bluuebunny/crossref_metadata_2025 huggingface dataset:
Granular flow behaviour in the transverse plane of a partially

filled rotating cylinder
Authors: BOATENG A. A. , BARR P. V.
Published: July 2002

Material flow in partially filled rotating cylinders (rotary

kilns) is encountered in many practical applications of material

processing, for example incineration, calcination, grain drying, etc.

The flow behaviour in the cross-section is important to other

transport mechanisms such as mixing and energy distribution within

the bed material. The paper describes an experimental study which was

carried out with the objective of understanding and improving our

predictive capabilities of the rheological behaviour of granular

materials in rotary cylinders. Measurement techniques similar to that

used in chute flows have been employed to measure flow

characteristics, e.g. particle velocities, granular temperature, and

solid concentration (in the shear layer developed between the free

surface and the bulk of the bed) for different materials having a

wide range of coefficients of restitution. The results of the

experiments provide the necessary assumptions, constraints, and data

for granular flows in partially filled rotating cylinders.
The horizontal scale of rotating convection in the

geostrophic regime
Authors: SAKAI SATOSHI
Published: July 2002

The horizontal scale of rotating convection with rigid boundary

conditions is studied.

The range of Rayleigh number concerned is moderate, i.e. large enough

to induce a finite-amplitude convection but small enough so that the

geostrophic processes are significant.

On considering an experimental law of the Nusselt number and some

constraints of elemental geostrophic processes, the horizontal scale

of the convection can be estimated. This estimation strongly depends

on the ratio between the thicknesses of the Ekman layer and the

thermal boundary layer, and does not depend monotonically on the

Rayleigh number. This dependency is compatible with the experimental

results of Rossby (1969).

The estimated horizontal scale was checked by laboratory experiments.

The horizontal temperature distribution was visualized by thermal

liquid-crystal capsules dispersed in the working fluid. The horizontal

scale was measured by counting vortices. The experimental results

agree fairly well with the estimated scale.
On the structures in the near-wake region of an

elevated turbulent jet in a crossflow
Authors: EIFF OLIVIER S. , KEFFER JAMES F.
Published: July 2002

A pattern-recognition technique, applied to multi-point

simultaneous velocity measurements obtained with 45°

X-wire anemometer probes, is used to extract and

characterize the underlying organized motions, i.e. coherent

structures, within the near-wake region of a turbulent round

jet discharged perpendicularly from a pipe into

a crossflow. This flow has been found to be quite complex owing to its

three-dimensional nature and the interactions between several flow regions.

Analyses of the underlying coherent structures, which play an important

role in the physics of the flow,

are still rare and are mostly based on flow-visualization techniques. Using

a pattern-recognition technique in conjunction with hot-wire measurements,

we recently examined the wake regions of the pipe and jet at levels near

the tip of the pipe, and found that Kármán-like vortex

structures in the wake of the pipe are locked to similar

structures in the jet-wake. In this paper we expand upon our previous

work and characterize these structures throughout the wake of the jet

up into the region of the bent-over jet – a region where they

have not been identified previously. The complex

geometry of these structures in the wake of the jet as well as their

interaction with the bent-over jet are discussed. The results show

that these structures split before they link to similar structures

on the opposite side of the symmetry plane in the jet region. The

results further suggest that the vorticity due to the structures in

the wake of the jet contributes to the motion of the well-known

counter-rotating vortex pair.
Strongly turbulent Rayleigh–Bénard

convection in mercury: comparison with results

at moderate Prandtl number
Authors: CIONI S. , CILIBERTO S. , SOMMERIA J.
Published: July 2002

An experimental study of Rayleigh–Bénard convection
in

the strongly turbulent

regime is presented. We report results obtained at low Prandtl number (in
mercury,

Pr = 0.025), covering a range of Rayleigh numbers

5 × 106 < Ra < 5 × 109, and

compare them with results at Pr∼1.

The convective chamber consists of a cylindrical cell of aspect ratio 1.

