407 research outputs found
A Heuristic Framework for Next-Generation Models of Geostrophic Convective Turbulence
Many geophysical and astrophysical phenomena are driven by turbulent fluid
dynamics, containing behaviors separated by tens of orders of magnitude in
scale. While direct simulations have made large strides toward understanding
geophysical systems, such models still inhabit modest ranges of the governing
parameters that are difficult to extrapolate to planetary settings. The
canonical problem of rotating Rayleigh-B\'enard convection provides an
alternate approach - isolating the fundamental physics in a reduced setting.
Theoretical studies and asymptotically-reduced simulations in rotating
convection have unveiled a variety of flow behaviors likely relevant to natural
systems, but still inaccessible to direct simulation. In lieu of this, several
new large-scale rotating convection devices have been designed to characterize
such behaviors. It is essential to predict how this potential influx of new
data will mesh with existing results. Surprisingly, a coherent framework of
predictions for extreme rotating convection has not yet been elucidated. In
this study, we combine asymptotic predictions, laboratory and numerical
results, and experimental constraints to build a heuristic framework for
cross-comparison between a broad range of rotating convection studies. We
categorize the diverse field of existing predictions in the context of
asymptotic flow regimes. We then consider the physical constraints that
determine the points of intersection between flow behavior predictions and
experimental accessibility. Applying this framework to several upcoming devices
demonstrates that laboratory studies may soon be able to characterize
geophysically-relevant flow regimes. These new data may transform our
understanding of geophysical and astrophysical turbulence, and the conceptual
framework developed herein should provide the theoretical infrastructure needed
for meaningful discussion of these results.Comment: 36 pages, 8 figures. CHANGES: in revision at Geophysical and
Astrophysical Fluid Dynamic
The role of Stewartson and Ekman layers in turbulent rotating Rayleigh-B\'enard convection
When the classical Rayleigh-B\'enard (RB) system is rotated about its
vertical axis roughly three regimes can be identified. In regime I (weak
rotation) the large scale circulation (LSC) is the dominant feature of the
flow. In regime II (moderate rotation) the LSC is replaced by vertically
aligned vortices. Regime III (strong rotation) is characterized by suppression
of the vertical velocity fluctuations. Using results from experiments and
direct numerical simulations of RB convection for a cell with a
diameter-to-height aspect ratio equal to one at ()
and we identified the characteristics of the
azimuthal temperature profiles at the sidewall in the different regimes. In
regime I the azimuthal wall temperature profile shows a cosine shape and a
vertical temperature gradient due to plumes that travel with the LSC close to
the sidewall. In regime II and III this cosine profile disappears, but the
vertical wall temperature gradient is still observed. It turns out that the
vertical wall temperature gradient in regimes II and III has a different origin
than that observed in regime I. It is caused by boundary layer dynamics
characteristic for rotating flows, which drives a secondary flow that
transports hot fluid up the sidewall in the lower part of the container and
cold fluid downwards along the sidewall in the top part.Comment: 21 pages, 12 figure
Geostrophic convective turbulence: The effect of boundary layers
Rayleigh--B\'enard (RB) convection, the flow in a fluid layer heated from
below and cooled from above, is used to analyze the transition to the
geostrophic regime of thermal convection. In the geostrophic regime, which is
of direct relevance to most geo- and astrophysical flows, the system is
strongly rotated while maintaining a sufficiently large thermal driving to
generate turbulence. We directly simulate the Navier--Stokes equations for two
values of the thermal forcing, i.e. and , a
constant Prandtl number~, and vary the Ekman number in the range
to which satisfies both requirements of
super-criticality and strong rotation. We focus on the differences between the
application of no-slip vs. stress-free boundary conditions on the horizontal
plates. The transition is found at roughly the same parameter values for both
boundary conditions, i.e. at~ for~ and at~ for~. However,
the transition is gradual and it does not exactly coincide in~ for
different flow indicators. In particular, we report the characteristics of the
transitions in the heat transfer scaling laws, the boundary-layer thicknesses,
the bulk/boundary-layer distribution of dissipations and the mean temperature
gradient in the bulk. The flow phenomenology in the geostrophic regime evolves
differently for no-slip and stress-free plates. For stress-free conditions the
formation of a large-scale barotropic vortex with associated inverse energy
cascade is apparent. For no-slip plates, a turbulent state without large-scale
coherent structures is found; the absence of large-scale structure formation is
reflected in the energy transfer in the sense that the inverse cascade, present
for stress-free boundary conditions, vanishes.Comment: Submitted to JF
The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians in three lettuce cultivars.
