Transiting aircraft parameter estimation using underwater acousticsensor data

Sound from an airborne source travels to a receiver beneath the sea surface via a geometric path that is most simply described using ray theory, where the atmosphere and the sea are assumed to be isospeed sound propagation media separated by a planar surface (the air-sea interface). This theoretical approach leads to the development of a time-frequency model for the signal received by a single underwater acoustic sensor and a time-delay model for the signals received by a pair of spatially separated underwater acoustic sensors. The validity of these models is verified using spatially averaged experimental data recorded from a linear array of hydrophones during various transits of a turboprop aircraft. The same approach is used to solve the inverse time-frequency problem, that is, estimation of the aircraft's speed, altitude, and propeller blade rate given the observed variation with time of the instantaneous frequency of the received signal. Similarly, the inverse time-delay problem is considered whereby the speed and altitude of the aircraft are estimated using the differential time-of-arrival information from each of two adjacent pairs of widely spaced hydrophones (with one hydrophone being common to each pair). It is found that the solutions to each of the inverse problems provide reliable estimates of the speed and altitude of the aircraft, with the inverse time-frequency method also providing an estimate that closely matches the actual propeller blade rate     

Time-frequency signal analysis of hydrophone data

The acoustic spectrum of a transiting aircraft, when received by a hydrophone located beneath the sea surface, changes with time due to the acoustical Doppler effect. The traditional method for analysing signals whose frequency content changes with time is the short-time Fourier transform that selects only a short segment of the signal (or window of data) for spectral analysis at any one time. The short-time Fourier transform requires the frequency content of the signal to be stationary during the analysis window, otherwise the frequency information will be smeared by the transformation. Recently, joint time-frequency distributions, which highlight the temporal localisation of a signal's spectral components, have been used to analyse nonstationary signals whose spectra are time dependent. In this paper, the short-time Fourier transform and the Wigner-Ville time-frequency distribution are applied to time-series data from a hydrophone so that the instantaneous frequency of the propeller blade rate of a turbo-prop aircraft can be estimated at short time intervals during the aircraft's transit over the hydrophone. The variation with time of the estimates of the Doppler-shifted blade rate is then compared with the corresponding temporal variation predicted using a model that assumes the sound propagates from the airborne acoustic source to the subsurface receiver through two distinct isospeed media (air and water) separated by a plane boundary (the air-sea interface). The results for five transits are presented in which the altitude of the aircraft ranged from 350 to 6050 ft with the speed of the aircraft varying from 232 to 245 kn     

Sensor position estimation and source ranging in a shallow water environment

Underwater acoustic transient signals are generated mechanically at known positions along a wharf. These signals are received by a wide aperture planar array of four underwater acoustic sensors, whose positions relative to the wharf are unknown. A method is described that enables the positions of the sensors to be estimated from accurate differential time-of-arrival measurements (with 0.1 /spl mu/s precision) as the signal wavefronts traverse the array. A comparison of the estimated positions with the nominal positions of the first three sensors, which form a 20-m-wide aperture horizontal line array, reveals a 2-cm displacement of the middle sensor from the line array axis. This slight bowing of the line array results in overranging (bias error of 3%) when the wavefront curvature method is used with the nominal collinear sensor positions to locate a static source of active sonar transmissions at a range of 59.2 m. The use of the spherical intersection method coupled with the estimated sensor positions of the line array provides an order of magnitude improvement in the range estimate (within 0.3% of the actual value). However, systematic ranging errors are observed when the sound propagation medium becomes nonstationary. Next, the differences in the arrival times of the direct path and boundary-reflected path signals at the middle sensor of the wide aperture line array are estimated using the differential phase residue of the analytic signal at the sensor output. These multipath delays are used to estimate the range and depth of the source. Although the average value of the multipath range estimates is within 0.5% of the actual value, the variance of the range estimates is 50 times larger when compared with the results of the spherical intersection and wavefront curvature methods. The multipath delay data are also processed to provide a reliable estimate of the temporal variation in the water depth enabling the tidal variation to be observed.     

