1,721,048 research outputs found
Diapycnal transport and pattern formation in double-diffusive convection
Get PDFThis work analyzes the role of double-diffusive convection in constraining diapycnal velocity in the mid-latitude thermocline and in the initiation and maintenance of the deep convection associated with polynya and sea ice thinning events. Previously, no comprehensive high-resolution modeling studies of the possible role of double-diffusion in these areas have been conducted. A series of simulations using a numerical, multi-scale, MPI-based general circulation model is presented to remedy this dearth of knowledge. The effects of turbulent-dominated and purely double-diffusive regimes are compared to dual turbulent/double-diffusive systems and results are used to assess the likely roles of double-diffusion in constraining diapycnal velocity and delaying convection onset in high-latitude regions of marginal water column stability. High-resolution numerical modeling indicates that when both double-diffusion and turbulence are present, the constraints on diapycnal velocity loosen (tighten) with the increase of the fraction of the overall mixing attributed to turbulence (double-diffusion). The results of this study also indicate that double-diffusion could play an important role in delaying the onset of deep convection in the vicinity of Maud Rise in the eastern Weddell Sea, and may contribute to polynya formation and the persistence of interannual sea ice thinning.Approved for public release; distribution is unlimited.Lieutenant Commander, United States Navyhttp://archive.org/details/diapycnaltranspo109454793
Dynamics, heat transport, spectral composition and acoustic signatures of mesoscale variability in the ocean
Get PDFThe general circulation of the global ocean is turbulent rather than laminar. Mesoscale eddies contribute to the transport of tracers like heat, salt, and oxygen, and affect large-scale ocean dynamics. The problem of representing mesoscale variability stems from the nonlinear character of eddy dynamics that makes it difficult to predict equilibrated fluxes. The most intuitive solution is to apply a parameterization based on the eddy-driven transport observed in a global ocean that has been spinning up for centuries, which may not be feasible at present. An alternative approach involves constructing relatively simple analytically tractable equilibration models. In this study, the equilibration mechanism called the Growth Rate Balance (GRB) model proposes an explanation to the eddy dynamics as a competition between primary and secondary instabilities. The GRB model is validated in two configurations: in a two-layer model, and in a continuously stratified model. They identify the dependences of equilibrated fluxes on the characteristics of the background flow, and the applicability range of the GRB model. Finally, acoustic signatures of a fully developed eddy field predicted by the GRB model characterize the role of mesoscale variability in the important naval problem of acoustic propagation.Approved for public release; distribution is unlimited.Outstanding ThesisLieutenant Commander, Brazilian Navyhttp://archive.org/details/dynamicsheattran109453889
Exploring turbulent wakes in a non-uniformly stratified environment for submarine detection
Get PDFThis study aims to explore the behavior of turbulent wakes generated by a
spherical submerged body propagating with constant speed in a non-uniformly stratified
fluid. The investigation is based on a series of high-resolution numerical simulations in
which the background stratification is systematically varied. We consider one linear and
five non-linear temperature profiles and two sets of Froude numbers (Fr), Fr = 1.0 and Fr
= 3.2. The analysis of dissipation of thermal variance (χ) shows that the shape of the
wake for non-uniform profiles is more horizontally spread, and internal waves are much
stronger than in linear stratification. Experiments with Fr = 1.0 show a rather asymmetric
energy distribution caused by internal wave reflections from low-gradient regions. An
idealized model demonstrates that internal waves emitted at horizontal angles shallower
than roughly 64 degrees are reflected. For Fr = 3.2, internal waves are radiated at steeper
angles and transmitted more. Using decay rates of χ, the maximum detection time of the
wake can be estimated, showing that for Fr = 3.2, the thermal signal lasts four to five
times longer than for Fr = 1.0. Furthermore, concave profiles produce signals lasting
approximately twice as long as those for linear profiles, whereas low-gradient types have
half the duration. This research is expected to assist in the development of non-traditional
detection algorithms for undersea warfare
