bluuebunny/crossref_metadata_2025 huggingface dataset:
The response of a mixing layer formed between parallel streams to a concomitant excitation at two frequencies
Authors: DE ZHOU MING , WYGNANSKI I.
Published: July 2002

Simultaneous excitation of a turbulent mixing layer by two frequencies, a fundamental
and a subharmonic, was investigated experimentally. Plane perturbations were
introduced to the flow at its origin by a small oscillating flap. The results describe
two experiments that differ mainly in the amplitudes of the imposed perturbations
and both are compared to the data acquired while the mixing layer was forced at a
single frequency.

Conventional statistical quantities such as: mean velocity profiles, widths of the
flow, turbulent intensities, spectra, phase-locked velocity and vorticity fields, as well
as streaklines were computed. The rate of spread of the flow under concomitant
excitation at the two frequencies was much greater than under a single frequency,
although it remained dominated by two-dimensional eddies. The Reynolds stresses
and turbulence production are associated with the deformation and orientation of the
large coherent vortices. When the major axis of the coherent vortices starts leaning
forward on the high-speed side of the flow, the production of turbulent energy changes
sign (i.e. becomes negative) and this results in the flow thinning in the direction of
streaming. It also indicates that energy is extracted from the turbulence to the mean
motion. Resonance phenomena play an important role in the evolution of the flow. A
vorticity budget showed that the change in mean vorticity was mainly caused by the
nonlinear interaction between coherent vorticities. Nevertheless, the locally dominant
frequency scales the mean growth rate, the inclination and distortion of the mean
velocity profiles as well as the phase-locked vorticity contours.
Derivative moments in stationary homogeneous shear turbulence
Authors: SCHUMACHER JÖRG
Published: July 2002

A statistically stationary and nearly homogeneous turbulent shear flow is established
by an additional volume forcing in combination with stress-free boundary conditions
in the shear direction. Both turbulent energy and enstrophy are stationary to a
much better approximation than in previous simulations that use remeshing. The
temporal fluctuations decrease with increasing Reynolds number. Energy spectra and
shear-stress cospectra show that local isotropy is satisfactorily obeyed at the level of
second-order moments. However, derivative moments of high order up to n = 7 yield
increasing moments for n [ges ] 4 for the spanwise vorticity and the transverse derivative
of the streamwise velocity in the range of Taylor Reynolds numbers 59 [les ] Rλ [les ] 99.
These findings, which are in apparent violation of local isotropy, agree with recent
measurements.
Mechanisms and passive control of crossflow-vortex-induced transition in a three-dimensional boundary layer
Authors: WASSERMANN PETER , KLOKER MARKUS
Published: July 2002

Crossflow-vortex-induced laminar breakdown in a three-dimensional flat-plate
boundary-layer flow is investigated in detail by means of spatial direct numerical
simulations. The base flow is generic for an infinite swept wing, with decreasing
favourable chordwise pressure gradient. First, the downstream growth and nonlinear
saturation states initiated by a crossflow-vortex-mode packet as well as by single
crossflow-vortex modes with various spanwise wavenumbers are simulated. Second,
the secondary instability of the flow induced by the saturated crossflow vortices is
scrutinized, clearly indicating the convective nature of the secondary instability and
strengthening knowledge of the conditions for its onset. Emphasis is on the effect of
crossflow-vortex-mode packets and of the spanwise vortex spacing on the secondary
stability properties of the saturation states. Saturated uniform crossflow vortices initiated
by single crossflow-vortex modes turn out to be less unstable than vortices
initiated by a packet of vortex modes, and closely spaced saturated vortices are
even stable. Third, we investigate the transition control strategy of upstream flow
deformation by appropriate steady nonlinear vortex modes as applied in wind tunnel
experiments at the Arizona State University. A significant transition delay is shown
in the base flow considered here, and the underlying mechanisms are specified.
On the propagation of internal bores
Authors: KLEMP JOSEPH B. , ROTUNNO RICHARD , SKAMAROCK WILLIAM C.
Published: July 2002

According to classical hydraulic theory, the energy losses
within an external bore must occur within the expanding layer. However,
the application of this theory to describe the propagation of internal
bores leads to contradiction with accepted gravity-current behaviour in
the limit as the depth of the expanding layer ahead of the bore becomes
small. In seeking an improved expression for the propagation of
internal bores, we have rederived the steady front condition for a bore
in a two-layer Boussinesq fluid in a channel under the assumption that
the energy loss occurs within the contracting layer. The resulting
front condition is in good agreement with available laboratory data and
numerical simulations, and has the appropriate behaviour in both the
linear long-wave and gravity-current limits. Analysis of an idealized
internal bore assuming localized turbulent stresses suggests that the
energy within the expanding layer should, in fact, increase. Numerical
simulations with a two-dimensional non-hydrostatic model also reveal a
slight increase of energy within the expanding layer and suggest that
the structure of internal bores is fundamentally different from
classical external bores, having the opposite circulation and little
turbulence in the vicinity of the leading edge. However, if there is
strong shear near the interface between layers, the structure and
propagation of internal jumps may become similar to their counterparts
in classical hydraulic theory. The modified jump conditions for
internal bores produce some significant alterations in the traditional
Froude-number dependence of Boussinesq shallow-water flow over an
obstacle owing to the altered behaviour of the upstream-propagating
internal bore.
Two-fluid flow in sedimentary rock:

simulation, transport and complexity
Authors: OLSON JOHN F. , ROTHMAN DANIEL H.
Published: July 2002