Heat flux measurements indicate a regime with Nusselt number increasing
as

Ra0.26, close to the 2/7 power observed at

Pr∼1, but with a smaller prefactor, which

contradicts recent theoretical predictions. A transition to a new turbulent

regime is suggested for Ra ≃ 2 × 109,

with significant increase of the Nusselt number. The

formation of a large convective cell in the bulk is revealed by its thermal

signature on the bottom and top plates. One frequency of the temperature

oscillation is related

to the velocity of this convective cell. We then obtain the typical temperature
and

velocity in the bulk versus the Rayleigh number, and compare them with
similar

results known for Pr∼1.

We review two recent theoretical models, namely the mixing zone model
of Castaing

et al. (1989), and a model of the turbulent boundary layer by

Shraiman & Siggia

(1990). We discuss how these models fail at low Prandtl number, and propose

modifications for this case. Specific scaling laws for fluids at low Prandtl

number are then obtained, providing an interpretation of our experimental

results in mercury, as well as extrapolations for other liquid metals.
Three-dimensional effects in turbulent bluff-body wakes
Authors: PRASAD ANIL , WILLIAMSON CHARLES H. K.
Published: July 2002

There has recently been a surge in activity concerning the

development of three-dimensionality in the wakes of nominally two-dimensional

bluff bodies, yielding the

realization that end effects can influence the

wake vortex shedding pattern over long

spanlengths. Much of this work has been focused on low Reynolds

numbers (Re),

but virtually no studies have investigated to what extent it is possible
to control

shedding patterns at higher Reynolds numbers, through the use of end manipulation.

In the present paper, we demonstrate that it is possible to induce parallel
shedding,

oblique shedding and vortex dislocations, by manipulation of the end conditions,
over

a large range of Reynolds number. Such patterns affect the frequency of
primary

wake instability and its amplitude of fluctuation, as they do at low Reynolds
number,

although distinct differences are found at the higher Reynolds numbers.

We find that imposition of oblique shedding conditions at high Reynolds
number

leads to a spatial variation of both the oblique shedding

angle and shedding frequency

across the span, and to sparse dislocations which are not

restricted to the spanwise end

regions, as they are at low Reynolds numbers (under similar geometrical
conditions).

In the wake transition regime (Re=190–250), it is

confirmed that the spontaneous

appearance of vortex dislocations in mode-A shedding precludes the control
of

shedding patterns using end manipulation. However, it has proven possible
to extend

the regime of Reynolds number where dislocations ‘naturally’

exist to Re>250,

by introducing them artificially through end control, where they would
otherwise

not occur. The possibility of introducing dislocations and of inducing
oblique vortex

shedding at higher Reynolds numbers has practical significance,

if one can deliberately

decorrelate the vortex shedding, and hence reduce the spanwise-integrated
unsteady

fluid forces on the body.

We confirm the existence of a transition in the mode of shedding at

Re≈5000

(originally found by Norberg 1987) under conditions where parallel shedding
is

attempted. This mode transition displays similarities to an inverse

of the mode A→mode B transition that is found in the

wake transition regime. It is clear that vortex

dislocations occur beyond Re=5000, although it is

not clear why the flow is unstable

to such a mode. Furthermore, there appears to be some support for the suggestion

that vortex dislocations may be a feature of the flow for Re

at least up to 30×103,

as evidenced by the work of Norberg (1994).
Numerical study of inviscid shock interactions

on double-wedge geometries
Authors: OLEJNICZAK JOSEPH , WRIGHT MICHAEL J. , CANDLER GRAHAM V.
Published: July 2002

Computational fluid dynamics has been used to study inviscid

shock interactions on

double-wedge geometries with the purpose of understanding the fundamental
gas

dynamics of these interactions. The parameter space of the interactions
has been

explored and the different types of interactions that occur have been identified.

