235 research outputs found
Simulation of a Pendulum-Type Wave Energy Converter for Oceanic Drifters
Drifters (floating devices) are Lagrangian instrumentation widely used in oceanography and climate research. They are designed to obtain marine data by passively following the water currents, providing information about the ocean surface such as currents or water temperature. One of the main challenges faced at drifterâs design is their autonomy. Therefore, some studies tried to deal with this issue by embedding a Wave Energy Converter (WEC) on the drifter: waves excite the drifter, and its motion is used to generate power through an inner mechanism, so no battery exchange is needed. In this work, a 4 Degree of Freedom (DoF) analytical model has been developed to explain the coupled motion of a floating spherical buoy with an inner single pendulum under realistic sea conditions. Lagrange equations have been used to obtain the motion model while Morrison formulation has been selected to describe the interaction between the buoy and the fluid. The model has been tested considering both a regular and an irregular sea state. Airyâs theory has been chosen to describe the motion of linear waves. The model has been implemented in Matlab to run the simulations and to obtain the drifterâs motion. The validation of the results has been carried out by comparing the modelâs output with the one obtained by OrcaFlex (Orcina), the worldâs leading package for dynamic analysis of offshore marine systems. In both simulations, considering the sea state regular and irregular, results show a high match between the analytical model and OrcaFlex. Once the analytical model has been validated, we have studied the possibility to increase the pendulumâs rotation which is directly related to the power generation. A pendulum whose articulation has an oscillatory movement is prone to undergo parametric instability. In our case, that oscillatory motion corresponds to the buoyâs vertical motion. We have optimized the pendulum design to operate in resonance with the sea excitation, and increase the rotation and therefore, the energy generated by the device. As a first optimization step of the drifter design, several simulations have been performed covering a large range of pendulum length. Parametric resonance occurs at a pendulumâs length of 155 cm. The pendulum is not directly connected to the drifterâs generator, there is a gear system between them. In this work, the motion of the gear system has been studied separately. An extension of the present work would be the design of a more complex pendulum in order to achieve its parametric resonance at a realistic length, as the common drifterâs diameter is around 10 cm and does not fit that pendulum length.Objectius de Desenvolupament Sostenible::14 - Vida Submarin
Estimating a mean transport velocity in the marginal ice zone using ice-ocean prediction systems
Understanding the transport of objects and material in the marginal ice zone (MIZ) is critical for human operations in polar regions. This can be the transport of pollutants, such as spilled oil, or the transport of objects, such as drifting ships and search and rescue operations. For emergency response, the use of environmental prediction systems are required which predict ice and ocean parameters and are run operationally by many centres in the world. As these prediction systems predict both ice and ocean velocities, as well as ice concentration, it must be chosen how to combine these data to best predict the mean transport velocities. In this paper we present a case study of four drifting buoys in the MIZ deployed at four distinct ice concentrations. We compare short-term trajectories, i.e. up to 48âh lead times, with standard transport models using ice and ocean velocities from two operational prediction systems. A new transport model for the MIZ is developed with two key features aimed to help mitigate uncertainties in iceâocean prediction systems: first, including both ice and ocean velocities and linearly weighting them by ice concentration, and second, allowing for a non-zero leeway to be added to the ice velocity component. This new transport model is found to reduce the error by a factor of 2 to 3 for drifters furthest in the MIZ using ice-based transport models in trajectory location after 48âh.publishedVersio
The physical oceanography of the transport of floating marine debris
