格林函数解卷积处理的阵不变量浅海无源定位

水下声源无源定位是声呐技术重要的研究方向。针对水下声源无源定位问题,本文提出了一种基于格林函数解卷积处理的阵不变量无源定位方法。该方法使用盲解卷积算法从水平阵接收信号中提取时域格林函数,然后采用空域解卷积方法处理得到的时域格林函数,获得波束时间偏移,从波束时间偏移中计算得到阵不变量,解算目标距离,从而实现声源定位。区别于常规阵不变量方法,该方法可以得到更精确的波束时间偏移,从而提升了声源定位精度。仿真和实验数据结果表明,在小孔径水平阵情况下,基于格林函数解卷积处理的阵不变量浅海无源定位方法相较于常规时域处理方法的距离估计精度提高近 40%。     

水声通信加权反馈双向Turb均衡器

为了提升自适应双向Turbo均衡器的收敛速度及降低误比特率,提出了采用加权反馈的双向Turbo均衡算法。首先在单个均衡器反馈输入中采用后验均值与先验均值混合的反馈方案,有效提升一轮迭代中均衡器输出的准确性;其次通过后验均值与先验均值的加权合并作为另一均衡器反馈的非因果项输入,在提升反馈输入准确性的同时提升了数据的利用率;最后在权值迭代中采用优化的比例归一化最小均方算法,提升训练阶段均衡器收敛速度。千岛湖试验中,在同样3.75 kbps通信速率的情况下,该方法误比特率仅为传统双向Turbo均衡器的1/3。仿真和试验数据表明,均衡器要达到同样的误比特率,本方法所需迭代轮数更少,在时变信道中系统稳定性更好,误比特率更低,提升了水声通信效率。     

水声无源材料插入损失虚拟聚焦测量方法

提出了一种适合在有限尺寸水池中测量水声材料插入损失的方法,称为\     

Outage performance analysis for relay-assisted UWOC systems over GGD weak turbulence with nonzero boresight pointing errors

In this paper, we mainly investigate the outage performance of serial and parallel relay-assisted underwater wireless optical communication (UWOC) systems based on a newly proposed aggregated underwater fading model. In order to overcome the deficiency of the traditional underwater weak turbulence models, that is, they could not accurately fit the measured data in the laboratory, the generalized gamma distribution (GGD) which has been verified by a series of experiments is chosen for the first time to characterize the weak oceanic turbulence. Then, we establish a new receiving signal model which has integrated the implicit path loss plus multipath propagation effect shown by fading free impulse response (FFIR), GGD weak oceanic turbulence, and nonzero boresight pointing errors. Next, we deduce the closed-form expression of the probability density function (PDF) of the hybrid fading considering GGD weak turbulence and nonzero boresight errors based on the new receiving signal model above through double-exponential Taylor expansion and higher transcendental Whittaker function. Finally, the analytical expressions of the outage probabilities for point-to-point (P2P) link, serial and parallel relay-assisted UWOC systems are further derived respectively under the proposed aggregated channel. Numerical simulations are also provided to validate the accuracy of the theoretical formulae derived above, and to show the effects of the key system parameters on the outage performance of relaying UWOC systems.     

Underwater acoustic localization by principal components analyses based probabilistic approach

In this paper, the underwater localization is given from wireless acoustic communication signals by probabilistic pattern recognition in eigenspace of PCA (principal components analyses). It should be emphasized that our underwater localization is from existing wireless acoustic communication signals, but not from additional localization systems. Our underwater localization scheme is based on fingerprinting and contains two stages, i.e., the off-line (i.e., training) and on-line (i.e., predicting) stages. In general, the received acoustic signals fluctuate seriously in underwater environments. To reduce the complexity and noise effects, all received signals are projected onto the eigenspace of PCA. Each projected feature is assumed to have Gaussian probabilistic distributions. Therefore, the location information can be easily obtained by probabilistic pattern recognition of projected features in PCA space. Note that our underwater localization scheme is not affected by reflected signals. To illustrate such a benefit, experiments were conducted in a bounded water pool where reflected signals exist near the walls. Experimental results show that the proposed underwater localization scheme is efficient and accurate. The proposed localization scheme is useful for underwater acoustic communication networks, and then in underwater technologies.     