The macroscopic properties and structure of the flow of two immiscible

fluids through

Fontainebleau sandstone are studied by numerical simulation. The pore space

geometry was obtained by X-ray microtomography (Kinney et al.
1993)

and the numerical simulations were performed by a new lattice-gas cellular

automaton method

(Olson & Rothman 1995). We first validate the numerical method by showing
that

the drag on a cubic array of spherical drops matches theoretical predictions.
As a

further test, we present a comparison between computed relative permeability
and

experimental measurements on the same rock. We then present a study of

fluid–fluid

coupling; we find that it is significant, and that it appears to be reciprocal:

the flux of

one fluid due to forcing on the other is the same, regardless of which

fluid is forced.

Lastly, we characterize the complexity and organization of the flow by
means of a

statistical parameter, the skewness of the distribution of local velocities.
Topographically forced long waves on a

sheared coastal current. Part 2. Finite amplitude waves
Authors: CLARKE S. R. , JOHNSON E. R.
Published: July 2002

This paper analyses the finite-amplitude flow of a

constant-vorticity current past

coastal topography in the long-wave limit. A forced finite-amplitude

long-wave equation is derived to describe the evolution of the vorticity

interface. An analysis of

this equation shows that three distinct near-critical regimes occur.

In the first the

upstream flow is restricted, with overturning of the vorticity interface

for sufficiently

large topography. In the second quasi-steady nonlinear waves form downstream
of

the topography with weak upstream influence. In the third regime the upstream

rotational fluid is partially blocked. Blocking and overturning are

enhanced at headlands

with steep rear faces and decreased at headlands with steep forward

faces. For certain

parameter values both overturning and partially blocked solutions are possible
and

the long-time evolution is critically dependent on the initial conditions.

The reduction

of the problem to a one-dimensional nonlinear wave equation allows solutions
to be

followed to much longer times and parameter space to be explored more finely
than

in the related pioneering contour-dynamical integrations of Stern (1991).
Natural convection during solidification of an

alloy from above with application to the evolution of sea ice
Authors: WETTLAUFER J. S. , WORSTER M. GRAE , HUPPERT HERBERT E.
Published: July 2002

We describe a series of laboratory experiments in which aqueous

salt solutions were

cooled and solidified from above. These solutions serve as model systems
of metallic

castings, magma chambers and sea ice. As the solutions freeze they form
a matrix

of ice crystals and interstitial brine, called a mushy layer.

The brine initially remains

confined to the mushy layer. Convection of brine from the interior of the
mushy layer

begins abruptly once the depth of the layer exceeds a critical value.

The principal path

for brine expelled from the mushy layer is through ‘brine channels’,

vertical channels

of essentially zero solid fraction, which are commonly observed in sea

ice and metallic

castings. By varying the initial and boundary conditions in the experiments,
we have

been able to determine the parameters controlling the critical depth of
the mushy

layer. The results are consistent with the hypothesis that

brine expulsion is initially

determined by a critical Rayleigh number for the mushy layer. The convection
of salty

fluid out of the mushy layer allows additional solidification

within it, which increases

the solid fraction. We present the first measurements of the temporal

evolution of the

solid fraction within a laboratory simulation of growing sea ice. We show
how the

additional growth of ice within the layer affects its rate of growth.
Lateral boundary contributions to wave-activity

invariants and nonlinear stability theorems for balanced dynamics
Authors: REN SHUZHAN , SHEPHERD THEODORE G.
Published: July 2002

The slow advective-timescale dynamics of the atmosphere and

oceans is referred

to as balanced dynamics. An extensive body of theory for disturbances to
basic

flows exists for the quasi-geostrophic (QG) model of balanced dynamics,
based

on wave-activity invariants and nonlinear stability theorems associated
with exact

symmetry-based conservation laws. In attempting to extend this theory to
the

semi-geostrophic (SG) model of balanced dynamics, Kushner &

Shepherd discovered lateral

boundary contributions to the SG wave-activity invariants which are not
present in

the QG theory, and which affect the stability theorems. However, because
of technical

difficulties associated with the SG model, the analysis of Kushner &
Shepherd was

not fully nonlinear.

This paper examines the issue of lateral boundary contributions to wave-activity

invariants for balanced dynamics in the context of Salmon's

nearly geostrophic model

of rotating shallow-water flow. Salmon's model has

certain similarities with the SG

model, but also has important differences that allow

the present analysis to be carried

to finite amplitude. In the process, the way in which constraints produce
boundary

contributions to wave-activity invariants, and additional

conditions in the associated

stability theorems, is clarified. It is shown that Salmon's

model possesses two kinds

of stability theorems: an analogue of Ripa's small-amplitude

stability theorem for shallow-water flow, and a finite-amplitude analogue
of Kushner

& Shepherd's SG

stability theorem in which the ‘subsonic’ condition of

Ripa's theorem is replaced by a

condition that the flow be cyclonic along lateral boundaries.

As with the SG theorem,

this last condition has a simple physical interpretation involving the
coastal Kelvin

waves that exist in both models.

