A novel array of elliptic ring radiators

An array of elliptic ring radiators is proposed which generates a fan-type beam with controllable sidelobe levels. Such an array has applications in specialized acoustic systems such as sidescan sonars, sonars for fish finding and stock assessment, obstacle avoidance systems, and others. Two possible approaches to the design are presented. One approach utilizes a mapping which transforms a radiation pattern generated by a circular ring array to that of an elliptic ring array. This approach takes advantage of methodologies developed for the design of circular ring arrays. The other approach uses the concept of an equivalent linear array. This approach benefits from existing design techniques developed for linear arrays. It requires only simple matrix operations and does not involve any optimization. The design examples presented demonstrate that a fan-type beam with sidelobes suppressed to more than 40 dB in all possible directions is achievable     

A novel array of ring radiators

An array of ring radiators which generates a symmetric search-light-type narrow beam with greatly reduced sidelobes is proposed. Such a narrow beam can find several applications related to acoustic remote sensing, telemetry, and specialized sonars. A design procedure which benefits from design techniques developed for linear arrays is detailed. The developed methodology requires only simple matrix operation and does not involve nonlinear optimization. The results indicate that radiation patterns with arbitrary sidelobe suppression can be achieved     

The effect of sub-optimal control and the spectral wave climate on the performance of wave energy converter arrays

A linear hydrodynamic model is used to assess the sensitivity of the performance of a wave energy converter (WEC) array to control parameters. It is found that WEC arrays have a much smaller tolerance to imprecision of the control parameters than isolated WECs and that the increase in power capture of WEC arrays is only achieved with larger amplitudes of motion of the individual WECs. The WEC array radiation pattern is found to provide useful insight into the array hydrodynamics. The linear hydrodynamic model is used, together with the wave climate at the European Marine Energy Centre (EMEC), to assess the maximum annual average power capture of a WEC array. It is found that the maximum annual average power capture is significantly reduced compared to the maximum power capture for regular waves and that the optimum array configuration is also significantly modified. It is concluded that the optimum configuration of a WEC array will be as much influenced by factors such as mooring layout, device access and power smoothing as it is by the theoretical optimum hydrodynamic configuration.     

声相关计程仪基阵设计方法

针对声相关计程仪的测速特点,研究了一维、二维基阵的设计方法。一维基阵采用基于约束最小冗余的设计方法,可以获得比均匀线阵大得多的阵列孔径,从而提高基阵的利用率,但约束最小冗余线阵(RMRLA)的设计方法计算量巨大,并不适合二维基阵的设计。在重新定义冗余因子,建立理想位置矢量图模型,提出位置矢量重合率等概念的基础上,实现了适用于声相关测速需求的二维基阵的快速设计。仿真结果证明了该设计方法的可行性。     

Code-fed omnidirectional arrays

The synthesis of linear and planar arrays of desired omnidirectional intensity patterns is considered. A novel approach that utilizes the relation between the array intensity pattern and the correlation function of the used feeding sequence, or code, is adopted. The basic theory of such code-fed arrays is reviewed, and it is shown that almost omnidirectional patterns result when codes with sharp autocorrelation functions are used as the feeding sequences. Examples of omnidirectional linear and planar arrays fed with Barker codes, Kuttruff-Quadt trial-and-error two-dimensional binary codes, and nonbinary Huffman-type codes are presented. The results have direct application in underwater communication systems, public address systems, and acoustical imaging systems     

Stochastic optimization of linear sparse arrays

In conventional beamforming systems, the use of aperiodic arrays is a powerful way to obtain high resolution employing few elements and avoiding the presence of grating lobes. The optimized design of such arrays is a required task in order to control the side-lobe level and distribution. In this paper, an optimization method aimed at designing aperiodic linear sparse arrays with great flexibility is proposed. Simulated annealing, which is a stochastic optimization methodology, has been utilized to synthesize the positions and the weight coefficients of the elements of a linear array in order to minimize the peak of the sidelobes and to obtain a beam pattern that meets given requirements. An important novelty is the fact that the latter goal can be achieved in parallel to the minimization of both the number of elements and the spatial aperture, resulting in a “global” optimization of the array characteristics. The great freedom that simulated annealing allows in defining the energy function to be minimized is the main reason for the notable versatility and the good results of the proposed method. Such results show an improvement in the array characteristics and performances over those reported in the literature     

