Lagrangian Numerical Methods for Ocean Biogeochemical Simulations

We propose two closely--related Lagrangian numerical methods for the simulation of physical processes involving advection, reaction and diffusion. The methods are intended to be used in settings where the flow is nearly incompressible and the P\'eclet numbers are so high that resolving all the scales of motion is unfeasible. This is commonplace in ocean flows. Our methods consist in augmenting the method of characteristics, which is suitable for advection--reaction problems, with couplings among nearby particles, producing fluxes that mimic diffusion, or unresolved small-scale transport. The methods conserve mass, obey the maximum principle, and allow to tune the strength of the diffusive terms down to zero, while avoiding unwanted numerical dissipation effects

Non-Gaussian buoyancy statistics in fingering convection

We examine the statistics of active scalar fluctuations in high-Rayleigh number fingering convection with high-resolution three-dimensional numerical experiments. The one-point distribution of buoyancy fluctuations is found to present significantly non-Gaussian tails. A modified theory based on an original approach by Yakhot (1989) is used to model the active scalar distributions as a function of the conditional expectation values of scalar dissipation and fluxes in the flow. Simple models for these two quantities highlight the role of blob-like coherent structures for scalar statistics in fingering convection

Modeling and Control of a Multibody Hinge-BargeWave Energy Converter

Wave Energy Converters (WECs) are devices used to extract energy from the waves. The particular WEC considered in this thesis is a three-body hinge-barge WEC, which is an articulated floating structure composed of 3 rectangular bodies interconnected by hinges, and it operates longitudinally to the direction to the incoming wave. The relative motion between each pair of bodies drives a Power Take-Off (PTO) system, which extracts the energy from the waves. The objective of this thesis is to increase the energy that can be extracted by a three-body hinge-barge WEC using an optimal control strategy, which computes the optimal loads applied by the PTOs driven by the relative motion between the bodies. The optimal control is formulated in the time domain, and computes the PTO loads in a coordinated way, so that the total energy extracted by the device is maximized. The optimal control strategy is formulated for a three-body hinge-barge WEC that is equipped with either passive or active PTOs. In this thesis, an optimal control strategy, for the maximization of the energy extracted by a three-body hinge-barge WEC, is derived with Pseudo-Spectral (PS) methods, which are a subset of the class of techniques used for the discretisation of integral and partial differential equations known as mean weighted residuals. In particular, PS methods based on Fourier basis functions, are used to derive an optimal control strategy, for a finite time horizon. Therefore, an optimal control strategy, with PS methods based on Fourier basis functions, cannot be applied for realtime control of the WEC, as Fourier basis functions can only represent the steady-state response of the WEC. However, PS methods based on Fourier basis functions provide a useful framework for the evaluation of the achievable power absorption performance of the WEC, with both active and passive PTOs. The Receding Horizon (RH) real-time optimal control of a three-body hingebarge WEC is derived with PS methods based on Half-Range Chebyshev-Fourier (HRCF) basis functions. The RH optimal real-time controller, with PS methods based on HRCF basis functions, maximizes the energy extracted by the WEC at each time step over a moving control horizon. In contrast to Fourier basis functions, HRCF basis functions are well suited for the approximation of non-periodic signals, allowing the representation of both the transient and steady-state response of the WEC. The optimal control strategy, with PS methods based on either Fourier or HRCF basis functions, is based on a dynamic model of the device, which is derived with two different modeling methodologies, that can be also applied to other types of multiple body WECs. The modeling methodologies are validated against wave-tank tests carried out on a 1/7th scale two-body hingebarge device, and a 1/25th and 1/20th scale three-body hinge-barge device

Filling Gaps in Chaotic Time Series

We propose a method for filling arbitrarily wide gaps in deterministic time series. Crucial to the method is the ability to apply Takens' theorem in order to reconstruct the dynamics underlying the time series. We introduce a functional to evaluate how compatible is a filling sequence of data with the reconstructed dynamics. An algorithm for minimizing the functional with a reasonable computational effort is then discussed.Comment: 14 pages (REVTeX preprint), 4 figure

Receding Horizon Pseudo-Spectral Control for Energy Maximization of a 1/25th Scale Hinge-Barge Wave Energy Converter

This paper addresses the real-time optimal control of a1/25thscale three-body hinge-barge wave energy device. The objective of the control is to maximize the power extracted by the device under given constraints on the maximum displacements,velocities and control forces. An optimal pseudo-spectral control based on the Half-Range Chebyshev-Fourier basis functions is presented. HRCF basis functions are well suited for the approximation of non-periodic signals, allowing the representation of both the transient and steady-state response of the device.A reduced equivalent dynamic model of the device, which is computationally more advantageous than a full dynamic model,is obtained for the optimal control problem formulation. Results show that pseudo-spectral control outperforms a simple control strategy based on the optimal constant passive damping for both monochromatic and polychromatic waves

Dynamics of (4+1)-Dihedrally Symmetric Nearly Parallel Vortex Filaments

We give a detailed analytical and numerical description of the global dynamics of 4+1 points interacting through the singular logarithmic potential and subject to the following symmetry constraint: at each instant 4 of them form an orbit of the Klein group D 2 of order 4. The main device in order to achieve our results is to use a McGehee-like transformation introduced in (Paparella and Portaluri in Global dynamics of the dihedral singular logarithmic potential and nearly parallel vortex filaments, 2011) for a problem analogous to the present one. After performing this transformation in order to regularize the total collision, we study the rest-points of the flow, the invariant (stable and unstable) manifolds and we derive some interesting information about the global dynamics

A unifying approach to allometric scaling of resource ingestion rates under limiting conditions

Individual resource ingestion rates depend on both individual body size and resource supply. A component of the latter, namely resource availability, is also body-size dependent. This raises the question of the adequacy of simple scaling laws to describe the body-size dependency of resource ingestion. Here we propose a model which integrates resource ingestion drivers by merging a scaling law for feeding metabolism and Holling's functional responses into a single mathematical framework. At any fixed level of resource supply, the model predicts a log-log concave-down relationship between resource ingestion rates and body size, rather than a simple scaling law. Deviations from the latter are accounted for by the body size dependency of resource limitations. Experimental and literature data describing patterns of perceived resource availability and individual intake rates under limiting conditions with increasing individual body size are used to validate the model's assumptions and predictions. The model inc..