Salmon's model has recently emerged as an important prototype for
constrained

Hamiltonian balanced models. The extent to which the present analysis applies
to

this general class of models is discussed.
Inertial Bénard–Marangoni convection
Authors: BOECK THOMAS , THESS ANDRÉ
Published: July 2002

Two-dimensional surface-tension-driven Bénard convection
in

a layer with a free-slip

bottom is investigated in the limit of small Prandtl number using accurate
numerical

simulations with a pseudospectral method complemented by linear stability
analysis

and a perturbation method. It is found that the system attains a

steady state consisting of counter-rotating convection rolls. Upon increasing
the

Marangoni number Ma

the system experiences a transition between two typical convective regimes.
The

first one is the regime of weak convection characterized by only slight
deviations

of the isotherms from the linear conductive temperature profile. In contrast,
the

second regime, called inertial convection, shows significantly deformed
isotherms.

The transition between the two regimes becomes increasingly sharp as the
Prandtl

number is reduced. For sufficiently small Prandtl number the transition
from weak to

inertial convection proceeds via a subcritical bifurcation

involving weak hysteresis. In

the viscous zero-Prandtl-number limit the transition manifests itself in
an unbounded

growth of the flow amplitude for Marangoni numbers beyond a critical

value Mai. For

Ma<Mai the zero-Prandtl-number

equations provide a reasonable approximation for

weak convection at small but finite Prandtl number. The possibility of
experimental

verification of inertial Bénard–Marangoni convection

is briefly discussed.
Wrap, tilt and stretch of vorticity lines

around a strong thin straight vortex tube in a simple shear flow
Authors: KAWAHARA GENTA , KIDA SHIGEO , TANAKA MITSURU , YANASE SHINICHIRO
Published: July 2002

The mechanism of wrap, tilt and stretch of vorticity lines around
a

strong thin straight vortex tube of circulation Γ starting with a
vortex

filament in a simple shear flow

(U=SX2Xˆ1,

S being a shear rate) is investigated analytically. An asymptotic

expression for the vorticity field is obtained at a large Reynolds number

Γ/ν[Gt ]1, ν being the kinematic viscosity of
fluid,

and during the initial time St[Lt ]1 of evolution as well as

St[Lt ](Γ/ν)1/2. The vortex tube,
which

is inclined from the streamwise (X1) direction both
in the

vertical (X2) and spanwise (X3)
directions,

is tilted, stretched and diffused under the action of the uniform shear
and

viscosity. The simple shear

vorticity is on the other hand, wrapped and stretched around the vortex
tube by a

swirling motion, induced by it to form double spiral vortex layers of high
azimuthal

vorticity of alternating sign. The magnitude of the azimuthal vorticity
increases

up to O((Γ/ν)1/3S) at
distance

r=O((Γ/ν)1/3

(νt)1/2) from the vortex tube. The spirals induce
axial flows

of the same spiral shape with alternate sign in adjacent spirals which
in turn tilt

the simple shear vorticity toward the axial direction. As

a result, the vorticity lines wind helically around the vortex tube accompanied
by

conversion of vorticity of the simple shear to the axial direction.

The axial vorticity increases in time as S2t,
the

direction of which is opposite to that of the vortex tube at

r=O((Γ/ν)1/2

(νt)1/2) where the vorticity magnitude is strongest.
In the

near region r[Lt ](Γ/ν)1/3

(νt)1/2, on the other hand, a viscous cancellation
takes

place in tightly wrapped vorticity of alternate sign, which leads

to the disappearance of

the vorticity normal to the vortex tube. Only the axial component of the
simple

shear vorticity is left there, which is stretched by the simple shear

flow itself. As a

consequence, the vortex tube inclined toward the direction of the simple

shear vorticity (a cyclonic vortex) is intensified, while the one oriented
in the

opposite direction (an

anticyclonic vortex) is weakened. The growth rate of vorticity due to this
effect

attains a maximum (or minimum) value of

±S2/33/2 when the vortex
tube

is oriented in the direction of

Xˆ1+Xˆ2∓

Xˆ3. The present asymptotic solutions are expected

to be closely related to the flow structures around intense vortex tubes
observed

in various kinds of turbulence such as helical winding of vorticity lines
around a

vortex tube, the dominance of cyclonic vortex tubes, the appearance of

opposite-signed vorticity around streamwise vortices and a zig-zag

arrangement of streamwise vortices in homogeneous isotropic turbulence,
homogeneous

shear turbulence and near-wall turbulence.
A viscoelastic model for turbulent flow over
undulating topography
Authors: ZOU QINGPING
Published: July 2002

Aviscoelastic model (a mixing-length model with
relaxation) is developed to investigate
the effect of turbulent advection on the mean flow
perturbation and the drag force
induced by turbulent shear flow over an undulating surface.
The relaxation term is
proportional to the ratio of eddy turnover time to travelling
time; accordingly, near the
surface, the relaxation model reduces to an eddy-viscosity or
mixing-length model,
while far from the surface it reduces to a rapid-distortion model.

