Wavelet-based vortical structure detection and length scale estimate for laboratory spilling waves

Turbulent flow fields under spilling breaking waves are measured by particle image velocimetry and analyzed using the wavelet techniques in a laboratory surf zone. The turbulent vortical structures and corresponding length scales in the flow are detected through the eduction of the most excited mode with local intermittency measure that is found to correlate with the passage of the structure. Distributions and evolution of the educed vortical structures are presented and discussed. Packets of vortical structures with high intermittency is observed to stretch downward below the initially low-intermittency trough level, indicating these structures play a crucial role in turbulent mixing below the trough level. It is found that the probability density functions of the intermittent energy of the educed structures, vorticity and swirl strength display an exponential decay. Ensemble-averaged length scales of the educed vortical structures are found to be about 0.1 to 0.2 times the local water depth, close to the turbulent mixing length reported in the surf zone. The Kolmogorov microscale is evaluated and the turbulent mixing length is estimated using the k − ε relation and mixing length hypothesis. The k − ε relation may overestimate the mixing length scale for energetic descending eddies.     

URANS investigation of the interaction between the free surface and a shallowly submerged underwater vehicle at steady drift

An axisymmetric underwater vehicle (UV) at a steady drift angle experiences the complex three-dimensional crossflow separation. This separation arises from the unfavorable circumferential pressure gradient developed from the windward side toward the leeward side. As is well known, the separated flow in the leeward side gives rise to the formation of a pair of vortices, which affects considerably the forces and moments acting on the UV. In this regard, the main purpose of the present study is to evaluate the role of the leeward vortical flow structure in the hydrodynamic behavior of a shallowly submerged UV at a moderate drift angle traveling beneath the free surface. Accordingly, the static drift tests are performed on the SUBOFF UV model using URANS equations coupled with a Reynolds stress turbulence model. The simulations are carried out in the commercial code STARCCM+ at a constant advance velocity based on Froude number equal to F_n = 0.512 over submergence depths and drift angles ranging from h = 1.1D to h = ∞ and from β = 0 to β = 18.11°, respectively. The validation of the numerical model is partially conducted by using the existing experimental data of the forces and moment acting on the totally submerged bare hull model. Significant interaction between the low-pressure region created by the leeward vortical flow structure and the free surface is observed. As a result of this interaction, the leeward vortical flow structure appears to be largely responsible for the behavior of the forces and moments exerted on a shallowly submerged UV at steady drift.     

Suspension of sediment particles over a ripple due to turbulent convection under unsteady flow conditions

We analyzed the motions of small sediment particles over a sinusoidal ripple due to an unsteady turbulent boundary layer flow using Large Eddy Simulation. The motions of sediment particles are described in terms of the Lagrangian framework as it is helpful in studying the structure of sediment suspension in detail. Strong coherent vortical structures are well developed along the upslope of the ripple surface during the accelerating flow phase, which effectively drag the particles to the ripple crest. At the maximum flow rate and at the decelerating flow phase, a cloud of vortical structures is developed vertically in the lee area of the ripple. Sediment particles render strong dispersion in the vertical direction when they are captured by these turbulent vortices, causing convective sediment flux that cannot be explained by the mean flows. The convective sediment suspension is strongest at the time of flow deceleration, while over a flat bed at the time of flow reversal. This observation suggests that bed form effect should be considered in modeling convective sediment flux.     

Dynamics of a Wave Packet on the Surface of an Inhomogeneously Vortical Fluid (Lagrangian Description)

A nonlinear Schrödinger equation (NSE) describing packets of weakly nonlinear waves in an inhomogeneously vortical infinitely deep fluid has been derived. The vorticity is assumed to be an arbitrary function of Lagrangian coordinates and quadratic in the small parameter proportional to the wave steepness. It is shown that the modulational instability criteria for the weakly vortical waves and potential Stokes waves on deep water coincide. The effect of vorticity manifests itself in a shift of the wavenumber of high-frequency filling. A special case of Gerstner waves with a zero coefficient at the nonlinear term in the NSE is noted.     