Although the interactions are produced by a different geometry, all but
one of them

may be identified as an Edney Type I, IV, V, or VI interaction. The previously

unidentified interaction occurs because of the geometrical constraints
imposed by

the double wedge. The physical mechanisms for transition have been studied,
and

the transition criteria have been identified. An important result

is that there are two

different regimes of the parameter space in which the state of the flow
downstream

of the interaction point is fundamentally different. At high Mach numbers
this flow

is characterized by an underexpanded jet which impinges on the wedge and
produces

large-amplitude surface pressure variations. At low Mach numbers, the jet
becomes

a shear layer which no longer impinges on the wedge surface.
The anomalous wake accompanying bubbles

rising in a thin gap: a mechanically forced Marangoni flow
Authors: BUSH JOHN W. M.
Published: July 2002

A novel wake structure, observed as penny-shaped air bubbles rise
at moderate

Reynolds number through a thin layer of water bound between parallel glass
plates

inclined at a shallow angle relative to the horizontal, is reported. The
structure of

the wake is revealed through tracking particles suspended in the water.
The wake

completely encircles the rising bubble, and is characterized by a reverse

surface flow or ‘edge jet’ which transports fluid in a thin
boundary

layer along the bubble surface from

the tail to the nose at speeds which are typically an order of magnitude

larger than the

bubble rise speed. A consistent physical explanation for the wake structure

is proposed.

The wake is revealed to be a manifestation of the three-dimensionality
of the flow

in the suspending fluid. The bubble surface advances through a rolling
motion, thus

generating regions of surface divergence and convergence at, respectively,

the leading and trailing edges of the bubble. A nose-to-tail gradient in
surfactant

concentration is

thus established, and the associated surface tension gradient drives the
edge

jet. The dependence of the wake structure on the suspending fluid is

examined experimentally.

Surfactants play an anomalous role in the reported flow, serving to
promote rather

than suppress surface motions. The wake structure is an example of a mechanically

forced Marangoni flow, and so represents a mechanical analogue of that

accompanying thermocapillary drop motion in microgravity. A theoretical
model

is developed

which reproduces the salient features of the flow, and on the basis of
which an

estimate is made of the mechanically induced surface tension gradient

along the bubble surface.
Motion of an array of drops through a cylindrical tube
Authors: COULLIETTE C. , POZRIKIDIS C.
Published: July 2002

We study the pressure-driven transient motion of a periodic file
of deformable liquid
drops through a cylindrical tube with circular cross-section, at vanishing
Reynolds
number. The investigations are based on numerical solutions of the equations
of Stokes
flow obtained by the boundary-integral method. It is assumed that the viscosity
and
density of the drops are equal to those of the suspending fluid, and the
interfaces have
constant tension. The mathematical formulation uses the periodic Green's
function of
the equations of Stokes flow in a domain that is bounded externally by
a cylindrical
tube, which is computed by tabulation and interpolation. The surface of
each drop is
discretized into quadratic triangular elements that form an unstructured
interfacial
grid, and the tangential velocity of the grid-points is adjusted so that
the mesh remains
regular for an extended but limited period of time. The results illustrate
the nature of
drop motion and deformation, and thereby extend previous studies for axisymmetric

flow and small-drop small-deformation theories. It is found that when the
capillary
number is sufficiently small, the drops start deforming from a spherical
shape, and then
reach slowly evolving quasi-steady shapes. In all cases, the drops migrate
radially
toward the centreline after an initial period of rapid deformation. The
apparent
viscosity of the periodic suspension is expressed in terms of the effective
pressure
gradient necessary to drive the flow at constant flow rate. For a fixed
period of
separation, the apparent viscosity of a non-axisymmetric file is found
to be higher than
that of an axisymmetric file. In the case of non-axisymmetric motion, the
apparent
viscosity reaches a minimum at a certain ratio of the drop separation to
tube radius.
Drops with large effective radii to tube radius ratios develop slipper
shapes, similar to
those assumed by red blood cells in flow through capillaries, but only
for capillary
numbers in excess of a critical value.
Strongly nonlinear convection cells in a rapidly
rotating fluid layer: the tilted f-plane
Authors: JULIEN KEITH , KNOBLOCH EDGAR
Published: July 2002

Investigation of the linear stability problem for rapidly rotating
convection on an
f-plane has revealed the existence of two distinct scales in the
vertical structure of
the critical eigenfunctions: a small length scale whose vertical wavenumber

kz is
comparable with the large horizontal wavenumber k⊥ selected at onset,
and a
large-scale modulation which forms an envelope on the order of the layer
depth d. The
small-scale structure in the vertical results from a geostrophic balance
imposed by the
Taylor–Proudman constraint. This primary balance forces rotational
alignment and
confines fluid motions to planes perpendicular to the rotation axis. For
convective
transport in the vertical this constraint must be relaxed. This is achieved
by molecular
dissipation which allows weak upward (downward) spiralling of hot (cold)
fluid
elements across the Taylor–Proudman planes and results in a large-scale
vertical
modulation of the Taylor columns.

