bluuebunny/crossref_metadata_2025_split huggingface dataset:
On general transformations and variational
principles for the magnetohydrodynamics of
ideal fluids. Part 4. Generalized isovorticity
principle for three-dimensional flows
Authors: VLADIMIROV V. A. , MOFFATT H. K. , ILIN K. I. 
Published: July 2002

The equations of magnetohydrodynamics (MHD) of an ideal fluid have two families
of topological invariants: the magnetic helicity invariants and the cross-helicity
invariants. It is first shown that these invariants define a natural foliation (described
as isomagnetovortical, or imv for short) in the function space in which solutions
{u(x, t), h(x, t)}
of the MHD equations reside. A relaxation process is constructed
whereby total energy (magnetic plus kinetic) decreases on an imv folium (all magnetic
and cross-helicity invariants being thus conserved). The energy has a positive lower
bound determined by the global cross-helicity, and it is thus shown that a steady
state exists having the (arbitrarily) prescribed families of magnetic and cross-helicity
invariants.The stability of such steady states is considered by an appropriate generalization
of (Arnold) energy techniques. The first variation of energy on the imv folium is
shown to vanish, and the second variation δ2E
is constructed. It is shown that δ2E is
a quadratic functional of the first-order variations
δ1u, δ1h of u and h (from a steady
state U(x), H(x)), and that δ2E is an invariant of the linearized MHD equations.
Linear stability is then assured provided δ2E is either positive-definite or negative-definite for all imv perturbations. It is shown that the results may be equivalently
obtained through consideration of the frozen-in ‘modified’ vorticity field introduced
in Part 1 of this series.Finally, the general stability criterion is applied to a variety of classes of steady
states {U(x), H(x)}, and new sufficient conditions for stability to three-dimensional
imv perturbations are obtained.
Visualization and measurement of internal waves
by ‘synthetic schlieren’. Part 1. Vertically
oscillating cylinder
Authors: SUTHERLAND BRUCE R. , DALZIEL STUART B. , HUGHES GRAHAM O. , LINDEN P. F. 
Published: July 2002

We present measurements of the density and velocity fields produced when an oscillating
circular cylinder excites internal gravity waves in a stratified fluid. These
measurements are obtained using a novel, non-intrusive optical technique suitable
for determining the density fluctuation field in temporally evolving flows which are
nominally two-dimensional. Although using the same basic principles as conventional
methods, the technique uses digital image processing in lieu of large and expensive
parabolic mirrors, thus allowing more flexibility and providing high sensitivity:
perturbations of the order of 1% of the ambient density gradient may be detected.
From the density gradient field and its time derivative it is possible to construct the
perturbation fields of density and horizontal and vertical velocity. Thus, in principle,
momentum and energy fluxes can be determined.In this paper we examine the structure and amplitude of internal gravity waves
generated by a cylinder oscillating vertically at different frequencies and amplitudes,
paying particular attention to the role of viscosity in determining the evolution of the
waves. In qualitative agreement with theory, it is found that wave motions characterized
by a bimodal displacement distribution close to the source are attenuated by
viscosity and eventually undergo a transition to a unimodal displacement distribution
further from the source. Close quantitative agreement is found when comparing our
results with the theoretical ones of Hurley & Keady (1997). This demonstrates that
the new experimental technique is capable of making accurate measurements and
also lends support to analytic theories. However, theory predicts that the wave beams
are narrower than observed, and the amplitude is significantly under-predicted for
low-frequency waves. The discrepancy occurs in part because the theory neglects the
presence of the viscous boundary layers surrounding the cylinder, and because it
does not take into account the effects of wave attenuation resulting from nonlinear
wave–wave interactions between the upward and downward propagating waves near
the source.
Simulation of boundary layer transition induced
by periodically passing wakes
Authors: WU XIAOHUA , JACOBS ROBERT G. , HUNT JULIAN C. R. , DURBIN PAUL A. 
Published: July 2002

