The acoustic radiation from an immersed cylindrical shell, periodically stiffened by internal axisymmetric frames, has been studied in the past due to its interest in underwater applications. Particularly, it has been shown that Bloch–Floquet waves induced by the periodic arrangement of stiffeners can lead to significant radiation of the shell in the far field. However, the fluid domain in these studies was generally unbounded, which is not representative of practical applications when the submerged structure is close to the sea surface. This work, therefore, investigates the influence on radiated pressure of a free surface close to a periodically stiffened cylindrical shell immersed in water. The shell is excited by a harmonic point force. The free surface corresponds to a pressure released boundary condition. A semi-analytical model is developed based on a frequency–wavenumber decomposition. The cylindrical shell and stiffeners are represented, respectively, by Flügge's analytical model and finite element models. The radiation impedance of the fluid domain including the free surface is evaluated using the image source method. Radiated pressure results are presented as a function of angle and frequency to study the effects of the free surface, specifically on Bloch–Floquet waves.

The characterization of the scattered pressure field of a submerged cylindrical target is of interest for underwater acoustics applications. The aim of this work is to study the vibroacoustic behavior of an immersed cylindrical shell excited by a plane wave, particularly in close proximity to a free surface. To achieve this goad, finite element simulation and experimental investigation are carried out on a mock-up. The effect of the free surface is highlighted by comparing results to those obtained in a free field situation, in both time and frequency domains. Additional echoes are observed on the time signals due to the presence of the free surface. In fact, after being scattered by the target, these echoes reflect on the surface, implying a time shift between echoes directly from the target and those that have travelled a longer path. The spectrum is also impacted as the form function shows additional oscillations. These observations are depth-dependent. To determine wave trajectories, a study based on flight time and ray theory is conducted.

Metagratings are flat periodic assemblies of small scatterers, engineered to achieve effects such as steering, beamforming, or absorption. They have recently received attention in both electromagnetism and acoustic fields. In this work, we report the design, modeling, and experimental characterization of metagratings to steer underwater acoustic wavefronts towards anomalous directions (i.e. not classically allowed by Snell-Decartes relationships) with high efficiency (close to 100%). They are build from simple assemblies of small brass cylinders, hold by 3D-printed plastic supports, and can redirect ultrasonic wavefronts either in reflection mode (when placed in front of a reflective surface like e.g. the water / air interface) or in transmission mode. Furthermore, we demonstrate how metagratings build from an asymmetrical pattern of multiple basic elements can achieve near perfect asymmetrical transmission: a wavefront incident from side of the structure is fully transmitted, while a similar wavefront incoming from the other side is fully reflected, achieving an acoustic one-way mirror effect. This work combine theoretical analysis, finite element modeling, and experimentation in water tanks to explore and demonstrate the potential of such metagratings for various applications in underwater acoustics, like communication, noise mitigation, and stealth.

Acoustic asymmetrical transmission is a theoretical and engineering challenge because of the reciprocity of the linear acoustic wave equation. It can be achieved by systems breaking reciprocity or by reciprocal systems relying solely on spatial symmetry breaking. Metagratings are planar structures relying on Bragg's diffraction to reroute wave energy toward a desired direction and are eventually able to achieve asymmetrical transmission when build from an asymmetrical pattern of multiple basic elements. The challenge for water-like media is to combine the geometrical complexity of the structure with good acoustic impedance contrast and practical feasibility. In this work, we build a reciprocal metagrating from brass cylinders arranged according to a numerically optimized pattern and obtain highly efficient asymmetrical transmission for underwater acoustic waves. Around 200 kHz, the structure transmits nearly all incident energy toward a 45° angle when insonified from one side, but act as a near perfect reflector when insonified from the other. The effect relies entirely on the simple phenomena of linear wave diffraction and interference. The generality and efficiency of this device could be of interest for applications in underwater acoustics or medical ultrasounds.