Shock initiated instabilities in underwater cylindrical structures

An experimental investigation to understand the mechanisms of dynamic buckling instability in cylindrical structures due to underwater explosive loadings is conducted. In particular, the effects of initial hydrostatic pressure coupled with a dynamic pressure pulse on the stability of metallic cylindrical shells are evaluated. The experiments are conducted at varying initial hydrostatic pressures, below the critical buckling pressure, to estimate the threshold after which dynamic buckling will initiate. The transient underwater full-field deformations of the structures during shock wave loading are captured using high-speed stereo photography coupled with modified 3-D Digital Image Correlation (DIC) technique. Experimental results show that increasing initial hydrostatic pressure decreases the natural vibration frequency of the structure indicating loss in structural stiffness. DIC measurements reveal that the initial structural excitations primarily consist of axisymmetric vibrations due to symmetrical shock wave loading in the experiments. Following their decay after a few longitudinal reverberations, the primary mode of vibration evolves which continues throughout later in time. At the initial hydrostatic pressures below the threshold value, these vibrations are stable in nature. The analytical solutions for the vibration frequency and the transient response of cylindrical shell are discussed in the article by accounting for both (1) the added mass effect of the surrounding water and (2) the effect of initial stress on the shell imposed by the hydrostatic pressure. The analytical solutions match reasonably well with the experimental vibration frequencies. Later, the transient response of a cylindrical shell subjected to a general underwater pressure wave loading is derived which leads to the analytical prediction of dynamic stability.     

Heaving and pitching oscillating foil propulsion in ground effect

A detailed series of experiments is performed to investigate the ‘ground effect’ experienced by propulsive flapping foils operating near a solid boundary. A high aspect ratio foil is towed at constant speed and oscillated in pitch and heave at varying distances from a rigid wall. It is shown that this distance has a significant impact on the lift and thrust forces generated by the foil, both in the time averaged mean forces and the phase averaged periodic forces. For some thrust producing kinematics, the instantaneous force profile may change significantly without altering the time averaged mean force; thus, mean force measurements alone are not sufficient to indicate the proximity, or the effect, of the solid boundary. Results are presented across a wide range of thrust generating kinematics, showing that the strength of the ground effect can be modulated, for any achievable level of thrust, through appropriate selection of kinematics. This finding in particular has significance for underwater vehicles propelled by oscillating foil thrusters, as it follows that the sensitivity of the thrusters to ground effect can be controlled independently of the desired thrust. While propulsive efficiency is increased slightly near the wall for some kinematics, in general this does not occur for kinematics where a strong ground cushion (repulsion) effect is observed. Finally, the results suggest that span-wise flow around the tip of the foil is important in determining whether the foil is repelled from or pulled into the wall.     

One-dimensional analytical model for the response of elastic coatings to water blast

In previous studies, the response of sandwich structures to water blast is solved by envisaging the plate adjacent to water as a rigid one, while the effects of the elasticity in the longitudinal direction of the plate are rarely studied. In this paper, a monolithic elastic coating with varying stiffness and thickness is investigated by a one-dimensional analytical approach, based on linear wave motion theory, to reveal the elastic effect of the plate on the incident wave. One side of the coating is loaded by a planar shock wave; on the opposite side, rigid boundary or air-backed boundary is imposed. The fluid–structure interaction (FSI), cavitation phenomenon and large deformation of the coating are taken into account. In particular, the initiation and evolution of cavitation, including the propagations of breaking fronts and closing fronts, as well as the pressure histories of radiated wave by the closing front, are examined. The analytical solution has been compared with finite element (FE) predictions. The results are found to be in excellent agreement for the propagation of breaking front and closing front, as well as the pressure and particle velocity histories at the wet face before the cavitation reaches the wet face. However, when the wet face cavitates, the predictions provided by the analytical method are less accurate and the analytically-computed particle velocity can only be compared in an average sense with the FE predictions. For air-backed case, Taylor׳s model prior to cavitation becomes a trivial case of the analytical model and the comparison also indicates the validity of the analytical model. The validated analytical model is used to determine the dependence of the peak pressure at the wet face and the impulse transmitted to the coating on the coating properties, including the wave impedance and thickness.     

Direct regressions for underwater acoustic source localization in fluctuating oceans

In this paper, we show the potential of machine learning regarding the task of underwater source localization through a fluctuating ocean. Underwater source localization is classically addressed under the angle of inversion techniques. However, because an inversion scheme is necessarily based on the knowledge of the environmental parameters, it may be not well adapted to a random and fluctuating underwater channel. Conversely, machine learning only requires using a training database, the environmental characteristics underlying the regression models. This makes machine learning adapted to fluctuating channels. In this paper, we propose to use non linear regressions for source localization in fluctuating oceans. The kernel regression as well as the local linear regression are compared to typical inversion techniques, namely Matched Field Beamforming and the algorithm MUSIC. Our experiments use both real tank-based and simulated data, introduced in the works of Real et al. Based on Monte Carlo iterations, we show that the machine learning approaches may outperform the inversion techniques.     

基于波数积分的水声传播建模实现方法研究

研究了基于波数积分的水声传播建模数值实现方法,利用局部系数矩阵的映射,建立了全局系数矩阵,分析了保持数值稳定性的方法。用FFP方法实现了数值积分,并讨论了实现过程中的一些细节。结合仿真实例与基于简正波的模型KrakenC和Kraken的计算结果进行了比较分析。结果表明,波数积分方法在近场计算时更为准确,同时还具有容易扩展到复数域计算衰减的问题、容易求解粘弹性介质中声场的特点。