The interaction between an initially laminar boundary layer developing spatially on
a flat plate and wakes traversing the inlet periodically has been simulated numerically.
The three-dimensional, time-dependent Navier–Stokes equations were solved
with 5.24×107 grid points using a message passing interface on a scalable parallel
computer. The flow bears a close resemblance to the transitional boundary layer on
turbomachinery blades and was designed following, in outline, the experiments by Liu
& Rodi (1991). The momentum thickness Reynolds number
evolves from Reθ = 80
to 1120. Mean and second-order statistics downstream of Reθ = 800 are of canonical
flat-plate turbulent boundary layers and are in good agreement with Spalart (1988).In many important aspects the mechanism leading to the inception of turbulence is
in agreement with previous fundamental studies on boundary layer bypass transition,
as summarized in Alfredsson & Matsubara (1996). Inlet wake disturbances inside
the boundary layer evolve rapidly into longitudinal puffs during an initial receptivity
phase. In the absence of strong forcing from free-stream vortices, these structures exhibit
streamwise elongation with gradual decay in amplitude. Selective intensification
of the puffs occurs when certain types of turbulent eddies from the free-stream wake
interact with the boundary layer flow through a localized instability. Breakdown of
the puffs into young turbulent spots is preceded by a wavy motion in the velocity
field in the outer part of the boundary layer.Properties and streamwise evolution of the turbulent spots following breakdown,
as well as the process of completion of transition to turbulence, are in agreement with
previous engineering turbomachinery flow studies. The overall geometrical characteristics
of the matured turbulent spot are in good agreement with those observed in the
experiments of Zhong et al. (1998). When breakdown occurs in the outer layer, where
local convection speed is large, as in the present case, the spots broaden downstream,
having the vague appearance of an arrowhead pointing upstream.The flow has also been studied statistically. Phase-averaged velocity fields and skin-friction coefficients in the transitional region show similar features to previous cascade
experiments. Selected results from additional thought experiments and simulations are
also presented to illustrate the effects of streamwise pressure gradient and free-stream
turbulence.
A study of turbulence under conditions of transient flow in a pipe
Authors: HE S. , JACKSON J. D. 
Published: July 2002

A detailed investigation of fully developed transient flow in a pipe has been undertaken
using water as the working fluid. Linearly increasing or decreasing excursions of
flow rate were imposed between steady initial and final values. A three-beam, two-component laser Doppler anemometer was used to make simultaneous measurements
of either axial and radial, or axial and circumferential, components of local velocity.
Values of ensemble-averaged mean velocity, root-mean-square velocity fluctuation
and turbulent shear stress were found from the measurements.Being the first really detailed study of ramp-type transient turbulent flow, the
present investigation has yielded new information and valuable insight into certain
fundamental aspects of turbulence dynamics. Some striking features are evident in
the response of the turbulence field to the imposed excursions of flow rate. Three
different delays have been identified: a delay in the response of turbulence production;
a delay in turbulence energy redistribution among its three components; and
a delay associated with the propagation of turbulence radially. The last of these is the most
pronounced under the conditions of the present study. A dimensionless delay parameter
τ+[= √2τUτ0/D] is
proposed to describe it. The first response of
turbulence is found to occur in the region near the wall where turbulence production
peaks. The axial component of turbulence responds earlier than the other two components
and builds up faster. The response propagates towards the centre of the pipe
through the action of turbulent diffusion at a speed which depends on the Reynolds
number at the start of the excursion. In the core region, the three components of
turbulence energy respond in a similar manner. Turbulence intensity is reduced in
the case of accelerating flow and increased in decelerating flow. This is mainly as a
result of the delayed response of turbulence. A dimensionless ramp rate parameter
γ[= (dUb/dt)
(1/Ub0)(D/Uτ0]
is proposed, which determines the extent to which the
turbulence energy differs from that of pseudo-steady flow as a result of the delay in
the propagation of turbulence.
The propagation of a liquid bolus along a liquid-lined flexible tube
Authors: HOWELL P. D. , WATERS S. L. , GROTBERG J. B. 
Published: July 2002