Dynamic and Kinematic Signatures of Propagating Bodies in Thermohaline Staircases
Get PDFSelf-propelled, submerged bodies travelling in thermohaline staircases generate evident temperature and velocity perturbations on the surface of the ocean, which may lead to possible airborne detection methods of underwater vehicles from above. This paper presents numerical simulations of a submerged body traveling in a staircase environment, to better understand the dynamics that occur underwater and on the surface. Previous studies of selfpropelled submerged bodies focused on the surface signatures of the late wake in a homogeneous or uniformly stratified fluid. However, this study will concentrate on the effects of the late wake in thermohaline staircases, an environment in which thermal signatures are more pronounced due to higher vertical heat transport compared to smooth gradients in the absence of staircases.Approved for public release; distribution is unlimited.Lieutenant Commander, United States Navyhttp://archive.org/details/dynamicndkinemat109455249
NUMERICAL MODELING OF THE VERTICAL HEAT TRANSPORT THROUGH THE DIFFUSIVE LAYER OF THE ARCTIC OCEAN
Get PDFThe Arctic Ocean has been a subject of increasing interest in recent years due to the reduction of the sea-ice thickness and spatial coverage and its implications for climate change. The future state of the Arctic is likely to be linked to vertical heat transport by microscale processes, specifically, double-diffusive convection. A series of realistic three-dimensional direct numerical simulations (DNS) were conducted to assess the vertical heat transport through thermohaline staircases in the Arctic region. Results revealed that vertical fluxes exceeded those of extant four-thirds flux laws by as much as a factor of two, and suggest that the 4/3 exponent requires downward revision. Results also showed that two-dimensional DNS can provide an accurate approximation of heat fluxes when the density ratio is sufficiently large. DNS results also reveal that the models with rigid boundaries result in heat flux estimates that are lower than those from models with periodic boundary conditions. Finally, the DNS-derived flux law was applied to Arctic data and results supported the conclusion that lab-derived flux laws significantly underestimate heat flux. All of these results suggest that vertical heat transport due to double-diffusive convection is a significant contributor to the measured reduction of Arctic sea-ice.Lieutenant Commander, United States Navyhttp://archive.org/details/numericalmodelin109453285
Analytical solutions for the ACC and its overturning circulation
Get PDFAn explicit analytical model of the Antarctic Circumpolar Current (ACC) is presented in which the
key feature is the balance between the Eulerian circulation acting to overturn the isopycnals and
geostrophic eddies which tend to flatten them. Solutions for the stratification and overturning
circulation are obtained by expanding the governing residual mean equations of motion in a small
parameter (ε) which measures the relative strength of the surface buoyancy flux and mechanical
forcing by winds. Our balanced asymptotic model extends and reconciles the earlier views on the
dynamics of the (streamwise-averaged) ACC by demonstrating that the zero order balance between
the eddy-induced circulation and the mean flow determines gross features of the buoyancy
distribution in the interior of the ACC. The diabatic buoyancy fluxes are essential in driving the
secondary (residual) circulation which is, however, largely steered along the pathways set by the
dominant adiabatic balance. Model assumptions are supported by a close agreement of the analytical
solutions and fully nonlinear numerical calculations
Surface Signatures of Submerged Bodies Propagating in Stratified Fluids
Get PDFA possible means of submersible detection is through the presence of surface signatures generated by a submerged body propagating in a stratified fluid. Direct numerical simulations (DNS) of perturbations generated by a submerged body can provide insight into how and when surface signatures occur based upon environmental conditions realized in the world's oceans. The use of realistic background stratifications is key to determining the significance of the phenomena to Navy operations and future research. This study employs a systematic DNS approach to diagnose the relationships between source speed/depth, mixed layer depth, temperature gradient, and Brunt-VΦaisΦalΦa frequency effects on resultant thermal and momentum surface signatures. Scope is limited to modeling of near-field wakes and analysis of resulting thermal and dynamic response. DNS is an extremely computationally expensive method for determination of surface signature occurrence and strength. Therefore, a predictive analytical algorithm, developed through dimensional analysis, is presented as an alternative to DNS.Approved for public release; distribution is unlimited.Lieutenant, United States Navyhttp://archive.org/details/surfacesignature109455252