On the fluid structure interaction of a marine cycloidal propeller

Marine cycloidal propulsion system is efficient in maneuvering ships like tugs, ferries, etc. It is capable of vectoring thrust in all direction in a horizontal plane. When used in pair, the system enables a vessel to perform maneuvers like moving sideways, perform rotation about a point, i.e. turning diameter of its own length, etc. In this system, the propeller blades have to change their angle of attack at different angular position of the disc. Due to this reason, the inflow velocity vector to propeller blades changes continuously. The propeller blade oscillates about a vertical axis passing through its body and at the same time rotates about a point. Superposed on these motions is the dynamics of the ship on which the propulsion system is installed. This results in a formidable and challenging hydrodynamics problem. Each of the propeller blade sections could be considered as an aerofoil operating in combined heave and pitch oscillation mode. Due to the constantly varying inflow velocity, the hydrodynamic flow is unsteady. The unsteady hydrodynamic flow is simulated by incorporating the effect of shed vortices at different time instant behind the trailing edge. Due to the kinematics of the problem, the blade is subjected to higher structural deformation and vibration load. The structural deformation and vibration when coupled with the hydrodynamic loading add another level of complexity to the problem. In this paper, the variation of hydrodynamic load on the propeller blade due to steady and unsteady flow is compared. We also model the structural dynamics of the blade and study its effect on the hydrodynamic loading. Finally, we couple the structural dynamics with hydrodynamics loading and study its influence on the propeller blade for different operating regimes.     

Numerical investigation on the hydrodynamic characteristics of a marine propeller operating in oblique inflow

The hydrodynamic characteristics of a marine propeller operating in oblique inflow are investigated by using CFD method. Two propellers with different geometries are selected as the study subjects. RANS simulation is carried out for the propellers working at a wide range of advance coefficients and incidence angles. The effects of axial inflow and lateral inflow are demonstrated with the hydrodynamic force on the propeller under different working conditions. Based on the obtained flow field details, the hydrodynamic mechanism of propeller operating in oblique inflow is analyzed further. The trailing vortex wake of propeller is highly affected by the lateral inflow, resulting in the deflected development path and the circumferentially non-uniform structure, as well as the enhanced axial velocity in slipstream. Different flow patterns are observed on the propeller blade with the variation of circumferential position. Combined with the computed hydrodynamic forces and pressure distribution on propeller, the mechanism resulting in the increase of propulsive loads and the generation of propeller side force is explored. Finally, a systematic analysis is carried out for the propulsive loads and propeller side force as a function of axial and lateral advance coefficients. The major terms that play a dominant role in the modeling of propulsive loads and propeller side force are determined through the sensitivity analysis. This study provides a deeper insight into the hydrodynamic characteristics of propeller operating in oblique inflow, which is useful to the investigation of propeller performance during ship maneuvers.     

块状冰对螺旋桨水动力性能的影响

基于重叠网格模型,通过非定常RANS数值模拟与结果分析,研究了块状冰的尺寸、轴向运动和冰桨位置对螺旋桨水动力性能的影响。选用切割体网格绘制整体静止计算域的背景网格,之后结合棱柱层网格绘制螺旋桨子计算域和冰块子计算域的重叠网格,不同的计算域之间通过两者的重叠区域进行数据传递和插值。计算结果显示,当冰块固定在桨前时,螺旋桨产生的非定常推力和扭矩均以叶频为基频进行周期性变化,而且两者的时间平均值和振幅主要受冰块在螺旋桨盘面内的轴向投影面积、冰桨轴向位置和冰桨水平位置的影响;当冰块在桨前沿轴向匀速靠近螺旋桨时,冰桨轴向距离逐渐变小,冰桨周向相对位置发生周期性的变化,使得推力和扭矩两者均以叶频振荡,而且两者的时间平均值和振幅均随着冰桨轴向距离减小而增加。     