Marine plastic debris floating on the ocean surface is a major environmental problem. However, its distribution in the ocean is poorly mapped, and most of the plastic waste estimated to have entered the ocean from land is unaccounted for. Better understanding of how plastic debris is transported from coastal and marine sources is crucial to quantify and close the global inventory of marine plastics, which in turn represents critical information for mitigation or policy strategies. At the same time, plastic is a unique tracer that provides an opportunity to learn more about the physics and dynamics of our ocean across multiple scales, from the Ekman convergence in basin-scale gyres to individual waves in the surfzone. In this review, we comprehensively discuss what is known about the different processes that govern the transport of floating marine plastic debris in both the open ocean and the coastal zones, based on the published literature and referring to insights from neighbouring fields such as oil spill dispersion, marine safety recovery, plankton connectivity, and others. We discuss how measurements of marine plastics (both in situ and in the laboratory), remote sensing, and numerical simulations can elucidate these processes and their interactions across spatio-temporal scales
Search and rescue at sea aided by hidden flow structures
Every year hundreds of people die at sea because of vessel and airplane
accidents. A key challenge in reducing the number of these fatalities is to
make Search and Rescue (SAR) algorithms more efficient. Here we address this
challenge by uncovering hidden TRansient Attracting Profiles (TRAPs) in
ocean-surface velocity data. Computable from a single velocity-field snapshot,
TRAPs act as short-term attractors for all floating objects. In three different
ocean field experiments, we show that TRAPs computed from measured as well as
modelled velocities attract deployed drifters and manikins emulating people
fallen in the water. TRAPs, which remain hidden to prior flow diagnostics, thus
provide critical information for hazard responses, such as SAR and oil spill
containment, and hence have the potential to save lives and limit environmental
disasters
Near-Surface Oceanic Kinetic Energy Distributions From Drifter Observations and Numerical Models
The geographical variability, frequency content, and vertical structure of near-surface oceanic kinetic energy (KE) are important for air-sea interaction, marine ecosystems, operational oceanography, pollutant tracking, and interpreting remotely sensed velocity measurements. Here, KE in high-resolution global simulations (HYbrid Coordinate Ocean Model; HYCOM, and Massachusetts Institute of Technology general circulation model; MITgcm), at the sea surface (0 m) and at 15 m, are compared with KE from undrogued and drogued surface drifters, respectively. Global maps and zonal averages are computed for low-frequency
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Temporal Variability in Ocean Mesoscale and Submesoscale Turbulence
Turbulence in the Ocean is characterized by a highly nonlinear interaction of waves, eddies and jets drawing energy from instabilities of the large-scale flow and spans a wide range of scales.
Turbulent mesoscale eddies are well known as the dominant reservoir of kinetic energy in the ocean and are suspected to contribute significantly to the transport of heat, momentum, and chemical tracers, thereby playing an important role in the global climate system. The intermediate-scale flow structures (i.e. the submesoscale), often manifest as fronts, filaments, wakes and coherent vortices and pose considerable theoretical challenges due to the breakdown of balanced dynamics and the overlapping of scales with inertia-gravity waves. The full role of these submesoscale motions in transport and mixing, therefore remains unknown.
This thesis is divided into three chapters focusing on different aspects of turbulence in the mesoscale and submesoscale range.
In Chapter 1, we develop an analytical framework for understanding the time dependent mesoscale eddy equilibration process in the Southern Ocean using theory and idealized numerical simulations. In the Southern Ocean, conventional wisdom dictates that the equilibrium stratification is determined by a competition between westerly-wind-driven Ekman upwelling and baroclinic eddy restratification. The transient picture however, is not well established. To study the time dependent response of the stratification in the Southern Ocean to changing winds, we derive a simple theoretical framework describing the energetic pathways between wind input, available potential energy (APE), eddy kinetic energy (EKE), and dissipation. By characterizing the phase and amplitude of the APE and EKE response to oscillating wind stress, with a transfer function, we show that the transient ocean response lies between - a high frequency (Ekman) limit, characterized by the isopycnal slopes responding directly to wind stress, and a low frequency ("eddy saturation") limit, wherein a large fraction of the anomalous wind work goes into mesoscale eddies. Both the phase and amplitude responses of EKE and APE predicted by the linear theory agrees with results from numerical simulations using an eddy resolving isopycnal-coordinate model. Furthermore, this theory can be used to explain certain features, like the lagged EKE response to winds, observed in previous modeling studies and observations.
