bluuebunny/crossref_metadata_2025_split huggingface dataset:
Turbulent plane Couette flow subject to strong system rotation
Authors: BECH KNUT H. , ANDERSSON HELGE I. 
Published: July 2002

System rotation is known to substantially affect the mean

flow pattern as well as the

turbulence structure in rotating channel flows. In a numerical

study of plane Couette

flow rotating slowly about an axis aligned with the mean vorticity, Bech

& Andersson

(1996a) found that the turbulence level was damped in the

presence of anticyclonic

system rotation, in spite of the occurrence of longitudinal counter-rotating

roll cells.

Moreover, the turbulence anisotropy was practically unaffected by the weak
rotation,

for which the rotation number Ro, defined as the

ratio of twice the imposed angular

vorticity Ω to the shear rate of the corresponding laminar flow,
was

±0.01. The aim

of the present paper is to explore the effects of stronger

anticyclonic system rotation

on directly simulated turbulent plane Couette flow. Turbulence

statistics like energy,

enstrophy and Taylor lengthscales, both componental and directional, were
computed

from the statistically steady flow fields and supplemented by structural
information

obtained by conditional sampling.The designation of the imposed system rotation as ‘high’

was associated with a

reversal of the conventional Reynolds stress anisotropy so that the velocity

fluctuations perpendicular to the wall exceeded those in the streamwise

direction. It was

observed that the anisotropy reversal was accompanied by an appreciable
region of

the mean velocity profile with slope ∼2Ω,

i.e. the absolute mean vorticity tended to

zero. It is particularly noteworthy that these characteristic

features were shared by

two fundamentally different flow regimes. First, the two-dimensional

roll cell pattern

already observed at Ro=0.01 became more regular and energetic
at

Ro=0.10 and

0.20, whereas the turbulence level was reduced by about 50%. Then, when

Ro was

further increased to 0.50, a disordering of the predominant roll cell pattern
set in

during a transient period until the flow field settled at a

new statistically steady state

substantially less affected by the roll cells. This was accompanied by
a substantial

amplification of the streamwise turbulent vorticity and an anomalous variation
of the

mean turbulent kinetic energy which peaked in the middle of the channel
rather than

near the walls. While the predominant flow structures of the non-rotating
flow were

longitudinal streaks, system rotation generated streamwise vortices, either
ordered

secondary flow or quasi-streamwise vortices. Eventually, at

Ro=1.0, the turbulent

fluctuations were completely suppressed and the flow field relaminarized.
Moderate Reynolds number flows through periodic

and random arrays of aligned cylinders
Authors: KOCH DONALD L. , LADD ANTHONY J. C. 
Published: July 2002

The effects of fluid inertia on the pressure drop required to drive
fluid

flow through

periodic and random arrays of aligned cylinders is investigated. Numerical

simulations

using a lattice-Boltzmann formulation are performed for Reynolds numbers
up to

about 180.The magnitude of the drag per unit length on cylinders in a square array

at moderate

Reynolds number is strongly dependent on the orientation of the drag (or
pressure

gradient) with respect to the axes of the array; this contrasts with Stokes

flow through

a square array, which is characterized by an isotropic permeability. Transitions
to

time-oscillatory and chaotically varying flows are observed at critical
Reynolds

numbers that depend on the orientation of the pressure gradient and the
volume

fraction.In the limit Re[Lt ]1, the mean drag per unit length,

F, in both periodic and random arrays, is given by

F/(μU)

=k1+k2Re2,

where μ is the fluid viscosity, U is the mean

velocity in the bed, and k1 and k2

are functions of the solid volume fraction ϕ.