The linear governing equations are transformed into
streamline coordinates and
solved through matched asymptotic expansions. According to
order-of-magnitude
estimates in Belcher, Newley & Hunt (1993), the drag
force contributed by nonlinear
shear stress is of the same order as that contributed by
asymmetric pressure arising
from the leeward thickening of the perturbed boundary
layer. The nonlinear analysis
in the present model confirms this estimate in most cases.
Our analytical results show
a dip in shear stress at the interface between the inner
and outer layers and provide
evidence that this dynamical feature is related to eddy
advection. Numerical calculation
using a shooting method gives results that compare well
with the analysis.
On the equation for spherical-particle motion: effect of Reynolds

and acceleration numbers
Authors: KIM INCHUL , ELGHOBASHI SAID , SIRIGNANO WILLIAM A.
Published: July 2002

The existing model equations governing the accelerated motion of
a spherical particle
are examined and their predictions compared with the results of the numerical

solution of the full Navier–Stokes equations for unsteady, axisymmetric
flow around
a freely moving sphere injected into an initially stationary or
oscillating fluid. The
comparison for the particle Reynolds number in the range of 2 to 150 and
the particle
to fluid density ratio in the range of 5 to 200 indicates that the existing
equations
deviate considerably from the Navier–Stokes equations. As a result,
we propose a
new equation for the particle motion and demonstrate its superiority to
the existing
equations over a range of Reynolds numbers (from 2 to 150) and particle
to fluid
density ratios (from 5 to 200). The history terms in the new equation account
for
the effects of large relative acceleration or deceleration of the particle
and the initial
relative velocity between the fluid and the particle. We also examine the
temporal
structure of the near wake of the unsteady, axisymmetric flow around a
freely
moving sphere injected into an initially stagnant fluid. As the sphere
decelerates, the
recirculation eddy size grows monotonically even though the instantaneous
Reynolds
number of the sphere decreases.
Cellular detonation stability. Part 1. A normal-mode linear analysis
Authors: SHORT MARK , STEWART D. SCOTT
Published: July 2002

A detailed investigation of the hydrodynamic stability to transverse
linear disturbances
of a steady, one-dimensional detonation in an ideal gas undergoing an irreversible,

unimolecular reaction with an Arrhenius rate constant is conducted via
a normal-mode analysis. The method of solution is an iterative shooting technique
which
integrates between the detonation shock and the reaction equilibrium point.
Variations
in the disturbance growth rates and frequencies with transverse wavenumber,
together
with two-dimensional neutral stability curves and boundaries for all unstable
low- and
higher frequency modes, are obtained for varying detonation bifurcation
parameters.
These include the detonation overdrive, chemical heat release and reaction
activation
energy. Spatial perturbation eigenfunction behaviour and phase and group
velocities
are also obtained for selected sets of unstable modes. Results are presented
for
both Chapman–Jouguet and overdriven detonation velocities. Comparisons
between
the earlier pointwise determination of stability and interpolated neutral
stability
boundaries obtained by Erpenbeck are made. Possible physical mechanisms
which
govern the wavenumber selection underlying the initial onset of either
regular or
irregular cell patterns are also discussed.
General solution of the particle momentum equation in unsteady Stokes
flows
Authors: COIMBRA C. F. M. , RANGEL R. H.
Published: July 2002

The general solution of the particle momentum equation for unsteady
Stokes flows is
obtained analytically. The method used to obtain the solution consists
of applying a
fractional-differential operator to the first-order integro-differential
equation of motion
in order to transform the original equation into a second-order non-homogeneous

equation, and then solving this last equation by the method of variation
of parameters.
The fractional differential operator consists of a three-time-scale linear
operator that
stretches the order of the Riemann–Liouville fractional derivative
associated with the
history term in the equation of motion. In order to illustrate the application
of the
general solution to particular background flow fields, the particle velocity
is calculated
for three specific flow configurations. These flow configurations correspond
to the
gravitationally induced motion of a particle through an otherwise quiescent
fluid, the
motion of a particle caused by a background velocity field that accelerates
linearly in
time, and the motion of a particle in a fluid that undergoes an impulsive
acceleration.
The analytical solutions for these three specific cases are analysed and
compared to
other solutions found in the literature.
Experimental investigations of
quasi-two-dimensional vortices in a stratified
fluid with source–sink forcing
Authors: DE ROOIJ FRANS , LINDEN P. F. , DALZIEL STUART B.
Published: July 2002

This paper describes laboratory experiments on the self-organizing
character of
geophysical turbulence. The experiments were carried out in a linearly
stratified fluid,
forced horizontally with sources and sinks around a horizontal ring. The
flow was
visualized with small particles illuminated by a horizontal light sheet,
recorded with
a camera and analysed with an advanced particle tracking system. Qualitative
and
quantitative data, such as flow patterns, velocity and vorticity fields,
were obtained. In
the experiments, the inverse energy cascade was clearly observed: the flow
organized
into a single quasi-steady, coherent vortex structure of the largest available
scale. This
vortex is maintained against diffusion of momentum by entrainment of vorticity
from
its exterior. In this process, patches of vorticity of the same sign as
the core of this
large structure intermittently cross the vorticity barrier around the vortex.
Patches of
opposite vorticity were observed to be effectively blocked by the barrier.
The direction
of rotation of the vortex was set by a slight bias in the experimental
apparatus and
could be changed by imposing a small initial circulation in the opposite
sense, the
magnitude of which suggested a measure for the bias. A detailed study of
the effects
of changing forcing parameters was carried out. The number of sinks (which
play
only a passive role) does not affect the flow, whereas the number of sources
sets the
lengthscale of the forcing and thereby determines the size of the vortices
that are
created close to the ring, as well as that of the large central vortex
that emerges.
However, after longer times of forcing, the vortex size also depends on
the strength
of the forcing. The velocities in the large vortex structure scale with
the mean velocity
from the sources, and with the square root of their number. Measurements
were also
taken of the decay of the vortex. After switching off the forcing it quickly
becomes
axisymmetric and a linear functional relationship is established between
the vorticity
and streamfunction. The spin-down time was observed to be much shorter
than can
be accounted for by vertical viscous diffusion alone: initially the short
horizontal
scale of the vorticity barrier causes a relatively fast decay, whereas
at later times the
size of the vortex as a whole is important.
Experiments on the stabilization of the no-motion
state of a fluid layer heated from below and cooled from above
Authors: TANG JIE , BAU H. H.
Published: July 2002

