Laboratory study of a shear instability of an alongshore sea current

A laboratory study of the alongshore current shear instability in a rotating and nonrotating homogeneous fluid was carried out with special attention paid to the conditions of the coherent eddy structure formation in a shear flow. The cases with both cyclonic and anticyclonic velocity shear between the current core and the coast were reproduced in a wide range of nondimensional velocity shear variations. No coherent eddy-like structure formation was observed in the nonrotating fluid; the flow was always chaotic or turbulent. However, chains of coherent eddy-like structures were formed in the rotating fluid in the case with cyclonic velocity shear in a broad range of its variations. In the case with anticyclonic velocity shear, a chain of eddies was observed only when the velocity shear was quite low. When it was high, the flow was chaotic or turbulent. A physical model that explained the asymmetry in the conditions of the coherent eddy-like structure formation in the rotating fluid with cyclonic and anticyclonic velocity shear was considered. The laboratory results agreed with the observations of coherent submesoscale eddies in the Black Sea coastal zone.     

基于拉格朗日拟序结构的黑潮区域海流运输结构提取和分析

海流的拉格朗日运动对于研究物质输送有着重要意义,拉格朗日拟序结构(LCSs)作为研究海流结构的新型方法,相比于传统欧拉方法更为客观。本文提出了一种新的计算LCSs束的方法,基于25年的平均速度场,利用变分方法计算得到黑潮区域的气候态LCSs,并通过简化合并的方法得到了气候态LCSs束,该LCSs束能够突出地显示出海流特性和运输模式,其代表的平均拉格朗日环流有很强的约束作用,且具有鲁棒性。最终我们获得了气候态下12个月份的流场结构图,揭示了月周期性拉格朗日环流规律。本文还利用虚拟粒子输运、多年浮标轨迹以及气候态温盐异常3种方法进行了验证,与拉格朗日运输模式相吻合,证明了海流拉格朗日拟序结构的准确性和可靠性。     

Large-scale turbulence under a solitary wave

The structure of large-scale turbulence under a broken solitary wave on a 1 in 50 plane slope was studied. Three-component velocity measurements were taken at different heights above a smooth bed in the middle surf zone using an acoustic Doppler velocimeter. The measured data showed that turbulent velocity components were well correlated in the middle part of the water column. The velocity correlations could be produced by an oblique vortex similar to the obliquely descending eddy observed previously by other investigators. The vertical distributions of the relative values of the components of the Reynolds stress tensor showed that the structure of turbulence evolved continuously between the free surface and the bottom. The evolution was related to transition from two-dimensional to three-dimensional flow structures and the effect of the solid bottom on flow structures. Time histories of measured turbulent kinetic energy and turbulence stresses showed episodic turbulent events near the free surface but more sporadic turbulence in the lower layer. Large or intense turbulent events were found to have short duration and time lag relative to the wave crest point. These events also maintained good correlations between the turbulence velocity components close to the bottom.Instantaneous turbulent velocity fields were measured near the bottom at the same cross-shore location by using a stereoscopic particle image velocimetry system. These measurements showed that the near-bed flow field was characterized by large-scale, coherent flow structures that were the sources of most of the turbulent kinetic energy and turbulence stresses. The types of organized flow structures observed included vortices and downbursts of turbulence descending directly from above, lateral spreading of turbulent fluid along the bed, and formation of vortices in shear layers between fluid streams. A common feature of the organized flow structures near the bed was the large turbulence velocities in the longitudinal and transverse directions, which reflected the influence of a solid bottom on the breaking-wave-generated turbulence arriving at the bed.     

Sloshing in a three-dimensional rectangular tank: Numerical simulation and experimental validation

Pressure variations and three-dimensional effects on liquid sloshing loads in a moving partially filled rectangular tank have been carried out numerically and experimentally. A numerical algorithm based on the volume of fluid (VOF) technique is used to study the non-linear behavior and damping characteristics of liquid sloshing. A moving coordinate system is used to include the non-linearity and avoid the complex boundary conditions of moving walls. The numerical model solves the complete Navier–Stokes equations in primitive variables by using of the finite difference approximations. In order to mitigate a series of discrete impacts, the signal computed is averaged over several time steps. In order to assess the accuracy of the method used, computations are compared with the experimental results. Several configurations of both baffled and unbaffled tanks are studied. Comparisons show good agreement for both impact and non- impact type slosh loads in the cases investigated.     

