than any other habitat, and conservation concerns should extend to nearby coastal ecosystems and the possible impacts of underwater sounds emanating from land.

Low-frequency fiber optic hydrophone based on weak value amplification

A high-sensitivity low-frequency fiber optic hydrophone based on weak value amplification (WVA) is proposed and demonstrated. A polarization maintaining (PM) fiber with a length of 0.8 m wound around a polycarbonate (PC) tube is used as the sensing element. Theoretical analysis shows that the PM fiber in a WVA measurement scheme responds to underwater acoustic pressure with unprecedented sensitivity. The prototypical hydrophone based on such a scheme can sense underwater acoustic disturbance as weak as 1.3×10-6 Pa/Hz1/2 at 10 Hz, with a flat frequency response in the low-frequency band of 0.1-50 Hz. The experimental result agrees well with the theoretical prediction to within 0.5 dB.

Machine learning post processing of underwater vehicle pressure sensor array for speed measurement

An array of pressure sensors can be used to correct the drift in inertial navigation systems for underwater vehicles (UVs) in absence of other navigation support systems such as acoustic positioning, GPS and Doppler velocity measurements. To date, multiple pressure sensor arrays have been designed, proposed, and tested to prove the concept. However, it has not been researched the inclusion of non-linearities is required in the post-processing. This paper focuses on the use of machine learning as a novel approach to improve the post-processing accuracy, including non-linearities caused by the vehicle acceleration on the estimated speed compared to the linear parametric equation methodology. A series of towing tank experiments have been conducted over an array of pressure sensors located on an UV platform. The results show that pressure measurement array requires the use of non-linear post-processing methodologies as linear methodologies are not able to accurately account for vehicle acceleration effects.

An experimental study of bellows-type fluidic soft bending actuators under external water pressure

Soft actuators which are composed of low-modulus material have broad application prospects in marine exploration tasks such as biological sampling due to their inherent compliance and adaptability. However, the influence of underwater pressure on the soft actuators remains to be studied. In this work, an experimental study of bellows-type fluidic soft bending actuators fabricated with 3D printing technology is implemented. Deep sea test environment is simulated by adjustable external water pressure. The response of the soft actuator to the input pressure under different external pressure (from 1 atm to 15 MPa) is presented and discussed. The results show that the external water pressure can cause the actuator to bend more in both static and dynamic conditions. Moreover, the increase of the bending angle is positively related with the environment pressure. In general, the feasibility of the soft actuators and the fluid power system for underwater applications are verified. This work can provide experimental reference for the design and control of soft manipulators in marine operation.