In the limit of fast rotation (i.e. large Taylor number) a multiple-scales
analysis leads
to the determination of a critical Rayleigh number as a function of wavenumber,
roll
orientation and the tilt angle of the f-plane. The corresponding

critical eigenfunction
represents the core solution; matching to passive Ekman boundary layers
is required
for a complete solution satisfying boundary conditions.

An extension of this analysis, introduced by Bassom & Zhang (1994),
is used to
describe strongly nonlinear two-dimensional convection, characterized by
significant
departures of the mean thermal field from its conduction profile. The analysis
requires
the solution of a nonlinear eigenvalue problem for the Nusselt number (for
steady
convection) and the Nusselt number and oscillation frequency (for the overstable

problem). The solutions of this problem are used to calculate horizontal
and vertical
heat fluxes, as well as Reynolds stresses, as functions of both the latitude
and roll
orientation in the horizontal, and these are used to calculate self-consistently

north–south and east–west mean flows. These analytical predictions

are in good agreement with the results of three-dimensional simulations
reported
by Hathaway & Somerville (1983).
Lagrangian accelerations in geostrophic turbulence
Authors: HUA BACH LIEN , McWILLIAMS JAMES C. , KLEIN PATRICE
Published: July 2002

A distinctive property of Lagrangian accelerations in geostrophic
turbulence is that
they are governed by the large and intermediate scales of the flow, both
in time
and space, so that the inertial part of the dynamics plays a much larger
role than
in three-dimensional turbulence where viscous effects are stronger. For
the case
of geostrophic turbulence on a β-plane, three terms contribute to
the Lagrangian
accelerations: the ageostrophic pressure gradient which often is the largest
term, a
meridional acceleration due to the β-effect, and an acceleration due
to horizontally
divergent ageostrophic motions. Both their spectral characteristics and
patterns in
physical space are studied in this paper. In particular the total accelerations
field has
an inertial spectrum slope which is identical to the geostrophic velocity
field inertial
slope.

The accelerations gradient tensor is shown to govern the topology of
quasi-geostrophic
stirring and transport properties. Its positive eigenvalues locate accurately

the position of extrema of potential vorticity gradients. The three-dimensional

distribution of tracer gradients is such that the vertical distribution
is entirely constrained
by the horizontal one, while the reverse is not true. We make explicit
analytically
their dependence on the three-dimensional accelerations gradient.
Turbulence measurements in a three-dimensional boundary layer in supersonic
flow
Authors: KONRAD WOLFGANG , SMITS ALEXANDER J.
Published: July 2002

Turbulence measurements were obtained in a three-dimensional supersonic
turbulent
boundary layer. A 20° curved fin was used to generate a three-dimensional
compression
of a boundary layer at Mach 3 in the absence of shock waves. Data include
hot-wire
measurements of five components of the Reynolds stress tensor. The results
are
interpreted in terms of the mean flow field history of the turbulence.
It is demonstrated
that in-plane curvature can have a strong stabilizing effect on the turbulence.
A model for near-wall dynamics in turbulent Rayleigh–Bénard
convection
Authors: THEERTHAN S. ANANDA , ARAKERI JAYWANT H.
Published: July 2002

