Direct numerical simulation of the oscillatory flow around a sphere resting on a rough bottom

The oscillatory flow around a spherical object lying on a rough bottom is investigated by means of direct numerical simulations of continuity and Navier-Stokes equations. The rough bottom is simulated by a layer/multiple layers of spherical particles, the size of which is much smaller that the size of the object. The period and amplitude of the velocity oscillations of the free stream are chosen to mimic the flow at the bottom of sea waves and the size of the small spherical particles falls in the range of coarse sand/very fine gravel. Even though the computational costs allow only the simulation of moderate values of the Reynolds number characterizing the bottom boundary layer, the results show that the coherent vortex structures, shed by the spherical object, can break-up and generate turbulence, if the Reynolds number of the object is sufficiently large. The knowledge of the velocity field allows the dynamics of the large scale coherent vortices shed by the object to be determined and turbulence characteristics to be evaluated. Moreover, the forces and torques acting on both the large spherical object and the small particles, simulating sediment grains, can be determined and analysed, thus laying the groundwork for the investigation of sediment dynamics and scour developments.Comment: 35 pages, 21 figure

Linear evolution of sandwave packets

We investigate how a local topographic disturbance of a flat seabed may become morphodynamically active, according to the linear instability mechanism which gives rise to sandwave formation. The seabed evolution follows from a Fourier integral, which can generally not be evaluated in closed form. As numerical integration is rather cumbersome and not transparent, we propose an analytical way to approximate the solution. This method, using properties of the fastest growing mode only, turns out to be quick, insightful, and to perform well. It shows how a local disturbance develops gradually into a sandwave packet, the area of which increases roughly linearly with time. The elevation at the packet¿s center ultimately tends to increase, but this may be preceded by an initial stage of decrease, depending on the spatial extent of the initial disturbance. In the case of tidal asymmetry, the individual sandwaves in the packet migrate at the migration speed of the fastest growing mode, whereas the envelope moves at the group speed. Finally, we apply the theory to trenches and pits and show where results differ from an earlier study in which sandwave dynamics have been ignored

Small - Scale Morphology and Boundary Layer Processes: Measurement And Modeling

LONG-TERM GOALS: The primary goal of the research project is to contribute to a better understanding of the basic mechanisms controlling sediment transport in the nearshore regions. In particular, the structure of flow in the bottom boundary layer is studied along with its interaction with sediment dynamics and bottom morphology. Attention is also devoted to the steady streamings generated both within the bottom boundary layer and throughout the water column up to the free surface.Award Number: N00014-97-1-079

The Stability Balloon for Two-dimensional Vortex Ripple Patterns

Patterns of vortex ripples form when a sand bed is subjected to an oscillatory fluid flow. Here we describe experiments on the response of regular vortex ripple patterns to sudden changes of the driving amplitude a or frequency f. A sufficient decrease of f leads to a "freezing" of the pattern, while a sufficient increase of f leads to a supercritical secondary "pearling" instability. Sufficient changes in the amplitude a lead to subcritical secondary "doubling" and "bulging" instabilities. Our findings are summarized in a "stability balloon" for vortex ripple pattern formation.Comment: 4 pages, 5 figure

A simple model of wave-current interaction

The interaction between a steady current and propagating surface waves is investigated by means of a perturbation approach, which assumes small values of the wave steepness and considers current velocities of the same order of magnitude as the amplitude of the velocity oscillations induced by wave propagation. The problems, which are obtained at the different orders of approximation, are characterized by a further parameter which is the ratio between the thickness of the bottom boundary layer and the length of the waves and turns out to be even smaller than the wave steepness. However, the solution is determined from the bottom up to the free surface, without the need to split the fluid domain into a core region and viscous boundary layers. Moreover, the procedure, which is employed to solve the problems at the different orders of approximation, reduces them to one-dimensional problems. Therefore, the solution for arbitrary angles between the direction of the steady current and that of wave propagation can be easily obtained. The theoretical results are compared with experimental measurements; the fair agreement found between the model results and the laboratory measurements supports the model findings

Starved versus alluvial river bedforms: an experimental investigation

Laboratory experiments were conducted to investigate the formation of river bedforms under sediment supply-limited conditions, i.e. when a motionless substratum is bared by the dynamics of the mobile sediments. Three series of experiments were organized in a laboratory flume by fixing all the hydrodynamic and morphodynamic parameters but varying the thickness Delta of the initial layer of mobile sediments which covers the rigid bottom of the flume. At the end of all the experiments, which lasted for the same amount of time, the formation of transverse sand dunes was observed. For decreasing , the rigid bottom of the flume was bared progressively earlier during the experiment and the measurements showed a clear tendency of the bedforms to lengthen, i.e. to increase F their crest-to-crest distance. Moreover, under strong supply lim- itation, the two-dimensional transverse dunes turned into three-dimensional barchanoid forms and into isolated barchan dunes characterised by an abrupt reduction in bedform heights. two-dimensional Fourier analysis r of the bottom profile was performed, providing the amplitude of the main streamwise and spanwise harmonic components of the bottom morphology as a function of Delta

Direct Numerical Simulation of Oscillatory Flow Over a Wavy, Rough, and Permeable Bottom

The results of a direct numerical simulation of oscillatory flow over a wavy bottom composed of different layers of spherical particles are described. The amplitude of wavy bottom is much smaller in scale than typical bed forms such as sand ripples. The spherical particles are packed in such a way to reproduce a bottom profile observed during an experiment conducted in a laboratory flow tunnel with well-sorted coarse sand. The amplitude and period of the external forcing flow as well as the size of the particles are set equal to the experimental values and the computed velocity field is compared with the measured velocity profiles. The direct numerical simulation allows for the evaluation of quantities, which are difficult to measure in a laboratory experiment (e.g., vorticity, seepage flow velocity, and hydrodynamic force acting on sediment particles). In particular, attention is focused on the coherent vortex structures generated by the vorticity shed by both the spherical particles and the bottom waviness. Results show that the wavy bottom triggers transition to turbulence. Moreover, the forces acting on the spherical particles are computed to investigate the mechanisms through which they are possibly mobilized by the oscillatory flow. It was found that forces capable of mobilizing surface particles are strongly correlated with the particle position above the mean bed elevation and the passage of coherent vortices above them

Improving Adjuvant Endocrine Treatment Tailoring in Premenopausal Women With Hormone Receptor-Positive Breast Cancer

The Oncology Grand Rounds series is designed to place original reports published in the Journal into clinical context. A case presentation is followed by a description of diagnostic and management challenges, a review of the relevant literature, and a summary of the authors' suggested management approaches. The goal of this series is to help readers better understand how to apply the results of key studies, including those published in Journal of Clinical Oncology, to patients seen in their own clinical practice

Comparing field observations of sorting patterns along tidal sand waves with theoretical predictions

We present a site-by-site comparison between field observations and model predictions of grains size variations over tidal sand waves at six sites in the North Sea. To drive the model, at each location, local sediment characteristics are derived from the described field data, while hydrodynamic conditions are determined from a numerical model for tidal wave propagation in the North Sea. It is found that the theoretical model providesreasonable estimates of the occurring tidally generated bed forms. Moreover, at five of the six locations, the modeldescribes a sorting pattern which concurs with the observed sediment grain size variation, indicating that the model provides a fair description of the processes governing the phenomenon