Flow structures of gaseous jets injected into water for underwater propulsion

Gaseous jets injected into water are typically found in underwater propulsion,and the flow is essentially unsteady and turbulent.Additionally,the high water-to-gas density ratio can result in complicated flow structures;hence measuring the flow structures numerically and experimentally remains a challenge.To investigate the performance of the underwater propulsion,this paper uses detailed Navier-Stokes flow computations to elucidate the gas-water interactions under the framework of the volume of fluid(VOF) model.Furthermore,these computations take the fluid compressibility,viscosity,and energy transfer into consideration.This paper compares the numerical results and experimental data,showing that phenomena including expansion,bulge,necking/breaking,and back-attack are highlighted in the jet process.The resulting analysis indicates that the pressure difference on the rear and front surfaces of the propulsion system can generate an additional thrust.The strong and oscillatory thrust of the underwater propulsion system is caused by the intermittent pulses of the back pressure and the nozzle exit pressure.As a result,the total thrust in underwater propulsion is not only determined by the nozzle geometry but also by the flow structures and associated pressure distributions.     

MAP-regularized robust reconstruction for underwater imaging detection

In order to enhance the visual quality of underwater images, applications such as enhancement and restoration can be applied, but the resolution is still limited. Super-resolution reconstruction is a widely used technique for improving resolution beyond the limit of imaging system. With knowledge of the point spread function and techniques of regularization, the performance of reconstruction can be further enhanced. The presented effort proposed a robust image super-resolution reconstruction method under maximum a posteriori framework with regularization by the point spread function for underwater imaging detection. Objective image quality metrics are used to quantify the effectiveness of the reconstruction. Experimental results showed that the proposed method can effectively improve the resolution and quality of underwater imaging detection.     

不同水深水下湿法焊接电弧等离子体数密度研究

水下湿法焊接技术应用日益广泛,由于特殊的焊接条件,导致深水下其焊接质量亟待改善。通过搭建水下湿法焊接实验平台,压力罐调节气压分别模拟0.3, 20和40 m水深,界定焊接引弧阶段,分别采集三个水深环境条件下焊接引弧阶段的光谱信息及电压电流数据,采集光谱信息时利用光谱仪的延时触发功能,分别采集引弧5,10,15,20和25 ms时刻的光谱数据,对采集到的电弧光谱数据整理后进行诊断分析。诊断分析时结合NIST原子光谱数据库以及特征谱线的相关数据,得到各元素粒子的识别结果。对于高价态的元素离子态,因为其电离能比较大,激发电离程度会受到电弧温度变化的影响,不能仅靠光谱图进行识别诊断,还需要进一步对其组分进行数密度计算。结合水下湿法焊接电弧光谱诊断的信息和水下湿法焊接反应过程,确定出计算中要考虑的电弧等离子组分的18种粒子,求解由沙哈方程、解离电离方程、准中性方程、气体压力平衡方程等组成的方程组,采用牛顿迭代法对方程组进行联立求解,对于求解非线性方程组,采取分段赋值的方法,得到等离子体组分在三个水深环境下的数密度,并对其变化规律进行分析,探究不同水深环境对焊接电弧等离子体数密度影响及因素。研究表明各个粒子数密度在不同水深条件下的变化是非线性的,随着水深加大电弧数密度变化幅度也快速增大。随着水深的增加,电弧会受到压缩,但电弧不能无限制被压缩;粒子的电离受温度的影响,温度越大电离作用越强烈,但当温度升高到一定程度时,各个电离作用有其电离极限,粒子数密度也不会无限增大。通过不同水深条件下焊接电弧引弧阶段数密度的计算,对水下焊接电弧引弧阶段粒子产生的机理进行了研究,为提高水下焊接电弧稳定性及电弧模拟仿真计算等提供了理论依据。     

Determination of the best carrier frequency based on the system and environmental parameters in underwater acoustic systems

One of the most challenging features of underwater acoustic (UWA) channel in comparison with its terrestrial radio frequency counterpart is highly frequency-dependent path loss. Thus, utilizing efficient carrier frequencies in UWA systems can considerably reduce the path loss. In this context, this paper presents an approximate formula for determining the best carrier frequency based on both the system and environmental parameters. To achieve this goal, this research first addresses a simple algorithm including general steps for tuning the parameters of Francois and Garrison (FG) formula in the frequency range of 10 to 100 kHz based on the appropriate experimental data which can be acquired from any interest region. Second, for a more accurate modeling of path loss, this paper considers the loss due to the reflections of sound from both the rough surface and bottom of the sea by employing the ray theory. Third, this study presents a general algorithm for modification of the power spectral density (PSD) of ambient noise based on Wenz formula in the frequency range of 10 to 100 kHz and the required experimental measurements which can be simply collected from any interest channel. Moreover, it is mathematically demonstrated that the ambient noise in the frequency range of 10 to 100 kHz, can be generally approximated with a strict sense stationary (SSS) colored normal stochastic process which is ergodic not only in mean and covariance but also in distribution. Finally, an approximate formula for the best carrier frequency is derived by maximizing the sound to noise intensity ratio (SNR). To verify the validity of simplifications and approximations utilized in this study and to assess the performance of our proposed algorithms and formulas, experimental results obtained in the Strait of Hormuz (SoH) are compared with the original, simplified, and modified models under different scenarios.     

