Rui Tang

Passive acoustic monitoring of ocean surface wind speed and rainfall faces severe degradation in shallow waters due to multipath propagation and non-meteorological noise interference. To address these challenges within the Internet of Underwater Things (IoUT), this study proposes a sequential acoustic sensing framework for vertical linear array (VLA) nodes to enhance meteorological inversion accuracy. First, an incoherent averaging method is introduced to mitigate acoustic field fluctuations by accumulating vertical spatial information across the sensing channels. Second, the Correlation Matrix of Energy Spectrum (CMES) and the higher-order bispectrum are analyzed to decouple cross-frequency characteristics and identify the dominant frequency bands of the acoustic sources. On this basis, a Principal Component Analysis (PCA)-assisted bispectral entropy method is implemented after spectral reconstruction, enabling accurate environmental scenario classification and interference suppression at the sensor level. Experimental validation using ambient noise data recorded by an IoUT VLA node in the South China Sea demonstrates that the proposed framework effectively stabilizes the acoustic field, reducing continuous spectrum fluctuations by 7–8 dB. Consequently, the applicability and robustness of empirical wind speed inversion models are significantly improved, providing a resilient and high-fidelity sensing solution for interference-prone shallow sea IoUT applications.

To realize flow field monitoring without interference to the flow field, this study proposes a new passive flow velocity and flow direction measurement system. This method utilizes piezoelectric lead zirconium titanate sensor arrays to measure the flow velocity and direction based on the relevant physical properties of the pressure fluctuation field of the turbulent boundary layer (TBL). Moreover, it can be widely used in flow field monitoring in limited and complex areas with strong interferences. First, the intensity and spatiotemporal characteristics of the TBL pressure fluctuation field are discussed, and the physical principles of three flow velocity measurement methods and two flow direction measurement methods are given according to the different physical characteristics. Second, the effectiveness of each method is verified using numerical simulations. Then, two basic array configurations are given, sensors are produced, and the flow velocity and flow direction measurements under interference environment are conducted in a water tunnel to verify the effectiveness of each measurement method and the noise reduction strategy. The advantages and disadvantages of each measurement method are discussed. Furthermore, neural networks are introduced to reduce the need for manual calibration. Finally, to improve the measurement accuracy and precision, a fusion strategy of the three flow velocity measurement methods is presented. The results prove that the flow velocity and flow direction measurement system can realize effective measurement without interfering with the flow field movement under interference environment.

In order to design a low-noise water-filled pipeline system, it is necessary to obtain knowledge of the dispersion characteristics of axial propagation modes in different water-filled elastic tubes. In this work, an algorithm is developed based on the spectral method, which has previously been used to solve the dispersion of axisymmetric modes in cylindrical structures but has not yet been applied to non-axisymmetric modes. The algorithm can obtain the dispersion characteristics, modal displacement, and stress distribution of axial propagation modes in a fluid-filled elastic multi-layer tube. The algorithm behaves well both at low and ultrasonic frequencies, and it is suitable for any tube dimensions, wall thickness and layers. The results of a water-filled PMMA tube obtained using the spectral method algorithm were verified using a COMSOL simulation, while the dispersion curves of the same tube from the literature were found to be missing some low-order modes. In addition, the dispersion curves of a water-filled three-layer tube are given. The spectral method algorithm has the advantages of fast calculation speed, less computational resources consumed, accurate results, and no modal omission.

Similar to air reverberation chambers, non-anechoic water tanks are important acoustic measurement devices that can be used to measure the sound power radiated from complex underwater sound sources using diffusion field theory. However, the problem of the poor applicability of low-frequency measurements in these tanks has not yet been solved. Therefore, we propose a low-frequency acoustic measurement method based on sound-field correction (SFC) in an enclosed space that effectively solves the problem of measuring the sound power from complex sound sources below the Schroeder cutoff frequency in a non-anechoic tank. Using normal mode theory, the transfer relationship between the mean-square sound pressure in an underwater enclosed space and the free-field sound power of the sound source is established, and this is regarded as a correction term for the sound field between this enclosed space and the free field. This correction term can be obtained based on previous measurements of a known sound source. This term can then be used to correct the mean-square sound pressure excited by any sound source to be tested in this enclosed space and equivalently obtain its free-field sound power. Experiments were carried out in a non-anechoic water tank (9.0 m × 3.1 m × 1.7 m) to confirm the validity of the SFC method. Through measurements with a spherical sound source (whose free-field radiation characteristics are known), the correction term of the sound field between this water tank and the free field was obtained. On this basis, the sound power radiated from a cylindrical shell model under the action of mechanical excitation was measured. The measurement results were found to have a maximum deviation of 2.9 dB from the free-field results. These results show that the SFC method has good applicability in the frequency band above the first-order resonant frequency in a non-anechoic tank. This greatly expands the potential low-frequency applications of non-anechoic tanks.

