Computer aided piloting of a deeply towed vehicle

An instrumented vehicle is towed at the end of a 5 km long cable, gathering data about the deep sea floor and near-bottom water column. Although bottom-moored acoustic transponders are used to determine the vehicle and ship positions precisely and in real time, predicting the ship manoeuvers required to bring the vehicle over an area of interest on the sea floor is far from trivial for the ship driver. Computer software has been developed which recommends courses for the ship to steer so that the vehicle will pass near a desired target. In trials at sea, the computer steered the vehicle 80, 40 and 85 m from pre-selected targets. Analysis of the causes of the misses suggests future developments which may reduce the miss distance, provide information on current structure of the water column, and reduce the level of skill and attention required of the vehicle pilot.     

Stream-wise and cross-flow vortex induced vibrations of single tapered circular cylinders: An experimental study

Tapered circular cylinders are employed in a variety of ocean engineering applications. While being geometrically simple, this configuration creates a complex flow pattern in the near wake of the structure. Most previous experimental studies on tapered circular cylinders were dealing with stationary cylinders to explore the wake flow field and vortex shedding patterns past the cylinder. Few studies paid attentions to the vortex induced vibration of the tapered cylinders. This paper reports some results from in-water towing-tank experiments on the vortex-excited vibrations of tapered circular cylinders in a uniform flow. Cylinders with different mean diameters (28 and 78 mm), mass ratios (6.1 and 2.27) and tapers (5–20), along with their equivalent uniform cylinders, have been examined. The single degree of freedom vibrating system has a low mass-damping parameter (m*ξ = 0.0084–0.0279). The Reynolds number, based on mean diameter of the cylinders, ranges from 1400 to 70,200. The reduced velocities vary from 1.5 to 22. Effects of variations in the taper and mass ratios on the lock-in range, the reduced response amplitude, the reduced velocity for the peak vibration response and other stream-wise and cross-flow VIV parameters are reported and discussed.     

An algorithm of image processing for underwater range finding by active triangulation

In this paper, we present an algorithm for range image processing to reduce effects of underwater environments on the quality of range finding. This algorithm of image processing is based on the principle of the least-squares error method, which fits discrete image data to continuous piecewise curves. To simply the fitting of image data, the interval of each row and column is subdivided into several subintervals. Then a straight line is used to fit the image data within the subinterval. To merge two adjacent lines together, a weighting technique with a linear weighting factor is imposed. After the image is processed, it will provide a better imaging quality than the original one if values of the design parameters are properly assigned. Thus, a series of design of experiment process runs are carried out to study effects of the design parameters on quality of range finding. To make the proposed algorithm robust against noises, both environmental illumination and turbidity are forced into the experiments by utilizing an outer array. From the results of the range finding experiments, it was found that the proposed algorithm of image processing in truth has potential of increasing quality of range finding. The results also show that the proposed algorithm can achieve high quality of range finding only having the processing of row elements. Also, the quality of range finding by using the proposed algorithm of image processing is superior to that of using a bandpass optical filter.     

海洋工程中双柔性圆柱流激振动响应特性

海洋立管、海底管线和浮式平台系泊缆等柔性圆柱结构的流激振动会造成严重的疲劳损伤。目前,关于单根柔性圆柱流激振动的研究较多。为研究多根柔性圆柱系统的流激振动特性,开展了双柔性圆柱流激振动的模型试验。采用模态分析法将试验测量应变转化为位移。通过分析双柔性圆柱在并列、串列和交错排布方式下的响应位移、控制频率,揭示了双柔性圆柱的流激振动特性。研究结果表明:柔性圆柱并列排布时,两圆柱的相互作用使结构顺流向位移增大;柔性圆柱串列排布时,受遮蔽效应的影响,下游圆柱控制模态较低。由于柔性圆柱的多模态响应特性,"锁频"区后下游圆柱的位移并未显著增大;柔性圆柱交错排布时,下游圆柱发生"Wake-flutter",横流向和顺流向的控制频率接近,顺流向的位移显著增大。     

正弦激励下零浮力海洋拖缆动力特性与实验研究

海洋地震拖缆是海洋油气资源勘探中的关键设备,为研究海洋地震拖缆的动力特性及其在不同频率正弦激励干扰下的动力响应,建立了零浮力拖缆的数学模型,设计了一套完整的实验方案,通过弹性测力机构放大张力信号以排除环境干扰,测量了不同拖曳速度与激振频率条件下拖缆首部的张力变化及缆上位移响应情况,初步分析了正弦激励作用下零浮力拖缆的动力学响应。弹性测力机构具有良好的线性度与可重复性,拖缆受到的水阻力与拖曳速度的平方成正比,数值计算与实验结果对比验证了数学模型的正确性,初步分析了正弦激励作用下零浮力拖缆的动力学响应。     