It is demonstrated experimentally that through the use of feedback
control, it is
possible to stabilize the no-motion (conductive) state of a fluid layer
confined in a
circular cylinder heated from below and cooled from above (the Rayleigh–Bénard

problem), thereby postponing the transition from a no-motion state to cellular

convection. The control system utilizes multiple sensors and actuators.
The actuators
consist of individually controlled heaters microfabricated on a silicon
wafer which
forms the bottom of the test cell. The sensors are diodes installed at
the fluid's
midheight. The sensors monitor the deviation of the fluid temperatures
from preset,
desired values and direct the actuators to act in such a way as to eliminate
these
deviations.
Secondary flow induced by riblets
Authors: GOLDSTEIN D. B. , TUAN T.-C.
Published: July 2002

The effects of riblets on one wall of a channel bounding fully
developed turbulent
flow are investigated. Various perturbation elements including wires, fins
and slots are
modelled in order to understand the effects of riblets. It is found that
widely spaced
riblets, fins and wires create a substantial increase in turbulent activity
just above the
element. These elements are also found to produce a remarkable pattern
of secondary
mean flows consisting of matched pairs of streamwise vortices. The secondary
flows
occur only if the bulk flow is turbulent and their characteristics depend
on element
geometry. It is suggested that these secondary flows are strongly linked
with the
increase in drag experienced by widely spaced riblets in experimental studies.
The
secondary flows are probably caused by two-dimensional spanwise sloshing
of the flow,
inherent in a turbulent boundary layer, interacting with the stream-aligned
element.
This two-dimensional mechanism is investigated with a series of two-dimensional

simulations of sloshing flow over isolated elements. Grid resolution and
domain size
checks are made throughout the investigation.
On the stability of liquid ridges
Authors: ROY R. VALÉRY , SCHWARTZ LEONARD W.
Published: July 2002

We consider the stability of a rectilinear liquid region whose boundary is composed
of a solid cylindrical substrate of arbitrary shape and a free surface whose cross-section, in the absence of gravity, is a circular arc. The liquid–solid contact angle is
a prescribed material property. A variational technique, using an energy functional,
is developed that predicts the minimum wavelength for transverse instability under
the action of capillarity. Conversely, certain configurations are absolutely stable and
a simple stability criterion is derived. Stability is guaranteed if, for given substrate
geometry and given contact angle, the unperturbed meniscus pressure is an increasing
function of the liquid cross-sectional area. The analysis is applied to a variety of
liquid/substrate configurations including (i) a liquid ridge with contact lines pinned
to the sharp edges of a slot or groove, (ii) liquid ridges with free contact lines on
flat and wedge-shaped substrates as well as substrates of circular or elliptical cross-section. Results are consistent with special cases previously treated including those
that employ a slope-small-slope approximation.
Hydrothermal wave instability of thermocapillary-driven convection
in a transverse magnetic field
Authors: PRIEDE JĀNIS , GERBETH GUNTER
Published: July 2002

We study the linear stability of thermocapillary-driven convection in a planar
unbounded layer of an electrically conducting low-Prandtl-number liquid heated from
the side and subjected to a transverse magnetic field. The thresholds of convective
instability for both longitudinal and oblique disturbances are calculated numerically
and also asymptotically by considering the Hartmann and Prandtl numbers as large
and small parameters, respectively. The magnetic field has a stabilizing effect on the
flow with the critical temperature gradient for the transition from steady to oscillatory
convection increasing as square of the field strength, as also does the critical
frequency, while the critical wavelength reduces inversely with field strength. These
asymptotics develop in a strong enough magnetic field when the instability is entirely
due to the jet of the base flow confined in the Hartmann layer at the free surface. In
contrast to the base flow, the critical disturbances, having a long wavelength at small
Prandtl numbers, extend from the free surface into the bulk of the liquid layer over
a distance exceeding the thickness of the Hartmann layer by a factor
O(Pr−1/2). For
Ha [lsim ] Pr−1/2 the instability is influenced by the
actual depth of the layer. For such moderate magnetic fields the instability threshold is
sensitive to the thermal properties of the bottom of the layer and the
dependences of the critical parameters on the
field strength are more complicated. In the latter case, various instability modes are
possible depending on the thermal boundary conditions and the relative magnitudes
of Prandtl and Hartmann numbers.
Enhanced dissipation for quasi-geostrophic motion over small-scale topography
Authors: VANNESTE JACQUES
Published: July 2002