We use lubrication theory and matched asymptotic expansions to model the quasi-steady propagation of a liquid plug or bolus through an elastic tube. In the limit
of small capillary number, asymptotic expressions are found for the pressure drop
across the bolus and the thickness of the liquid film left behind, as functions of the
capillary number, the thickness of the liquid lining ahead of the bolus and the elastic
characteristics of the tube wall. These results generalize the well-known theory for
the low capillary number motion of a bubble through a rigid tube (Bretherton 1961).
As in that theory, both the pressure drop across the bolus and the thickness of the
film it leaves behind vary like the two-thirds power of the capillary number. In our
generalized theory, the coefficients in the power laws depend on the elastic properties
of the tube.For a given thickness of the liquid lining ahead of the bolus, we identify a critical
imposed pressure drop above which the bolus will eventually rupture, and hence
the tube will reopen. We find that generically a tube with smaller hoop tension or
smaller longitudinal tension is easier to reopen. This flow regime is fundamental to
reopening of pulmonary airways, which may become plugged through disease or by
instilled/aspirated fluids.
On the vorticity transport due to dissipating or breaking waves in shallow-water flow
Authors: BÜHLER OLIVER 
Published: July 2002

Theoretical and numerical results are presented on the transport of vorticity (or
potential vorticity) due to dissipating gravity waves in a shallow-water system with
background rotation and bottom topography. The results are obtained under the
assumption that the flow can be decomposed into small-scale gravity waves and a
large-scale mean flow. The particle-following formalism of ‘generalized Lagrangian-mean’ theory is then used to derive an ‘effective mean force’ that captures the vorticity
transport due to the dissipating waves. This can be achieved without neglecting
other, non-dissipative, effects which is an important practical consideration. It is then
shown that the effective mean force obeys the so-called ‘pseudomomentum rule’, i.e.
the force is approximately equal to minus the local dissipation rate of the wave's
pseudomomentum. However, it is also shown that this holds only if the underlying
dissipation mechanism is momentum-conserving. This requirement has important
implications for numerical simulations, and these are discussed.The novelty of the results presented here is that they have been derived within a
uniform theoretical framework, that they are not restricted to small wave amplitude,
ray-tracing or JWKB-type approximations, and that they also include wave dissipation
by breaking, or shock formation. The theory is tested carefully against shock-capturing
nonlinear numerical simulations, which includes the detailed study of a wavetrain
subject to slowly varying bottom topography. The theory is also cross-checked in the
appropriate asymptotic limit against recently formulated weakly nonlinear theories. In
addition to the general finite-amplitude theory, detailed small-amplitude expressions
for the main results are provided in which the explicit appearance of Lagrangian
fields can be avoided. The motivation for this work stems partly from an on-going
study of high-altitude breaking of internal gravity waves in the atmosphere, and some
preliminary remarks on atmospheric applications and on three-dimensional stratified
versions of these results are given.
Descriptive adequacy in phonology: a variationist

perspective
Authors: DOCHERTY GERARD J. , FOULKES PAUL , MILROY JAMES , MILROY LESLEY , WALSHAW DAVID 
Published: July 2002

This paper offers a variationist critique of aspects of phonological

theory and method,

focusing on advances in descriptive methods and highlighting the problems
that need

to be addressed in explaining phonological variation. On the one hand,
socially

situated language samples which have been systematically collected and
analysed

constitute a legitimate – indeed often vital – source of

evidence to be utilised by

linguists for assessing and refining theoretical models. On the other

hand, variationists

cannot operate in isolation from theoretical concerns, and can benefit
from an

evaluation of the competing theoretical frameworks available to them.The paper begins with a brief review of the philosophical foundations
underlying

the tension between ‘external’ and ‘internal’

methodology. We then focus on a

particular phonological example – glottalisation in English.