马钱子碱透皮贴剂致突变实验研究

目的：探讨马钱子碱透皮贴剂的安全性。方法：采用小鼠骨髓细胞微核实验和姐妹染色单体互换实验研究马钱子碱透皮贴剂对小鼠骨髓细胞的致突变影响。2组实验均分为空白对照组,马钱子碱透皮贴剂小、中、大剂量组,环磷酰胺组,均分别予以空白基质贴皮,马钱子碱透皮贴剂120、240、480mg/kg体质量贴皮,腹腔注射环磷酰胺30、20mg/kg体质量,分别计算各组小鼠骨髓细胞微核（MN）发生率、姐妹染色单体互换（SCE）数。结果：与空白对照组比较,马钱子碱透皮贴剂3个剂量组小鼠骨髓细胞MN发生率和SCE数差异均无统计学意义（P>0.05）。结论：马钱子碱透皮贴剂无致突变作用,在一定剂量下具有安全性。     

规则波作用下水下拖缆数值分析研究

研究了水面波浪对水下拖缆的干扰。首先基于集中质量法给出了三维非均匀拖缆在规则波作用下的数学模型,然后通过四阶龙格库塔方法进行数值求解,并根据仿真计算讨论了拖缆在不同波浪作用下的稳态响应特性。仿真结果表明,水下拖缆在规则波的作用下,不仅作近似的简谐振动,还会在横向、垂向产生一个偏移量,且随浪向角的变化呈现不同的响应特征。     

多个淹没水平圆柱诱发高阶谐波特性的数值与试验研究

基于势流理论采用时域高阶边界元方法建立了模拟非线性波浪与淹没水平双、三圆柱作用的数值水槽模型,其中采用混合欧拉-拉格朗日方法更新自由水面,四阶Runga-Kutta方法进行时间步进。利用两点法分离得到潜体下游高阶自由波,进而研究潜体间距、布置形式等对高倍频自由波的影响。同时在物理水槽内开展多潜体高阶谐波物理模型试验,并将试验结果与数值结果进行对比,吻合较好。研究发现:淹没双圆柱下游高阶谐波幅值随潜体间距呈现周期性振荡变化,其振荡的重现距离约为波长一半。而淹没三圆柱下游的高阶谐波随第一个间距呈周期性振荡变化,随第二个间距的增大而减小。     

The experimental verification of a towed body and cable dynamic response theory

Measurements of towed body motion in a laboratory towing tank have validated the mathematical model used as a basis for predicting body and cable dynamic behaviour under seagoing conditions.     

Measurements of the wall pressure spectra on a full-scale experimental towed array

Turbulent wall pressure data acquired during tests of a full-scale experimental towed array over a range of tow speeds in straight tows and turns is presented. The experimental towed array contained a linear array of sensors mounted at the fluid–solid interface to measure the spectra of the wall pressure fluctuations due to the cylindrical turbulent boundary layer. The physics are dominated by the growth of a thick, high Reynolds number turbulent boundary layer at arc length Reynolds numbers as high as 9×10⁸. The measured wavenumber-frequency spectra, autospectra, cross-spectral decay and convection velocities are presented. A well-defined convective ridge exists in the wavenumber-frequency spectra obtained during straight tows and turns. Turns give rise to a complicated fluid–structure interaction problem, but do not lead to the separation of the turbulent boundary layer. As the array moves through a turn, flow-induced vibrations of the array are shown to dominate the spectra at low frequencies, with more rapid decay in the measured coherence occurring at higher frequencies. The use of tow speed as a velocity scale is shown to collapse autospectra and convection velocities.     

On splitter plate coverage for suppression of vortex-induced vibrations of flexible cylinders

Experimental results show how vortex-induced vibration (VIV) amplitudes of flexible cylinders can be reduced up to a 90% by covering less than half of the length of the cylinder with splitter plates elastically mounted to the surface of the cylinder.The VIV amplitude reduction takes place along with drag coefficient reductions of up to a 50% for the reduced velocities investigated.     