Coupled SPHS–BEM method for transient fluid–structure interaction and applications in underwater impacts

Coupled SPHS–BEM method is proposed for transient fluid–structure interaction problems: SPH shell (SPHS) is selected to discretize shell structures, the second-order doubly asymptotic approximations (DAA₂) of boundary element method (BEM) is chosen to analyze flow-field. BEM can remedy the expensive costs for three-dimensional SPH (smoothed particle hydrodynamics), yet SPHS provides a structural solver for BEM. The coupled method is attractive, since only a layer of SPHS particles and a piece of flow-field boundary elements are needed to be modeled; the compatibility conditions of the coupled surface are performed with moving least square (MLS) function. The final two benchmarks on underwater impacts prove the feasibility, stability and accuracy of the proposed method.     

Effects of backfill on seismic behavior of rectangular tanks

The reliability and/or stability of the lifeline structures against failure under seismic loads are of critical concern, and must be studied carefully. Therefore, the main objective of this paper is to demonstrate the commonly encountered backfill effects on the dynamic response of rectangular tanks. However, only the exterior wall of the tank which interacts with both the backfill and fluid is tackled, as each part of the structure shows considerable differences in terms of both the load bearing mechanisms and the geometrical and positional differences. Finite element analyses are employed, taking into consideration the fluid-wall-backfill interaction. The analyses are conducted to observe whether or not both backfill and wall behavior can be affected by variation of the internal friction angle. For that purpose, some comparisons are made on vertical displacements of the backfill, roof displacements, stress responses, etc., by means of internal friction angle variations of the backfill from 25° to 40°. Consequently, it is observed that the variations on maximum vertical displacements are affected considerably. In contrast, the maximum stress responses are affected partially. However, the inertial effects of the backfill show that pseudo-static approximations may be insufficient to understand the dynamic behavior of the backfill-wall-fluid system.     

VORTEX SHEDDING FROM A CIRCULAR CYLINDER IN OSCILLATORY FAR FLOW

By using a process of successive approximations, the Boundary-Layer equations are solved to determine the separation points of a circular cylinder in oscillatory flow under the conditions of vortex existing. Combining with the discrete vortex model, the separation points and the fluid force coefficients are calculated at different KC numbers and Re numbers, A modified Morison equation is used in calculating the inline forces, and good agreements are obtained between the calculated results and those from other's experiments.     

Acoustic plane-wave reflection from a rough surface over a random fluid half-space

Plane-wave reflection from a rough surface overlying a fluid half-space, with a sound speed distribution subject to a small and random perturbation, is considered. A theory based upon a boundary perturbation method in conjunction with a formulation derived from Green's function for the coherent field in the random medium have been applied to a typical oceanic environment to study their effects on the plane-wave reflection. By considering the coherent field itself, the plane-wave reflection may be obtained straightforwardly through a procedure consistent with the formalisms currently employed in rough surface scattering. The results show that both the rough surface and medium inhomogenieties may reduce the plane-wave reflection, however, the characteristics of the curves representing their effects are different, enabling us to identify the dominant scattering mechanism. The results for the coherent reflection due to the individual scattering mechanism are compatible with those found in the existing literature.     

Coherent reflection of an acoustic plane wave from a random sediment layer over an elastic sea floor

The problem of coherent reflection of an acoustic plane wave from a seabed consisting of a randomly inhomogeneous sediment layer overlying a uniform elastic sea floor is considered in this analysis. The random perturbation in the sediment layer is attributable to the sound-speed variations, resulting in volume scattering due to medium inhomogeneities. An approach based upon perturbation theory, combining with a derived Green's function for a slab bounded above and below, respectively, by a fluid and an elastic half-space, is employed to obtain an analytic solution for the coherent field in the sediment layer. A linear system is then constructed to facilitate the computation of the coherent reflection field. The results of the coherent reflection coefficient for various sediment randomness, frequency, sediment thickness, and sea floor elasticity have been numerically generated and analyzed. It is found that the higher/larger the randomness, frequency, thickness, and shear-wave speed, the lower is the coherent reflection. Physical interpretations for the characteristics of the various results are provided.     