Theoretical analyses based on point-particle and lubrication approximations
are used

to determine these coefficients in the limits of small and large concentration,

respectively.In random arrays, the drag makes a transition from a quadratic to a
linear

Re-dependence at Reynolds numbers of between 2 and 5. Thus,

the empirical Ergun formula, F/(μU)

=c1+c2Re,
is

applicable for Re>5. We determine the constants c1

and c2 over a wide range of ϕ. The relative

importance of inertia becomes smaller as

the volume fraction approaches close packing, because the largest contribution
to the

dissipation in this limit comes from the viscous lubrication flow in the
small gaps

between the cylinders.
Ringing of a vertical cylinder in waves
Authors: CHAPLIN J. R. , RAINEY R. C. T. , YEMM R. W. 
Published: July 2002

This paper analyses the results of two series of experiments

concerned with the response of a single vertical cylinder in the inertia
regime in

steep non-breaking waves.

We recorded first the loading on a cylinder when it was held stationary,

and secondly,

its response in the same waves when it was pivoted just above the floor
of the wave

flume, and supported at the top by springs in the horizontal plane.

Spring stiffnesses were set to achieve natural frequencies (measured in
still

water) in the range between

3 and 11 times the dominant wave frequency. The experiments were repeated
with

cylinders of three different diameters.Peak loading on stationary cylinders was found to exceed the predictions
of a

Morison model (based on kinematics computed from a numerical model of the

measured waves), though improvements were achieved through the inclusion
of

slender-body terms. Measured ringing responses are generally in good agreement
with

those computed on a quasi-static basis from the measured loading

history, but in some

conditions, particularly at low frequency ratios, there is clearly

some feedback from the

motion to the excitation. Peak accelerations in the steepest waves are
found to be

limited approximately to those that would occur if the maximum loading
were applied

as a step change. Particular attention is given to a rapid cycle of

loading that occurs

after the crest has passed the cylinder's axis, and to images of

the flow around the cylinder at the water surface.
Internal wave excitation from stratified flow over a thin barrier
Authors: SUTHERLAND BRUCE R. , LINDEN PAUL F. 
Published: July 2002

We perform laboratory experiments in a recirculating shear flow
tank of non-uniform
salt-stratified water to examine the excitation of internal gravity waves
(IGW) in
the wake of a tall, thin vertical barrier. The purpose of this study is
to characterize
and quantify the coupling between coherent structures shed in the wake
and internal
waves that radiate from the mixing region into the deep, stationary fluid.
In agreement
with numerical simulations, large-amplitude internal waves are generated
when the
mixing region is weakly stratified and the deep fluid is sufficiently strongly
stratified.
If the mixing region is unstratified, weak but continuous internal wave
excitation
occurs. In all cases, the tilt of the phase lines of propagating waves
lies within a
narrow range. Assuming the waves are spanwise uniform, their amplitude
in space
and time is measured non-intrusively using a recently developed ‘synthetic
schlieren’
technique. Using wavelet transforms to measure consistently the width and
duration
of the observed wavepackets, the Reynolds stress is measured and, in particular,
we
estimate that when large-amplitude internal wave excitation occurs, approximately

7% of the average momentum across the shear depth and over the extent of
the
wavepacket is lost due to transport away from the mixing region by the
waves.We propose that internal waves may act back upon the mean flow modifying
it
so that the excitation of waves of that frequency is enhanced. A narrow
frequency
spectrum of large-amplitude waves is observed because the feedback is largest
for
waves with phase tilt in a range near 45°. Numerical simulations and
analytic theories
are presented to further quantify this theory.
Turbulent shear flow over fast-moving waves
Authors: COHEN J. E. , BELCHER S. E. 
Published: July 2002