Some peculiarities of the sea surface temperature in the vicinity of western boundary currents

The sea surface temperature Laplacian and related parameters have been applied to identify the basic elements of water circulation and wave and vortical features. Sources of quasistationary meander generation and areas, where cyclonic and anticyclonic vorticities dominate are determined.Translated by Vladimir A. Puchkin.     

About a limiting vortical wave on a fluid surface

A non-linear stabilized vortical wave on the surface of an infinitely-deep fluid is considered. The angle between the tangents at the apex of the limiting symmetric wave, similar to the Stokes limiting, is shown to equal 120°.Translated by V. Puchkin.     

Unsteady air bubble entrainment and detrainment at a plunging breaker: dominant time scales and similarity of water level variations

At plunging breakers, air bubbles are entrained at the impingement of the water jet, formed at the top of the wave, with the water free surface in front. During the present study, air bubble entrainment at a pseudo-plunging breaker was investigated at near full-scale and further experimental work studied the bubble detrainment process. Experimental observations included the generation and propagation of waves downstream of the plunge point. Experimental results highlighted a number of unsteady air–water flow patterns and emphasise high levels of aeration: i.e., depth-averaged void fraction of more than 10% next to jet impact in shallow waters. Unsteady bubble injection experiments showed a strong vortical motion induced by the rising bubbles. Altogether, the results suggest that a dominant time scale is the bubble rise time d₁/u_r, which cannot be scaled properly with an undistorted Froude model. The study contributes to a better understanding of unsteady bubble entrainment at a pseudo-plunging breaker and the associated vortical circulation.     

Stationary arrays of vortical patches near linear boundaries

This paper addresses the problem of plane stationary currents generated in an ideal incompressible fluid by an infinite array of identical areas of constant vorticity located near a linear boundary. Eddy shapes are constructed analytically and numerically for diverse vortical areas and ranges off the boundary. The limits of applicability of the asymptotic solution derived are given. The results are matched within situ observations.Translated by V. Puchkin.     

Geophysical hydrodynamics of vortical patches

This paper reviews the works on vortical patch dynamics, carried out at the Pacific Oceanological Institute, the Russian Academy of Sciences, from 1983 to 1992, as applied to dynamic oceanography modelling. Quasi-geostrophic barotropic and baroclinic models, plane gravitational axisymmetric models, and models considering entrainment effects, are examined.Translated by Vladimir A. Puchkin. UDK 551.465.11.     

Internal flow structure of short wind waves

For wind waves generated in a wind-wave tunnel, the surface pressure and also the pressure distribution along the internal streamlines were calculated from the measured internal velocity field. In distinct waves, with wave height comparable with or larger than the mean, the surface pressure is found to vary drastically in a narrow region around the crest, showing a dominant minimum near the crest. On the other hand, the pressure distribution along the streamline shows systematic variations that are nearly in phase with the streamline profile. It is shown that the occurrence of the pressure in phase with the streamline profile is linked with the internal vorticity distribution, especially with the presence of a high vorticity region below the crest described in Part I of this study. As a result of the occurrence of such pressure variations, the dispersion relation is modified by about 10% from that for linear irrotational waves. It is argued from the present measurements that the dispersion relation and also the energy transfer from wind into wind waves are strongly affected by the internal vortical structure so that the assumption of irrotational gravity waves cannot be applied to the wind waves being studied.     

Collocation numerical model for synoptic eddy evolution in the ocean

A quasi-geostrophic model for synoptic eddy evolution in a confined oceanic area is examined. The numerical scheme is based on the combination of spline collocation, splitting, and Fourier series expansion by the dynamic operator's eigenfunctions over the vertical. The functions and their derivatives are approximated with cubic splines in a basis constituted by B-spline functions. A series of experiments focusing on eddy transformation were conducted. The acquired data confirmed the inference regarding the oscillatory nature of the energy exchange between the primary eddy and the secondary ones and showed how vortical formations affect the currents. The evidence obtained indicates that, vertically, the pattern of eddy propagation is complicated.Translated by Vladimir A. Puchkin.     