Dynamics of arrays of floating point-absorber wave energy converters with inter-body and bottom slack-mooring connections

If point absorbers are employed in the extensive exploitation of the offshore wave energy resource, they should be deployed in arrays, the distance between the elements in the array being possibly tens of meters. In such cases, it may be more convenient that the array is spread moored to the sea bottom through only some of its elements, located in the periphery, while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. An array of identical floating point absorbers located at the grid points of an equilateral triangular grid is considered in the paper. A spread set of slack-mooring lines connect the peripheric floaters to the bottom. A weight is located at the centre of each triangle whose function is to pull the three floaters towards each other and keep the inter-body mooring lines under tension. The power take-off system (PTO) is a linear damper activated by the buoy heaving motion. The whole system–buoys, moorings and power take-off systems–is assumed linear, so that a frequency domain analysis may be employed. Hydrodynamic interference between the oscillating buoys in array is accounted for. Equations are presented for a set of three identical point absorbers. This is then extended to more complex equilateral triangular grid arrays. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of hemispherical converters in arrays of three and seven elements and for different mooring parameters and wave incidence angles. Comparisons are given with the unmoored and independently-moored buoy situations.     

Technical Note: Successful instrument array deployments in soft mud bottoms adjacent to the Mississippi delta

An anchor design for instrument arrays was developed for use in the very soft muddy sediments adjacent to the Mississippi Delta. Because of low sediment shear strengths and high sensitivities combined with large dynamic loadings due to strong currents, problems in successful deployments and recoveries were anticipated. A total of six deployments were attempted; four were sediment trap arrays and two were current meter/transmissometer arrays. All six deployments and recoveries were successful. The array design presented herein is a safe, easy and cost effective method for deploying instrument arrays in areas where soft bottoms and strong currents are a consideration.     

Static Design of Cable Mooring Arrays for Offshore Guyed Towers

The key consideration in controlling the dynamic response of an offshore guyed tower subjected to environmental loading is the selection of the stiffness characteristics of the cable mooring array. In the present study, each cable in the array consists of three segments: a leading segment attached to the tower, a short heavy middle segment, and a seabed trailing segment attached at the extremity to an anchor pile. A nonlinear static model is presented in algorithmic form that defines the stiffness characteristics of the cable array. The model involves the inelastic static catenary equations coupled with conditions of geometric compatibility, formulated in terms of seven nondimensional system parameters. The effects of varying each of the system parameters on the stiffness characteristics of example cable arrays are presented and the implications for cable design are discussed.     

Clear water scour around a finite array of cylinders

This experimental study presents clear-water scour and deposition patterns around hexagonal arrays of circular cylinders in steady flow conditions. Understanding the scour processes around such configurations could facilitate the design of several hydraulic and marine engineering structures, such as bridge piers and piles. The flow alteration caused by the examined porous obstacles depends on the solid volume fraction of the obstacles and on the angle of attack of the incoming flow, due to the limited number of cylinders constituting the array. Flume experiments with erodible bed were carried out for four array densities (solid volume fractions: 0.14, 0.20, 0.32 and 0.56) under three different orientations (regular, angled and staggered configurations). The scour/deposition characteristics were obtained by means of laser scanner and the results were compared to solid cylinders of equal circumambient diameter. Different angles of attack of the incoming flow lead to different blockage ratios, which have direct impact on the scour characteristics and deposition patterns. The arrays with the higher solid volume fraction generated scour/deposition patterns similar to solid cylinder, while in the arrays with the lower solid volume fractions, local scour around the individual small cylinders became evident. Finally, considering that the load bearing capacity of a pier basically depends on the area of its cross-section, a comparison of the maximum induced scour depth and volume by the cylinder arrays and the solid cylinder with equal solid cross-sectional area is presented, in order to introduce an alternative pier configuration that induces less scour. The results showed that the array of cylinders could generate 27% less scour volume and 22% less scour depth compared to its single solid cylinder counterpart.     

