On the onset of the instability of a three-dimensional jet in a stratified fluid

The onset of a three-dimensional jet flow in a stratified fluid is studied with the aid of a direct numerical simulation. An initially cylindrical jet with a Gaussian velocity profile is considered in a fluid with stable linear density stratification. The results indicate that, if an initial small perturbation of the velocity field has a wide spectrum, an exponential growth of the isolated quasi-two-dimensional mode occurs and its spectral maximum is shifted toward smaller wave numbers in comparison with the maximum of the helical mode of the instability of a nonstratified jet. The growth rate is proportional to Ri^0.5, where Ri is the global Richardson number. The onset of the instability leads to the formation of the flow’s vortex structure, which consists of a collection of different-polarity quasi-two-dimensional vortices located in a horizontal plane near the longitudinal axis of the jet. At sufficiently long times (Nt > 100, where N is the buoyancy frequency and t is time), the growth of instability reaches the saturation stage and further fluctuations in velocity and density decay under the effect of viscous diffusion. At this stage, the flow becomes self-similar and the time dependences of the transverse and vertical widths of the jet are consistent with the asymptotic behaviors of integral parameters of the flow that are observed experimentally in the far stratified wake. The results suggest that the onset of the instability of a quasitwo-dimensional mode can play the determining role in the dynamics of flow in the far stratified wake.     

Mechanism of the wave-induced seabed instability in the vicinity of a breakwater: a review

A detailed knowledge of the wave-induced seabed instability is particularly important for engineers involved in the design procedure of many marine structures and offshore installations. In this paper, the basic aspects of such instability will be examined. The current understanding of the mechanism of the wave–seabed interaction phenomenon and available approaches will be reviewed. Based on the framework of simplified analysis, the potential for such instability will be formulated that will help engineers to identify potential unstable sediments in the vicinity of a marine structure.     

Numerical modelling of the movement of a cyclonic eddy in the presence of a surrounding flow

The motion of an intensive cyclonic eddy on a β-plane in the presence of a zonal non-uniform flow is studied numerically in the, framework of an equivalent-barotropic quasi-geostrophic model. The main aspects influencing the cyclone motion, namely, the velocity of the surrounding flow and the gradient of potential vorticity gradient, are analysed. The main types of trajectories of tropical cyclones are thence simulated. Translated by Valdimir A. Puchkin.     

The effect of a current upon the generation of internal waves by a semi-diurnal tide in the vicinity of a ridge with a continuously changing height

In the framework of the linear theory for long waves, the paper studies internal waves generated by a semi-diurnal barotropic tide impinging on a bottom ridge at an arbitrary angle. The ocean is assumed to be double-layered. In the vicinity of the ridge, whose height is continuously changing, geostrophic flows occurring due to the inclination of the free surface and interface are considered. The dependencies of the generated wave's amplitudes on the angle of incidence of the tide and on the magnitude and direction of the geostrophic current velocity are determined, allowing a conclusion that the current contributes to the generation of internal waves. Translated by Vladimir A. Puchkin.     

On the interaction of a solitary wave and a submerged dike

The unsteady, two-dimensional Navier–Stokes equations and the exact free surface boundary conditions were solved to study the interaction of a solitary wave and a submerged dike. A piston-type wavemaker was set up in the computational domain to produce the incident solitary waves. The incident wave and the associated boundary layer flow in a wave tank with a flat bed were compared with the analytical solutions to verify the accuracy of this numerical scheme. Effects of the incident wave height and the size of the dike on the wave transformation, the flow fields, and the drag forces on the dike were discussed. Our numerical results showed that even though the induced local shear stress on the top surface of the dike is large at some particular locations, the resultant pressure drag is much larger than the friction drag. The primary vortex generated at the lee side of the dike and the secondary vortex at the right toe of the dike may scour the bottom and cause a severe problem for the dike.     

