829 research outputs found
Stochastic representation of the Reynolds transport theorem: revisiting large-scale modeling
We explore the potential of a formulation of the Navier-Stokes equations
incorporating a random description of the small-scale velocity component. This
model, established from a version of the Reynolds transport theorem adapted to
a stochastic representation of the flow, gives rise to a large-scale
description of the flow dynamics in which emerges an anisotropic subgrid
tensor, reminiscent to the Reynolds stress tensor, together with a drift
correction due to an inhomogeneous turbulence. The corresponding subgrid model,
which depends on the small scales velocity variance, generalizes the Boussinesq
eddy viscosity assumption. However, it is not anymore obtained from an analogy
with molecular dissipation but ensues rigorously from the random modeling of
the flow. This principle allows us to propose several subgrid models defined
directly on the resolved flow component. We assess and compare numerically
those models on a standard Green-Taylor vortex flow at Reynolds 1600. The
numerical simulations, carried out with an accurate divergence-free scheme,
outperform classical large-eddies formulations and provides a simple
demonstration of the pertinence of the proposed large-scale modeling
A future for intelligent autonomous ocean observing systems
Ocean scientists have dreamed of and recently started to realize an ocean observing revolution with autonomous observing platforms and sensors. Critical questions to be answered by such autonomous systems are where, when, and what to sample for optimal information, and how to optimally reach the sampling locations. Definitions, concepts, and progress towards answering these questions using quantitative predictions and fundamental principles are presented. Results in reachability and path planning, adaptive sampling, machine learning, and teaming machines with scientists are overviewed. The integrated use of differential equations and theory from varied disciplines is emphasized. The results provide an inference engine and knowledge base for expert autonomous observing systems. They are showcased using a set of recent at-sea campaigns and realistic simulations. Real-time experiments with identical autonomous underwater vehicles (AUVs) in the Buzzards Bay and Vineyard Sound region first show that our predicted time-optimal paths were faster than shortest distance paths. Deterministic and probabilistic reachability and path forecasts issued and validated for gliders and floats in the northern Arabian Sea are then presented. Novel Bayesian adaptive sampling for hypothesis testing and optimal learning are finally shown to forecast the observations most informative to estimate the accuracy of model formulations, the values of ecosystem parameters and dynamic fields, and the presence of Lagrangian Coherent Structures
Bayesian Learning of Coupled Biogeochemical-Physical Models
Predictive dynamical models for marine ecosystems are used for a variety of
needs. Due to sparse measurements and limited understanding of the myriad of
ocean processes, there is however significant uncertainty. There is model
uncertainty in the parameter values, functional forms with diverse
parameterizations, level of complexity needed, and thus in the state fields. We
develop a Bayesian model learning methodology that allows interpolation in the
space of candidate models and discovery of new models from noisy, sparse, and
indirect observations, all while estimating state fields and parameter values,
as well as the joint PDFs of all learned quantities. We address the challenges
of high-dimensional and multidisciplinary dynamics governed by PDEs by using
state augmentation and the computationally efficient GMM-DO filter. Our
innovations include stochastic formulation and complexity parameters to unify
candidate models into a single general model as well as stochastic expansion
parameters within piecewise function approximations to generate dense candidate
model spaces. These innovations allow handling many compatible and embedded
candidate models, possibly none of which are accurate, and learning elusive
unknown functional forms. Our new methodology is generalizable, interpretable,
and extrapolates out of the space of models to discover new ones. We perform a
series of twin experiments based on flows past a ridge coupled with
three-to-five component ecosystem models, including flows with chaotic
advection. The probabilities of known, uncertain, and unknown model
formulations, and of state fields and parameters, are updated jointly using
Bayes' law. Non-Gaussian statistics, ambiguity, and biases are captured. The
parameter values and model formulations that best explain the data are
identified. When observations are sufficiently informative, model complexity
and functions are discovered.Comment: 45 pages; 18 figures; 2 table
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Mini-Workshop: Innovative Trends in the Numerical Analysis and Simulation of Kinetic Equations
