bluuebunny/crossref_metadata_2025 huggingface dataset:
Collision statistics in an isotropic particle-laden
turbulent suspension. Part 1. Direct numerical simulations
Authors: SUNDARAM SHIVSHANKAR , COLLINS LANCE R.
Published: July 2002

Direct numerical simulations of heavy particles suspended in a
turbulent
fluid are performed to study the rate of inter-particle collisions as a
function of
the turbulence parameters and particle properties. The particle volume
fractions
are kept small (∼10−4) so that the system is well
within the
dilute limit. The fluid velocities are updated using a pseudo-spectral
algorithm
while the particle forces are approximated
by Stokes drag. One unique aspect of the present simulations is that the
particles
have finite volumes (as opposed to point masses) and therefore particle
collisions
must be accounted for. The collision frequency is monitored over several
eddy
turnover times. It is found that particles with small Stokes numbers behave
similarly to the
prediction of Saffman & Turner (1956). On the other hand, particles
with
very large
Stokes numbers have collision frequencies similar to kinetic theory (Abrahamson
1975). For intermediate Stokes numbers, the behaviour is complicated by
two effects:
(i) particles tend to collect in regions of low vorticity (high strain)
due
to a centrifugal
effect (preferential concentration); (ii) particle pairs are less strongly
correlated with
each other, resulting in an increase in their relative velocity. Both effects
tend to
increase collision rates, however the scalings of the two effects are
different, leading
to the observed complex behaviour. An explanation for the entire range
of Stokes
numbers can be found by considering the relationship between the collision
frequency
and two statistical properties of the particle phase: the radial distribution
function and the relative velocity probability density function. Statistical
analysis of the data, in
the context of this relationship, confirms the relationship and provides
a quantitative
description of how preferential concentration and particle decorrelation
ultimately
affect the collision frequency.
On the boundary layer arising in the spin-up of a

stratified fluid in a container with sloping walls
Authors: DUCK P. W. , FOSTER M. R. , HEWITT R. E.
Published: July 2002

In this paper we consider the boundary layer that forms on the
sloping

walls of a

rotating container (notably a conical container), filled with a stratified
fluid,

when flow conditions are changed abruptly from some initial (uniform) state.
The

structure of the solution valid away from the cone apex is derived, and
it is

shown that a

similarity-type solution is appropriate. This system, which is inherently
nonlinear

in nature, is solved numerically for several flow regimes, and the results
reveal

a number of interesting and diverse features.

In one case, a steady state is attained at large times inside the boundary
layer.

In a second case, a finite-time singularity occurs, which is fully analysed.
A third

scenario involves a double boundary-layer structure developing at large
times, most

significantly including an outer region that grows in thickness as the
square-root of

time.

We also consider directly the nonlinear fully steady solutions to the
problem, and

map out in parameter space the likely ultimate flow behaviour. Intriguingly,
we find

cases where, when the rotation rate of the container is equal to that of
the main

body of the fluid, an alternative nonlinear state is preferred, rather
than the

trivial (uniform) solution.

Finally, utilizing Laplace transforms, we re-investigate the linear

initial-value problem for small differential spin-up studied by MacCready

& Rhines (1991), recovering

the growing-layer solution they found. However, in contrast to earlier
work, we

find a critical value of the buoyancy parameter beyond which the solution
grows

exponentially in time, consistent with our nonlinear results.
Viscous dissipation in steep capillary–gravity waves
Authors: LONGUET-HIGGINS MICHAEL S.
Published: July 2002

Some simple but exact general expressions are derived for

the viscous stresses required

at the surface of irrotational capillary–gravity waves of

periodic or solitary type on

deep water in order to maintain them in steady motion. These expressions
are applied

to nonlinear capillary waves, and to capillary–gravity waves

of solitary type on deep

water. In the case of pure capillary waves some algebraic expressions are
found for

the work done by the surface stresses, from which it is possible to infer
the viscous

rate of decay of free, nonlinear capillary waves.

Similar calculations are carried out for capillary–gravity

waves of solitary type on

deep water. It is shown that the limiting rate of decay of a solitary wave
at low

amplitudes is just twice that for linear, periodic waves. This is

due to the spreading

out of the wave envelope at low wave steepnesses. At large wave steepnesses
the

dissipation increases by an order of magnitude, owing to the sharply increased

curvature in the wave troughs. The calculated rates of decay are in agreement
with

recent observations.
Turbulent plane Couette flow subject to strong system rotation
Authors: BECH KNUT H. , ANDERSSON HELGE I.
Published: July 2002

System rotation is known to substantially affect the mean

flow pattern as well as the

turbulence structure in rotating channel flows. In a numerical

study of plane Couette

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

& Andersson

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

presence of anticyclonic

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

roll cells.

