Plankton accumulation and transport in propagating nonlinear internal fronts

Author Posting. © Sears Foundation for Marine Research, 2007. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 65 (2007): 117-145, doi:10.1357/002224007780388702.Accumulation and transport of plankton in fronts propagating across-shore is a process of considerable ecological importance for many inhabitants of the littoral zone, since it links the offshore larval pool with the juvenile and adult inshore habitat. Several field studies have shown that larval plankton accumulates in fronts, but have failed to give a conclusive proof that effective Lagrangian transport takes place. A few process-oriented numerical studies have lent support to the idea, but the scope of their results is limited by the two-dimensional nature of the flows considered and by the simple model used to account for the behavior of plankton. In this paper, we relax both constraints. We solve the three-dimensional Navier-Stokes equation to compute the time dependent velocity field, and we use an empirically based model for the behavior of plankton. Our results show that accumulation and transport is possible, even for larvae characterized by sustained swimming speeds that are small compared with the speed of propagation of the front. We introduce a simple model to characterize the accumulation along the front, which includes both entrainment and detrainment. The model accurately represents accumulation calculated from the numerical runs, and provide a simple tool to estimate transport under a variety of circumstances. We also investigate the spatial distribution of plankton along and across the front and show that it is very patchy and dependent on the swimming speed of plankton, with important implications for interpreting results from field experiments

Numerical simulation of pulsating turbulent channel flow

Direct and large-eddy simulations of the Navier–Stokes equations are used to study the pulsating flow in a channel. The cases examined span a wide range of frequencies of the driving pressure gradient, and encompass different physical behaviors, from the quasi-Stokes flow observed at high frequencies, to a quasisteady behavior at the lowest ones. The validity of the dynamic Smagorinsky model to study this kind of unsteady flow is established by a posteriori comparison with direct simulations and experimental data. It is shown that the fluctuations generated in the near-wall region by the unsteady pressure gradient do not propagate beyond a certain distance l t from the wall, which can be estimated quite accurately by a simple eddyviscosity argument. No substantial departure from the Stokes regime at very high frequency (ω + as high as 0.1) is observed. The time-dependent characteristics of the flow are examined in detail, as well as the topology of the coherent structures

Observation of very large and steep internal waves of elevation near the Massachusetts coast

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 31 (2004): L22307, doi:10.1029/2004GL021052.We report on near-bottom waves of elevation with amplitude nearly half the 25 m water column, very steep, and propagating into a sheared turbulent wave-guide. The waves contained trapped cores transporting parcels of water shoreward. These large waves depart strongly from weakly-nonlinear and weakly-nonhydrostatic conditions and challenge established paradigms. They can also represent an important factor in the across-shore transport of plankton and contaminants.Funding provided by ONR, grants N00014-01-1-0172 and N00014-03-1-0553

An Efficient Method For Solving Highly Anisotropic Elliptic Equations

Solving elliptic PDEs in more than one dimension can be a computationally expensive task. For some applications characterised by a high degree of anisotropy in the coefficients of the elliptic operator, such that the term with the highest derivative in one direction is much larger than the terms in the remaining directions, the discretized elliptic operator often has a very large condition number - taking the solution even further out of reach using traditional methods. This paper will demonstrate a solution method for such ill-behaved problems. The high condition number of the D-dimensional discretized elliptic operator will be exploited to split the problem into a series of well-behaved one and (D-1)-dimensional elliptic problems. This solution technique can be used alone on sufficiently coarse grids, or in conjunction with standard iterative methods, such as Conjugate Gradient, to substantially reduce the number of iterations needed to solve the problem to a specified accuracy. The solution is formulated analytically for a generic anisotropic problem using arbitrary coordinates, hopefully bringing this method into the scope of a wide variety of applications.Comment: 37 pages, 11 figure

On the interpretation of energy and energy fluxes of nonlinear internal waves : an example from Massachusetts Bay

Author Posting. © Cambridge University Press, 2006. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 561 (2006):103–112, doi:10.1017/S0022112006000991A self-consistent formalism to estimate baroclinic energy densities and fluxes resulting from the propagation of internal waves of arbitrary amplitude is derived using the concept of available potential energy. The method can be applied to numerical, laboratory or field data.MBIWE98 was supported by the US Geological Survey and the Office of Naval Research. A.S. received support from the Office of Naval Research (N00014-05-1-0361), R.B. from the Walter A. and Hope Noyes Smith Chair on Coastal Oceanography and B.B. from the US Geological Survey

Euler angles for G2

We provide a simple parametrization for the group G2, which is analogous to the Euler parametrization for SU(2). We show how to obtain the general element of the group in a form emphasizing the structure of the fibration of G2 with fiber SO(4) and base H, the variety of quaternionic subalgebras of octonions. In particular this allows us to obtain a simple expression for the Haar measure on G2. Moreover, as a by-product it yields a concrete realization and an Einstein metric for H.Comment: 21 pages, 2 figures, some misprints correcte

Avaliação do vigor de sementes de milho através do teste de envelhecimento precoce

Seeds of three different sizes of four corn cultivars ('Ag-152', 'Hmd-7974', 'Centralmex’ and 'Cateto') were studied for vigor, by employing of three accelerated aging periods. The evaluation was made through germination tests in the laboratory and by the speed of emergence in the field. Large sized seeds showed significantly higher percentage of germination. Differences in vigor in laboratory as well as field tests were observed between varieties. Specific studies on period of aging for different cultivars are indicated.Sementes de quatro cultivares de milho (‘Ag-152’; Hmd-7974, 'Centralmex’ e 'Cateto'), classificadas em três tamanhos, foram comparadas quanto ao seu vigor, pela aplicação de diferentes períodos de envelhecimento precoce. A avaliação dos efeitos foi efetuada através de testes de germinação (em laboratório) e de velocidade de emergência (em campo). Nos trabalhos de laboratório, foram observados efeitos significativos, favorecendo sementes grandes; verificaram-se diferenças no vigor entre as cultivares testadas e a necessidade de estudos específicos de diferentes períodos de envelhecimento para as cultivares, em razão de diferenças encontradas na sensibilidade das mesmas ao teste