Beamforming on seismic interface waves with an array of geophoneson the shallow sea floor

The problem of locating very low frequency sound sources in shallow water is made difficult by the interaction of propagating acoustic waves with the sea floor. Slow wave speeds and the attendant short wavelengths suggest that low frequency beamforming and source localization with sea floor geophones can be accomplished with relatively small arrays when compared with hydrophone arrays in the water column. To test the feasibility of this approach, experiments were carried out in the shallow water of the Malta Channel of the Straits of Sicily where the Scholte wave speed was some 10 to 20 times slower that the speed of sound in water. A linear array of ten vertically gimballed geophones was deployed and measurements were made on propagating seismic wave fields generated by explosive shots. The resulting directivities, beam patterns, and sidelobe characteristics are in excellent agreement with array theory, which suggests that coherent processing is a viable technique on which to base new applications for seismic arrays on the sea floor. Supporting materials on the geophysics of Scholte waves and calculations of the wave field at the site are presented     

Reverberation rejection with a dual-line towed array

Low-frequency bottom reverberation from explosive sources was measured with a dual-line horizontal towed array during a 1991 system engineering trial. The receiver's parallel line arrays were processed coherently in order to unambiguously resolve the reverberation field in azimuth. Initial beamforming that assumed a straight-and-parallel array configuration led to poor or erroneous ambiguity resolution, suggesting the system was deformed significantly from the assumed shape. In the absence of reliable shape measurement data, the array shape was deduced acoustically using tow ship signals and the direct blast of an explosive source. The estimated shape was then used in a shape-compensated beamformer to produce azimuthally unambiguous estimates of the reverberation field. Backlobe rejection of more than 20 dB was attained for both direct blast and distant reverberation energy     

A reconfigurable real-time interpolation beamformer

This paper describes an architecture for a compact real-time time-delay sonar beamformer. The design is based on a time-domain interpolation concept, and includes shading, interpolation, and time-delay functions for up to 64 sensor data inputs. A one card per beam configuration has been achieved. The digital interpolation technique used improves the effective sampling rate by a factor of 128. The beamformer design features 16-bit arithmetic precision, up to approximately a 5-kHz sampling rate for each of the 64 sensor input channels, and adaptability to any type of array, e.g., linear, circular, or arrays with nonequal interelement spacings. A 31-beam beamformer system, performing more than 620-million arithmetic operations a second, is discussed.     

Approximations to directivity for linear, planar, and volumetricapertures and arrays

Even after decades of sonar design, approximations to the directivity factor (DF) or index of an array, are often used inappropriately. Many of the approximations commonly used provide accurate directivity approximations for only the simplest of array geometries. As the array's size, shape, weighting, and complexity increase, there is a renewed need for better directivity approximations. Directivity is defined as the ratio of the output signal-to-noise (SNR) of an array to the input SNR at an omnidirectional element in a spherically isotropic noise field. Calculation of directivity is obtained by integrating the magnitude-squared response of the array over all angles of incidence. In spherical coordinates, these arrival angles are denoted by an azimuthal angle &thetas; and a polar angle φ. Hence, calculation of the directivity requires a two-fold integration over the angular space defined by the azimuthal and polar angles. For complex, large-size arrays consisting of thousands of array elements, directivity calculations using numerical integration procedures can be time consuming, even on state-of-the-art computing systems. This report provides a number of accurate formulas for estimating the directivity of linear, planar, and volumetric apertures and arrays, which are allowed to have arbitrary shading coefficients, steering angles, and directional array element responses     

A unified approach to array-factor synthesis for line arrays with nonuniformly positioned elements

An approach for the design of line arrays which synthesizes array factors having desired attributes is developed. This approach draws upon, clarifies, and expands earlier developments in nonuniformly positioned line-array design. A new array with important characteristics, called the square-root design, is derived. The approach is also applied to formulation and evaluation of design tradeoff studies. Specific examples are given for exponential and square-root arrays.     