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

for which the rotation number Ro, defined as the

ratio of twice the imposed angular

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

±0.01. The aim

of the present paper is to explore the effects of stronger

anticyclonic system rotation

on directly simulated turbulent plane Couette flow. Turbulence

statistics like energy,

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

from the statistically steady flow fields and supplemented by structural
information

obtained by conditional sampling.

The designation of the imposed system rotation as ‘high’

was associated with a

reversal of the conventional Reynolds stress anisotropy so that the velocity

fluctuations perpendicular to the wall exceeded those in the streamwise

direction. It was

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

the mean velocity profile with slope ∼2Ω,

i.e. the absolute mean vorticity tended to

zero. It is particularly noteworthy that these characteristic

features were shared by

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

roll cell pattern

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

Ro=0.10 and

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

Ro was

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

during a transient period until the flow field settled at a

new statistically steady state

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

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

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

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

longitudinal streaks, system rotation generated streamwise vortices, either
ordered

secondary flow or quasi-streamwise vortices. Eventually, at

Ro=1.0, the turbulent

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

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

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

flow through

periodic and random arrays of aligned cylinders is investigated. Numerical

simulations

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

about 180.

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

at moderate

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

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

flow through

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

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

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

fraction.

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

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

F/(μU)

=k1+k2Re2,

where μ is the fluid viscosity, U is the mean

velocity in the bed, and k1 and k2

are functions of the solid volume fraction ϕ.

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

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

respectively.

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

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

the empirical Ergun formula, F/(μU)

=c1+c2Re,
is

applicable for Re>5. We determine the constants c1

and c2 over a wide range of ϕ. The relative

importance of inertia becomes smaller as

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

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

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

This paper analyses the results of two series of experiments

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

steep non-breaking waves.

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

and secondly,

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

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

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

water) in the range between

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

cylinders of three different diameters.

Peak loading on stationary cylinders was found to exceed the predictions
of a

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

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

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

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

history, but in some

conditions, particularly at low frequency ratios, there is clearly

some feedback from the

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

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

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

loading that occurs

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

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

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

7% of the average momentum across the shear depth and over the extent of
the
wavepacket is lost due to transport away from the mixing region by the
waves.

We propose that internal waves may act back upon the mean flow modifying
it
so that the excitation of waves of that frequency is enhanced. A narrow
frequency
spectrum of large-amplitude waves is observed because the feedback is largest
for
waves with phase tilt in a range near 45°. Numerical simulations and
analytic theories
are presented to further quantify this theory.
Experimental assessment of fractal scale
similarity in turbulent flows. Part 4. Effects of
Reynolds and Schmidt numbers
Authors: FREDERIKSEN RICHARD D. , DAHM WERNER J. A. , DOWLING DAVID R.
Published: July 2002

Experimental results are presented for the influence of Reynolds
number on multifractal scale similarity in turbulent flows. These are obtained from
single-point measurements of a dynamically passive Sc≈1
conserved scalar quantity ζ(t) in a turbulent shear flow at outer-scale Reynolds numbers of 14000[les ]Reδ[les ]110000. Statistical criteria based on the maximum allowable scale-to-scale variation L1(ε) in multiplier distributions P(Mε) from multifractal gauge sets
allow accurate discrimination between multifractal and non-multifractal scaling. Results show that the surrogate scalar energy dissipation rate χs(t)≡(dζ/dt)2is found to display a scale-invariant similarity consistent with
a random multiplicative cascade characterized by
a bilinear multiplier distribution P(Mε) over a range of scales extending downward from the outer scaleTδ. For a range of scales extending upward from the inner (diffusive) scale TD, the dissipation rate displays a different scale-invariant similarity characterized by a uniform multiplier distribution. The former scale-invariance becomes evident in the present Sc≈1 data only when Reδ is sufficiently large. Comparisons with results from Sc 1 data indicate that this
scale-invariant similarity applies when the outer-to-inner scale ratio
Tδ/TD≈0.09
Re3/4δSc1/2
is greater than about 400. In contrast to the scalar dissipation rate field, the scalar
field is found to lack any multifractal scale similarity.
A study of the evolution and characteristics of
the invariants of the velocity-gradient tensor in
isotropic turbulence
Authors: OOI ANDREW , MARTIN JESUS , SORIA JULIO , CHONG M. S.
Published: July 2002