About the relationship between acoustic fields and hydrological fields in the area of a mesoscale eddy in the Black Sea

This paper discusses the application of acoustic sounding with the purpose of identifying vortical features in the sea. Analysis of the results of a multidisciplinary acoustic/hydrological experiment conducted in the Black Sea has revealed a stable recurrent correlation between the amplitudinal variance of echo signals, on the one hand, and the peculiarities of water temperature fields, on the other. Given these characteristics, the peripheral and the central areas of a mesoscale eddy essentially differed. This allows us to apply acoustic sounding to identify mesoscale eddies in the sea. Translated by Vladimir A. Puchkin.     

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.     

Wave Hindcast for the Neighbouring Seas of Korea Based on Loosely Coupled Wave-Tide-Surge Model

A hindcast simulation of 75 typhoons and winter monsoons which affected the coastal areas of Korean Peninsula is performed by use of a third generation ocean wave prediction model, WAM-cycle 4 model, loosely coupled with a com-bined tide and surge model. Typhoon wind fields are derived from the planetary marine boundary layer model for effective neutral winds embedding the vortical storm wind from the parameterized Rankin vortex type model in the limited areas of the overall modeled region. The hindcasted results illustrate that significant wave heights (SWH) considering the wave-tide-surge coupled process are significantly different from the results via the decoupled case especially in the region of the estuaries of the Changjiang Estuary, The Hangzhou Bay, and the southwestern tip of Korean Peninsula. This extensive model simulation is the first attempt to investigate the strong wave-tide-surge interaction for the shallow depth area along the coasts of the Yellow Sea and the East China Sea Continental     

Residual currents in plume front zone of the Hangzhou Bay

It is of some limitations to analyse residual currents by means of the 25 h anchored current measurements. On the basis of the drift tracking and the mooring system data, here, analyses have been done aiming at the structural characteristics and the dynamical mechanisms of the frontal residual current field of the Hangzhou Bay. Especially a theoritical model is given focusing on the frontal density-driven currents. The results indicate that there exists obviously the upper-layer front-driven flow along the orientation of the front during neap tides in the research area of the Hangzhou Bay. But the flow is restrained by the strong vortical viscosity during spring tides. In the lower layer, the effect of the front is little and the subtidal movements are dominated by the tide-induced residual currents. In addition, the influences of wind forcing to the residual current field are also suggested to be important.     

Review of experimental work in biomimetic foils

Significant progress has been made in understanding some of the basic mechanisms of force production and flow manipulation in oscillating foils for underwater use. Biomimetic observations, however, show that there is a lot more to be learned, since many of the functions and details of fish fins remain unexplored. This review focuses primarily on experimental studies on some of the, at least partially understood, mechanisms, which include 1) the formation of streets of vortices around and behind two- and three-dimensional propulsive oscillating foils; 2) the formation of vortical structures around and behind two- and three-dimensional foils used for maneuvering, hovering, or fast-starting; 3) the formation of leading-edge vortices in flapping foils, under steady flapping or transient conditions; 4) the interaction of foils with oncoming, externally generated vorticity; multiple foils, or foils operating near a body or wall.     

Fractal structure formation at various stages of the evolution of the oceanic lithosphere: Premises, examples, and problems

Premises according to which the lithosphere represents a nonlinear dynamical system with particular fractal objects are considered. A series of examples helps to show that, at the principal stages of the evolution of the lithosphere, from continental rifts to passive continental margins and spreading centers on the crests of mid-ocean ridges, scale-invariant structures are generated associated with pull-apart features under the presence of a shear component. An hypothesis is posed according to which the latter structures are formed due to vortical movements similar to those in the hydrosphere and atmosphere at their origin. The diverse properties of the geological medium, which may be referred to as fractal, together with the examples assessed, allow us to make a conclusion concerning the development of the fractal (syneregetic) line in the structural-tectonic analysis. Within the frameworks of this line, selected problems are specified: the scales of the compressional deformations and horizontal stratification of the oceanic crust and lithosphere and the necessity for the development of a special terminological basis.     

