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

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

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

Note di Meccanica dei Fluidi

Queste note sono state scritte per fornire un testo di riferimento per approfondire gli argomenti trattati durante i corsi di Idrodinamica e Meccanica dei Fluidi off erti agli studenti delle lauree magistrali in Ingegneria Navale, Civile e Ambientale attive presso la Scuola Politecnica. Tuttavia anche gli studenti di altri corsi di laurea magistrale possono trovarle utili per la loro formazione. Partendo dai principi fondamentali della fi sica, nelle note vengono ricavate le equazioni puntuali che descrivono la dinamica dei fl uidi. Successivamente vengono descritte alcune soluzioni esatte del problema del moto di un fl uido e gli approcci che sono utilizzati per determinare in forma approssimata il campo di moto, quando la complessit\ue0 del problema ne impedisce la soluzione in forma chiusa. In particolare vengono esaminati i moti caratterizzati da bassi e alti valori del numero di Reynolds. In quest\u2019ultimo caso vengono descritti il modello di fl uido ideale e quello di moto irrotazionale insieme alla teoria dello strato limite. L\u2019ultima parte delle note \ue8 dedicata ai moti turbolenti, anche se la vastit\ue0 di questo argomento ne impedisce una trattazione approfondita.These notes were written to provide a reference text for the students of the master degrees in Naval, Civil and Environmental Engineering of the Polytechnic School of the University of Genoa, who are interested in learning more about the topics covered in the courses of Hydrodynamics and Fluid Mechanics. However, also students of other master degrees might fi nd them useful for their formation. In the notes, starting from the basic principles of physics, the equations describing fl uid dynamics are derived. Some exact solutions of the problem of fl uid motion are determined and the approaches to be used to derive approximate solutions when the complexity of the problem prevents fi nding its solution in closed-form, are described. In particular, the fl ows at both low and high Reynolds numbers are considered. In the latter case, the inviscid fl uid and the irrotational fl ow models are described along with the the boundary layer which develops close to a rigid wall. The last part of the notes is devoted to turbulent fl ows, even though this topic is too broad to be discussed in depth in these brief note

Introduction to Morphodynamics of Sedimentary Patterns

Morphodynamics is a new discipline that investigates the formation and development of sedimentary patterns, i.e. the shapes of the cohesionless or cohesive boundaries of water bodies, evolving in response to the action of flowing water. Sedimentary patterns occur in fluvial, transitional, coastal and submarine environments. Their fascinating forms (e.g. dunes, meanders, alluvial fans, deltas, lagoons, coastal bars, tidal ridges, submarine fans) have attracted the attention of scientists. They also play a major role in fluvial, coastal and offshore engineering. The present monograph is the first of a series planned by an Editorial Committee comprising the four Authors. It provides a phenomenological introduction to the variety of patterns that will be investigated in the future Monographs. It also introduces to the mathematical theory of Morphodynamics, clarifying its nature of free boundary problem for the interface between a flowing water-sediment mixture and an erodible boundary

Role of the vertical pressure gradient in wave boundary layers

The pressure field in an oscillatory boundary layer is obtained by means of direct numerical simulations (DNS). The vertical pressure gradient is treated as any other turbulence quantity and its statistical properties are calculated from the DNS data. Moreover, a criterion involving the vertical pressure gradient is used to detect spots. The large fluctuations of the vertical pressure gradient, which take place in the turbulent flow, have significant implications for sediment transport

The boundary layer at the bottom of a solitary wave and implications for sediment transport

The present paper summarizes the theoretical and numerical results of recent studies of the bottom boundary layer generated by the propagation of a solitary wave which is often used as a model of a tsunami wave. The flow and the bottom shear stress are discussed as function of the parameters of the problem, i.e. (i) the ratio between the height H* of the wave and the local water depth h*, (ii) the ratio between the thickness delta* of the bottom boundary layer and h*, (iii) the relative bottom roughness. In particular, the conditions leading to turbulence appearance, which are obtained by means of a linear stability analysis, are presented along with those obtained by means of direct numerical simulations of Navier-Stokes equations and the integration of the RANS equations. It is shown that turbulence tends to appear during the decelerating phase of the wave cycle, if the wave height is larger than a critical value which depends on the ratio between the thickness of the bottom boundary layer and h* and the relative bottom roughness. As the height of the wave increases, turbulence appears earlier and becomes more intense, thus enhancing mixing phenomena and the sediment transport rate