An experimental study of rip channel flow

A laboratory study of the flow over a bar with a single rip channel has been performed. First, the well-known pattern of a bar circulation cell with a strong offshore-directed current out through the rip channel and a weaker onshore-directed return flow over the bar is documented. Then measurements of the three-dimensional structure of the flow in the area where the rip channel, the bar and the trough meet and well inside the rip channel are presented. These measurements reveal that 3D effects play an important role, and that a depth-integrated viewpoint may not always be sufficient for predicting the flow in the near bed region. Particle-tracking experiments illustrate the near bed flow pattern over the entire area. These demonstrate how the overall trajectory pattern changes as a function of the distance of wave breaking from the bar crest: For some conditions, the rip current is fed from the trough and for other conditions it is fed directly from the bar. Both the 3D measurements and the trajectory tests show the existence of a weaker onshore-directed near-bed drift in the area where the rip current ceases. Finally, in a series of sensitivity tests, measurements of the rip current intensity for different wave climate and water level conditions reveal a strong correlation between the rip current intensity and the wave height (both normalized).     

Image processing for video images of buoy motion

In this paper, image processing technique that reduces video images of buoy motion to yield time series of image coordinates of buoy objects will be investigated. The buoy motion images are noisy due to time-varying brightness as well as non-uniform background illumination. The occurrence of boats, wakes, and wind-induced white caps interferes significantly in recognition of buoy objects. Thus, semiautomated procedures consisting of object recognition and image measurement aspects will be conducted. These offer more satisfactory results than a manual process. Spectral analysis shows that the image coordinates of buoy objects represent wave motion well, indicating its usefulness in the analysis of wave characteristics.     

An iterative procedure for the diffraction of water waves by multiple cylinders

Diffraction of water waves by an array of vertical cylinders of circular cross section is studied. In order to account for first order interaction among the cylinders, the body boundary condition is satisfied for each cylinder considering the scattered wave field from other cylinders in an iterative way. After each iteration, coefficients in the partial wave decomposition of the wave potential are modified. Convergence is fast for the whole range of frequencies and for a large number of bodies, compared with exact algebraic methods of Linton and Evans [J. Fluid Mech. 46 (1990) 549] and Kagemoto and Yue [J. Fluid Mech. 166 (1) (1986) 189].     

An efficient algorithm for simulating the dynamics of towed cable systems

An efficient algorithm is presented which simulates the three-dimensional motion of a towed cable. The formulation is based on an implicit, second-order finite difference approximation of the equations of motion. The results of the computation are compared both to those obtained by another numerical model and to experimental measurements.     

An experimental study of a pulse compression scatterometer system

The ideal scatterometer, operating from either an aircraft or a satellite platform, should be capable of making rapid, accurate estimates of the sea backscatter cross sectionsigmaover as wide a range of grazing anglespsias possible. Efficient operation over a large range of grazing angles is desirable because 1)sigmabehavior for90deg geq psi geq 70degyields rms gravity wave slope information and is an indicator of sea state and 2)sigmabehavior for70deg geq psi geq 0degyields data on surface wind magnitude and direction as well as information about the power spectrum of the sea. A "hybrid" estimation procedure has been developed for pulse compression radars which uses both frequency and spatially decorrelated samples ofsigmato provide an unbiased estimate ofsigmahaving minimum variance over the entire range of grazing angles for which radar reception is not noise-limited.     

An experimental study on water entry of asymmetric wedges

The aim of the paper is to provide an experimental reference for investigation of asymmetric water entry of wedges. Parameters of the study include initial deadrise angle, inclination angle and impact speed. Initial deadrise angles of the wedges were 20° and 30° with inclination angles ranging from 0° to 15° in 5° increments. Wedges were freely fallen from three different heights. Time histories of impact pressure and body acceleration were recorded. Sampling rate of measurements were set to 25 KHz. Main configuration of each test including mass of the wedge and water level were kept unchanged during all experiments. Additionally, several calibration tests were conducted to assess the repeatability and accuracy of the recorded data. The experimental results are compared with different entry theories and other available experiments. The comparison shows a reasonable agreement and indicates that the inclination angle can dramatically affect the impact pressure experienced by the wedges. Finally, the results show that the traditional asymmetric theories are not appropriate for all inclination angles.     

波浪水槽中振动格栅湍流特征的实验研究

在波浪水槽中采用格栅振动的方式产生湍流,研究振动格栅产生湍流的特征。本文开展了4类实验,调节测量点到格栅平均位置的距离、格栅振动频率、振动冲程,采用ADV测量水体中单点的脉动速度。实验结果显示,湍流强度在一定范围内随离格栅距离的增大而逐渐减小,随着振动频率的增大而呈幂指数增大,随着振动冲程的增大而呈幂指数增大。同时还比较了两个不同格栅产生湍流的不同。结果显示,在波浪水槽中,振动格栅产生湍流的强度还与格栅的长度尺寸有关,这与在水箱中振动格栅产生湍流的特征不同。