We divide the interaction between wind and ocean surface waves into three parameter
regimes, namely slow, intermediate and fast waves, that are distinguished by the ratio
c/u∗ (c is the wave phase speed and
u∗ is the friction velocity in the wind). We develop
here an analytical model for linear changes to the turbulent air flow caused by waves
of small slope that is applicable to slow and to fast waves. The wave-induced turbulent
shear stress is parameterized here with a damped mixing-length model, which tends to
the mixing-length model in an inner region that lies close to the surface, and is then
damped exponentially to zero in an outer region that lies above the inner region.
An adjustable parameter in the damped mixing-length model controls the rate of
decay of the wave-induced stress above the inner region, and shows how the results
vary from a model with no damping, which corresponds to using the mixing-length
model throughout the flow, to a model with full damping, which, following previous
suggestions, correctly represents rapid distortion of the wave-induced turbulence in
the outer region.Solutions for air flow over fast waves are obtained by analysing the displacement of
streamlines over the wave; they show that fast waves are damped, thereby giving their
energy up to the wind. There is a contribution to this damping from a counterpart
of the non-separated sheltering mechanism that gives rise to growth of slow waves
(Belcher & Hunt 1993). This sheltering contribution is smaller than a contribution
from the wave-induced surface stress working against the orbital motions in the water.
Solutions from the analysis for both slow and fast waves are in excellent agreement
with values computed by Mastenbroek (1996) from the nonlinear equations of motion
with a full second-order closure model for the turbulent stresses. Comparisons with
data for slow and intermediate waves show that the results agree well with laboratory
measurements over wind-ruffled paddle-generated waves, but give results that are a
factor of about two smaller than measurements of purely wind-generated waves. We
know of no data for fast waves with which to compare the model. The damping rates
we find for fast waves lead to e-folding times for the decay of the waves that are a
day or longer. Although this wind-induced damping of fast waves is small, we suggest
that it might control low-frequency waves in a fully-developed sea.
Self-similarity and internal structure of turbulence induced by
Rayleigh–Taylor instability
Authors: DALZIEL S. B. , LINDEN P. F. , YOUNGS D. L. 
Published: July 2002

This paper describes an experimental investigation of mixing due to Rayleigh–Taylor
instability between two miscible fluids. Attention is focused on the gravitationally
driven instability between a layer of salt water and a layer of fresh water with
particular emphasis on the internal structure within the mixing zone. Three-dimensional
numerical simulations of the same flow are used to give extra insight into the
behaviour found in the experiments.The two layers are initially separated by a rigid barrier which is removed at the start
of the experiment. The removal process injects vorticity into the flow and creates a
small but significant initial disturbance. A novel aspect of the numerical investigation
is that the measured velocity field for the start of the experiments has been used to
initialize the simulations, achieving substantially improved agreement with experiment
when compared with simulations using idealized initial conditions. It is shown that the
spatial structure of these initial conditions is more important than their amplitude for
the subsequent growth of the mixing region between the two layers. Simple measures
of the growth of the instability are shown to be inappropriate due to the spatial
structure of the initial conditions which continues to influence the flow throughout
its evolution. As a result the mixing zone does not follow the classical quadratic time
dependence predicted from similarity considerations. Direct comparison of external
measures of the growth show the necessity to capture the gross features of the initial
conditions while detailed measures of the internal structure show a rapid loss of
memory of the finer details of the initial conditions.Image processing techniques are employed to provide a detailed study of the internal
structure and statistics of the concentration field. These measurements demonstrate
that, at scales small compared with the confining geometry, the flow rapidly adopts
self-similar turbulent behaviour with the influence of the barrier-induced perturbation
confined to the larger length scales. Concentration power spectra and the fractal
dimension of iso-concentration contours are found to be representative of fully
developed turbulence and there is close agreement between the experiments and
simulations. Other statistics of the mixing zone show a reasonable level of agreement,
the discrepancies mainly being due to experimental noise and the finite resolution of
the simulations.
Non-axisymmetric magnetohydrodynamic shear layers in a rotating spherical shell
Authors: SOWARD ANDREW M. , HOLLERBACH RAINER 
Published: July 2002

