452 research outputs found
An acoustic view of ocean mixing
Knowledge of the parameter K (turbulent diffusivity/"mixing intensity") is a key to understand transport processes of matter and energy in the ocean. Especially the almost vertical component of K across the ocean stratification
(diapycnal diffusivity) is vital for research on biogeochemical cycles or greenhouse gas budgets.
Recent boost in precision of water velocity data that can be obtained from vessel-mounted acoustic instruments (vmADCP) allows identifying ocean regions of elevated diapycnal diffusivity during research cruises - in high horizontal resolution and without extra ship time needed.
This contribution relates acoustic data from two cruises
in the Tropical North East Atlantic Oxygen Minimum Zone
to simultaneous field observations of diapycnal diffusivity:
pointwise measurements by a microstructure profiler
as well as one integrative value from a large scale Tracer Release Experiment
Mesh-adaptive simulations of horizontal-axis turbine arrays using the actuator line method
Numerical models of the flow and wakes due to turbines operating within a
real-scale offshore wind farm can lead to a prohibitively large computational
cost, particularly when considering blade-resolved simulations. With the
introduction of turbine parametrizations such as the actuator disk (AD) or the
actuator line (AL) models, this problem has been partially addressed, yet the
computational cost associated with these simulations remains high. In this
work, we present an implementation and validation of an AL model within the
mesh-adaptive three-dimensional fluid dynamics solver, Fluidity, under a
unsteady Reynolds-averaged Navier-Stokes based turbulence modelling approach. A
key feature of this implementation is the use of mesh optimization techniques,
which allow for the automatic refinement or coarsening of the mesh locally
according to the resolution needed by the fluid flow solver. The model is first
validated against experimental data from wind tunnel tests. Finally, we
demonstrate the benefits of mesh-adaptivity by considering flow past the
Lillgrund offshore wind farm
Enhancing the performance of malleable MPI applications by using performance-aware dynamic reconfiguration
The work in this paper focuses on providing malleability to MPI applications by using a novel performance-aware dynamic reconfiguration technique. This paper describes the design and implementation of Flex-MPI, an MPI library extension which can automatically monitor and predict the performance of applications, balance and redistribute the workload, and reconfigure the application at runtime by changing the number of processes. Unlike existent approaches, our reconfiguring policy is guided by user-defined performance criteria. We focus on iterative SPMD programs, a class of applications with critical mass within the scientific community. Extensive experiments show that Flex-MPI can improve the performance, parallel efficiency, and cost-efficiency of MPI programs with a minimal effort from the programmer.This work has been partially supported by the Spanish Ministry of Economy and Competitiveness under the project TIN2013-
41350-P, Scalable Data Management Techniques for High-End Computing Systems, and EU under the COST Program Action IC1305, Network for Sustainable Ultrascale Computing (NESUS)Peer ReviewedPostprint (author's final draft
Simulating ice processes using the finite element, unstructured, adaptive model fluidity
The cryosphere impacts global climate in various ways. Snow and ice have a higher albedo than land or the open ocean and therefore affect the total reflectivity of the earth. Sea ice forms an insulating layer over the polar oceans controlling both heat and water vapour fluxes between the atmosphere and polar ocean. Ice sheets hold around 77% of Earth's freshwater reserves, and recent increases in ice loss from the Earth's ice sheets are cause for concern. This thesis develops numerical tools that can be used for the study of various ice processes such as ocean -- sea ice interaction, ice sheet and glacier dynamics.
A coupled ocean - sea ice model is developed, using the open source, unstructured, adaptive ocean model Fluidity and a finite element sea ice model developed at the Alfred Wegener Institute of Polar and Marine Research, FESIM. The tightly coupled model is verified and validated through a series of tests, demonstrating its dynamical capabilities. The sea ice dynamics are a model of Elastic-Viscous-Plastic rheology, as described in Hunke and Dukowicz. The thermodynamic parameterisation is similar to the 1D simplest model of Parkinson and Washington which is based on the zero-layer approach of Semtner.
