774 research outputs found
European Railway Comparisons: Final Report
The Institute for Transport Studies (ITS), University of Leeds and the British Railways Board (BRB) carried out a major comparative study of Western European railways in the late 1970s (BRB and University of Leeds, 1979). Follow-up work was carried out by ITS financed by the Social Science Research Council and reported by Nash (1985). It was deaded to revive this work at ITS for a number of reasons:
It is over ten years since the last set of comparisons (for 1981) were made at ITS and therefore a review of the changes in costs and productivity may be timely.
There has been a number of technical developments that make the use of statistical cost analysis more promising. These developments include the use of more flexible functional forms such as the translog, and the development of comprehensive total factor productivity indices (see, for example, Dodgson, 1985 and, more recently, Hensher and Waters, 1993).
There is increasing interest in the organisational structure of railway industries as a result of the 1988 Transport Act in Sweden, the EC directive 91/4-40 and the publication of proposals for privatising British Rail in July 1992 (see, for example, ECMT, 1993).
Given the explosion in information technology, there were some hopes that data availability would have improved.
(Continues..
Stick-slip instability for viscous fingering in a gel
The growth dynamics of an air finger injected in a visco-elastic gel (a
PVA/borax aqueous solution) is studied in a linear Hele-Shaw cell. Besides the
standard Saffmann-Taylor instability, we observe - with increasing finger
velocities - the existence of two new regimes: (a) a stick-slip regime for
which the finger tip velocity oscillates between 2 different values, producing
local pinching of the finger at regular intervals, (b) a ``tadpole'' regime
where a fracture-type propagation is observed. A scaling argument is proposed
to interpret the dependence of the stick-slip frequency with the measured
rheological properties of the gel.Comment: 7 pages, 4 figures. Submitted to Europhysics Letter
Growth of Perturbation in Gravitational Collapse and Accretion
When a self-gravitating spherical gas cloud collapses or accretes onto a
central mass, the inner region of the cloud develops a density profile
and the velocity approaches free-fall. We show that in
this region, nonspherical perturbations grow with decreasing radius. In the
linear regime, the tangential velocity perturbation increases as ,
while the Lagrangian density perturbation, , grows as
. Faster growth occurs if the central collapsed object maintains a
finite multiple moment, in which case increases as ,
where specifies the angular degree of the perturbation. These scaling
relations are different from those obtained for the collapse of a homogeneous
cloud. Our numerical calculations indicate that nonspherical perturbations are
damped in the subsonic region, and that they grow and approach the asymptotic
scalings in the supersonic region. The implications of our results to
asymmetric supernova collapse and to black hole accretion are briefly
discussed.Comment: 23 pages including 6 ps figures; Minor changes and update; To appear
in ApJ, 200
Evolution of transonicity in an accretion disc
For inviscid, rotational accretion flows driven by a general pseudo-Newtonian
potential on to a Schwarzschild black hole, the only possible fixed points are
saddle points and centre-type points. For the specific choice of the Newtonian
potential, the flow has only two critical points, of which the outer one is a
saddle point while the inner one is a centre-type point. A restrictive upper
bound is imposed on the admissible range of values of the angular momentum of
sub-Keplerian flows through a saddle point. These flows are very unstable to
any deviation from a necessarily precise boundary condition. The difficulties
against the physical realisability of a solution passing through the saddle
point have been addressed through a temporal evolution of the flow, which gives
a non-perturbative mechanism for selecting a transonic solution passing through
the saddle point. An equation of motion for a real-time perturbation about the
stationary flows reveals a very close correspondence with the metric of an
acoustic black hole, which is also an indication of the primacy of
transonicity.Comment: 18 page
Secular instability in quasi-viscous disc accretion
A first-order correction in the -viscosity parameter of Shakura and
Sunyaev has been introduced in the standard inviscid and thin accretion disc. A
linearised time-dependent perturbative study of the stationary solutions of
this "quasi-viscous" disc leads to the development of a secular instability on
large spatial scales. This qualitative feature is equally manifest for two
different types of perturbative treatment -- a standing wave on subsonic
scales, as well as a radially propagating wave. Stability of the flow is
restored when viscosity disappears.Comment: 15 pages, 2 figures, AASTeX. Added some new material and upgraded the
reference lis
Perturbations on steady spherical accretion in Schwarzschild geometry
The stationary background flow in the spherically symmetric infall of a
compressible fluid, coupled to the space-time defined by the static
Schwarzschild metric, has been subjected to linearized perturbations. The
perturbative procedure is based on the continuity condition and it shows that
the coupling of the flow with the geometry of space-time brings about greater
stability for the flow, to the extent that the amplitude of the perturbation,
treated as a standing wave, decays in time, as opposed to the amplitude
remaining constant in the Newtonian limit. In qualitative terms this situation
simulates the effect of a dissipative mechanism in the classical Bondi
accretion flow, defined in the Newtonian construct of space and time. As a
result of this approach it becomes impossible to define an acoustic metric for
a conserved spherically symmetric flow, described within the framework of
Schwarzschild geometry. In keeping with this view, the perturbation, considered
separately as a high-frequency travelling wave, also has its amplitude reduced.Comment: 8 pages, no figur
Realizability of stationary spherically symmetric transonic accretion
The spherically symmetric stationary transonic (Bondi) flow is considered a
classic example of an accretion flow. This flow, however, is along a
separatrix, which is usually not physically realizable. We demonstrate, using a
pedagogical example, that it is the dynamics which selects the transonic flow.Comment: 4 pages in REVTeX with 2 figures. Typos have been corrected and some
alterations have been made in the version published in Physical Review
Protein turnover measured in vivo and in vitro in muscles undergoing compensatory growth and subsequent denervation atrophy
The metabolic state of muscle in the isolated perfused rat hemicorpus in relation to rates of protein synthesis
Acute effects of corticosterone on tissue protein synthesis and insulin-sensitivity in rats in vivo
- …