In Chapter 2, we investigate the role of submesoscale flows and inertia-gravity waves (IGW) on lateral transport, and lagrangian coherence, using velocity fields and particle trajectories from a high resolution ocean general circulation model (MITgcm llc4320). We use a temporal filter to partially filter the fast timescale processes, which results in a largely rotational/geostrophic flow, with a rapid drop off in energy at scales away from the mesoscales. We calculate and compare various Lagrangian diagnostics from particle advection simulations with these filtered/unfiltered velocities.At large length/time scales, dispersion by filtered and unfiltered velocities is comparable, while at short scales, unfiltered velocities disperse particles much faster. For the temporally filtered velocity fields, we observe strong material coherence similar to previous studies with altimetry derived velocities. When temporal filtering is reduced/removed, this material coherence breaks down with the particles experiencing enhanced vertical motion, which indicates that vertical advection is mainly associated with small scale, high frequency motions embedded within the larger scale flows. This study suggests that Lagrangian diagnostics based on satellite-derived surface geostrophic velocity fields, even with improved spatial resolution, as in the upcoming SWOT mission, may overestimate the presence of coherent structures and underestimate small scale dispersion.
These high-frequency unbalanced motions are likely to alias the estimation of surface currents from low temporal resolution satellite altimetry, and the high-wavenumber sea surface height (SSH) variability may represent a dynamically different ageostrophic regime, where geostrophy might not be the best route to infer velocities. In Chapter 3, we explore statistical models based on machine learning (ML) algorithms, as an alternate route to infer surface currents from satellite observable quantities like SSH, wind and temperature. Our model is simply a regression problem with sea surface height, sea surface temperature, windstress (quantities that are directly observable by satellites) as input (regressors) and the surface currents (which are typically inferred by physical models like geostrophy, Ekman etc.) as the output (regressands). To help the model learn physical principles like geostrophy (which relies on taking spatial gradients), we also provide the spatial coordinates and information in the neighboring gridpoints as additional features. Using output from an ocean general circulation model (CESM POP) simulation as data, we first train a linear rigression model on small domains and show that linear models only work up to a certain extent in small localized regions far from the equator (no large variation in the Coriolis parameter f). We then train a deep neural network on the whole globe for a relatively short period of time and use it to make predictions. Even with a short training period, the NN can make fairly accurate predictions of surface currents over most of the global ocean just as well as the physical models themselves. At its present state the NN fails to pick up on some mesoscale and submesoscale turbulent flow features. We discuss some possible ways to address these present problems in future studies
Building a Maxey--Riley framework for surface ocean inertial particle dynamics
A framework for the study of surface ocean inertial particle motion is built
from the Maxey--Riley set. A new set is obtained by vertically averaging each
term of the original set, adapted to account for Earth's rotation effects,
across the extent of a sufficiently small spherical particle that floats at an
assumed unperturbed air--sea interface with unsteady nonuniform winds and ocean
currents above and below, respectively. The inertial particle velocity is shown
to exponentially decay in time to a velocity that lies close to an average of
seawater and air velocities, weighted by a function of the seawater-to-particle
density ratio. Such a weighted average velocity turns out to fortuitously be of
the type commonly discussed in the search-and-rescue literature, which alone
cannot explain the observed role of anticyclonic mesoscale eddies as traps for
marine debris or the formation of great garbage patches in the subtropical
gyres, phenomena dominated by finite-size effects. A heuristic extension of the
theory is proposed to describe the motion of nonspherical particles by means of
a simple shape factor correction, and recommendations are made for
incorporating wave-induced Stokes drift, and allowing for inhomogeneities of
the carrying fluid density. The new Maxey--Riley set outperforms an ocean
adaptation that ignored wind drag effects and the first reported adaption that
attempted to incorporate them.Comment: To appear in Phys. Fluid
Contributions to the design of energy harvesting systems for autonomous sensors in low power marine applications
Tesi en modalitat de compendi de publicacionsOceanographic sensor platforms provide biological and meteorological data to help understand changes in marine environment and help to preserve it. Lagrangian drifters are autonomous passive floating platforms used in climate research to obtain surface marine data. They are low-cost, versatile, easy-to-deploy and can cover large extensions of the ocean when deployed in group.