Furthermore, a new computational framework for carrying out ice sheet simulations is presented. A thermo-mechanical, non-linear, full-Stokes model is used to carry out the exercises of the Ice Sheet Model Intercomparison Project for higher--order models (ISMIP--HOM). The results presented here show that Fluidity compares favourably with other ice sheet models. Further tests are performed to demonstrate the use of dynamic adaptive remeshing in lowering the computational cost of models compared to their structured, fixed-mesh counterparts, by focusing resolution only where and when required. Finally, initial simulations of the full Greenland ice sheet are performed demonstrating the potential utility of adaptive meshes for large-scale, full-Stokes modelling.Open Acces
Aeronautical Engineering. A continuing bibliography with indexes, supplement 135, May 1981
This bibliography lists 536 reports, articles, and other documents introduced into the NASA scientific and technical information system in April 1981
Climate-Driven Stock Shifts And Expansions In The U.S. Northeast Shelf: Identifying Challenges, Opportunities, And Barriers Through Fishermen And Manager Perspectives
Climate-driven warming in the U.S. Northeast Shelf (NES) has led to changes in the spatial distributions of many marine resources. Shifts and expansions of commercially important fish stocks pose major challenges to fishermen and fisheries managers in this region. American lobster (Homarus americanus) in the Gulf of Maine (GOM) is one of these impacted stocks and is projected to continue its shift towards more northern and offshore areas. Continued ocean warming could potentially reduce the GOM lobster stock by up to 60% over the next several decades. Given Maine’s reliance on its lobster fishery—which contributes over 80% of the value of Maine’s commercially harvested marine resources—building climate resilience into the fisheries social-ecological system is critical. Southern New England (SNE) serves as an example of a region that has already experienced much of the changes posed to impact the GOM. Through semi-structured interviews with SNE and GOM fishermen and a focus group of NES fisheries managers, black sea bass (Centropristis striata) was identified as a potential opportunity for fishermen to adapt to climate-driven changes. However, existing barriers—such as permitting, quota allocations, and bycatch regulations—prohibit the region’s fishermen from actualizing emerging opportunities. Results indicated that these barriers are not insurmountable and implementing “social-ecological management” approaches could provide viable pathways to facilitate opportunities and bolster climate resilience in the GOM
DisPar Methods and Their Implementation on a Heterogeneous PC Cluster
Esta dissertação avalia duas áreas cruciais da simulação de advecção-
difusão.
A primeira parte é dedicada a estudos numéricos. Foi comprovado que
existe uma relação directa entre os momentos de deslocamento de uma partícula
de poluente e os erros de truncatura. Esta relação criou os fundamentos teóricos
para criar uma nova família de métodos numéricos, DisPar.
Foram introduzidos e avaliados três métodos. O primeiro é um método
semi-Lagrangeano 2D baseado nos momentos de deslocamento de uma partícula
para malhas regulares, DisPar-k. Com este método é possível controlar
explicitamente o erro de truncatura desejado. O segundo método também se
baseia nos momentos de deslocamento de uma partícula, 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 partícula. O método 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 possível verificar que o número de máquinas vizinhas se
apresentou como o mais limitativo em termos de escalabilidade, mesmo utilizando
comunicações assíncronas. As ferramentas utilizadas para as comunicações
foram a principal causa deste fenómeno. Aparentemente, o Microsoft .Net remoting 1.0 não funciona de forma apropriada nos ambientes de concorrência
criados pelas comunicações assíncronas. Este facto não permitiu a obtenção de
conclusões acerca dos níveis 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 assíncronas. A técnica 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 possíveis 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
Proceedings, MSVSCC 2014
Proceedings of the 8th Annual Modeling, Simulation & Visualization Student Capstone Conference held on April 17, 2014 at VMASC in Suffolk, Virginia
Research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics, numerical analysis, and computer science
Research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics, numerical analysis, and computer science is summarized
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