The effect of a small-scale topography on large-scale, small-amplitude oceanic motion
is analysed using a two-dimensional quasi-geostrophic model that includes free-surface and β effects, Ekman friction and viscous (or turbulent) dissipation. The
topography is two-dimensional and periodic; its slope is assumed to be much larger
than the ratio of the ocean depth to the Earth's radius. An averaged equation of
motion is derived for flows with spatial scales that are much larger than the scale
of the topography and either (i) much larger than or (ii) comparable to the radius
of deformation. Compared to the standard quasi-geostrophic equation, this averaged
equation contains an additional dissipative term that results from the interaction
between topography and dissipation. In case (i) this term simply represents an
additional Ekman friction, whereas in case (ii) it is given by an integral over the
history of the large-scale flow. The properties of the additional term are studied in
detail. For case (i) in particular, numerical calculations are employed to analyse the
dependence of the additional Ekman friction on the structure of the topography and
on the strength of the original dissipation mechanisms.
Stability of fluid flow in a flexible tube to non-axisymmetric disturbances
Authors: SHANKAR V. , KUMARAN V.
Published: July 2002

The stability of fluid flow in a flexible tube to non-axisymmetric perturbations is
analysed in this paper. In the first part of the paper, the equivalents of classical
theorems of hydrodynamic stability are derived for inviscid flow in a flexible tube
subjected to arbitrary non-axisymmetric disturbances. Perturbations of the form
vi = v˜i exp
[ik(x − ct) + inθ] are imposed on a
steady axisymmetric mean flow U(r) in a flexible
tube, and the stability of mean flow velocity profiles and bounds for
the phase velocity of the unstable modes are determined for arbitrary values of
azimuthal wavenumber n. Here r, θ and x are
respectively the radial, azimuthal and axial coordinates, and
k and c are the axial wavenumber and phase velocity
of disturbances. The flexible wall is represented by a standard constitutive relation
which contains inertial, elastic and dissipative terms. The general results indicate
that the fluid flow in a flexible tube is stable in the inviscid limit if the quantity
Ud[Gscr ]/dr [ges ] 0, and could be unstable for
Ud[Gscr ]/dr < 0, where [Gscr ] ≡
rU′/(n2 + k2r2).
For the case of Hagen–Poiseuille flow, the general result implies that the flow is stable
to axisymmetric disturbances (n = 0), but could be unstable to non-axisymmetric
disturbances with any non-zero azimuthal wavenumber (n ≠ 0). This is in marked
contrast to plane parallel flows where two-dimensional disturbances are always more
unstable than three-dimensional ones (Squire theorem). Some new bounds are derived
which place restrictions on the real and imaginary parts of the phase velocity for
arbitrary non-axisymmetric disturbances.

In the second part of this paper, the stability of the Hagen–Poiseuille flow in
a flexible tube to non-axisymmetric disturbances is analysed in the high Reynolds
number regime. An asymptotic analysis reveals that the Hagen–Poiseuille flow in a
flexible tube is unstable to non-axisymmetric disturbances even in the inviscid limit,
and this agrees with the general results derived in this paper. The asymptotic results
are extended numerically to the moderate Reynolds number regime. The numerical results reveal
that the critical Reynolds number obtained for inviscid instability to non-axisymmetric
disturbances is much lower than the critical Reynolds numbers obtained in the previous studies
for viscous instability to axisymmetric disturbances when the dimensionless parameter
Σ = ρGR2/η2 is large.
Here G is the shear modulus of the elastic medium,
ρ is the density of the fluid, R is the radius of the tube and
η is the viscosity of the fluid. The viscosity of the wall medium is found to have a
stabilizing effect on this instability.
Intermittency and cascades
Authors: JIMÉNEZ JAVIER
Published: July 2002

The theory of intermittency in multiplicative cascades is reviewed, with special, but not
exclusive, emphasis on its applications to turbulence. It is noted that, in many physical
systems, this theory is incomplete, and two of its limitations are discussed in some
detail. It is first argued that large fluctuations will in most cases behave differently
from the lower-level background, since the overall mean introduces an intensity scale
that breaks self-similarity, and that they will, under the right conditions, evolve into
coherent structures decoupled from the rest of the system. The effect of non-local
interactions is then addressed. It is shown that the results depend on the nature of
the interaction, and that it is possible to generate non-local cascades which are less
intermittent, as intermittent, or even more intermittent, than local ones. It is finally
stressed that the multiplicative theory of cascades is a kinematic description, and that
its relation with the real dynamics is not straightforward.
Bosses and Clients: Municipal Employment in the Buenos Aires of
the Radicals, 1916–30
Authors: HOROWITZ JOEL
Published: July 2002

The success of the Radical Party in Argentina under Hipólito
Yrigoyen has frequently been attributed to the creation of an effective political
machine. While the machine did exist, it cannot explain Yrigoyen's overwhelming
popularity with many sectors. Such political organisations and client–patron
relations flourished in many cities throughout the world. This article examines
employment with the municipality of Buenos Aires, and concludes not only did
governments both before and after Yrigoyen have similar policies but all major
political parties shared in the spoils. However, they failed to reap similar political
benefits. The roots of Yrigoyen's popularity lie elsewhere.
The puzzle-puddle-pickle problem and the Duke-of-York gambit in acquisition
Authors: DINNSEN DANIEL A. , O’CONNOR KATHLEEN M. , GIERUT JUDITH A.
Published: July 2002