Constant-density electrically conducting fluid is confined to a rapidly rotating
spherical shell and is permeated by an axisymmetric magnetic field. Slow steady non-axisymmetric motion is driven by a prescribed non-axisymmetric body force; both
rigid and stress-free boundary conditions are considered. Linear solutions of the governing
magnetohydrodynamic equations are derived in the small Ekman number E
limit analytically for values of the Elsasser number Λ less than order unity and
they are compared with new numerical results. The analytic study focuses on the nature
of the various shear layers on the equatorial tangent cylinder attached to the inner
sphere. Though the ageostrophic layers correspond to those previously isolated by
Kleeorin et al. (1997) for axisymmetric flows, the quasi-geostrophic layers have a new
structure resulting from the asymmetry of the motion.In the absence of magnetic field, the inviscid limit exhibits a strong shear singularity
on the tangent cylinder only removeable by the addition of viscous forces. With
the inclusion of magnetic field, large viscous forces remain whose strength [Zscr ] was
measured indirectly by Hollerbach (1994b). For magnetic fields with dipole parity,
cf. Kleeorin et al. (1997), [Zscr ] increases throughout the range
Λ [Lt ] 1; whereas, for quadrupole parity, cf. Hollerbach (1994b),
[Zscr ] only increases for Λ [Lt ] E1/5.The essential difference between the dipole and quadrupole fields is the magnitude
of their radial components in the neighbourhood of the equator of the inner sphere. Its
finite value for the quadrupole parity causes the internal shear layer – the
Hartmann–Stewartson layer stump – to collapse and merge with the equatorial Ekman
layer when Λ = O(E1/5).
Subsequently the layer becomes an equatorial Hartmann layer,
which thins and spreads polewards about the inner sphere surface as Λ increases
over the range E1/5 [Lt ] Λ [Lt ] 1.
Its structure for the stress-free boundary conditions
employed in Hollerbach's (1994b) model is determined through matching with a new
magnetogeostrophic solution and the results show that the viscous shear measured by
[Zscr ] decreases with increasing Λ. Since [Zscr ]
depends sensitively on the detailed boundary layer structure, it provides a sharp diagnostic
of new numerical results for Hollerbach's model; the realized [Zscr ]-values compare
favourably with the asymptotic theory presented.
The influences of bathymetry and flow regime upon the morphology of sublittoral sponge communities
Authors: Bell James J. , Barnes David K.A. 
Published: July 2002

Sponge communities were sampled at 3 m depth intervals at six sites experiencing different flow regimes at Lough Hyne, Ireland. Sponges were identified and classified within the following morphological groups: encrusting, massive, globular, pedunculate, tubular, flabellate, arborescent, repent and papillate morphological types on both vertical (≈90°) and inclined (≈45°) surfaces.Differences in the proportional abundance of the sponge body forms and density (sponge m−2) were observed between sites and depths. The density of sponges increased with depth at sites of slight to moderate current flow, but not at the site where current flow was turbulent. Morphological diversity of sponge communities decreased with increasing current flow due to the removal of delicate forms such as pedunculate and arborescent shaped sponges. Massive and encrusting morphologies dominated at the high-energy sites (fast and turbulent flow regimes) due to a high basal area to volume ratio, which prevents removal from cliff surfaces. However, pedunculate, papillate and arborescent types dominated at the low current sites as these shapes may help to prevent the settlement of sediment on sponge surfaces. Bray–Curtis Similarity analysis and Correspondence Analysis were used to distinguish between five different morphological communities.
Trauma and loss as determinants of medically unexplained epidemic illness in a Bhutanese refugee camp
Authors: VAN OMMEREN M. , SHARMA B. , KOMPROE I. , POUDYAL B. N. , SHARMA G. K. , CARDEÑA E. , DE JONG J. T. V. M. 
Published: July 2002

Background. We sought to identify personal factors that placed people at risk during an epidemic of medically unexplained illness in a Bhutanese refugee camp in southeastern Nepal.Methods. We conducted a case–control study, involving 68 cases and 66 controls. Caseness was defined as experiencing at least one attack of medically unexplained fainting or dizziness during the time of the epidemic. We performed hierarchical logistic regression analysis to identify significant predictors of case status.Results. In terms of Western psychiatric constructs, the illness involved somatoform symptoms of both acute anxiety and dissociation. Sixty per cent reported visual and 28% reported auditory hallucinatory experiences. Cases and controls were similar on all demographic variables, school performance, number of attacks witnessed and psychopathology before the onset of the epidemic. Recent loss, early loss, childhood trauma and pulse-rate were predictors of case status.Conclusion. We identified trauma, early loss and, especially, recent loss as predictors of attacks during medically unexplained epidemic illness in a Bhutanese refugee community.
The Interview for Recent Life Events
Authors: PAYKEL E. S. 
Published: July 2002