In multiscale modeling hierarchy, kinetic theory plays a vital role in connecting microscopic Newtonian mechanics and macroscopic continuum mechanics. As computing power grows, numerical simulation of kinetic equations has become possible and undergone rapid development over the past decade. Yet the unique challenges arising in these equations, such as highdimensionality, multiple scales, random inputs, positivity, entropy dissipation, etc., call for new advances of numerical methods. This mini-workshop brought together both senior and junior researchers working on various fastpaced growing numerical aspects of kinetic equations. The topics include, but were not limited to, uncertainty quantification, structure-preserving methods, phase transitions, asymptotic-preserving schemes, and fast methods for kinetic equations
Stochastic acoustic ray tracing with dynamically orthogonal equations
Submitted in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 2020.Developing accurate and computationally efficient models for ocean acoustics is inherently challenging due to several factors including the complex physical processes and the need to provide results on a large range of scales. Furthermore, the ocean itself is an inherently dynamic environment within the multiple scales. Even if we could measure the exact properties at a specific instant, the ocean will continue to change in the smallest temporal scales, ever increasing the uncertainty in the ocean prediction. In this work, we explore ocean acoustic prediction from the basics of the wave equation and its derivation. We then explain the deterministic implementations of the Parabolic Equation, Ray Theory, and Level Sets methods for ocean acoustic computation. We investigate methods for evolving stochastic fields using direct Monte Carlo, Empirical Orthogonal Functions, and adaptive Dynamically Orthogonal (DO) differential equations. As we evaluate the potential of Reduced-Order Models for stochastic ocean acoustics prediction, for the first time, we derive and implement the stochastic DO differential equations for Ray Tracing (DO-Ray), starting from the differential equations of Ray theory. With a stochastic DO-Ray implementation, we can start from non-Gaussian environmental uncertainties and compute the stochastic acoustic ray fields in a reduced order fashion, all while preserving the complex statistics of the ocean environment and the nonlinear relations with stochastic ray tracing. We outline a deterministic
Ray-Tracing model, validate our implementation, and perform Monte Carlo stochastic computation as a basis for comparison. We then present the stochastic DO-Ray methodology with detailed derivations. We develop varied algorithms and discuss implementation challenges and solutions, using again direct Monte Carlo for comparison. We apply the stochastic DO-Ray methodology to three idealized cases of stochastic sound-speed profiles (SSPs): constant-gradients, uncertain deep-sound channel, and a varied sonic layer depth. Through this implementation with non-Gaussian examples, we observe the ability to represent the stochastic ray trace field in a reduced order fashion.Office of Naval Research Grants N00014-19-1-2664 (Task Force Ocean: DEEP-AI) and N00014-19-1-2693 (INBDA
DisPar Methods and Their Implementation on a Heterogeneous PC Cluster
Esta dissertação avalia duas áreas cruciais da simulação de advecção-
difusĂŁo.
A primeira parte é dedicada a estudos numéricos. Foi comprovado que
existe uma relação directa entre os momentos de deslocamento de uma partĂcula
de poluente e os erros de truncatura. Esta relação criou os fundamentos teóricos
para criar uma nova famĂlia de mĂ©todos numĂ©ricos, DisPar.
Foram introduzidos e avaliados três métodos. O primeiro é um método
semi-Lagrangeano 2D baseado nos momentos de deslocamento de uma partĂcula
para malhas regulares, DisPar-k. Com este mĂ©todo Ă© possĂvel controlar
explicitamente o erro de truncatura desejado. O segundo método também se
baseia nos momentos de deslocamento de uma partĂcula, sendo, contudo,
desenvolvido para malhas uniformes não regulares, DisParV. Este método
também apresentou uma forte robustez numérica. Ao contrário dos métodos
DisPar-K e DisParV, o terceiro segue uma aproximação Eulereana com três
regiões de destino da partĂcula. O mĂ©todo foi desenvolvido de forma a manter um
perfil de concentração inicial homogéneo independentemente dos parâmetros
usados. A comparação com o método DisPar-k em situações não lineares realçou
as fortes limitações associadas aos métodos de advecção-difusão em cenários
reais.
A segunda parte da tese é dedicada à implementação destes métodos num
Cluster de PCs heterogéneo. Para o fazer, foi desenvolvido um novo esquema de
partição, AORDA. A aplicação, Scalable DisPar, foi implementada com a
plataforma da Microsoft .Net, tendo sido totalmente escrita em C#. A aplicação foi
testada no estuário do Tejo que se localiza perto de Lisboa, Portugal.