Passive acoustic synthetic aperture processing techniques

Various parameters associated with the track of a stable CW source moving with constant velocity are estimated using synthetic aperture and Doppler processing techniques. These include the source frequency before Doppler distortion by its motion, the relative speed between the source and a constant velocity receiver, the range at closest approach to the source track, and the relative bearing to the source. Different processing techniques are suggested for a range of signal stabilities and observation times. Frequency analysis, or Doppler processing, supplements conventional synthetic aperture processing, and for relatively unstable signals a synthetic Doppler method is recommended. This method makes use of a rapid scan of signals from a succession of sensors in a horizontal line array to stimulate a higher speed motion of the array     

Multimegameter-range acoustic data obtained by bottom-mountedhydrophone arrays for measurement of ocean temperature

Acoustic signals transmitted from the ATOC source on Pioneer Seamount off the coast of California have been received at various sites around the Pacific Basin since January 1996. We describe data obtained using bottom-mounted receivers, including US Navy Sound Surveillance System arrays, at ranges up to 5 Mm from the Pioneer Seamount source. Stable identifiable ray arrivals are observed in several cases, but some receiving arrays are not well suited to detecting the direct ray arrivals. At 5-Mm range, travel-time variations at tidal frequencies (about 50 ms peak to peak) agree well with predicted values, providing verification of the acoustic measurements as well as the tidal model. On the longest and northernmost acoustic paths, the time series of resolved ray travel times show an annual cycle peak-to-peak variation of about 1 s and other fluctuations caused by natural oceanic variability. An annual cycle is not evident in travel times from shorter acoustic paths in the eastern Pacific, though only one realization of the annual cycle is available. The low-pass-filtered travel times are estimated to an accuracy of about 10 ms. This travel-time uncertainty corresponds to errors in range- and depth-averaged temperature of only a few millidegrees, while the annual peak-to-peak variation in temperature averaged horizontally over the acoustic path and vertically over the upper 1 km of ocean is up to 0.5°C     

Experimental measurement of the array bearing error caused bybathymetric refraction

Experimental measurements have been carried out to identify the effects of a realistic bottom bathymetry on detecting a source with a horizontal line array. The measurements have been conducted over a 1:10000 scale model of the Santa Lucia Escarpment. The experiments measured the bearing error obtained in locating a CW source with a horizontal line array using plane-wave beamforming. The error in the detected bearing is caused by the bending of sound rays in the horizontal plane, commonly referred to as horizontal or bathymetric refraction. The results of the experiment demonstrate large bearing errors which vary rapidly and do not increase monotonically with range. The rapid variation of the bearing errors was unexpected and was a result which has not been previously identified. The magnitude of the errors was also a function of the across slope look direction and frequency, with bearing errors as large as 26° for low frequencies. At higher frequencies, the bearing error is reduced. An analytical solution for the acoustic field over a shear supporting sloping bottom has been used to approximate the results of the experiment. The simulation confirmed the trends in the experimental results by showing bearing errors of the same order of magnitude with the same dependence on across slope direction and frequency. Most importantly, the theoretical results also showed a rapidly varying bearing error as a function of across slope range     

A brief description of the fundamental difficulties in passive ranging

The use of passive sensors to estimate range and bearing to an acoustic source is investigated. The variance of the estimators is shown to depend on the number of sensors, integration time, signal-to-noise ratio, and more significantly, on the available array length. The variances for four different array configurations are discussed.     

Low-frequency bottom loss

The shallow refracted path through sea floor sediments plays a significant role in the transmission of acoustic energy at low frequencies. For bottom grazing angles of 90/spl deg/ to 25/spl deg/, low-frequency acoustic energy was observed to come from reflected paths. For bottom grazing angles of 25/spl deg/ to 10/spl deg/ the dominant source of low-frequency acoustic energy is from shallow refracted paths through the sediments. At angles less than 10/spl deg/, low-frequency acoustic energy is received from both the refracted and the reflected paths. The refracted path is possible because of the positive gradient within the sediment. The sudden emergence of the refracted arrival is related to the overall sound path length in the sediment and sediment absorption of sound. Since sediment absorption is directly proportional to frequency, only low-frequency energy is transmitted via this path. The refracted path may well exist where unconsolidated sediments of at least a few hundred feet are present.     