基于Boussinesq水波模型的聚焦波模拟

基于最高导数为3阶的单层Boussinesq方程,建立了聚焦波的时域波浪计算模型.数值模型求解采用了预报-校正的有限差分法.对于时间差分格式,预报和校正分别采用3阶Adams-Bashforth格式和4阶Adams-Moulton格式.首先,针对不同水深条件下水槽中传播的强非线性波进行模拟,并将数值结果与流函数的数值解...     

Analysis of the response of a non-linear data assimilation algorithm in a one-dimensional model of the upper ocean

A one-dimensional (1D) integral dynamico-stochastic model of the upper ocean with a non-linear assimilation algorithm is considered. The accuracy of computing the characteristics of the upper layer depends essentially on the values of the empirical coefficients. A numerical experiment was carried out which verified the efficiency of the model's adaptive mechanism operation when different values of the empirical coefficients and their variances were preset. Recommendations on assigment of the model's initial parameters are derived.Translated by Mikhail M. Trufanov.     

On the equations of rolling motion of a ship with a flooded compartment

This paper deals with obtaining the governing equations of rolling motion of a ship with a flooded compartment. The equations of motion are obtained through the variational formulation in the form of Hamilton-Ostrogradskii equation by taking the ship, the fluid in the flooded compartment and the sea as a single mechanical system. Since no specification concerning ships or flooded compartments has been made, the obtained equations are applicable to any sea-going vessel. As an application, the equation of rolling motion of a ship with a prismatic flooded compartment is obtained by choosing a suitable velocity potential for the fluid motion in the compartment.     

Topographic effect of a marine ridge on the spin-down of a cyclonic eddy

The topographic effect of a meridional marine ridge on the spin-down of a cyclonic eddy is discussed by use of a two layer model. It is shown that cyclonic eddies on the eastern side of a marine ridge decay in a shorter time in comparison with those on the western side. The fast decay on the eastern side is mainly due to the planetary effect, which carries the eddy westward to a shallower region on the ridge, where remarkable frictional spin-down is carried out. Conversely, westward shift of the similar cyclonic eddy yields a weak bottom effect in the western side. It is inferred that the resulting remarkable spin-down on the eastern side of a marine ridge is one of the causes of the observed fast decay of the cyclonic cold water mass accompanied by the large meander of the Kuroshio on the eastern side of the Izu Ridge south of Japan.     

Characteristics of a Cup Anemometer and a Procedure of Measuring the Wind Velocity

On the basis of the data of laboratory, model, and field investigation of the characteristics of a proposed cup anemometer, we obtain the following results: the distance constant is equal to 0.32 m with a mean-square deviation of 0.05 m, which is smaller than for the other anemometers of this type by an order of magnitude, the dynamic characteristics of the anemometer are linear for typical conditions of atmospheric turbulence, the variance and spectrum of noises (as a result of transformation of pulsed signals at the output of the anemometer) are described by universal functions with known parameters, the transmission bandwidth is approximately equal to 2 Hz for wind velocities lower than 5 m/sec and up to 5 Hz for higher wind velocities, and the errors of measurements of the mean wind velocity and the characteristics of the longitudinal component of velocity are approximately equal to 1 and 3%, respectively. The dimensions and weight of the anemometer are quite small, which enables one to perform long-term measurements of the characteristics of wind velocity for all types of platforms within broad ranges of hydrometeorological conditions.     

Why a corrugated coast only influences a longshore current within a wave length of the coast

An inviscid zone of influence is postulated from a physical model to occur when a steady homogeneous fluid flows by a two-dimensional corrugated wall with small slopes. The thickness of the influence zone is calculated to be a constant times the wave length of the wall. The method used involves balancing pressure differences between extreme points along the wall and between the wall and the outer edge of the influence zone but does not involve the irrotational assumption. An influence zone forms because the curvature of the boundary induces cross-stream accelerations and pressure gradients in the fluid. The result for the thickness of the influence zone agrees qualitatively with the exponential decay scale of an available potential flow solution to a similar problem, which suggests that the influence zone may occur in some rotational (as well as irrotational) flows.     