MIMO-OFDM underwater acoustic communication systems—A review

The ever increasing demand for bandwidth, efficiency, spatial diversity and performance of underwater acoustic (UWA) communication has opened doors for the use of Multi-Input Multi-Output (MIMO). A combination of MIMO and Orthogonal Frequency Division Multiplexing (OFDM) has proved to be a promising solution for many scenarios in UWA communication; on the contrary, it also amplifies the design challenges for implementing such schemes to acquire the required bandwidth efficiency. The goal of this study is to provide a comprehensive survey of the latest researches in the field of UWA MIMO-OFDM communication. The previous works are summarized, reviewed and compared according to their years of publication while problems faced by UWA MIMO-OFDM communication are highlighted. The articles are classified according to the focused techniques like channel estimation, equalization, coding and detection. Furthermore the works are compared based on the complexity and performance of the algorithms while some future research issues are identified.     

Underwater Oil Wettability on Nanostructured Superamphiphobic Surface Tuned by Trapped Air Layer Continuity

When the superamphiphobic meshes are immersed in water, the rough structures on steel wires are filled with air. The nanostructured superamphiphobic surfaces were prepared on the stainless-steel mesh. By adjusting the mesh size of the surface, the continuity of trapped air layer on the superamphiphobic surface underwater could be controlled. Then the underwater oil-wetting behavior on the prepared superamphiphobic mesh was investigated. The oil droplet spread out on the superamphiphobic surface without mesh and exhibited an oil contact angle of about 0° under water. But the oil contact angle formed on the superamphiphobic mesh surfaces and extended with increasing mesh size. We thought the discontinuity of trapped air layer on the surface and the entry of water into interval between the steel wires should be responsible for these behaviors.     

Dynamic compression of metallic sandwich structures during planar impulsive loading in water

The compressive response of rigidly supported stainless steel sandwich panels subject to a planar impulsive load in water is investigated. Five core topologies that spanned a wide range of crush strengths and strain-dependencies were investigated. They included a (i) square-honeycomb, (ii) triangular honeycomb, (iii) multi-layer pyramidal truss, (iv) triangular corrugation and (v) diamond corrugation, all with a core relative density of approximately 5%. Quasi-statically, the honeycombs had the highest peak strength, but exhibited strong softening beyond the peak strength. The truss and corrugated cores had significantly lower strength, but a post yield plateau that extended to beyond a plastic strain of 60% similar to metal foams. Dynamically, the transmitted pressures scale with the quasi-static strength. The final transmitted momentum increased slowly with core strength (provided the cores were not fully crushed). It is shown that the essential aspects of the dynamic response, such as the transmitted momentum and the degree of core compression, are captured with surprising fidelity by modeling the cores as equivalent metal foams having plateau strengths represented by the quasi-static peak strength. The implication is that, despite considerable differences in core topology and dynamic deformation modes, a simple foam-like model replicates the dynamic response of rigidly supported sandwich panels subject to planar impulsive loads. It remains to ascertain whether such foam-like models capture more nuanced aspects of sandwich panel behavior when locally loaded in edge clamped configurations.     

基于滑模理论的水下机器人定深控制算法研究

水下机器人操控性能指标中对定深控制性能有较高要求,而水下机器人的运动具有强的非线性和耦合性,使得不同航速下的定深控制成为难点。建立了水下航行器的运动学模型,基于滑模理论设计了深度控制器和纵倾控制器,在MATLAB SIMULINK环境下搭建了深度控制仿真系统,数值仿真结果表明：滑模变结构控制器对于不同航行条件具有较强的适应性,同时深度的控制效果明显优于PID控制器。     

Durable,water-cleanable,superhydrophilic coatings for oil/water separation under harsh conditions

A simple, economical, and efficient method for fabricating stable hydrophilic/underwater superoleophobic coating under harsh conditions remains a significant challenge. Here, by the hydrolysis of 3-(Methacryloyloxy) propyltrimethoxysilane (TMSPMA) on cotton fabric and the free radical polymerization of [2-(Methacryloyloxy) ethyl] dimethyl-(3-sulfonic acid propyl) ammonium hydroxide (SBMA) and TMSPMA, a superhydrophilic coating was fabricated. The coating can withstand harsh environments, such as strong acid and alkali. In addition, the coated cotton fabrics show an effective separation of surfactant-stabilized oil-in-water emulsions with extreme flux as high as 1500 Lm^?2 h^?1 only under gravity. Importantly, the oil-contaminated coated cotton fabrics can be cleaned only by water washing. The outstanding properties of the coating including durability, recyclability and resistance to harsh environment, highlight its practical application in emulsion separation and oily wastewater purification.     

各种水下声源的发声机理及其特性

扼要综述了现有的七种水下声源产生技术,包括炸药爆炸声源、电声换能器声源、参量阵声源、流体动力式声源、电磁式声源、激光声源和等离子体声源。分析了这些声源的发声机理,结合其性能特点阐述了在实际中的不同应用,并对等离子体声源的机理、特点和实际应用进行了重点分析。列表归纳总结了各种水下声源的原理、主要性能和应用范围,为水下声源的应用拓展提供了依据。