Since the availability of data from direct numerical simulation
(DNS) of turbulence,
researchers have utilized the joint PDFs of invariants of the velocity
gradient tensor
to study the geometry of small-scale motions of turbulence. However, the
joint PDFs
only give an instantaneous static representation of the properties of fluid
particles and
dynamical Lagrangian information cannot be extracted. In this paper, the
Lagrangian
evolution of the invariants of the velocity gradient tensor is studied
using conditional
mean trajectories (CMT). These CMT are derived using the concept of the
conditional
mean time rate of change of invariants calculated from a numerical simulation
of
isotropic turbulence. The study of the CMT in the invariant space
(RA, QA) of
the velocity-gradient tensor, invariant space
(RS, QS) of
the rate-of-strain tensor, and invariant space
(RW, QW) of
the rate-of-rotation tensor show that the mean evolution in the
(Σ, QW) phase plane,
where Σ is the vortex stretching, is cyclic with a
characteristic period similar to that found by Martin et al. (1998)
in the cyclic mean
evolution of the CMT in the (RA, QA)
phase plane. Conditional mean trajectories in
the (Σ, QW) phase plane suggest
that the initial
reduction of QW in regions of high
QW is due to viscous diffusion and that vorticity
contraction only plays a secondary
role subsequent to this initial decay. It is also found that in regions
of the flow with
small values of QW, the local values of
QW do not begin to increase, even in the
presence of self-stretching, until a certain self-stretching rate threshold
is reached, i.e.
when Σ≈0.25 〈QW〉1/2.

This study also shows that in regions where the kinematic
vorticity number (as defined by Truesdell 1954) is low, the local value
of dissipation
tends to increase in the mean as observed from a Lagrangian frame of reference.

However, in regions where the kinematic vorticity number is high, the local
value of
enstrophy tends to decrease. From the CMT in the
(−QS, RS
phase plane,
it is also deduced that for large values of dissipation, there is a tendency
for fluid particles
to evolve towards having a positive local value of the intermediate principal
rate of
strain.
Large- and small-scale vortical motions in
a shear layer perturbed by tabs
Authors: FOSS JUDITH K. , ZAMAN K. B. M. Q.
Published: July 2002

The large- and small-scale vortical motions produced by
‘delta tabs’ in a two-stream
shear layer have been studied experimentally. An increase in mixing was
observed
when the base of the triangular shaped tab was affixed to the trailing
edge of the
splitter plate and the apex was pitched at some angle with respect to the
flow
axis. Such an arrangement produced a pair of counter-rotating streamwise
vortices.
Hot-wire measurements detailed the velocity, time-averaged vorticity
(Ωx) and small-scale turbulence features in the three-dimensional space downstream of
the tabs. The
small-scale structures, whose scale corresponds to that of the peak in
the dissipation
spectrum, were identified and counted using the peak-valley-counting technique.
The
optimal pitch angle, θ, for a single tab and the
optimal spanwise spacing, S, for a
multiple tab array were identified. Since the goal was to increase mixing,
the optimal
tab configuration was determined from two properties of the flow field:
(i) the large-scale motions with the maximum Ωx,
and (ii) the largest number of small-scale
motions in a given time period. The peak streamwise vorticity magnitude

[mid ]Ωx−max[mid ] was
found to have a unique relationship with the tab pitch angle. Furthermore,
for all
cases examined, the overall small-scale population was found to correlate
directly with
[mid ]Ωx−max[mid ]. Both quantities peaked
at
θ≈±45°. It is interesting to note that the peak
magnitude of the corresponding circulation in the cross-sectional plane
occurred
for θ≈±90°. For an array of tabs, the two
quantities also depended on the tab
spacing. An array of contiguous tabs acted as a solid deflector producing
the weakest
streamwise vortices and the least small-scale population. For the measurement
range
covered, the optimal spacing was found to be S≈1.5 tab widths.
Instabilities of a horizontal shear flow with a free surface
Authors: LONGUET-HIGGINS MICHAEL S.
Published: July 2002

