12,412 research outputs found
Turbulence energetics in stably stratified geophysical flows: strong and weak mixing regimes
Traditionally, turbulence energetics is characterized by turbulent kinetic
energy (TKE) and modelled using solely the TKE budget equation. In stable
stratification, TKE is generated by the velocity shear and expended through
viscous dissipation and work against buoyancy forces. The effect of
stratification is characterized by the ratio of the buoyancy gradient to
squared shear, called Richardson number, Ri. It is widely believed that at Ri
exceeding a critical value, Ric, local shear cannot maintain turbulence, and
the flow becomes laminar. We revise this concept by extending the energy
analysis to turbulent potential and total energies (TPE and TTE = TKE + TPE),
consider their budget equations, and conclude that TTE is a conservative
parameter maintained by shear in any stratification. Hence there is no
"energetics Ric", in contrast to the hydrodynamic-instability threshold,
Ric-instability, whose typical values vary from 0.25 to 1. We demonstrate that
this interval, 0.25<Ri<1, separates two different turbulent regimes: strong
mixing and weak mixing rather than the turbulent and the laminar regimes, as
the classical concept states. This explains persistent occurrence of turbulence
in the free atmosphere and deep ocean at Ri>>1, clarify principal difference
between turbulent boundary layers and free flows, and provide basis for
improving operational turbulence closure models.Comment: 23 pages, 4 figures, Quarterly Journal of Royal Meteorological
Society, in pres
Supermassive Black Hole Feedback
Understanding the processes that drive galaxy formation and shape the
observed properties of galaxies is one of the most interesting and challenging
frontier problems of modern astrophysics. We now know that the evolution of
galaxies is critically shaped by the energy injection from accreting
supermassive black holes (SMBHs). However, it is unclear how exactly the
physics of this feedback process affects galaxy formation and evolution. In
particular, a major challenge is unraveling how the energy released near the
SMBHs is distributed over nine orders of magnitude in distance throughout
galaxies and their immediate environments. The best place to study the impact
of SMBH feedback is in the hot atmospheres of massive galaxies, groups, and
galaxy clusters, which host the most massive black holes in the Universe, and
where we can directly image the impact of black holes on their surroundings. We
identify critical questions and potential measurements that will likely
transform our understanding of the physics of SMBH feedback and how it shapes
galaxies, through detailed measurements of (i) the thermodynamic and velocity
fluctuations in the intracluster medium (ICM) as well as (ii) the composition
of the bubbles inflated by SMBHs in the centers of galaxy clusters, and their
influence on the cluster gas and galaxy growth, using the next generation of
high spectral and spatial resolution X-ray and microwave telescopes.Comment: 10 pages, submitted to the Astro2020 decada
[Report of] Specialist Committee V.4: ocean, wind and wave energy utilization
The committee's mandate was :Concern for structural design of ocean energy utilization devices, such as offshore wind turbines, support structures and fixed or floating wave and tidal energy converters. Attention shall be given to the interaction between the load and the structural response and shall include due consideration of the stochastic nature of the waves, current and wind
Atmospheric Circulation of Terrestrial Exoplanets
The investigation of planets around other stars began with the study of gas
giants, but is now extending to the discovery and characterization of
super-Earths and terrestrial planets. Motivated by this observational tide, we
survey the basic dynamical principles governing the atmospheric circulation of
terrestrial exoplanets, and discuss the interaction of their circulation with
the hydrological cycle and global-scale climate feedbacks. Terrestrial
exoplanets occupy a wide range of physical and dynamical conditions, only a
small fraction of which have yet been explored in detail. Our approach is to
lay out the fundamental dynamical principles governing the atmospheric
circulation on terrestrial planets--broadly defined--and show how they can
provide a foundation for understanding the atmospheric behavior of these
worlds. We first survey basic atmospheric dynamics, including the role of
geostrophy, baroclinic instabilities, and jets in the strongly rotating regime
(the "extratropics") and the role of the Hadley circulation, wave adjustment of
the thermal structure, and the tendency toward equatorial superrotation in the
slowly rotating regime (the "tropics"). We then survey key elements of the
hydrological cycle, including the factors that control precipitation, humidity,
and cloudiness. Next, we summarize key mechanisms by which the circulation
affects the global-mean climate, and hence planetary habitability. In
particular, we discuss the runaway greenhouse, transitions to snowball states,
atmospheric collapse, and the links between atmospheric circulation and CO2
weathering rates. We finish by summarizing the key questions and challenges for
this emerging field in the future.Comment: Invited review, in press for the Arizona Space Science Series book
"Comparative Climatology of Terrestrial Planets" (S. Mackwell, M. Bullock,
and J. Harder, editors). 56 pages, 26 figure
Nonlinear physics of electrical wave propagation in the heart: a review
The beating of the heart is a synchronized contraction of muscle cells
(myocytes) that are triggered by a periodic sequence of electrical waves (action
potentials) originating in the sino-atrial node and propagating over the atria and
the ventricles. Cardiac arrhythmias like atrial and ventricular fibrillation (AF,VF)
or ventricular tachycardia (VT) are caused by disruptions and instabilities of these
electrical excitations, that lead to the emergence of rotating waves (VT) and turbulent
wave patterns (AF,VF). Numerous simulation and experimental studies during the
last 20 years have addressed these topics. In this review we focus on the nonlinear
dynamics of wave propagation in the heart with an emphasis on the theory of pulses,
spirals and scroll waves and their instabilities in excitable media and their application
to cardiac modeling. After an introduction into electrophysiological models for action
potential propagation, the modeling and analysis of spatiotemporal alternans, spiral
and scroll meandering, spiral breakup and scroll wave instabilities like negative line
tension and sproing are reviewed in depth and discussed with emphasis on their impact
in cardiac arrhythmias.Peer ReviewedPreprin
The Transition to Superrotation in Terrestrial Atmospheres
We show that by changing a single non-dimensional number, the thermal Rossby
number, global atmospheric simulations with only axisymmetric forcing pass from
an Earth-like atmosphere to a superrotating atmosphere that more resembles the
atmospheres of Venus or Titan. The transition to superrotation occurs under
conditions in which equatorward-propagating Rossby waves generated by
baroclinic instability at intermediate and high latitudes are suppressed, which
will occur when the deformation radius exceeds the planetary radius. At large
thermal Rossby numbers following an initial, nearly axisymmetric phase, a
global baroclinic wave of zonal wavenumber one generated by mixed
barotropic-baroclinic instability dominates the eddy flux of zonal momentum.
The global wave converges eastward zonal momentum to the equator and deposits
westward momentum at intermediate latitudes during spinup and before
superrotation emerges, and the baroclinic instability ceases once superrotation
is established. A global barotropic mode of zonal wavenumber one generated by a
mix of high- and low-latitude barotropic instability is responsible for
maintaining superrotation in the statistically steady state. At intermediate
thermal Rossby numbers, momentum flux by the global baroclinic mode is
subdominant relative to smaller baroclinic modes, and thus strong superrotation
does not develop.Comment: accepted for publication in JGR-Planet
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