Influence of Ice Size Parameter Variation on Hydrodynamic Performance of Podded Propulsor

During ice-breaking navigation, a massive amount of crushed ice blocks with different sizes is accumulated under the hull of an ice-going ship. This ice slides into the flow field in the forward side of the podded propulsor, affecting the surrounding flow field and aggravating the non-uniformity of the propeller wake. A pulsating load is formed on the propeller, which affects the hydrodynamic performance of the podded propulsor. To study the changes in the propeller hydrodynamic performance during the ice podded propulsor interaction, the overlapping grid technique is used to simulate the unsteady hydrodynamic performance of the podded propulsor at different propeller rotation angles and different ice block sizes. Hence, the hydrodynamic blade behavior during propeller rotation under the interaction between the ice and podded propulsor is discussed. The unsteady propeller loads and surrounding flow fields obtained for ice blocks with different sizes interacting with the podded propulsor are analyzed in detail. The variation in the hydrodynamic performance during the circular motion of a propeller and the influence of ice size variation on the propeller thrust and torque are determined. The calculation results have certain reference significance for experiment-based research, theoretical calculations and numerical simulation concerning ice podded propulsor interaction.     

带前缘结节叶片导管桨尾流不稳定性分析

导管桨的尾流不稳定性在其性能评价中非常重要,不但是其能否提供稳定推力的保证,而且也与螺旋桨的尾流噪声直接相关。为了改善导管桨的尾流,提高尾流稳定性,并优化导管桨的流场脉动,根据座头鲸鳍肢前缘结节的仿生原理,对导管桨叶片的导边进行改进,提出了两种仿生桨型,采用IDDES湍流模型对低进速系数下常规导管桨和仿生叶片导管桨进行数值模拟,探究叶片构型对导管桨性能和尾流不稳定性的影响。计算结果表明,前缘结节可以有效降低叶片受力波动的幅值和叶片所受合力的主频域峰值,具有较大结节的叶片对导管桨尾流有明显的优化作用,在尾流远场中扩大了流动稳定区,延后了尾流处涡破碎的发生,改善了能量谱密度的频域分布。进一步,大前缘结节叶片导管桨应用在低速工况下时,可以大量减少尾流泄涡区域的二次涡产生,这是由于前缘结节提升了相邻涡互感的强度,使得尾流更加稳定,而小结节叶片仿生桨型对导管桨尾流则无明显优化作用。研究方法和成果可为螺旋桨尤其是导管桨尾流不稳定性研究提供参考,不仅验证了前缘结节在导管桨叶片应用的合理性,而且揭示了其优化尾流稳定性的机理。     

Direct scantling assessment of propeller blades

Traditionally, propeller design has been focused on all activities necessary to obtain a propeller featuring a high efficiency, avoiding erosive cavitation for given operating conditions and having adequate structural strength. In recent years, more and more challenging requirements have been imposed, such as the reduction of radiated noise and pressures pulses, requiring more precise analyses and methods in the optimization of the propeller performance. On the other hand, the evaluation of the propeller strength still relies on simplified methods, which basically consider the blade as a cantilever beam subjected to characteristic static forces. Since the loads acting on a blade are variable in the blade revolution and in different operating conditions throughout the ship life, a procedure to account for the influence of fatigue phenomena is proposed. The fatigue assessment could reduce the safety factor in the propeller scantling rules and allow improving the quality of propeller design (e.g. obtaining higher efficiency, margin on cavitation phenomena, less noise).     

