10,644 research outputs found
Resonant drag instability of grains streaming in fluids
We show that grains streaming through a fluid are generically unstable if
their velocity, projected along some direction, matches the phase velocity of a
fluid wave (linear oscillation). This can occur whenever grains stream faster
than any fluid wave. The wave itself can be quite general--sound waves,
magnetosonic waves, epicyclic oscillations, and Brunt-V\"ais\"al\"a
oscillations each generate instabilities, for example. We derive a simple
expression for the growth rates of these "resonant drag instabilities" (RDI).
This expression (i) illustrates why such instabilities are so virulent and
generic, and (ii) allows for simple analytic computation of RDI growth rates
and properties for different fluids. As examples, we introduce several new
instabilities, which could see application across a variety of physical systems
from atmospheres to protoplanetary disks, the interstellar medium, and galactic
outflows. The matrix-based resonance formalism we introduce can also be applied
more generally in other (nonfluid) contexts, providing a simple means for
calculating and understanding the stability properties of interacting systems.Comment: 5 Pages. Published in ApJ
Physical models of streaming instabilities in protoplanetary discs
We develop simple, physically motivated models for drag-induced dustâgas streaming instabilities, which are thought to be crucial for clumping grains to form planetesimals in protoplanetary discs. The models explain, based on the physics of gaseous epicyclic motion and dustâgas drag forces, the most important features of the streaming instability and its simple generalization, the disc settling instability. Some of the key properties explained by our models include the sudden change in the growth rate of the streaming instability when the dust-to-gas mass ratio surpasses one, the slow growth rate of the streaming instability compared to the settling instability for smaller grains, and the main physical processes underlying the growth of the most unstable modes in different regimes. As well as providing helpful simplified pictures for understanding the operation of an interesting and fundamental astrophysical fluid instability, our models may prove useful for analysing simulations and developing non-linear theories of planetesimal growth in discs
The distribution of density in supersonic turbulence
We propose a model for the density statistics in supersonic turbulence, which
play a crucial role in star-formation and the physics of the interstellar
medium (ISM). Motivated by [Hopkins, MNRAS, 430, 1880 (2013)], the model
considers the density to be arranged into a collection of strong shocks of
width , where is the turbulent Mach
number. With two physically motivated parameters, the model predicts all
density statistics for turbulence: the density probability
distribution and its intermittency (deviation from log-normality), the density
variance-Mach number relation, power spectra, and structure functions. For the
proposed model parameters, reasonable agreement is seen between model
predictions and numerical simulations, albeit within the large uncertainties
associated with current simulation results. More generally, the model could
provide a useful framework for more detailed analysis of future simulations and
observational data. Due to the simple physical motivations for the model in
terms of shocks, it is straightforward to generalize to more complex physical
processes, which will be helpful in future more detailed applications to the
ISM. We see good qualitative agreement between such extensions and recent
simulations of non-isothermal turbulence
Australia's Prosperous 2000s: Housing and the Mining Boom
The 2000s was a particularly eventful decade for both the international and Australian economies. There were: two recessions in many countries; the largest international financial crisis since the Great Depression; the ongoing rapid development of Asia; asset booms and busts; and, Australia experienced the longest sustained increase in commodity prices and the terms of trade in the nation's history. This paper provides an overview of the Australian economy's performance in the decade. Several key topics are elaborated on, including the development of Asia and implications for Australia, policy frameworks, and the opportunities and challenges facing the Australian economy, with a particular focus on the expansion of household balance sheets and the rapid growth in the mining economy.Australian macroeconomy; economic performance; household balance sheets; terms of trade; monetary policy; fiscal policy
Geometry and Dynamics with Time-Dependent Constraints
We describe how geometrical methods can be applied to a system with
explicitly time-dependent second-class constraints so as to cast it in
Hamiltonian form on its physical phase space. Examples of particular interest
are systems which require time-dependent gauge fixing conditions in order to
reduce them to their physical degrees of freedom. To illustrate our results we
discuss the gauge-fixing of relativistic particles and strings moving in
arbitrary background electromagnetic and antisymmetric tensor fields.Comment: 8 pages, Plain TeX, CERN-TH.7392/94 and MPI-PhT/94-4
Demographic vital rates and population growth: rethinking the relationship in a harvested elk population
Understanding the nature of the relationship between demographic vital rates and the rate of population change (λ) is important for determining effective strategies for population management and conservation. We examined the relative impacts of various demographic vital rates on λ within the range of temporal vital rate variability observed in a harvested population to test the hypotheses that adult survival rates in ungulates are relatively invariant when compared to other vital rates and that variability in calf survival has a greater influence on rates of population change than adult survival. Vital rates were estimated for an elk (_Cervus elaphus_) population at Fort Riley, Kansas from October 2003 – February 2007. The magnitude of adult survival rates were similar to other harvested populations and models including a negative relationship between survival and age received the highest levels of support. Prime-age adult survival had the highest stage-specific elasticity values, indicating a high contribution of these matrix elements to λ. Life-stage simulation analysis indicated that variation in calf survival had the highest correlation with variation in λ (r^2^ = 0.61). Our results suggest that adult survival rates in harvested populations may experience increased levels of variability, but that calf survival rates have the greatest relative influence on λ due to the wider range of variability observed for this vital rate
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