The acoustic scattering technique is a powerful tool in the non-destructive control and evaluation of structures. In many real applications, it is important to study the acoustic scattering in bistatic configuration (the emitter is far from the receiver) since it allows to give multiple viewpoints of the considered structure. The present paper investigates both theoretically and experimentally the acoustic scattering in different viewpoints of a cylindrical shell. To analyse the bistatic evolution of the acoustic scattering, we call upon the clockwise and counterclockwise waves propagating on the cylindrical shell. The use of time–angle representation gives a map of the different waves circumnavigating around the shell. This representation allows to show the chronological evolution of the acoustic scattering as a function of the azimuthal angle. The conducted spectral analysis reveals that the frequency content of the scattered acoustic response varies from one location to another. The majority of the resonant frequencies can be detected in the monostatic configuration (the emitter and receiver are co-located). The application of the time–frequency method on the experimental acoustic signals recorded in different viewpoints of the shell allows to analyse and follow the time and frequency shifts of the clockwise and counterclockwise waves propagating on the cylindrical structure. The echo patterns observed in the time–frequency plane are used to estimate the arrival times of the clockwise and counterclockwise waves as a function of the azimuthal angle. Based on the obtained arrival times, we estimate the delay between the clockwise and counterclockwise waves and therefore their group velocities.

Metagratings are periodic arrays of subwavelength scatterers, or atoms, engineered to refract or reflect waves toward anomalous directions with unit efficiency. Here, we design and build a metagrating to control the reflection direction of waterborne ultrasound waves impinging on a rigid or free surface. The grating and its atoms are designed to cancel the specular reflection and to redirect acoustic power toward a negative-reflection direction, through the first-negative-order Floquet mode. Despite a simple design, based on C-shaped brass particles acting as Helmholtz resonators, the grating is efficient (> 90%) over a relatively broad range of frequencies (74-103 kHz) for a broad range of incidence angles (14 degrees-54 degrees). This good performance is obtained by tuning the distance between the atoms and the reflective surface. A multiple-scattering analytical model is presented to explain the phenomenon and a finite-element model is developed to further investigate the performance of the proposed design. Predictions from the model are confirmed experimentally in a water tank. The simplicity, reconfigurability, and scalability of the design, as well as its high efficiency, broadband behavior, and robustness to the incidence angle are all features that make the grating potentially useful for various applications in underwater acoustics, such as telemetry, communication, or noise mitigation.

The optimization of stealth (response to an active sonar) and of discretion (noise radiated from the vehicle) performances for underwater vehicles at very low frequencies is currently an important research topic in underwater acoustics. The solution presented here consists in the introduction of slight modifications of the shell internal structure.

Double layer structures consisting of stainless steel and polymer rods are designed to blur and attenuate Bragg and Bloch-Floquet scattering from a periodically ribbed plate in a given frequency bandwidth. These structures can be considered as ribbed plate-spring-mass systems, the resonance frequencies of which are obtained from random and circular permutations of five basic oscillators. Analytical and finite element methods are used to find their parameters and tank tests have been carried out to ensure the accuracy of the numerical results and validate the relevance of such a model. It has been found that the typical features associated with the periodicity of the ribs are replaced by a "Christmas tree" appearance when the far field pressure backscattered from the model is displayed in the (frequency, aspect angle) plane. An analysis of backscattering patterns is presented and comparisons with Naval Research Laboratory results are made.

Forward acoustic scattering of an immersed solid LINE (cylinder bounded by hemispherical endcaps) in water is investigated in our study. The object is made of stainless steel and its L/2a ratio is equal to 2 (L: Length of the cylinder part and a: the radius is equal to 60 mm). An impulse measurement method is used in the experimentation. Most of results are obtained experimentally, in bistatic configuration. Mobile receiver transducer is located in a distinct position from the emitter. The polar diagram patterns of the scattered pressure shows an important amplitude of pressure in the shadow side of this object. Analysis of this phenomenon is based on theoretical and experimental results obtained for a sphere with the help of elasticity theory. Moreover, this study relies on the grey-level representation of the angular position of the receiver in function of recorded time signals. Thus on forward acoustic time signals, it is possible to identify echoes due to propagation paths of waves on this object.

Non-destructive testing and structural health monitoring are essential for safety and reliability of marine kinetic energy related fields. In this work we address the problem of damage detection of underwater finite length plates using acoustic inverse scattering and image processing methods. Time series signals and the 2D images obtained from these signals have been studied to improve detection accuracy. Optimal parameters are selected for near-end edge echoes elimination and binarization is used to reduce computational complexity. A robust and simple method was proposed to detect and localize a possible damage in 2D images based on image processing and analysis. Experimental results show that the detection rate for crack damage reaches 100% and localization accuracy reaches 96% on average.