The simple shear-flow model of Stern & Adam (1973), in which
a layer of uniform
vorticity and depth overlies an infinitely deep fluid, is here extended
by the addition
of an upper fluid layer of uniform thickness and constant velocity. In
this way many
experimentally observed velocity profiles can be approximated. The normal
mode
instabilities of such a model can be found analytically, and their properties
calculated
through the solution of a quartic polynomial equation. The dispersion relation
is here
determined and illustrated in its dependence on the Froude number and on
the ratio
H1/H2, where H1

and H2 denote the mean depths of the surface layer
and the base
of the shear layer, respectively. It is found that two branches of instability
which are
distinct when H1/H2 is moderate
or
small can become merged when H1/H2[ges ]0.4924.

Also calculated are the fastest-growing modes, and their wavelengths. The
results are
applied to some examples of surface flows generated by towed bodies, and
to steady
spilling breakers.
Turbulent shear flow over fast-moving waves
Authors: COHEN J. E. , BELCHER S. E.
Published: July 2002

We divide the interaction between wind and ocean surface waves into three parameter
regimes, namely slow, intermediate and fast waves, that are distinguished by the ratio
c/u∗ (c is the wave phase speed and
u∗ is the friction velocity in the wind). We develop
here an analytical model for linear changes to the turbulent air flow caused by waves
of small slope that is applicable to slow and to fast waves. The wave-induced turbulent
shear stress is parameterized here with a damped mixing-length model, which tends to
the mixing-length model in an inner region that lies close to the surface, and is then
damped exponentially to zero in an outer region that lies above the inner region.
An adjustable parameter in the damped mixing-length model controls the rate of
decay of the wave-induced stress above the inner region, and shows how the results
vary from a model with no damping, which corresponds to using the mixing-length
model throughout the flow, to a model with full damping, which, following previous
suggestions, correctly represents rapid distortion of the wave-induced turbulence in
the outer region.

Solutions for air flow over fast waves are obtained by analysing the displacement of
streamlines over the wave; they show that fast waves are damped, thereby giving their
energy up to the wind. There is a contribution to this damping from a counterpart
of the non-separated sheltering mechanism that gives rise to growth of slow waves
(Belcher & Hunt 1993). This sheltering contribution is smaller than a contribution
from the wave-induced surface stress working against the orbital motions in the water.
Solutions from the analysis for both slow and fast waves are in excellent agreement
with values computed by Mastenbroek (1996) from the nonlinear equations of motion
with a full second-order closure model for the turbulent stresses. Comparisons with
data for slow and intermediate waves show that the results agree well with laboratory
measurements over wind-ruffled paddle-generated waves, but give results that are a
factor of about two smaller than measurements of purely wind-generated waves. We
know of no data for fast waves with which to compare the model. The damping rates
we find for fast waves lead to e-folding times for the decay of the waves that are a
day or longer. Although this wind-induced damping of fast waves is small, we suggest
that it might control low-frequency waves in a fully-developed sea.
A simple criterion for the sign of
the pseudomomentum of modes in shallow water systems
Authors: IGA KEITA
Published: July 2002

A simple criterion is derived for determining the sign of the pseudomomentum
of neutral modes in shallow water systems. The sign of the pseudomomentum is
determined by the gradient of the dispersion curve on a wavenumber vs. phase-speed
plane: a mode has pseudomomentum with the opposite sign to that of the gradient
of the dispersion curve. In most cases, the sign of the pseudomomentum is also
determined only from the value of its phase speed: the pseudomomentum of a mode
is positive if its phase speed is faster than the velocity of the basic flow at any point
and vice versa, but with a few exceptions.
Characteristics of high Rayleigh number
two-dimensional convection in an open-top
porous layer heated from below
Authors: CHERKAOUI ABDELLAH S. M. , WILCOCK WILLIAM S. D.
Published: July 2002

