1,618 research outputs found
Valuing agricultural externalities in Canterbury rivers and streams
Water quality and quantity concerns in Canterbury are intrinsically related to agriculture. Monetary values for impacts on streams and rivers is lacking in policy debate. This paper employs choice modelling to estimate values of three impacts on rivers and streams in Canterbury associated with agriculture: health risks of E coli from animal waste, ecological effects of excess nutrients, and low-flow impacts of irrigation. This study provides a valuation of outcomes for public policy implemented in Canterbury such as The Dairy and Clean Streams Accord, Living Streams, and The Restorative Programme for Lowland Streams.non-market-valuation, choice experiment, agricultural externalities, New Zealand, Agricultural and Food Policy, Environmental Economics and Policy, Food Consumption/Nutrition/Food Safety, Health Economics and Policy,
The Numerical Simulation of Radiative Shocks I: The elimination of numerical shock instabilities using a localized oscillation filter
We address a numerical instability that arises in the directionally split
computation of hydrodynamic flows when shock fronts are parallel to a grid
plane. Transverse oscillations in pressure, density and temperature are
produced that are exacerbated by thermal instability when cooling is present,
forming post--shock `stripes'. These are orthogonal to the classic post--shock
'ringing' fluctuations. The resulting post--shock `striping' substantially
modifies the flow. We discuss three different methods to resolve this problem.
These include (1) a method based on artificial viscosity; (2) grid--jittering
and (3) a new localized oscillation filter that acts on specific grid cells in
the shock front. These methods are tested using a radiative wall shock problem
with an embedded shear layer. The artificial viscosity method is unsatisfactory
since, while it does reduce post--shock ringing, it does not eliminate the
stripes and the excessive shock broadening renders the calculation of cooling
inaccurate, resulting in an incorrect shock location. Grid--jittering
effectively counteracts striping. However, elsewhere on the grid, the shear
layer is unphysically diffused and this is highlighted in an extreme case. The
oscillation filter method removes stripes and permits other high velocity
gradient regions of the flow to evolve in a physically acceptable manner. It
also has the advantage of only acting on a small fraction of the cells in a two
or three dimensional simulation and does not significantly impair performance.Comment: 20 pages, 6 figures, revised version submitted to ApJ Supplement
Serie
Space station integrated propulsion and fluid systems study
The program study was performed in two tasks: Task 1 addressed propulsion systems and Task 2 addressed all fluid systems associated with the Space Station elements, which also included propulsion and pressurant systems. Program results indicated a substantial reduction in life cycle costs through integrating the oxygen/hydrogen propulsion system with the environmental control and life support system, and through supplying nitrogen in a cryogenic gaseous supercritical or subcritical liquid state. A water sensitivity analysis showed that increasing the food water content would substantially increase the amount of water available for propulsion use and in all cases, the implementation of the BOSCH CO2 reduction process would reduce overall life cycle costs to the station and minimize risk. An investigation of fluid systems and associated requirements revealed a delicate balance between the individual propulsion and fluid systems across work packages and a strong interdependence between all other fluid systems
Induced Compton Scattering in Gigahertz Peak Spectrum Sources
We revisit the shocked shell model for the class of Active Galactic Nuclei
known as Gigahertz Peak Spectrum sources, incorporating new observational data
on the radiation brightness temperatures. We argue that in addition to
free-free absorption, induced Compton scattering will also have an important
effect in forming the ~GHz peak and in shaping the radio spectra that
characterize these sources. Indeed, our arguments suggest that GPS sources may
provide the first real evidence for the role of induced Compton scattering in
extragalactic radio sources.Comment: 12 pages, 1 figure, AAS LaTeX style with epsf, to appear in ApJ
Letter
Adiabatic relativistic models for the jets in the radio galaxy 3C 31
We present a general approach to the modelling of the brightness and
polarization structures of adiabatic, decelerating relativistic jets, based on
the formalism of Matthews & Scheuer (1990). We compare the predictions of
adiabatic jet models with deep, high-resolution observations of the radio jets
in the FR I radio galaxy 3C 31. Adiabatic models require coupling between the
variations of velocity, magnetic field and particle density. They are therefore
more tightly constrained than the models previously presented for 3C 31 by
Laing & Bridle (2002). We show that adiabatic models provide a poorer
description of the data in two crucial respects: they cannot reproduce the
observed magnetic-field structures in detail, and they also predict too steep a
brightness decline along the jets for plausible variations of the jet velocity.
We find that the innermost regions of the jets show the strongest evidence for
non-adiabatic behaviour, and that the adiabatic models provide progressively
better descriptions of the jet emission at larger distances from the galactic
nucleus. We briefly discuss physical processes which might contribute to this
non-adiabatic behaviour. In particular, we develop a parameterized description
of distributed particle injection, which we fit to the observed total
intensities. We show that particles are preferentially injected where bright
X-ray emission is observed, and where we infer that the jets are
over-pressured.Comment: 16 pages, 11 figures, accepted for publication in MNRA
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