Lettuce yields can be reduced by the disease bacterial leaf spot (BLS) caused by the pathogen Xanthomonas campestris pv. vitians (Xcv) and host resistance is the most feasible method to reduce disease losses. The cultivars La Brillante, Pavane and Little Gem express an incompatible host-pathogen interaction as a hypersensitive response (HR) to California strains of Xcv resulting in resistance. Little was known about the inheritance of resistance; however, resistance to other lettuce pathogens is often determined by resistance gene candidates (RGCs) encoding nucleotide-binding leucine-rich repeat (NB-LRR) proteins. Therefore, we determined the inheritance of BLS resistance in the cultivars La Brillante, Little Gem and Pavane and mapped it relative to RGCs. The reaction to Xcv was analyzed in nine F1, F2 and recombinant inbred line populations of lettuce from HR×compatible or HR×HR crosses. The HR in La Brillante, Pavane and Little Gem is conditioned by single dominant genes, which are either allelic or closely linked genes. The resistance gene in La Brillante was designated Xanthomonas resistance 1 (Xar1) and mapped to lettuce linkage group 2. Xar1 is present in a genomic region that contains numerous NB-LRR encoding RGCs and functional pathogen resistance loci in the RGC2 family. The Xar1 gene confers a high level of BLS resistance in the greenhouse and field that can be introgressed into commercial lettuce cultivars to reduce BLS losses using molecular markers
Laboratory Exploration of Heat Transfer Regimes in Rapidly Rotating Turbulent Convection
We report heat transfer and temperature profile measurements in laboratory
experiments of rapidly rotating convection in water under intense thermal
forcing (Rayleigh number as high as ) and unprecedentedly
strong rotational influence (Ekman numbers as low as ).
Measurements of the mid-height vertical temperature gradient connect
quantitatively to predictions from numerical models of asymptotically rapidly
rotating convection, separating various flow phenomenologies. Past the limit of
validity of the asymptotically-reduced models, we find novel behaviors in a
regime we refer to as rotationally-influenced turbulence, where rotation is
important but not as dominant as in the known geostrophic turbulence regime.
The temperature gradients collapse to a Rayleigh-number scaling as
in this new regime. It is bounded from above by a critical convective Rossby
number independent of domain aspect ratio , clearly
distinguishing it from well-studied rotation-affected convection.Comment: 14 pages, 7 figure
Discontinuous Transitions Towards Vortex Condensates in Buoyancy-Driven Rotating Turbulence: Analogies with First-Order Phase Transitions
Using direct numerical simulations of rotating Rayleigh-B\'enard convection,
we explore the transitions between turbulent states from a 3D flow state
towards a quasi-2D condensate known as the large-scale vortex (LSV). We vary
the Rayleigh number as control parameter and study the system response
(strength of the LSV) in terms of order parameters assessing the energetic
content in the flow and the upscale energy flux. By sensitively probing the
boundaries of the domain of existence of the LSV, we find discontinuous
transitions and we identify the presence of a hysteresis loop as well as
nucleation & growth type of dynamics, manifesting a remarkable correspondence
with first-order phase transitions in equilibrium statistical mechanics. We
show furthermore that the creation of the condensate state coincides with a
discontinuous transition of the energy transport into the largest mode of the
system.Comment: 10 pages, 5 figure
Simultaneous Matrix Diagonalization for Structural Brain Networks Classification
This paper considers the problem of brain disease classification based on
connectome data. A connectome is a network representation of a human brain. The
typical connectome classification problem is very challenging because of the
small sample size and high dimensionality of the data. We propose to use
simultaneous approximate diagonalization of adjacency matrices in order to
compute their eigenstructures in more stable way. The obtained approximate
eigenvalues are further used as features for classification. The proposed
approach is demonstrated to be efficient for detection of Alzheimer's disease,
outperforming simple baselines and competing with state-of-the-art approaches
to brain disease classification
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