Using a control-volume method and the simpler algorithm, we computed steady-state
and time-dependent solutions for two-dimensional convection in an open-top porous
box, up to a Rayleigh number of 1100. The evolution of the convective system from
onset to high Rayleigh numbers is characterized by two types of transitions in the
flow patterns. The first type is a decrease in the horizontal aspect ratio of the cells.
We observe two such bifurcations. The first occurs at Ra = 107.8 when the convective
pattern switches from a steady one-cell roll to a steady two-cell roll. The second occurs
at Ra ≈ 510 when an unsteady two-cell roll evolves to a steady four-cell roll. The
second type of transition is from a steady to an unsteady pattern and we also observe
two of these bifurcations. The first occurs at Ra ≈ 425 in the two-cell convective
pattern; the second at Ra ≈ 970 in the four-cell pattern. Both types of bifurcations
are associated with an increase in the average vertical convective heat transport. In the
bi-cellular solutions, the appearance of non-periodic unsteady convection corresponds to
the onset of the expected theoretical scaling Nu ∝ Ra
and also to the onset of plume
formation. Although our highest quadri-cellular solutions show signs of non-periodic
convection, they do not reach the onset of plume formation. An important hysterisis
loop exists for Rayleigh numbers in the range 425–505. Unsteady convection appears
only in the direction of increasing Rayleigh numbers. In the decreasing direction,
steady quadri-cellular flow patterns prevail.
Self-similarity and internal structure of turbulence induced by
Rayleigh–Taylor instability
Authors: DALZIEL S. B. , LINDEN P. F. , YOUNGS D. L.
Published: July 2002

This paper describes an experimental investigation of mixing due to Rayleigh–Taylor
instability between two miscible fluids. Attention is focused on the gravitationally
driven instability between a layer of salt water and a layer of fresh water with
particular emphasis on the internal structure within the mixing zone. Three-dimensional
numerical simulations of the same flow are used to give extra insight into the
behaviour found in the experiments.

The two layers are initially separated by a rigid barrier which is removed at the start
of the experiment. The removal process injects vorticity into the flow and creates a
small but significant initial disturbance. A novel aspect of the numerical investigation
is that the measured velocity field for the start of the experiments has been used to
initialize the simulations, achieving substantially improved agreement with experiment
when compared with simulations using idealized initial conditions. It is shown that the
spatial structure of these initial conditions is more important than their amplitude for
the subsequent growth of the mixing region between the two layers. Simple measures
of the growth of the instability are shown to be inappropriate due to the spatial
structure of the initial conditions which continues to influence the flow throughout
its evolution. As a result the mixing zone does not follow the classical quadratic time
dependence predicted from similarity considerations. Direct comparison of external
measures of the growth show the necessity to capture the gross features of the initial
conditions while detailed measures of the internal structure show a rapid loss of
memory of the finer details of the initial conditions.

Image processing techniques are employed to provide a detailed study of the internal
structure and statistics of the concentration field. These measurements demonstrate
that, at scales small compared with the confining geometry, the flow rapidly adopts
self-similar turbulent behaviour with the influence of the barrier-induced perturbation
confined to the larger length scales. Concentration power spectra and the fractal
dimension of iso-concentration contours are found to be representative of fully
developed turbulence and there is close agreement between the experiments and
simulations. Other statistics of the mixing zone show a reasonable level of agreement,
the discrepancies mainly being due to experimental noise and the finite resolution of
the simulations.
Subgrid-scale interactions in a numerically
simulated planar turbulent jet and implications
for modelling
Authors: AKHAVAN R. , ANSARI A. , KANG S. , MANGIAVACCHI N.
Published: July 2002

The dynamics of subgrid-scale energy transfer in turbulence is investigated in a
database of a planar turbulent jet at
Reλ ≈ 110, obtained by direct numerical
simulation. In agreement with analytical predictions (Kraichnan 1976), subgrid-scale
energy transfer is found to arise from two effects: one involving non-local interactions
between the resolved scales and disparate subgrid scales, the other involving local
interactions between the resolved and subgrid scales near the cutoff. The former
gives rise to a positive, wavenumber-independent eddy-viscosity distribution in the
spectral space, and is manifested as low-intensity, forward transfers of energy in the
physical space. The latter gives rise to positive and negative cusps in the spectral eddy-viscosity distribution near the cutoff, and appears as intense and coherent regions of
forward and reverse transfer of energy in the physical space. Only a narrow band of
subgrid wavenumbers, on the order of a fraction of an octave, make the dominant
contributions to the latter. A dynamic two-component subgrid-scale model (DTM),
incorporating these effects, is proposed. In this model, the non-local forward transfers
of energy are parameterized using an eddy-viscosity term, while the local interactions
are modelled using the dynamics of the resolved scales near the cutoff. The model
naturally accounts for backscatter and correctly predicts the breakdown of the net
transfer into forward and reverse contributions in a priori tests. The inclusion of
the local-interactions term in DTM significantly reduces the variability of the model
coefficient compared to that in pure eddy-viscosity models. This eliminates the need
for averaging the model coefficient, making DTM well-suited to computations of
complex-geometry flows. The proposed model is evaluated in LES of transitional
and turbulent jet and channel flows. The results show DTM provides more accurate
predictions of the statistics, structure, and spectra than dynamic eddy-viscosity models
and remains robust at marginal LES resolutions.
Non-axisymmetric magnetohydrodynamic shear layers in a rotating spherical shell
Authors: SOWARD ANDREW M. , HOLLERBACH RAINER
Published: July 2002