These deployments can last for years, so one of the main design challenges is the autonomy of the drifter. Several energy harvesting (EH) sources are being explored to reduce costs in battery replacement maintenance efforts such as solar panels. Drifters must avoid the impact of the wind because this may compromise proper surface current tracking and therefore, should ideally be mostly submerged. This interferes with the feasibility of solar harvesting, so other EH sources are being explored such as the oscillatory movement of the drifter caused by ocean waves. Wave energy converters (WEC) are the devices that turn this movement into energy.
The motion of the drifter can principally be described by 3 oscillatory degrees of freedom (DoF); surge, heave and pitch. The heave motion includes the buoyancyâs response of the drifter, which can be explained by a mass-spring-damping model. By including the waveâs hydrodynamic load in this model, it is converted into a nonlinear system whose frequency response includes the waveâs frequency and the natural frequencies from the linear system. A smart option to maximize the captured energy is to design the inner WEC with a natural frequency similar to that of the drifter's movement. In this thesis, a 4 DoF model is obtained. This model includes the heave, the surge and the pitch motion of the drifter in addition to the inner pendulum motion relative to the buoy. Simultaneously, different pendulum-type WECs for small-size oceanic drifters are proposed. One of these converters consists of an articulated double-pendulum arm with a proof mass that generates energy through its relative motion with the buoy. Different experimental tests are carried out, with a prototype below 10 cm in diameter and 300 g of total mass, proving the capability of harvesting hundreds of microwatts in standard sea conditions EH sources require an additional power management unit (PMU) to convert their variable output into a constant and clean source to be able to feed the sensor electronics. PMUs should also ensure that the maximum available energy is harvested with a maximum power point tracking (MPPT) algorithm. Some sources, such as WECs, require fast MPPT as its output can show relatively rapid variations. However, increasing the sampling rate may reduce the harvested energy. In this thesis, this trade-off is analyzed using the resistor-based fractional open circuit voltage-MPPT technique, which is appropriate for low-power EH sources. Several experiments carried out in marine environments demonstrate the need for increasing the sampling rate. For this purpose, the use of a commercial PMU IC with additional low-power circuitry is proposed. Three novel circuits with a sampling period of 60 ms are manufactured and experimentally evaluated with a small-scale and low-power WEC. Results show that these
configurations improve the harvested energy by 26% in comparison to slow sampling rate configurations.
Finally, an EH-powered oceanographic monitoring system with a custom wave measuring algorithm is designed. By using the energy collected by a small-size WEC, this system is capable of transmitting up to 22 messages per day containing data on its location and measured wave parameters.Les plataformes dâobservaciĂł oceanogrĂ fiques integren sensors que proporcionen dades fĂsiques i biogeoquĂmiques de lâoceĂ que ajuden a entendre canvis en lâentorn marĂ. Un exemple dâaquestes plataformes sĂłn les boies de deriva (drifters), que sĂłn dispositius autĂČnoms i passius utilitzats en lâĂ mbit de la recerca climĂ tica per obtenir dades in-situ de la superfĂcie marina. Aquests instruments sĂłn de baix cost, versĂ tils, fĂ cils de desplegar i poden cobrir grans superfĂcies quan sâutilitzen en grup. Lâautonomia Ă©s un dels principals desafiaments en el disseny de drifters. Per tal dâevitar els costos en la substituciĂł de bateries, sâestudien diferents fonts de captaciĂł dâenergia com per exemple la solar. Els drifters utilitzats per lâestudi dels corrents marins superficials han dâevitar lâimpacte directe del vent ja que afecta al correcte seguiment de les corrents i, per tant, cal que estiguin majoritĂ riament submergides. AixĂČ compromet la viabilitat de lâenergia solar, fet que requereix lâestudi dâaltres fonts de captaciĂł com el propi moviment de la boia causat per les onades. Els