We demonstrate that

phonological models of this can be complemented by systematic and accountable

data collection and analysis of the kind associated with sociolinguistics.

It is suggested

that the patterns of variation produced by speakers are significantly more
complex

than has been indicated in the phonological literature. Consequently, these

approaches can be usefully expanded and extended as theoretical models.
We discuss

some desiderata for extending the range of phonological models, focusing
chiefly on

the need to account for variability and change in language.
Are there principles of grammatical change?
Authors: HASPELMATH MARTIN 
Published: July 2002

David Lightfoot,The development of language: acquisition, change, and
evolution. (Maryland Lectures in Language and Cognition 1.) Malden, MA
& Oxford: Blackwell, 1999. Pp. xii+287.The central thesis of The development of language is that there are no
principles of grammatical change, so that ‘historicist’ or deterministic
approaches to diachronic change are misguided. Instead, Lightfoot argues
that language change can only be understood by taking the perspective of the
‘growth’ (i.e. acquisition) of an individual's biological grammar, which may
end up with a different parameter setting from the parent's generation when
the trigger experience changes. Such events of grammatical change are
abrupt and unpredictable, and Lightfoot suggests that they can be
understood better from the point of view of catastrophe theory and chaos
theory than under a deterministic theory of history as was common in the
nineteenth century.
Hegesias; the Death-Persuader; or, the Gloominess of Hedonism
Authors: Matson Wallace I. 
Published: July 2002

Hegesias (3d c.BC), as hedonist, held that the sage will kill himself. For: One should pursue pleasure and avoid pain. But life is virtually certain to contain more pain than pleasure. Therefore death, which is neither pleasurable nor painful, is better than life.The flaw in the argument lies in the underlying game-theoretical model of life as a game in which play and payoff are distinct. Hegesias's conclusion, that life is not ‘worth living,’ is inescapable by any philosophy so based, including John Rawls's. Why shouldn't his rational persons behind the veil of ignorance opt for prenatal suicide?
Schizophrenia and familial idiopathic basal ganglia calcification: a case report
Authors: CHABOT B. , ROULLAND C. , DOLLFUS S. 
Published: July 2002

Background. Familial idiopathic basal ganglia calcification (FIBGC) is generally associated with neurological and psychiatric symptoms. An association between FIBGC and schizophrenia has been described but it remains uncertain. We studied the relationship between the presence and extent of basal ganglia calcification and schizophrenia in a multiply affected family.Method. Symmetrical basal ganglia calcifications (BGC) were detected on computerized tomography (CT) in a schizophrenic proband and led us to carry out CTs and standardized psychiatric evaluations (SADS – Endicott & Spitzer, 1978) in all available first-degree relatives (mother and six siblings).Results. Five subjects had BGC, including three subjects diagnosed as schizophrenic. Three subjects had no BGC and none of them was diagnosed as schizophrenic. We subdivided the BGC into three groups: massive (pallidum, striatum and dentate nuclei affected); medium (pallidum and striatum); and mild (pallidum only). The two subjects with massive BGC and one of the two with medium BGC had schizophrenia. The subject with mild BGC had no psychotic symptoms.Conclusion. Our results are consistent with the hypothesis that BGC favours the occurrence of a schizophrenia-like syndrome and that the risk of occurrence of this syndrome is proportional to the extent of calcification. These findings support the hypothesis that schizophrenia is determined by a disruption of thalamo–cortico–striatal circuits.
The association between psychosis and problematical drug use among Australian adults: findings from the National Survey of Mental Health and Well-Being
Authors: DEGENHARDT LOUISA , HALL WAYNE 
Published: July 2002