Constant-density electrically conducting fluid is confined to a rapidly rotating
spherical shell and is permeated by an axisymmetric magnetic field. Slow steady non-axisymmetric motion is driven by a prescribed non-axisymmetric body force; both
rigid and stress-free boundary conditions are considered. Linear solutions of the governing
magnetohydrodynamic equations are derived in the small Ekman number E
limit analytically for values of the Elsasser number Λ less than order unity and
they are compared with new numerical results. The analytic study focuses on the nature
of the various shear layers on the equatorial tangent cylinder attached to the inner
sphere. Though the ageostrophic layers correspond to those previously isolated by
Kleeorin et al. (1997) for axisymmetric flows, the quasi-geostrophic layers have a new
structure resulting from the asymmetry of the motion.

In the absence of magnetic field, the inviscid limit exhibits a strong shear singularity
on the tangent cylinder only removeable by the addition of viscous forces. With
the inclusion of magnetic field, large viscous forces remain whose strength [Zscr ] was
measured indirectly by Hollerbach (1994b). For magnetic fields with dipole parity,
cf. Kleeorin et al. (1997), [Zscr ] increases throughout the range
Λ [Lt ] 1; whereas, for quadrupole parity, cf. Hollerbach (1994b),
[Zscr ] only increases for Λ [Lt ] E1/5.

The essential difference between the dipole and quadrupole fields is the magnitude
of their radial components in the neighbourhood of the equator of the inner sphere. Its
finite value for the quadrupole parity causes the internal shear layer – the
Hartmann–Stewartson layer stump – to collapse and merge with the equatorial Ekman
layer when Λ = O(E1/5).
Subsequently the layer becomes an equatorial Hartmann layer,
which thins and spreads polewards about the inner sphere surface as Λ increases
over the range E1/5 [Lt ] Λ [Lt ] 1.
Its structure for the stress-free boundary conditions
employed in Hollerbach's (1994b) model is determined through matching with a new
magnetogeostrophic solution and the results show that the viscous shear measured by
[Zscr ] decreases with increasing Λ. Since [Zscr ]
depends sensitively on the detailed boundary layer structure, it provides a sharp diagnostic
of new numerical results for Hollerbach's model; the realized [Zscr ]-values compare
favourably with the asymptotic theory presented.
Oblique shock reflection from an axis of symmetry
Authors: HORNUNG H. G.
Published: July 2002

An exploratory computational study of the reflection of an inward-facing conical
shock wave from its axis of symmetry is presented. This is related to more complex
practical situations in both steady and unsteady flows. The absence of a length scale
in the problem studied makes features grow linearly with time. The ensuing flow is
related to the Guderley singularity in a cylindrical imploding shock. The problem is
explored by making a large number of computations of the Euler equations. Distinct
reflection configurations are identified, and the regions of parameter space in which
they occur are delineated.
Protestantism and Indigenous Mobilisation: The Moravian Church

among the Miskitu Indians of Nicaragua
Authors: HAWLEY SUSAN
Published: July 2002

This article examines the role of the Protestant Moravian

Church in the politicisation of Miskitu ethnic identity, and on the

mobilisation of the Miskitu against the Sandinistas during the 1980s.