Experiments indicate that turbulent free convection over a horizontal
surface (e.g.
Rayleigh–Bénard convection) consists of essentially line plumes
near the walls, at
least for moderately high Rayleigh numbers. Based on this evidence, we
propose
here a two-dimensional model for near-wall dynamics in Rayleigh–Bénard
convection
and in general for convection over heated horizontal surfaces. The model
proposes a
periodic array of steady laminar two-dimensional plumes. A plume is fed
on either
side by boundary layers on the wall. The results from the model are obtained
in two
ways. One of the methods uses the similarity solution of Rotem & Classen
(1969) for
the boundary layer and the similarity solution of Fuji (1963) for the plume.
We have
derived expressions for mean temperature and temperature and velocity fluctuations

near the wall. In the second approach, we compute the two-dimensional flow
field
in a two-dimensional rectangular open cavity. The number of plumes in the
cavity
depends on the length of the cavity. The plume spacing is determined from
the critical
length at which the number of plumes increases by one. The results for
average plume
spacing and the distribution of r.m.s. temperature and velocity fluctuations
are shown
to be in acceptable agreement with experimental results.
Vortex dynamics in a spatially accelerating shear layer
Authors: PAWLAK G. , ARMI L.
Published: July 2002

An experimental and analytical study into the vortex dynamics of
a stratified shear
layer subjected to a spatial acceleration is presented. The outer flow
is dictated by
a hydraulically controlled wedge flow which provides a spatially accelerating
shear
layer and baroclinic generation of vorticity along the inclined interface.
A new,
finite-amplitude mechanism is observed in which the core of the growing
vortex is separated
from the vorticity source at the interface. A secondary core develops and
an altered
vortex pairing interaction is observed. A spatial linear stability analysis
reveals that
one of two modified Kelvin–Helmholtz modes is dominant, resulting
in the centre of
the instability being offset from the density interface into the slower
moving stream.
Digital particle imaging velocimetry (DPIV) measurements are presented
along with
flow visualization which indicate that the mechanism is a result of the
offset in the
vortex core from the source of vorticity at the interface combined with
the effects of
spatial acceleration and buoyancy. The mixing induced by the interfacial
instabilities
is such that a sharp density interface remains near the high-momentum stream,
with
a low-gradient region extending into the low-momentum stream.
Examination of the flow near the
leading edge of attached cavitation.
Part 1. Detachment of two-dimensional
and axisymmetric cavities
Authors: LEGER A. TASSIN , CECCIO S. L.
Published: July 2002

The flow near the cavity detachment region of stable attached cavitation
was
examined using qualitative and quantitative flow visualization. The non-cavitating
and
cavitating flows around a hydrophilic brass and hydrophobic Teflon sphere
and cylinder
were examined. The location of non-cavitating boundary layer separation
and
cavity detachment was related to the free-stream Reynolds and cavitation
numbers.
The shape of the cavity near the detachment was greatly affected by the
material
of the cavitating object. The cavity interface on the hydrophilic test
objects curved
downstream to form a forward facing step. A region of recirculating fluid
existed
upstream of the cavity interface. The cavity detachment on the hydrophobic
test
objects was much closer to the location of boundary layer separation. The
forward
facing step and the recirculating region were nearly absent.

The measured flow field near the surface of the brass sphere, cylinder,
and hydrofoils
under cavitating and non-cavitating conditions was used to calculate the
position
of two-dimensional laminar boundary layer separation.
Thwaites' and Stratford's
methods were used to predict the location of boundary layer separation
upstream of
the cavity detachment. The predictions compared well with the observed
position of
separation.
Rising speed and dissolution rate of a carbon
dioxide bubble in slightly contaminated water
Authors: TAKEMURA F. , YABE A.
Published: July 2002