Three-dimensional reversed horseshoe vortex structures under broken solitary waves

Turbulent vortical structures under broken solitary waves are studied using three-dimensional smoothed particle hydrodynamics (SPH) method. The numerical model predicts water surface evolution and horizontal velocity very well in comparison with the experimental results. The numerical results detect organized coherent structures characterized as reversed horseshoe (hairpin) vortices being generated at the back of the broken spilling wave and traveling downward. The counter rotating legs of the reversed horseshoe structures appear to be a continuous form of the previously found obliquely descending eddies. The reversed horseshoe structures are associated with the turbulence motion of sweep events (downwelling motion) and transport momentum and turbulent kinetic energy downward into the water column. Vortex turning play an important role on the generation and evolution of three dimensional reversed horseshoe structures from the spanwise breaking wave rollers.     

On the properties of decaying two-dimensional turbulence: Characteristic features of the spectra and coherent structures

The main results of the theory of two-dimensional turbulence are presented. The conventional approach based on the Karman-Howarth equation is used to describe anomalous properties (sharply differing from those inherent in usual three-dimensional turbulence) of two-dimensional turbulent motion of an incompressible fluid, in particular, energy transfer across the spectrum from larger toward smaller wave numbers and the formation of coherent structures, whose origin is associated with the fact that the spectrum attains the form of a δ-function. A uniform method is proposed for obtaining self-similar spectra of two-dimensional turbulence in inertial ranges. The problem of turbulent diffusion of a passive tracer in the two-dimensional case is also considered. It turns out that the corresponding quantities, as well as those related to the dynamic characteristics of motion, can exhibit anomalous properties under certain conditions. The relationship between the results and experimental data is discussed. In particular, the experimentally observed inversion of the spectra of two-dimensional turbulence in the atmosphere is explained.     

Nonlinear modeling of liquid sloshing in a moving rectangular tank

A nonlinear liquid sloshing inside a partially filled rectangular tank has been investigated. The fluid is assumed to be homogeneous, isotropic, viscous, Newtonian and exhibit only limited compressibility. The tank is forced to move harmonically along a vertical curve with rolling motion to simulate the actual tank excitation. The volume of fluid technique is used to track the free surface. The model solves the complete Navier–Stokes equations in primitive variables by use of the finite difference approximations. At each time step, a donor–acceptor method is used to transport the volume of fluid function and hence the locations of the free surface. In order to assess the accuracy of the method used, computations are verified through convergence tests and compared with the theoretical solutions and experimental results.     

Coherent reflection of acoustic plane wave from a rough seabed, with a random sediment layer overlying an elastic basement

The problem of coherent reflection of an acoustic plane wave from a rough seabed with a randomly inhomogeneous sediment layer overlying a uniform elastic basement is considered in this analysis. The randomness of the sound field is attributable to the roughness of the seabed and the sound-speed perturbation in the sediment layer, resulting in a joint rough surface and volume scattering problem. An approach based upon perturbation theory, combined with a derived Green's function for a slab bounded above and below by a fluid and an elastic half-space, respectively, is employed to obtain an analytic solution for the coherent field in the sediment layer. Furthermore, a boundary perturbation theory developed by Kuperman and Schmidt (1989) is applied to treat the problem of rough surface scattering. A linear system is then established to facilitate the computation of the coherent reflection field. The coherent reflection coefficients for various surface roughness, sediment randomness, frequency, sediment thickness, and basement elasticity have been generated numerically and analyzed. It was found that the higher/larger size of surface and/or medium randomness, frequency, thickness, and shear-wave speed, the lower the coherent reflection. Physical interpretations of the various results are provided.     

A comparative mineralogical study of gas-related sediments of the Gulf of Cádiz

The Gulf of Cádiz area has been extensively surveyed in recent years and several gas-related fluid escape seafloor structures have been identified. In this study, gravity cores, collected during the ANASTASYA/00 and ANASTASYA/01 cruises, on mud volcanoes, hemipelagic sediments and dredged material from diapiric structures, have been studied. A comparative mineralogical analysis by XRD and SEM of samples from different areas has been performed in order to determine whether there is a characteristic mineralogy related to these fluid escape structures, and also to determine the origin of the mud matrix and constrain the depth of the parent units. The mineralogical analysis reflects the different origins of the different units described in the cores: hemipelagic material of the slope, clays that underlie the mud volcanoes and are discharged at the sea bottom surface, and authigenic and diagenetic minerals possibly involved in the anaerobic oxidation of methane in the mud volcano sediments.     