It argues that changes in the institution of the Church during the

1960s and 70s, as a result of state policy, socio-economic context and

internal conflicts within Miskitu society, led to Moravianism becoming

a cultural marker of Miskitu ethnicity. At the same time, the

encounter with and appropriation of the pastoral tactics of a Catholic

priest resulted in a radicalisation of Miskitu Moravian pastors on

indigenous issues. When the Miskitu came to mobilise against the

Sandinistas, the Moravian Church was the expressive vehicle and the

institutional means through which the mobilisation took place. The

article reveals how politicised ethnic identities find their

expression in religious institutions.
The morphosyntax of WH-extraction in Irish
Authors: McCLOSKEY JAMES
Published: July 2002

It has been widely assumed that the preverbal particles of Irish are complementizers. Given the distribution of the particle aL, this assumption provides support for two central claims about WH-movement – that its application is successive-cyclic, and that it is driven by a morphosyntactic feature of the complementizer. However, the claim that aL is a complementizer also has been widely challenged. This paper aims to (re)confirm the original analysis. It argues that the sceptical literature (i) underestimates the morphosyntactic heterogeneity of Irish complementizers, and (ii) restricts attention to an overly narrow subset of constructions in which aL appears.
Pastoral politics in the northeast periphery in Uganda: AK-47 as change agent
Authors: Mirzeler Mustafa , Young Crawford
Published: July 2002

Our aim in this article is to contribute to understanding the effects of firearms
on the relationship between the state and pastoral communities in contemporary
northeast Uganda. The Karamoja region has from early colonial
times been a peripheral zone. Although successive post-colonial regimes have
made episodic efforts to incorporate this zone more fully into Ugandan
national space, relations between Karamoja and the centre remain distant
and distrustful. The transformation of local modes of conflict by large-scale
infusion of the AK-47 has had far-reaching effects both on relationships with
the Ugandan state and its local representatives, and within Karamoja
societies. The younger men who possess these weapons elude the authority of
the elders, and entertain ambiguous relations with the state authorities,
whom they may serve as auxiliaries or resist.
Plasma-object diagnostics via resonant
transition radiation from an electron bunch
Authors: ANISIMOV I. O. , LYUBICH K. I.
Published: July 2002

The resonant transition radiation from an electron charge that moves
along the concentration gradient of a cold isotropic plasma is calculated. The possibility
of using this radio-emission for inhomogeneous plasma diagnostics is discussed.
Measurements of this kind of radiation in a planar-stratified cold isotropic
plasma give the possibility of finding out the profile function of the plasma concentration
profile (parts of the plasma density profile where the density is increased
over its previous maximum value along the direction of the plasma bunch velocity).
Growth of low-frequency waves due to a
photon beam in a magnetized electron–positron plasma
Authors: LUO QINGHUAN , MELROSE D. B.
Published: July 2002

The effect of a beam of radio waves of very high brightness passing
through a
cold, magnetized, electron–positron plasma is discussed. The properties
of the
natural wave modes in such a plasma are summarized, and approximate forms
for the nonlinear response tensor are written down. Photon-beam-induced
instabilities of low-frequency waves in the pair plasma are analysed in
the
random-phase approximation. When three-wave interactions involve two high-frequency
waves in the same mode and a low-frequency wave in a different mode, wave–wave
interactions are similar to wave–particle interactions in that photons
act like
particles that emit and absorb low-frequency waves. The absorption coefficients
for various low-frequency waves due to a photon beam are evaluated. In
a pure
electron–positron plasma, photon-beam-induced instabilities can be
effective only
when either the high-frequency or the low-frequency waves are strongly
modified
by the magnetic field. The growth of the low-frequency waves is most effective
when the high-frequency photon beam has a frequency close to the cyclotron
frequency.
Laser excitation of surface waves over a dense plasma
Authors: PARASHAR JETENDRA , PANDEY H. D. , TRIPATHI V. K.
Published: July 2002

An intense short laser pulse incident on a metallic
target produces an overdense
plasma and excites a plasma surface wave via stimulated Compton scattering.
The pump and the surface wave exert a beat-frequency ponderomotive force on
the electrons in the skin layer, driving a heavily damped quasimode. The
density perturbations due to the quasimode couple with the
oscillatory velocity
due to the pump to drive the surface wave. The growth period
for the process
turns out to be about 0.5 ps at a Nd[ratio ]glass laser intensity of
1016W cm−2 and a
plasma density of 1.5 times the critical value.
Three-dimensional stability of ion-acoustic
solitary waves in a magnetized plasma consisting of non-isothermal electrons
Authors: GHOSH G. , DAS K. P.
Published: July 2002