Background. This paper summarizes work with the
Interview for Recent Life Events (IRLE), an instrument that has been
used extensively but not previously reported in detail.Methods. Rationale, development, organization, content,
reliability, validity and applications are reviewed in full.Results. The instrument covers a comprehensive range of
recent life events, their timing and other important qualities. It has
been found reliable and valid, and has been translated into a number
of languages. It has been used by many research groups in different
countries, in studies of psychiatric patients, medical patients and
subjects in the general population.Conclusions. The instrument is useful, has been widely
applied and has the necessary qualities for ascertainment of life
events in research studies.
Genetic influences on childhood hyperactivity:
contrast effects imply parental rating bias, not sibling
interaction
Authors: SIMONOFF E. , PICKLES A. , HERVAS A. , SILBERG J. L. , RUTTER M. , EAVES L. 
Published: July 2002

Background. Previous twin studies of hyperactivity have

supported a ‘contrast effect’, whereby the
more hyperactive the rating of one twin, the less the rating of the other.

It has not been clear whether
contrast effects occur in the twins' behaviour or in the ratings
made of their behaviour but the
implications for hyperactivity are different under the two models.Method. We use hyperactivity ratings from mothers and teachers

for 1644 twin pairs in the Virginia
Twin Study of Adolescent Behavioral Development (VTSABD) to explore the
origin of contrast
effects, making use of independent teacher reports in a proportion
of twins. Models were fitted
separately for the two informants and jointly to ratings combined through

a latent variable, using structural equation modelling.Results. Models for maternal data confirm the contrast effect

previously reported. Teacher ratings
show a different form of bias, with both twin confusion and correlated
errors representing
alternative but not mutually exclusive explanations of the data.
Latent variable modelling of the
joint responses allowed comparison of a model in which the contrast
effect was placed on maternal
ratings, representing bias, versus one in which the contrast occurred
in the underlying ‘true’
phenotype. The fit of the former model was significantly better.Conclusions. Support is provided for the notion of
contrast effects as a form of rater bias in maternal
hyperactivity ratings. Different bias in teacher reports highlight that
no one report can be
considered a gold standard. The extent to which such biases may
distort information for other data
sources such as sib-pair studies of concordance/discordance is discussed.
The cultural context of caregiving: a comparison
of Alzheimer's caregivers in Shanghai, China
and San Diego, California
Authors: PATTERSON T. L. , SEMPLE S. J. , SHAW W. S. , YU E. , HE Y. , ZHANG M. Y. , WU W. , GRANT I. 
Published: July 2002

Background. Systematic comparisons of the psychological
and
physical responses of caregivers of
Alzheimer's disease (AD) patients in the US and China have not been
previously reported.Methods. Informal caregivers of community dwelling AD patients

in Shanghai, China and
demographically-matched non-caregiving Chinese controls were compared with
a sample of
American caregivers residing in San Diego, California and demographically-matched
American
controls.Results. Despite some demographic discrepancies, caregivers

from both China and the US were
similar. Caregivers from both cultures reported more depressive symptoms
and more physical
symptoms when compared with non-caregivers. Both groups of caregivers reported
that patients
required similar amounts of care and help with activities of daily
living (ADLs). However, Shanghai
caregivers reported less access to emotional support when compared with
the San Diego sample.
A conceptual model, guided by the stress process model of Pearlin
et al. (1990), was used to explore
multivariate relationships between caregiver characteristics and the physical
and psychological
health of our sample of AD caregivers in Shanghai, China. Results
from a path analytical procedure
revealed that the relationships among these variables and health
outcome did not differ significantly from those observed in the US sample.Conclusions. Although elderly family members are venerated

in the stereotypical Chinese family
unit, and informal caregiving of disabled family members is socially
mandated, the negative health
consequences of caregiving appear to be similar to those observed among
caregivers in the US.