The rising speed and dissolution rate of a carbon dioxide bubble
in slightly
contaminated water were investigated experimentally and numerically. We
developed an
experimental system that uses a charged-coupled device (CCD) camera coupled
with
a microscope to track the rising bubble. By precisely measuring the bubble
size and
rising speed, we were able to accurately estimate the drag coefficient
and the Sherwood
number for the dissolution rate of gas bubbles at Reynolds numbers below
100
in the transient regime, where the bubble changes from behaving as a fluid
sphere
to behaving as a solid particle. We also numerically estimated the drag
coefficient
and Sherwood number of the ‘stagnant cap model’ by directly
solving the coupled
Navier–Stokes and convection–diffusion equations. We compared
our experimental
results with our numerical results and proposed equations for estimating
the drag
coefficient and Sherwood number of the bubble affected by contamination
and clarified
that the gas–liquid interface of the carbon dioxide bubble in water
is immobile. We
also show that the experimental and numerical results are in good agreement
and the
stagnant cap model can explain the mechanism of the transient process where
the
bubble behaviour changes from that of a fluid sphere to that of a solid
particle.
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.
A DNS-based thermal second-moment closure
for buoyant convection at vertical walls
Authors: DOL H. S. , HANJALIĆ K. , VERSTEEGH T. A. M.
Published: July 2002

Direct numerical simulations (DNS) of natural convection in a vertical channel by
Versteegh & Nieuwstadt (1998) are used for assessing the budget of the turbulent
heat flux θui and the
temperature variance θ2, and for modelling the transport
equations governing these two properties. The analysis is confined to a simple fully
developed situation in which the gravitational vector, as the sole driving force, is
perpendicular to the only non-zero component of the mean temperature gradient.
Despite its simplicity, the flow displays many interesting features and represents a
generic case of the interaction of buoyancy-driven turbulent temperature and velocity
fields. The paper discusses the near-wall variation of the second moments and their
budgets, as well as possible scaling of
θui
and θ 2 both in the near-wall region and away
from the wall. Various proposals for the Reynolds-averaged modelling are analysed
and new models are proposed for these two transport equations using the term-by-
term approach. An a priori test (using the DNS data for properties other than
θui and θ 2)
reproduced very well all terms in the transport equations, as well as their
near-wall behaviours and wall limits, without the use of any wall-topology-dependent
parameters. The computational effort is still comparable to that for the ‘basic
model’. The new term-by-term model of the
θui and θ 2
equations was then used for a full simulation in conjunction with a low-Reynolds-number second-moment
velocity closure, which was earlier found to reproduce satisfactorily a variety of isothermal
wall flows. Despite excellent term-by-term reproduction of thermal turbulence, the
predictions with the full model show less satisfactory agreement with the DNS data
than a priori validation, indicating a further need for improvement of the modelling
of buoyancy effects on mechanical turbulence.
Channelized free-surface flow of
cohesionless granular avalanches in a chute
with shallow lateral curvature
Authors: WIELAND M. , GRAY J. M. N. T. , HUTTER K.
Published: July 2002

A series of laboratory experiments and numerical simulations have been performed
to investigate the rapid fluid-like flow of a finite mass of granular material down a
chute with partial lateral confinement. The chute consists of a section inclined at 40°
to the horizontal, which is connected to a plane run-out zone by a smooth transition.
The flow is confined on the inclined section by a shallow parabolic cross-slope profile.
Photogrammetric techniques have been used to determine the position of the evolving
boundary during the flow, and the free-surface height of the stationary granular
deposit in the run-out zone. The results of three experiments with different granular
materials are presented and shown to be in very good agreement with numerical
simulations based on the Savage–Hutter theory for granular avalanches. The basal
topography over which the avalanche flows has a strong channelizing effect on the
inclined section of the chute. As the avalanche reaches the run-out zone, where the
lateral confinement ceases, the head spreads out to give the avalanche a characteristic
‘tadpole’ shape. Sharp gradients in the avalanche thickness and velocity began to
develop at the interface between the nose and tail of the avalanche as it came to rest,
indicating that a shock wave develops close to the end of the experiments.
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.
Miscible displacement in a Hele-Shaw cell at high rates
Authors: LAJEUNESSE E. , MARTIN J. , RAKOTOMALALA N. , SALIN D. , YORTSOS Y. C.
Published: July 2002