Hydroelastic Analysis of Very Large Floating Structures Using Plate Green Functions

1 .IntroductionRecentlygreatinteresthasbeenshowninthedevelopmentofverylargefloatingstructuressuchasMegaFloatofJapan (Isobe ,1 999)andMOBofUSA (Remmers ,1 999) .Owingtotheirextremelargesizeandgreatflexibility ,thecouplingbetweenthestructuraldeformationandfluidmotionissignifi cant.Thisisatypicalproblemofhydroelasticity .Efficientandaccurateestimationofthehydroelasticresponseofverylargefloatingstructuresinwavesisveryimportantfordesign .Manymethodshavebeenproposedinliteratureforthepredictiono…     

Hadley and Rossby regimes in a simple model of convection of a rotating fluid

This paper analyzes the properties of solutions to the equations describing the motion of a stratified fluid in the class of velocity and temperature fields linear in coordinates. For an ideal fluid, these equations, on the one hand, are exact for the corresponding hydrodynamic problem and, on the other hand, are identical to the equations of motion for a heavy top. In a conservative case, the equations of motion of a top share common solutions with the equations of geophysical fluid dynamics and reproduce motions similar to those existing in the theory of the large-scale atmospheric circulation. This study considers the effects of viscosity and heat conduction in the fluid, which are, in a sense, similar to the effect of friction in the case of a top. The influence of deflections of the vectors of gravity and external rotation from their standard directions for a plane-parallel atmosphere is also considered. The regimes of motions that are described by the starting equations and approximations commonly used to model the atmospheric general circulation (the quasi-geostrophic approximation) are analyzed. It is shown that these equations correctly describe the Hadley and Rossby circulation regimes and transitions between them that are observed in numerical and laboratory experiments. Particular attention is given to the consistency between different regimes of the exact equations and their quasi-geostrophic approximations, which is manifested for small Rossby numbers and is generally absent for large Rossby numbers. The asymptotic behaviors of the curves of transition between the Hadley and Rossby regimes under the conditions of breaking the external symmetry of flows are obtained. These asymptotics explain the corresponding transition boundaries for the regimes observed in the known experiments in annuluses.     

Weakly non-linear free oscillations in a basin of variable depth

Weakly non-linear free oscillations in a basin of variable depth filled with non-viscous fluid are investigated in terms of the long waves theory using the perturbation technique. Solutions to the initial two approximations for the horizontal velocity component and free surface departure are numerically derived. An attempt is made to determine the area of applicability of the linear theory by applying a parabolic bottom profile to the basin.Translated by V. Puchkin.     

Environmental Effects of Waveguide Uncertainty on Coherent Aspects of Propagation, Scattering, and Reverberation

The robustness of the coherence of waveguide propagation to environmental uncertainty becomes an important consideration for systems that seek to exploit coherence for gain. Examples include matched field processing for passive localization and time reversal mirrors (TRMs) for active systems. Here, efficient normal mode representations of midfrequency time-domain propagation using the narrowband and adiabatic approximations are used to explore the deterioration of coherent active system predictability and performance in the presence sound-speed perturbations in the water column. Results show that for TRMs the reverberation level at the focal range is increased, and the scattering from an illuminated object is reduced for ensembles over oceanographic uncertainty. Results are obtained analytically as formal averages and are believed to represent a lower limit on the deterioration of TRM performance in the presence of environmental uncertainty for shallow-water waveguides.     

Commentary

This discussion on a report by Mascle and Mascle provides some information on the structure of the Apenninic orogenic belt. A number of structural scenarios can be recognized in the Apenninic fold belt; chaotic, melange-like, thrusted sheets are or are not associated with deeper compressional structures affecting coherent stratigraphic sequences at least as old as Trias. Variations and combinations of these structural frames cannot be simply summarized by a scheme, but can be used as basic category in a comparison of ocean ic-subduction complexes.