Background. The present paper aimed to: (a) provide Australian estimates of the population-level association between psychotic ‘caseness’ and substance use; (b) examine liability to problematical substance use according to ‘caseness’ via the conditional prevalence (prevalence among users); and (c) examine associations between problematical substance use and the number of psychotic symptoms using ordinal logistic regression.Method. Data were from the National Survey of Mental Health and Well-Being (NSMHWB), a stratified multi-stage probability sample of Australian adults, using a subset of persons under the age of 50 years (N = 6722). A screener assessed the presence of characteristic psychotic symptoms. Associations between ‘case’ status and DSM-IV alcohol, cannabis and other drug use disorders were examined. Ordinal logistic regressions predicting psychosis scores were carried out, including demographic, mental health and drug use variables.Results. Ninety-nine persons (1·2 %) screened positively for psychosis. Regular tobacco, alcohol and cannabis use were much more common among persons screening positively, as were alcohol, cannabis and other drug use disorders. Among alcohol and cannabis users, psychosis ‘cases’ were much more likely to be dependent. Ordinal logistic regressions revealed that regular tobacco use, cannabis and alcohol dependence, and opiate abuse were predictors of psychosis scores.Conclusions. The mental health risks of problematical substance use need to be disseminated to persons at risk of, or suffering from, psychotic illness, and to heavy substance users. Work is needed to develop effective treatment approaches for problematical substance use among persons with psychosis.
Treatment resistant depression and Axis I co-morbidity
Authors: PETERSEN T. , GORDON J. A. , KANT A. , FAVA M. , ROSENBAUM J. F. , NIERENBERG A. A. 
Published: July 2002

Background. Treatment resistant depression (TRD) continues to present a formidable challenge to clinicians, accounts for over half of the annual costs associated with treatment for depression and causes great frustration to patients. Although there have been studies attempting to define TRD, little information is available as to the cause of TRD. One suggestion is that patients with TRD have a greater frequency of co-morbid psychiatric disorders, which explains their resistance to standard antidepressant treatments. The objective of this study was to compare the co-morbidity of Axis I disorders between a sample of TRD patients and a sample of non-TRD patients.Methods. TRD and non-TRD patients, recruited from two separate antidepressant treatment studies, were assessed for Axis I co-morbidity using the SCID-P for the DSM-III-R. Patients for the two samples were then matched for baseline HAM-D-17 total score and gender.Results. Results reveal that non-TRD patients had a higher rate of both lifetime and current generalized anxiety disorder co-morbidity than did the TRD patients. No other statistically significant differences in Axis I co-morbidity were found.Conclusions. These findings do not support the idea that current or lifetime Axis I co-morbidity is more common in TRD than non-TRD patients. In fact, the only statistical difference showed non-TRD patients with higher co-morbidity rates.
Life events, difficulties and depression among women

in an urban setting in Zimbabwe
Authors: BROADHEAD JEREMY C. , ABAS MELANIE A. 
Published: July 2002

Background. A previous paper (Abas & Broadhead, 1997)

reported that among 172 women

randomly selected from a Zimbabwean township 30·8% had a

depressive or anxiety disorder during

the previous year. Compared with London, the higher annual

prevalence of disorders in Harare

could mostly be accounted for by an excess of onset cases in the

study year (annual incidence of

depression 18%). This paper reports on the role of life events and

difficulties in the aetiology of

depression among these women.Method. Randomly selected women (N=172) from a

township in Harare were interviewed with a

Zimbabwean modification of the Bedford College Life Events and

Difficulties Schedule (LEDS).Results. Events and difficulties proved critical in

provoking the onset of depression in Harare. Far

more events occurring in Harare were severe or disruptive. Furthermore,

a proportion of the Harare

severe events were more threatening than have been described in London.

As in London, certain

types of severe event were particularly depressogenic, i.e. those

involving the woman's humiliation,

her entrapment in an ongoing difficult situation, or bereavement.

However, more severe events in

Harare involved these specific dimensions.Conclusions. Results indicate a common mechanism for the

development of depression, as defined

by international criteria, between Zimbabwe and London. The high

frequency of severe events, and

their especially adverse qualities, offer an explanation for the

high incidence of depression in Harare.