Localization of a broadband source using a matched-mode procedurein the time-frequency domain

In this paper, we present a nonconventional matched-mode procedure for localizing a broadband source in the time-frequency domain. This hybrid coherent and incoherent approach exploits both the temporal and spatial characteristics of the multimode arrival structure at a receiving sensor array. In the previous work, a time-domain technique was developed to deal with narrowband signals coherently. It consists of the following three steps. The first step employs a receiving sensor array to separate the modes by the conventional modal filtering approach. The second step is to estimate the energy and relative arrival times of the various modes which arrive at the receiver. The last step uses the differences of modal travel times to estimate the source range, and uses the ratios of modal energies to estimate the source depth. Here, we employ bandpass filters to divide the received broadband signal into several subfrequency bands, and apply the first and second steps of the previously developed coherent narrowband technique to the subfrequency bands in the time domain. The results obtained from subfrequency bands are then combined incoherently in the frequency domain to produce an estimate of the source position. Numerical simulation of an experiment with explosive sources at the shallow water site of the Yellow Sea is presented     

Predicting the equilibrium depth of a body towed by a faired cable

Comprehensive wind tunnel measurements have been made of the drag coefficients of both the body and the faired cable of a towed system. The drag data has been embodied in theoretical calculations of the depth attained in tow, at speed up to 3 m s⁻¹ (6 knots) with scopes up to 135 m (440 ft). The agreement with sea trial observations is excellent.The work has highlighted the importance of accounting for the variation of faired cable drag with cable angle and has shown that the drag increments due to gaps and misalignments between individual fairing elements cannot be ignored.Methods are derived for estimating these effects for application to other faired cable towed systems.     

Ten years after the enactment of the Oil Pollution Act of 1990: a success or a failure

In response to the Exxon Valdez incident, the Oil Pollution Act of 1990 was enacted as allegedly comprehensive oil pollution legislation. There have been changes in the patterns of oil spills and the strategic response to the Act. The number and volume of oil spills from tank vessels in US waters have fallen considerably without an obvious increase in oil prices since the enactment of the Act. This implies that the Act has successfully served its purpose. The Act may also facilitate the development of other oil pollution regimes including the international regime beyond the US. However, it remains to be seen whether the reduction of oil spills in US waters will be sustained in the future. There is an irreducible risk irrespective of any policy controlling oil pollution risks. The assertion that the Act has been effective in deterring oil spills may be counteracted by statistics demonstrating that the number and quantity of oil spills have also declined worldwide since 1990. It is premature to evaluate the performance of the Act because it has not so far been tested by a major oil spill since its enactment. A major oil spill in US waters in the future would illuminate limitation of the Act and lead to a fundamental reconsideration of the structure of the Act.     

Coefficient of horizontal eddy diffusion of a tracer on the water surface as a function of the wave age

A relationship between the statistical parameters of horizontal diffusion and the parameters of the energy-containing part of the frequency spectrum of sea-surface elevations is found depending on the wave age Ω and the ratio between the wind speed at 10 m and the phase velocity of the peak of a wave. It has been observed in [1–7] that the diffusion coefficient K(r) of a patch of size r increases as r ^β, where 1.15 < β < 4/3, and the patch area S(t) increases with time as t ^γ, where 2 < γ < 3. As was calculated in [15], in the energy-containing part of the elevation frequency spectrum, S(ω) ～ ω^?n , where n = 13/3 for young waves with Ω > 2, n = 4 for waves with 1.2 < ψ < 2, and n = 11/3 for developed waves with 0.83 < Ω < 1.2. It is found that β = (n + 1)/4 and γ = 8/(7 ? n). These relations explain the entire set of observed exponents: β = 4/3 and γ = 3 for young waves and β = 1.15 and γ = 2.34 for large sizes (up to 1000 km) and times (up to a month) when it is found here that β = 7/6 and γ = 2.4.     