convertidors dâenergia de les onades (WEC, wave energy converters) compleixen aquesta funciĂł. El moviment dels drifters pot explicar-se bĂ sicament a travĂ©s de 3 graus de llibertat oscil·latoris: la translaciĂł vertical i la horitzontal i el balanceig. La translaciĂł vertical inclou la flotabilitat del dispositiu, que es pot descriure mitjançant el model massamolla- amortidor. Incloure la cĂ rrega hidrodinĂ mica de lâonada en aquest model el converteix en un sistema no lineal amb una resposta freqĂŒencial que inclou la de lâonada i les naturals del sistema lineal. Una opciĂł per maximitzar lâenergia captada Ă©s dissenyar el WEC amb una freqĂŒĂšncia natural similar a la del moviment de la boia. En aquesta tesis es proposa un model de 4 graus de llibertat per a lâestudi del moviment del drifter. Aquest inclou els 3 graus de llibertat de la boia i el moviment del pĂšndul relatiu a ella. En paral·lel, es proposen diferents WEC del tipus pendular per drifters de reduĂŻdes dimensions. Un dâaquests WEC consisteix en un doble braç articulat amb massa flotant que genera energia a travĂ©s del seu moviment relatiu al drifter. Sâhan dut a terme diferents proves experimentals amb un prototip inferior a 10 cm de diĂ metre i 300 g de massa, les quals demostren la seva capacitat de captar centenars de microwatts en condicions marines estĂ ndard. Utilitzar fonts de captaciĂł dâenergia requereix incloure una unitat gestora de potĂšncia (PMU, power management unit) per tal de convertir la seva sortida variable en una font constant i neta que alimenti lâelectrĂČnica dels sensors. Les PMU tambĂ© tenen la funciĂł dâassegurar que es recull la mĂ xima energia mitjançant un algoritme de seguiment del punt de mĂ xima potĂšncia. Els WEC requereixen un seguiment dâaquest punt rĂ pid perquĂš la seva sortida consta de variacions relativament rĂ pides. Tanmateix, augmentar la freqĂŒĂšncia de mostreig pot reduir lâenergia captada. En aquesta tesi, s'analitza a fons aquesta relaciĂł utilitzant la tĂšcnica de seguiment de la tensiĂł en circuit obert fraccionada basada en resistĂšncies, que Ă©s molt adequada per a fonts de baixa potĂšncia. Diversos experiments realitzats en el medi marĂ mostren la necessitat d'augmentar la freqĂŒĂšncia de mostreig, aixĂ que es proposa l'Ășs de PMU comercials amb una electrĂČnica addicional de baix consum. Sâhan fabricat tres circuits diferents amb un perĂode de mostreig de 60 ms i sâhan avaluat experimentalment en un WEC de reduĂŻdes dimensions. Els resultats mostren que aquestes configuracions milloren l'energia recollida en un 26% en comparaciĂł a PMU amb mostreig mĂ©s lent. Finalment, sâha dissenyat un sistema autĂČnom de monitoritzaciĂł marina que inclou un algoritme de mesura d'ones propi. Aquest sistema Ă©s capaç de transmetre fins a 22 missatges al diaPostprint (published version
Oil Spill Modeling in Sea Ice Covered Ocean
The ongoing reduction in extent and thickness of sea ice in the Arctic allows the expansion of shipping activity and oil exploration in the high north, and with that a potential increased risk of oil spill in ice covered areas. This thesis asses the response of two oil-in-ice surface drift models implemented in an open-source Lagrangian framework and forced by four dif- ferent ice-ocean products (RIOPS, TOPAZ4 real-time forecast system, TOPAZ4 reanalysis and SVIM). Both approaches were evaluated over three sets of simulations: (I) a field experiment conducted in the Barents Sea marginal ice zone in 2009; (II) observed trajectories of buoys in the ice pack and in the Barents Sea marginal ice zone; and (III) stochastic simulations (960 runs, from 1998 to 2017) to reproduce a hypothetical oil spill in the Kara Sea. Results from experiments (I) and (II) indicate that the two drift models provide similar response both in the ice pack and the marginal ice zone under the same forcing. It was also found that finer horizontal resolution ice-ocean products (RIOPS and SVIM) did not reproduce better the ob- served drifts. The experiment (III) revealed that the sea ice concentration (%) field dictates the spread, the predominant direction of trajectories and the distance (km) traveled by the cloud of particles (SVIM: -1.41 km/% and TOPAZ4 reanalysis: -1.24 km/%).Master's Thesis in Meteorology and OceanographyGEOF399MAMN-GEO
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