Two classic and previously unrelated problems are reconsidered for their implications for optimality theory and acquisition. The puzzle-puddle-pickle problem centers on the debate over children’s underlying representations and the characterization of interacting error patterns which, when lost, result in overgeneralizations. In response to the challenges that this problem poses, an optimality theoretic solution is offered that appeals to the second problem, the Duke-of-York gambit, which involves co-occurring generalizations with reverse effects. The solution avoids language-specific restrictions on input representations and characterizes the loss and introduction of errors by one mechanism. New insight is offered for when overgeneralization is (not) expected to occur.
Topological analysis of a perturbed MHD
equilibrium using magnetic field-line invariants
Authors: BATES JASON W. , LEWIS H. RALPH
Published: July 2002

A procedure has previously been developed for the iterative
construction of invariants associated with magnetic field-line Hamiltonians
that consist of an axisymmetric zeroth-order term plus a non-axisymmetric
perturbation. Approximate
field-line invariants obtained with this procedure are used to examine
the
topological properties of magnetic field lines in a parabolic-current
MHD equilibrium that
was slightly perturbed from axisymmetry in the limit of a periodic cylindrical
configuration. Excellent agreement between Poincaré maps and the
level curves of the
first-order invariant is found for small perturbations. A means of circumventing
the
‘small-divisor problem’ in some cases is identified and
implemented with outstanding results. These results indicate that this
perturbation method can have valuable
consequences for future investigations of magnetic field-line topology.
Nonlinear dynamics of obliquely propagating Alfvén waves
Authors: GAZOL A. , PASSOT T. , SULEM P. L.
Published: July 2002

The long-wave, small-amplitude dynamics of obliquely propagating
Alfvén waves
is shown, using a reductive perturbative expansion, to be purely linear
not only
in one space dimension but also in the dispersionless limit in higher dimensions.

Furthermore, in the context of multidimensional wave-train modulation,
all the
diffraction coefficients are found to tend to zero with the dispersion,
while the non-linear
terms in the envelope equation remain finite. In this ‘semiclassical’
limit, the
envelope dynamics results in the formation of growing regions of finite-amplitude

oscillations with a typical scale intermediate between the size of the
wave packet
and its wavelength.
The dielectric tensor for dusty magnetized
plasmas with variable charge on dust particles
Authors: de JULI M. C. , SCHNEIDER R. S.
Published: July 2002

We derive the dielectric tensor for multicomponent magnetized dusty
plasmas, including
the effect of capture of plasma electrons and ions by the dust particles.
For
propagation perpendicular to the external magnetic field and Maxwellian
distributions
of electrons and ions, we obtain compact expressions for the components
of
the dielectric tensor, which can be used to analyse wave propagation. An
application
to the magnetosonic wave is presented.
Nonlinear dynamics of electron bunches in
the presence of dissipative effects
Authors: KRAFFT C. , VOLOKITIN A. , FLÉ M.
Published: July 2002

The transition phase between two nonlinear regimes of electron-beam–wave interaction
depending on the amplitude and the nature of the effective dissipation is
investigated with the help of numerical simulations. Effective dissipation due to
wave escaping to infinity out of the beam–wave interaction region as well as to collisions
in the background plasma is considered. If the dissipation is strong enough,
the evolution of the electron beam proceeds in a general way, independently of the
type of dissipation and of the nature of the considered waves: structures of strongly
concentrated electron bunches are formed. These bunches are not trapped in the
wave, and decelerate continuously owing to friction on waves: in the presence of
dissipation, the usual quasiperiodic exchange of energy between the wave and the
trapped particles, which prevents the wave from collapsing, does not occur. Considering
beam interaction with a finite number of waves (modulated wave packet), it
is shown that, if the dissipation is strong enough, the structure of electron bunches
is dynamically stable in a range of times exceeding several characteristic times of
their formation.
To ‘errrr’ is human: ecology and acoustics of speech disfluencies
Authors: Shriberg Elizabeth
Published: July 2002

Unlike read or laboratory speech, spontaneous speech contains high rates of disfluencies (e.g. repetitions, repairs, filled pauses, false starts). This paper aims to promote ‘disfluency awareness’ especially in the field of phonetics –which has much to offer in the way of increasing our understanding of these phenomena. Two broad claims are made, based on analyses of disfluencies in different corpora of spontaneous American English speech. First, an Ecology Claim suggests that disfluencies are related to aspects of the speaking environments in which they arise. The claim is supported by evidence from task effects, location analyses, speaker effects and sociolinguistic effects. Second, an Acoustics Claim argues that disfluency has consequences for phonetic and prosodic aspects of speech that are not represented in the speech patterns of laboratory speech. Such effects include modifications in segment durations, intonation, voice quality, vowel quality and coarticulation patterns. The ecological and acoustic evidence provide insights about human language production in real-world contexts. Such evidence can also guide methods for the processing of spontaneous speech in automatic speech recognition applications.
The occurrence of Hymenopenaeus debilis (Decapoda: Aristeidae: Solenocerinae) in Mediterranean waters: a case of pseudopopulations of Atlantic origin?
Authors: Cartes Joan E. , Abelló Pere , Torres Pedro
Published: July 2002

The occurrence of the aristeid shrimp Hymenopenaeus debilis in the Mediterranean Sea is reported from two individuals collected in the Alborán Sea and off Eivissa (western Mediterranean) from depths of 668-679 and 1109-1140 m, respectively. Comments on the origin of the populations and biogeography of the species are related to the Mediterranean pseudopopulations hypothesis.
Microscale analysis of chlorophyll-a in cohesive, intertidal sediments: the implications of microphytobenthos distribution
Authors: Kelly Julie A. , Honeywill Claire , Paterson David M.
Published: July 2002