The aim of this work is to study theoretically and experimentally, the acoustic radiation of a cylindrical elastic shell excited by a point source with no internal loading and immersed in a water tank. We are primarily focused on the theoretical study of a cylindrical elastic shell receiving a point force in the form of a Dirac pulse and the acoustic pressure radiated along the tube into a point at far-field observation. The model used is based on the theory of elasticity. It is therefore interesting, in a second step, to conduct experiments to validate these theoretical calculations and to realize complementary measures corresponding to the case of a focalized observer. The used shell is a cylindrical stainless steel tube characterized by its ratio equal to 0.94. Two experiments are carried out. The first experiment is based on a bistatic method, and the transmission and reception are performed by two focalized transducers, characterized by a center frequency of 1 MHz. For the second experiment, the two transducers are different in nature; the transducer acting as the receiver is replaced by a plane transducer characterized by a center frequency identical to the transmitter focalized transducer

Health monitoring is investigated for immersed structures. The environment of these structures makes their monitoring and diagnosis very difficult. In this paper, the major challenge is to make easy and efficient the monitoring of this kind of structures. The proposed detection method is based on non contact measurements with acoustic scattering. It uses artificial intelligence, with gaussian neural networks and signal processing with wavelets transformation and principal component analysis. The method is validated with experimental measurements collected from immersed plates including surface cracks with different orientations. Such plates represent a simplified model of underwater turbine blades.

Lamb modes are widely used for non-destructive evaluation of plate-like structures and simple interpretation procedures for the analysis of the monitored structures are needed. In this study we apply the orthogonality relation based method for post-processing Finite Element (FE) predictions in order to separate Lamb modes at a plate edge. The reflected wave field from the free edge is a superposition of all the eigenmodes of an infinite plate. The eigenmode amplitudes of the reflected wave field are determined by applying the orthogonality-based method. Overlapping wavepackets of Lamb modes at a plate edge are simulated by using the FE model of the incident S0 mode in a plate with a crack. Time-domain signals of propagating and non-propagating modes are extracted.

The assessment of sound pressure levels generated by submerged structures is part of problems regarding underwater noise pollution. The growing number of offshore constructions related to energy generation by wind farms increases the importance of these studies. The understanding of physical mechanisms responsible for this radiation should allow us to reduce the noise by refining the construction of structures. The object of the present work is the study of underwater acoustic radiation of tube vibrations. The results presented were obtained on two stainless steel tubes. These tubes are placed vertically into water. The vibrations are generated by a shock obtained using a hammer with a force sensor hitting the tube on the emerged part. The vibrations of the tube placed in the air and in water are measured using accelerometers. The influence of water on the modes of resonances observed is then highlighted. The acoustic radiation measurements are made by a hydrophone located at a few meter distance of the tube. Results are presented as time signals and spectra and are compared to results of analytical and numerical calculations. Relationships between the resonances of the tube and the acoustic radiation in water are then interpreted.

A marine current turbine is made of a boss and several blades. An acoustic scattering method is appropriate to detect defects in this machine. The acoustic backscattering from a blade identical to a finite plane plate has previously been studied. The flight times of acoustic echoes are calculated taking into account the quasi Lamb waves in a plate. In order to connect with the reality, the shape of the chosen blade is a bent finite dimension sheet. To simplify the study, a half cylindrical shell obtained cutting a tube in two parts is used. This target, immersed in water, is excited with short impulses in a plan perpendicular to its surface with incidence angles between 0 to 360°. Successively, the convex part and the concave part of the half tube are excited. The backscattering signals are compared with those which are obtained from an air-filled cylindrical shell immersed in water. The flight times of the different acoustic signals are identified taking into account the circumferential guided waves in the shell. Spectra are computed with a Fourier transform and the resonances are identified. The convex part and the concave part do not give same results. This experimental result will be explained.

The evaluation of sound pressure levels produced by submerged structures is a part of regulations on underwater noise pollution. The purpose of this work is the study of the underwater acoustic radiation of a stainless steel tube subjected to vibrations generated by a shock obtained by using a hammer. The vibrations of the tube, placed successively in air and in water, are measured by using accelerometers. In water, the acoustic radiation measurements are performed by using a hydrophone. Results are presented as frequency spectra and are confronted with results of the elastic theory.

This paper is devoted to the study of acoustic vibrations induced by a flow upon an air-filled cylindrical tube vertically placed in water. A water pump with adapted piping generates a turbulent flow horizontally canalized in a large laboratory tank (6 m × 4 m × 3 m). The tube is located across this flow and an accelerometer measures vibrations. The signal processing performed on the recorded signals brings out resonance modes of the tube excited by the flow. A theoretical study (tube in air) and complementary experiments (tube in air and in water) are conducted to identify these modes.