Para superar os problemas de balanceamento de cargas provocados pelas
marés, foram implementados diversos esquemas de partição: “Scatter
Partitioning”, balanceamento dinâmico de cargas e uma mistura de ambos. Pelos
testes elaborados, foi possĂvel verificar que o nĂşmero de máquinas vizinhas se
apresentou como o mais limitativo em termos de escalabilidade, mesmo utilizando
comunicações assĂncronas. As ferramentas utilizadas para as comunicações
foram a principal causa deste fenĂłmeno. Aparentemente, o Microsoft .Net remoting 1.0 nĂŁo funciona de forma apropriada nos ambientes de concorrĂŞncia
criados pelas comunicações assĂncronas. Este facto nĂŁo permitiu a obtenção de
conclusões acerca dos nĂveis relativos de escalabilidade das diferentes
estratégias de partição utilizadas. No entanto, é fortemente sugerido que a melhor
estratégia irá ser “Scatter Partitioning” associada a balanceamento dinâmico de
cargas e a comunicações assĂncronas. A tĂ©cnica de “Scatter Partitioning” mitiga
os problemas de desbalanceamentos de cargas provocados pelas marés. Por
outro lado, o balanceamento dinâmico será essencialmente activado no inicio da
simulação para corrigir possĂveis problemas nas previsões dos poderes de cada
processador.This thesis assesses two main areas of the advection-diffusion simulation.
The first part is dedicated to the numerical studies. It has been proved that
there is a direct relation between pollutant particle displacement moments and
truncation errors. This relation raised the theoretical foundations to create a new
family of numerical methods, DisPar.
Three methods have been introduced and appraised. The first is a 2D semi-
Lagrangian method based on particle displacement moments for regular grids,
DisPar-k. With this method one can explicitly control the desired truncation error.
The second method is also based on particle displacement moments but it is
targeted to regular/non-uniform grids, DisParV. The method has also shown a
strong numerical capacity. Unlike DisPar-k and DisParV, the third method is a
Eulerian approximation for three particle destination units. The method was
developed so that an initial concentration profile will be kept homogeneous
independently of the used parameters. The comparison with DisPar-k in non-linear
situations has emphasized the strong shortcomings associated with numerical
methods for advection-diffusion in real scenarios.
The second part of the dissertation is dedicated to the implementation of
these methods in a heterogeneous PC Cluster. To do so, a new partitioning
method has been developed, AORDA. The application, Scalable DisPar, was
implemented with the Microsoft .Net framework and was totally written in C#. The
application was tested on the Tagus Estuary, near Lisbon (Portugal).
To overcome the load imbalances caused by tides scatter partitioning was
implemented, dynamic load balancing and a mix of both. By the tests made, it was
possible to verify that the number of neighboring machines was the main factor
affecting the application scalability, even with asynchronous communications. The
tools used for communications mainly caused this. Microsoft .Net remoting 1.0
does not seem to properly work in environments with concurrency associated with
the asynchronous communications. This did not allow taking conclusions about the
relative efficiency between the partitioning strategies used. However, it is strongly
suggested that the best approach will be to scatter partitioning with dynamic load
balancing and with asynchronous communications. Scatter partitioning mitigates
load imbalances caused by tides and dynamic load balancing is basically trigged
at the begging of the simulation to correct possible problems in processor power
predictions
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Mathematical Theory and Modelling in Atmosphere-Ocean Science
Mathematical theory and modelling in atmosphere-ocean science combines a broad range of advanced mathematical and numerical techniques and research directions. This includes the asymptotic analysis of multiscale systems, the deterministic and stochastic modelling of sub-grid-scale processes, and the numerical analysis of nonlinear PDEs over a broad range of spatial and temporal scales. This workshop brought together applied mathematicians and experts in the disciplinary fields of meteorology and oceanography for a wide-ranging exchange of ideas and results in this area with the aim of fostering fundamental interdisciplinary work in this important science area
Particles and fields in fluid turbulence
The understanding of fluid turbulence has considerably progressed in recent
years. The application of the methods of statistical mechanics to the
description of the motion of fluid particles, i.e. to the Lagrangian dynamics,
has led to a new quantitative theory of intermittency in turbulent transport.
The first analytical description of anomalous scaling laws in turbulence has
been obtained. The underlying physical mechanism reveals the role of
statistical integrals of motion in non-equilibrium systems. For turbulent
transport, the statistical conservation laws are hidden in the evolution of
groups of fluid particles and arise from the competition between the expansion
of a group and the change of its geometry. By breaking the scale-invariance
symmetry, the statistically conserved quantities lead to the observed anomalous
scaling of transported fields. Lagrangian methods also shed new light on some
practical issues, such as mixing and turbulent magnetic dynamo.Comment: 165 pages, review article for Rev. Mod. Phy
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