Microphytobenthos are significant primary producers in many coastal systems. It is therefore important to quantify their biomass and productivity. Chlorophyll-a is often used as an index for microphytobenthic biomass. However, complications arise as most studies of sediment properties have been on a millimetre scale, whilst chemical and biological gradients in the surface layers of sediment occur over a microscale. The development of a new technique, the Cryolander (Wiltshire et al., 1997; Wiltshire, 2000), now allows microscale analysis of the sediment surface. Areas of high and low diatom biomass were compared using two coring techniques of different vertical resolution; the Cryolander method, with a vertical resolution of 0·2 mm and plastic core tubes (coarse coring), with a vertical resolution of 5 mm. Results indicated that, except at extreme biomass levels, coarse coring does not detect statistically significant differences in chlorophyll-a between obviously diverse sample sites. This may lead to misinterpretation of seasonal and spatial data when coarse coring is used. Furthermore microscale sectioning allows distinctions to be made between chlorophyll-a measured in the photic zone (photosynthetically active biomass (PAB)) and chlorophyll-a measured below the photic zone (photosynthetically inactive biomass (PIB)), allowing accurate determination of biomass specific primary production.
The phylum Sipuncula in the eastern Mediterranean: composition, ecology, zoogeography
Authors: Murina G.V.V. , Pancucci-Papadopoulou M.A. , Zenetos A.
Published: July 2002

The bibliography concerning the Mediterranean Sipuncula is reviewed, particularly that from the eastern Mediterranean. Additional data are provided from macrobenthic samples collected between 1974 and 1997 in the Hellenic waters. A total of 17 species are recorded, five of which (Sipunculus norvegicus, Golfingia elongata, Nephasoma lilljeborgi, Thysanocardiacatharinae and Aspidosiphon (Akrikos) mexicanus) are reported for the first time in Hellenic waters. Three more species (Onchnesoma squamatum squamatum, Aspidosiphon (Aspidosiphon) elegans, Apionsoma (Apionsoma) trichocephalus) which are found exclusively in the extreme eastern basin (Israel, Egypt) raises the number of eastern Mediterranean Sipuncula species to twenty.
British Archaeologists, Hindu Abbots, and Burmese Buddhists: The Mahabodhi Temple at Bodh Gaya, 1811–1877
Authors: Trevithick Alan
Published: July 2002

The Mahabodhi Temple at Bodh Gaya represented to the British, in the nineteenth century, a conveniently ruined Indian past which could be protected, recovered and restored as both a duty and a validation of legitimacy. However, for the temple's Hindu proprietors, the shrine—aside from being a valuable estate in purely financial terms—functioned as a symbol of the triumph of ‘orthodox’ Hinduism over ‘heterodox’ Buddhism. Finally, for the Burmese Buddhists who undertook their own ‘restoration’ mission in 1874, the Mahabodhi Temple marked the site where, according to Buddhist tradition, Gautama Buddha first achieved enlightenment and thereby began the career of the religion that bears his name.
Revisiting 2 Corinthians: Rhetoric and the Case for Unity
Authors: AMADOR J. D. H.
Published: July 2002

The current work argues for the compositional unity of 2 Corinthians. First, it addresses the chronological and argumentative relationship of chapters 10–13 to chapters 1–9. It next rejects the presence of so-called ‘interpolations’ and ‘sources’ in chapters 1–9. Finally, it considers the integrity of chapters 8 and 9. The uniqueness of this effort lies in its emphasis upon rhetorical reasons and warrants against the widely accepted partition hypothesis of Pauline historical- and source-critical scholarship. It does so by demonstrating the complex integrity of the argumentative development and trajectories in 2 Corinthians. It concludes with a new, suggestive approach to the correspondence as a complex, but nevertheless coherent, act of persuasion.
Proverbs, Persuasion and People: A Three-Dimensional Investigation of 2 Cor 9.6–15
Authors: BRUEHLER BART B.
Published: July 2002

2 Cor 9.6–15 is crucial for understanding Paul's collection for Jerusalem, and this investigation exegetically examines the text while giving particular attention to three dimensions: sapiential material, rhetoric, and the socio-economic nature of the Corinthian church. The study concludes that the sapiential material comes from a Hellenistic Jewish background, though it shares affinities with Greco-Roman themes. Paul employs deliberative logos in vv. 6–10 as he addresses those in the lower socio-economic levels of the church, while he primarily employs pathos in vv. 12–14 and addresses those in the higher socio-economic levels of the church.
Seven Stades to Emmaus
Authors: REECE S.
Published: July 2002

Luke 24.13–35 narrates the story of how the recently crucified and risen Christ meets up with two of his perplexed and despondent disciples as they are walking from Jerusalem to the town of Emmaus, how he explains to them as they walk together that it had been foretold long ago by the prophets that Christ would suffer and die, how he is persuaded by them, when evening has come, to break his journey at their home in Emmaus, how he reveals himself to them at dinner in the breaking of the bread, and, finally, how the two disciples return immediately, even at this late hour, to Jerusalem in order to report these miraculous events to the 11 apostles.