Adaptive beamforming: spatial filter designed blocking matrix

Controlling the resolution in adaptive beamformers is often crucial. A simple method that works for both narrow-band and broad-band arrays is presented. This method is based on the normalized leaky LMS algorithm in conjunction with a generalized sidelobe canceller (GSC) structure, where the GSC is designed using a spatial filtering approach. In essence, the suppression of the spatial filters and the implicit noise of the leaky LMS algorithm together determine the adaptive beamformer. Analytical expressions are given for the Wiener filters and the output spectrum versus frequency and point source location. These expressions are employed in the design specification of the spatial filters and to obtain conditions for a controlled quiescent beamformer response. Simulation results are presented to illustrate the behavior of the array     

Mean drift forces on arrays of axisymmetric structures in a wave tank

An approximate method is presented for calculating drift forces on arrays of vertically axisymmetric bodies in a wave tank. It is assumed that the wave scattering properties of an isolated element and of the array in the open sea are known. The procedure described allows the open-sea results to be post-processed to give good estimates of drift forces when a structural array is placed in a wave tank. For the particular case of an array of vertical circular cylinders extending throughout the depth, the method is compared with accurate results from the full linear problem for scattering by the array both when the array is in the tank and when it is in the open sea. The results show how the mean forces on the array when in the tank may differ considerably from those experienced in the open sea.     

Track Parameter Estimation from Multipath Delay Information

It is desired to track the location of an underwater acoustic source with range difference measurements from a stationary passive array. Many times, the array has only one or two sensors, and the multipath and intersensor range difference measurements are insufficient to localize and track a source moving along an arbitrary path [1]. Here, we propose to track sources with one- or two-sensur stationary passive arrays by making the simplifying assumption that the source's path can be described by a small set of so-called track parameters. Range difference information can then be used to estimate the track parameter set rather than the source location as a function of time. In this paper, we choose the track parameters to specify a straight-line constant-velocity constant-depth path. Cramer-Rao bounds are presented for estimating these track parameters from the time history of multipath and intersensor range difference measurements. It is shown that this track parameter set cannot be accurately estimated from the time history of a single multipath range difference without side information (an independent velocity estimate, for instance), although multipath and intersensor range difference measurements from a two-sensor array are generally sufficient to estimate the track parameter set. Computationally efficient techniques are presented which estimate track parameters from range difference measurements taken from one- and two-sensor arrays. Monte-Carlo simulations are presented which show that these techniques have sample mean-square error approximately equal to the Cramer-Rao bound when a single multipath range difference and an independent velocity estimate are available. The sample mean-square error is shown to be in the range of two to ten times the corresponding Cramer-Rao bounds when these techniques are applied to two-sensor range difference data.     

On the use of multiple antenna arrays and corner reflector assemblies in radio positioning applications

Modern radio positioning systems often use multiple-antenna arrays or corner reflector arrays in order to increase the system range. A theoretical investigation of the movement of the antenna phase center on these arrays is carried out, and the resulting distance error is presented for several array configurations.     

Optimal configurations of wave energy device arrays

In this paper, the influence of the spatial configuration of a wave energy device array upon total power output is investigated. Hydrodynamic interactions are computed using a method capable of producing the linear wave theory solution to arbitrary accuracy. The overall performance of devices with two different power take-off arrangements is maximised at one incident wave frequency and direction by altering the formation of the array. Minimisation of the power is also carried out in a third case in order to demonstrate potential array-related losses. The optimisation is applied using two different approaches in each case: the Parabolic Intersection (PI) method and a Genetic Algorithm (GA). The former is a heuristic technique that has been devised for this study to enable rapid array construction using only simple calculations. The latter is an existing method, applied here with a novel crossover operator. Although considerably more computational effort is required, superior results may be obtained using the GA compared to the PI method. All of the arrays are subsequently analysed under incident waves of different frequency and direction, the resulting behaviour explained in terms of certain geometrical features of the arrangements.