Underwater reverberation environments that satisfy the conditions of uniformity and isotropy of the diffuse field can be used to measure the acoustic characteristics of underwater targets. This study combines two practical indicators — the standard deviation of the absolute sound pressure field (to indicate uniformity) and the analysis of the wavenumber spectrum in the spherical harmonics domain (to indicate isotropy) — for an accurate evaluation of the diffusion of the sound field in a reverberation tank. A method is proposed that can improve the narrow-band diffusion of the sound field by employing a randomly fluctuating surface. An acoustic experiment was performed in a reverberation water tank (1.2 m × 1 m × 0.8 m), where a randomly fluctuating surface was generated by making waves. The experimental results show that as the wave motion contributes effectively to the random reflection of sound rays in all directions, the uniformity and isotropy are improved significantly when the surface is fluctuating randomly. This work helps to ensure accurate measurements of the characteristics of underwater targets in reverberation tanks.

In order to perform flow velocity measurements without interfering in the flow field itself, a flow velocity measurement method based on the characteristics (the rule by which the power spectrum varies with the flow velocity) of turbulent pressure fluctuations is proposed. This method is based on the characteristics of turbulent pressure fluctuations in the turbulent boundary layer (TBL) and uses a polyvinylidene fluoride (PVDF) sensor to convert pressure fluctuations into voltage outputs. First, TBL pressure fluctuation characteristics are studied. Second, the piezoelectric characteristics of PVDF piezoelectric sensors are studied and a sensor array signal processing method that can improve the signal-to-noise ratio is provided. Then, numerical simulations are performed to verify the characteristics of turbulent pressure fluctuations in the TBL. Finally, a PVDF sensor is made and flow velocity measurement experiments are performed. The results show that the power spectrum of the PVDF sensor response to excitation by turbulent pressure fluctuations is proportional to the fourth power of the flow velocity. It is feasible to use this characteristic to measure the flow velocity.

Underwater noise produced by rainfall is an important component of underwater ambient noise. For example, the existence of rainfall noise causes strong disturbances to sonar performance. The underwater noise produced by a single raindrop is the basis of rainfall noise. Therefore, it is necessary to study the associated underwater noise when drops strike the water surface. Previous research focused primarily on the sound pressure and frequency spectrum of underwater noise from single raindrops, but the study on its sound energy is insufficient. The purpose of this paper is to propose a method for predicting the acoustic energy generated by raindrops of any diameter. Here, a formula was derived to calculate the underwater sound energy radiated by single raindrops based on a dipole radiation pattern. A series of experiments were conducted to measure the underwater sound energy in a 15 m × 9 m × 6 m reverberation tank filled with tap water. The analysis of the acoustic energy characteristics and conversion efficiency from kinetic to acoustic energy helped develop the model to predict the average underwater sound energy radiated by single raindrops. Using this model, the total underwater sound energy of all raindrops during a rainfall event can be predicted based on the drop size distribution.

This paper reports an experimental study on the electrical properties of five coal samples taken from various Chinese coal mines. The dielectric permittivity and specific resistivity of grinded coal samples subjected to electromagnetic (EM) fields in a wide frequency spectrum were determined. Based on the experimental data, a set of approximating equations of the change in electric properties the 100–1000 MHz frequency region was obtained. These equations, along with EM equations for EM speed and attenuation, were used for modeling and studying radar-wave propagation in a coal seam and radar-wave reflection from the body of miners trapped in collapsed tunnels. The modeling concept assumes that a radar transducer with the dominating frequency of 500 MHz is lowered through a vertical or inclined rescue borehole to the depth of the coal seam. It is assumed that only the miner is present in the part of the tunnel that did not collapse. Thus, in the path of the radar wave from the transducer to the human body, only one geological interface reflecting the radar signals is present (coal-air) and one is connected with human body. The human (acting as the reflector) can be located at various distances from the tunnel face; this factor was included in the analyzed geometrical model. Based on the modeling results of different thickness coal seams (2, 3, 4, 5, 6, 7, 8m), conclude that a radar wave reflected from a human body can be reliably measured, when the distance between the human and the transducer is not exceed 8m.

In this paper, a finite-element method for calculating the sound field in a water tank with impedance boundaries is proposed based on the theory of standing waves in a tube. The equivalent acoustic impedance of the tank walls is calculated by establishing a three-dimensional axisymmetric virtual standing-wave tube in finite-element software, whereupon boundaries with that impedance are used as the tank boundaries. Since the impedance is the property of the material itself, the calculated impedance value can be used for the calculation of the three-dimensional sound field. The sound field due to a point source in a glass tank is calculated using the proposed method, the correctness of which is assessed experimentally. By comparing the experimental and numerical results, the proposed method is shown to be correct.