Ekman drift and vortical structures

In this article, the authors study the influence of a constant wind on the displacement of a vortex. The well known Ekman current develops in the surface layer and is responsible for a transport perpendicular to the wind: the Ekman drift.An additional process is, however, evidenced, whose importance is as strong as the Ekman drift. There indeed exists a curl of the wind-driven acceleration along isopycnic surfaces when they are spatially variable (they enter and leave the depth where the wind stress acts), which generates potential vorticity anomalies. This diabatic effect is shown to generate potential vorticity anomalies which acts on the propagation of vortical waves and non linear vortices.It is shown that this effect drastically reduces the effect of the Ekman drift for linear waves and surface intensified vortices, while extending its effect to subsurface vortices. It also generates along wind propagation, whose sign depends on the vortex characteristics.     

大洋细结构的一种可能的机制Ⅲ.对混合过程动能耗散的估计

继第部分之后研究了惯性内波和近惯性内波由f～的作用所致的剪切不稳定引起的破碎机制。物理上,该机制很象存在由风应力所致薄表面涡旋漂流层时表面波的破碎与饱和过程。惯性内波和近惯性内波的破碎产物与小尺度湍流一起形成了混合块,它与Gregg等人(1986)的持久混合观测结果一致。依据Thorpe(1973)实验的结果作者提出了一个估计湍流动能耗散率和消衰时间的方法。结果表明,在剪切不稳定中近惯性内波在湍动耗散中起了关键作用,而惯性内波引起非常弱的湍动耗散。使用内波能量谱的标准总能量密度估计出的近惯性内波的耗散率和消衰时间与PATCHEX测量结果非常一致。文中还讨论了几个与此破碎机制有关的问题。     

Turbulence-plankton interactions: a new cartoon

Climate change redistributes turbulence in both space and time, adding urgency to understanding of turbulence effects. Many analytic and analog models used to simulate and assess effects of turbulence on plankton rely on simple Couette flow. There shear rates are constant and spatially uniform, and hence so is vorticity. Over the last decade, however, turbulence research within fluid dynamics has focused on the structure of dissipative vortices in space and time. Vorticity gradients, finite net diffusion of vorticity and small radii of curvature of streamlines are ubiquitous features of turbulent vortices at dissipation scales but are explicitly excluded from simple, steady Couette flows. All of these flow components contribute instabilities that cause rotation of particles and so are important to simulate in future laboratory devices designed to assess effects of turbulence on nutrient uptake, particle coagulation, motility and predator‐prey encounter in the plankton. The Burgers vortex retains these signature features of turbulence and provides a simplified “cartoon” of vortex structure and dynamics that nevertheless obeys the Navier‐Stokes equations. Moreover, this idealization closely resembles many dissipative vortices observed in both the laboratory and the field as well as in direct numerical simulations of turbulence. It is simple enough to allow both simulation in numerical models and fabrication of analog devices that selectively reproduce its features. Exercise of such numerical and analog models promises additional insights into mechanisms of turbulence effects on passive trajectories and local accumulations of both living and nonliving particles, into solute exchange with living and nonliving particles and into more subtle influences on sensory processes and swimming trajectories of plankton, including demersal organisms and settling larvae in turbulent bottom boundary layers. The literature on biological consequences of vortical turbulence has focused primarily on the smallest, Kolmogorov‐scale vortices of length scale η. Theoretical dissipation spectra and direct numerical simulation, however, indicate that typical dissipative vortices with radii of 7η to 8η, peak azimuthal speeds of order 1 cm s^?1 and lifetimes of order 10 s or longer (and much longer for moderate pelagic turbulence intensities) deserve new attention in studies of biological effects of turbulence.