We study experimentally and theoretically the downward vertical displacement of one
miscible fluid by another lighter one in the gap of a Hele-Shaw cell at sufficiently
high velocities for diffusive effects to be negligible. Under certain conditions on the
viscosity ratio, M, and the normalized flow rate, U, this results in the formation of
a two-dimensional tongue of the injected fluid, which is symmetric with respect to
the midplane. Thresholds in flow rate and viscosity ratio exist above which the two-
dimensional flow destabilizes, giving rise to a three-dimensional pattern. We describe
in detail the two-dimensional regime using a kinematic wave theory similar to Yang
& Yortsos (1997) and we delineate in the (M, U)-plane three different domains,
characterized respectively by the absence of a shock, the presence of an internal
shock and the presence of a frontal shock. Theoretical and experimental results are
compared and found to be in good agreement for the first two domains, but not for
the third domain, where the frontal shock is not of the contact type. An analogous
treatment is also applied to the case of axisymmetric displacement in a cylindrical
tube.
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.
Obstacles to the Consolidation of the
Venezuelan Neighbourhood
Movement: National and Local
Cleavages
Authors: ELLNER STEVE
Published: July 2002

The tenuous organisational structure of the Venezuelan neighbourhood

movement explains its failure to live up to the lofty expectations which
social
movements created in Latin America in the 1980s. To understand why structural

looseness prevails, it is necessary to examine social cleavages as well
as the
disparity between the theoretical model which underpins much discourse
and
legislation, on the one hand, and the daily practice of neighbourhood associations,

on the other. An additional area of enquiry is the division in the national

leadership between an apolitical current and one that views electoral politics
as
a natural avenue for the movement to pursue. Scholars, even those who have

recently emphasized the importance of links between social movements and

political structures, overlook the importance of organisational unity at
the
national level.
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.
A weak turbulence theory for incompressible magnetohydrodynamics
Authors: GALTIER S. , NAZARENKO S. V. , NEWELL A. C. , POUQUET A.
Published: July 2002

We derive a weak turbulence formalism for incompressible magnetohydrodynamics.
Three-wave interactions lead to a system of kinetic equations for the
spectral densities of energy and helicity. The kinetic equations conserve energy in
all wavevector planes normal to the applied magnetic field B0ê∥.
Numerically and analytically, we find energy spectra
E± ∼ kn±⊥,
such that n+ + n− = −4, where E±
are the spectra of the Elsässer variables z± = v ± b in the
two-dimensional case (k∥ = 0). The constants of the spectra are computed exactly and found to
depend on the amount of correlation between the velocity and the magnetic field. Comparison
with several numerical simulations and models is also made.
Biometric investigations on the cnidae of the Aegean colour morphs of Anemonia viridis
Authors: Chintiroglou Chariton C. , Karalis Panagiotis
Published: July 2002

The sea anemone Anemonia viridis is one of the most common species of the North Aegean Sea. The variety in colour morphs exhibited by this species, causes problems to taxonomy and ecology, and suggests that A. viridis should be subdivided into two distinct species according to the colour of the individuals. The approach used in this study was based on the biometrical cnidae characteristics of the two colour morphs (rustica and smaragdina), as it is known that cnidae biometry has been used as a taxonomic tool for the phylum Cnidaria. More than 20,000 nematocysts were measured to try and find a correlation between these measurements and some body parameters indicative of the maturity grade of the individuals, as potential taxonomic characteristics. Stable cnidae characteristics of the two different colour morphs were compared. However, they differed only in the biometry of the tentacles' a-basitrichs. This slight difference does not support the proposal to subdivide Anemonia viridis into two different species.
An analysis of the nematocysts of the beadlet anemone Actinia equina and the green sea anemone Actinia prasina
Authors: Watts Phillip C. , Allcock A. Louise , Lynch Sean M. , Thorpe John P.
Published: July 2002

Numerous studies of their population genetics have reported incipient reproductive isolation among sympatric populations of the common intertidal beadlet anemone Actinia equina. This has lead to certain morphs being raised to specific status. A study of the nematocysts of the green sea anemone Actinia prasina and three genetically isolated morphs of A. equina was undertaken to establish that mean nematocyst length could act as diagnostic phenotypic characters within a morphologically variable group. The results support genetic and ecological evidence for the specific status of the three red/brown coloured morphs of A. equina. The data are discussed with respect to the ecology of Actinia and concepts of species, but more work is required before the specific status of A. prasina can be confirmed.