自主回收UUV水声定位与遥测遥控导引技术研究

针对 UUV 任务结束或能源不足时自主回收的需求,深入研究了 UUV 中远程水声定位与遥测遥控导引技术,完成了水声定位与遥测遥控导引系统方案设计,重点研究了 MFSK、OFDM、扩频等水声遥测遥控调制方式。 通过分析和对比,设计了一种正交混合扩频调制方式,并采用相干二维搜索技术,提高扩频技术多普勒补偿能力。 开展了湖上静态与动态跑船试验,试验数据结果表明:水声水平定位精度优于 0. 5%,水声遥测遥控系统解算误码率达到了 10^-3 数量级,可有效引导 UUV 回收作业。     

上软下硬黏土中四筒基础抗倾覆承载特性研究

随着海上风电向深远海发展,四筒基础发展潜力巨大。中国大多数海域存在较厚的上覆软土层,对基础抗倾覆承载能力有着重大影响。采用有限元软件ABAQUS建立上软下硬分层黏土模型,对四筒基础在单向水平荷载和弯矩荷载作用下的承载力特性进行研究。研究结果表明：四筒基础在水平、弯矩荷载作用下主要运动模式为转动,对边加载时,转动轴靠近受压筒且随着软土层厚度增加不断靠近四筒基础平面中心,但变化幅度较小;水平和弯矩极限承载力对于不同的筒间距和长径比具有相同的变化趋势,当软土层厚度h/筒高L≤3/4时,水平和弯矩承载力随着软土层厚度的增加近似线性降低,当h/L＞3/4后,承载力降低速率明显减小。     

On the hydrodynamic loading of marine cycloidal propeller during maneuvering

In marine cycloidal propeller (MCP), the inflow velocity vector to the propeller blade continuously changes at different blade orbit angle. Earlier marine cycloidal propellers were installed on ships that mainly performed towing operations. Recently marine cycloidal propellers are being installed on large naval vessels, which spend lot of their operating hours in cruising. Therefore, the hydrodynamic loading on the blades both during cruising maneuvers need to be investigated. The flow characteristics around the propeller blade are computed numerically by panel method. Viscous effects on the flow are then estimated by boundary layer technique. The effect of rotating disc on viscous fluid is also investigated. The corrected flow characteristics are then used for estimating the hydrodynamic loading. The operating conditions that are critical for the loading of the blade and the support structure and some aspects of the maneuvering simulation at cruising speed are investigated.     

A computationally efficient Doppler compensation system forunderwater acoustic communications

A Doppler compensation system is presented which is suitable for high-data-rate acoustic communication between rapidly moving platforms such as autonomous underwater vehicles. The proposed approach provides a generic preprocessor to conventional adaptive receiver structures with only a marginal increase in computational load and hardware cost. The preprocessor employs a novel Doppler estimation technique and efficient sample rate conversion to remove Doppler shift induced by platform velocity and acceleration. Performance predicted by simulation is compared to that of sea trials of a prototype communication system in the North Sea. Successful communication is demonstrated at 16 kbit/s with a transmitting platform moving at up to ±2.6 m/s     

Prediction of non-cavitation propeller noise in time domain

The blade frequency noise of non-cavitation propeller in a uniform flow is analyzed in time domain. The unsteady loading (dipole source) on the blade surface is calculated by a potential-based surface panel method. Then the time- dependent pressure data is used as the input for Ffowcs Williams-Hawkings formulation to predict the acoustics pressure. The integration of noise source is performed over the true blade surface rather than the nothickness blade surface, and the effect of hub can be considered. The noise characteristics of the non-cavitation propeller and the numerical discretization forms are discussed.     

Passive Doppler-bearing tracking using a pseudo-linear estimator

In this paper an explicit pseudo-linear estimator for Doppler-bearing tracking is proposed. It overcomes the problems with the bias of earlier pseudo-linear estimators and with the nonlinear frequency measurement equation by using another representation than the Cartesian one and by using the logarithm of the frequency, respectively. It is fast, inherently stable, and easy to implement. The Doppler shift is, for a nonmoving own-ship, determined by the target velocities, while the bearing rate is determined by the same velocities divided by the range. The special representation in this paper uses this difference in behavior to give fast and bias free estimation of the range. Instead of iterating a weighted feast squares problem using bearing and frequency measurements simultaneously, the true bearings and the course are estimated in a bearings only step followed by a frequency only step, which estimates range and frequency. The range estimate then gives the speed estimate. Modifications for scenarios with multiple emitted frequency lines and/or for frequency lines that disappear during parts of a scenario are shown