Effects of mixed layer shear on vertical heat flux
Get PDFReissued 21 Feb 2017 with correction to spelling of Second Reader’s name.Measurements of salinity, temperature, and velocity shear profile time series were calculated from collocated AOFB and ITP buoys deployed in the Beaufort Sea from 2014–2015. Of interest was the effect ice speed has on MLD shear generation, Richardson number, and heat flux. The inertial components were also considered, as a large inertial event was present during the beginning of the data set. Data from the buoys show turbulent activity in the ocean during inertial wind events contributes to enhanced mixing in the mixed layer and entrainment of heat from the pycnocline. Data during non-inertial events has a much weaker correlation. Results demonstrated that during inertial events, ice speed was moderately correlated with heat flux (r = .56, p < .001). Non-inertial events saw a lower correlation of ice speed to heat flux (r = .312, p < .001). Relationships between ice speed and shear (r = .107, p < .001), ice speed and inverse Richardson number (r = .035, p = .256), inverse Richardson number and heat flux (r = .3, p < .001), heat content and heat flux (r = .084, p < .001) were also explored.Approved for public release; distribution is unlimited.Lieutenant Commander, United States Navyhttp://archive.org/details/effectsofmixedla109455169
Long-range interaction and elastic collisions of isolated vortices
Get PDFThe article of record as published may be found at http://dx.doi.org/10.1017/S0022112008002632This study explores the interaction of two nearly axisymmetric two-dimensional
vortices using a combination of numerical simulations and analytical arguments. We
consider isolated or ‘shielded’ eddies, characterized by zero net vorticity. The ability of
such vortices to propagate and interact is associated with the small dipolar component
that is introduced initially. Numerical contour dynamics experiments indicate that
the interaction of shielded eddies takes one of two forms, depending on their initial
separation and on the relative orientation of their dipolar components. Eddies can
influence each other by remotely modifying the dipolar moments of partner vortices,
an effect manifested in a gentle deflection of their trajectories from a straight course.
Strong interactions occur when eddies collide and rebound. The remote interaction
is explained by weakly nonlinear theory in which the basic state consists of identical
circularly symmetric eddies and the perturbation is assumed to be small. It is argued
that the elastic rebounds observed during direct collisions are induced by the exchange
of fluid between colliding vortices
Instabilities of a Time-Dependent Shear Flow
Get PDFThe article of record as published may be found at http://dx.doi.org/10.1175/JPO-D-19-0067.1This study offers a systematic stability analysis of unsteady shear flows representing large-scale, low-frequency internal waves in the ocean. The analysis is based on the unbounded time-dependent Couette model. This setup makes it possible to isolate the instabilities caused by uniform shear from those that can be attributed to resonant triad interactions or to the presence of inflection points in vertical velocity profiles. Linear analysis suggests that time-dependent spatially uniform shears are unstable regardless of the Richardson number (Ri). However, the growth rate of instability monotonically decreases with increasing Ri and increases with increasing frequency of oscillations. Therefore, models assuming a steady basic state—which are commonly used to conceptualize shear-induced instability and mixing—can be viewed as singular limits of the corresponding time-dependent systems. The present investigation is focused on the supercritical range of Richardson numbers (Ri . 1/4) where steady parallel flows are stable. An explicit relation is proposed for the growth rate of shear instability as a function of background parameters. For moderately supercritical Richardson numbers (Ri ; 1), we find that the growth rates obtained are less than, but comparable to, those expected for Kelvin–Helmholtz instabilities of steady shears at Ri , 1/4. Hence, we conclude that the instability of time-dependent flows could represent a viable mixing mechanism in the ocean, particular in regions characterized by relatively weak wave activity and predominantly supercritical large-scale shears.Grant OCE 1756491National Science Foundatio
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