围隔实验研究台湾海峡2005年夏季小型浮游动物对硅藻水华的摄食

为了研究小型浮游动物对近岸浮游植物藻华的摄食调控作用,于2005年7月,应用"稀释法"并结合高效液相色谱(HPLC)光合色素分析技术,研究了台湾海峡船基围隔实验条件下浮游植物生长率及小型浮游动物摄食率的日变动。结果表明:由于营养盐添加的影响,迅速形成了以尖刺伪菱形藻(Pseudo-nitzschia pungens)为优势种的藻华,生物量(叶绿素a)从实验初始7月6日的1.45μg/dm3迅速增加到7月8日的29.80μg/dm3,随后消退。镜检和光合色素分析的结果显示,实验期间一直以此硅藻占绝对优势。浮游植物的生长率在藻华峰值(7月8日)前保持了较高的生长速率(>1.0/d)且大于小型浮游动物的摄食率;小型浮游动物的摄食率也逐渐增加,7月7日时达到0.86/d,显示有57%以上的浮游植物现存量被摄食。7月8日后,水华迅速消退,摄食率除13日外,均大于浮游植物的生长率。小型浮游动物主要由急游虫(Strombidium spp.)、侠盗虫(Strobilidium spp.)等无壳纤毛虫、异养甲藻-螺旋环沟藻(Gyrodinium spirale)及砂壳纤毛虫等组成,其对浮游植物的生长迅速作出了反应,各类群的丰度在水华峰值后的7月9日均几达最大值,水华后期(11日)大型的无壳纤毛虫达最大值。小型浮游动物的这种组成及变动特点是其保持较高摄食率及一定程度上控制和促进藻华消退的原因之一。     

An Experimental Investigation on the Dynamic Response of a Damaged Ship with a realistic arrangement of the flooded compartment

The dynamics of a damaged ship in waves is a complex phenomenon regarding fluid and structure interactions. Flooded water motions in the damaged compartment could be influenced by decks, obstructions and obstacles in the compartment. This becomes particularly relevant in case of flooding in the engine room that is usually characterized by the presence of large objects such as engines and machineries. In such cases the possibility to better understand the behavior of a damaged ship, influenced by the fluid and structure interactions, could provide novel outcomes and thus enhance the damaged ship safety.In this paper an experimental campaign is conducted on a passenger ferry hull. The effects of obstacles in the engine room compartment, such as decks and engines, on ship roll responses, are studied. Roll decay in still water and steady roll responses in beam regular waves at zero speed are measured for the empty compartment and for the compartment with obstructions, as defined above.The main outcomes from the conducted experiments disclose a mitigation of the resonant behavior of the coupled system, ship with damaged compartment, by having engine shapes occupying the flooded engine room. Moreover it is possible to observe how the resonant frequency of the ship modifies having a more realistic arrangement of damaged compartment and how motion RAOs and roll decay characteristics modify accordingly.     

Development of the collision ratio to infer the time at which to begin a collision avoidance of a ship

A prior study attempted to solve the intrinsic problem of a critical collision condition including the slower ship’s dilemma by considering the maneuverability of the own ship and the International Regulations for Preventing Collisions at Sea (COLREGs). The collision ratio proposed in the previous study provided insight into the solution of the intrinsic problem, but it was not suitable as an index to determine the time at which to begin the maneuver to avoid the collision.This paper develops a collision ratio as an index that can be used to determine the time at which to begin the collision avoidance maneuver. The collision ratio is calculated by considering the maneuverability of the own ship, the COLREGs, and three virtual intrusion domains (VIDs). The validity of the inferred collision ratio is then assessed according to the dynamic characteristics of the own ship and the basic knowledge of the collision avoidance algorithm. The results indicate that the proposed collision ratio is suitable for use as a new index to determine the time at which to begin the collision avoidance maneuver.