A stability analysis is performed for solitary ion-acoustic waves
in a magnetized
plasma in which the electrons are non-isothermal. Including the effect
of ion
drift velocity and magnetic perturbation, a three-dimensional mKdV equation

is derived for ion-acoustic waves. The solitary-wave solution of this equation
is
found to have a sech4 profile. A stability analysis of this
solitary wave is
performed using the small-k perturbation expansion method of Rowlands
and
Infeld. A condition for the onset of instability is obtained. The growth
rate of
the instability is found to attain a maximum for perturbations in the plane

perpendicular to the direction of propagation of the solitary wave.
Magnetic-field generation and wave-breaking in collisionless plasmas
Authors: CALIFANO F. , PRANDI R. , PEGORARO F. , BULANOV S. V.
Published: July 2002

We discuss the nonlinear evolution of the Weibel instability of
two counter-streaming
electron beams in the relativistic and non-relativistic regimes in the
framework
of a two-fluid description. We show the presence of two singularities per
wavelength,
responsible for the formation of very large density spikes. In the case
of
non-symmetric beams, we observe the compressive wave-break of the fast
electron
population, and the generation of a dipolar magnetic field associated with
a central
fast thin current and two larger slow return currents. This structure is
very similar
to those observed in PIC simulations of laser-plasma interactions.
Excitation of ion acoustic solitons from grids
Authors: EL-ZEIN YASSER , SHERIDAN T. E. , LONNGREN KARL E. , HORTON WENDELL
Published: July 2002

Using a particle-in-cell code, we are able to simulate the excitation
of ion
acoustic solitons from a grid whose potential is suddenly increased. The

numerical results are in substantial agreement with previous laboratory

experiments.
The influences of bathymetry and flow regime upon the morphology of sublittoral sponge communities
Authors: Bell James J. , Barnes David K.A.
Published: July 2002

Sponge communities were sampled at 3 m depth intervals at six sites experiencing different flow regimes at Lough Hyne, Ireland. Sponges were identified and classified within the following morphological groups: encrusting, massive, globular, pedunculate, tubular, flabellate, arborescent, repent and papillate morphological types on both vertical (≈90°) and inclined (≈45°) surfaces.

Differences in the proportional abundance of the sponge body forms and density (sponge m−2) were observed between sites and depths. The density of sponges increased with depth at sites of slight to moderate current flow, but not at the site where current flow was turbulent. Morphological diversity of sponge communities decreased with increasing current flow due to the removal of delicate forms such as pedunculate and arborescent shaped sponges. Massive and encrusting morphologies dominated at the high-energy sites (fast and turbulent flow regimes) due to a high basal area to volume ratio, which prevents removal from cliff surfaces. However, pedunculate, papillate and arborescent types dominated at the low current sites as these shapes may help to prevent the settlement of sediment on sponge surfaces. Bray–Curtis Similarity analysis and Correspondence Analysis were used to distinguish between five different morphological communities.
Demersal fish assemblages and habitat characteristics on the continental shelf and upper slope of the north-western Mediterranean
Authors: Demestre Montserrat , Sánchez Pilar , Abelló Pere
Published: July 2002

Continental shelf and upper slope fish communities were studied along the Catalan coast based on 66 experimental bottom otter trawls. A total of 79 demersal fish species were studied by means of cluster analysis and multi-dimensional scaling (MDS) ordination for community structure. Analysis revealed the existence of five major location clusters. Similarity percentage analysis (SIMPER) was determined by comparing the dissimilarity between two groups of samples using the discriminating species. Geomorphological characteristics, bottom substratum and depth showed direct influences on species assemblages. High correlation between the biotic data samples and depth was observed. The fish species assemblages identified five main demersal fish associations which corresponded with the five location clusters and with five benthic sediments (mud of the upper slope, sand and gravel, mud of the shelf, muddy-sand and sand with rocky outcrops).
New species of holothurian (Echinodermata: Holothuroidea) from hydrothermal vent habitats
Authors: Smirnov A.V. , Gebruk A.V. , Galkin S.V. , Shank T.
Published: July 2002

A new holothurian species Chiridota hydrothermica (Apodida: Chiridotidae) is described, restricted to hydrothermal vent habitats. The new species is known from the west Pacific (Manus and North Fiji Basins) and the South East Pacific Rise, between 17 and 21°S. The unusual large, lobe-like tentacles of this holothurian, uncommon in the family Chiridotidae, could be an adaptation to facilitate shifts between suspension- and deposit-feeding in the hydrothermal environment. A brief review of all known records of holothurians and other echinoderms at hydrothermal vents is given.
The Fashioning of a Frontier: The Radcliffe Line and Bengal's Border Landscape, 1947–52
Authors: Chatterji Joya
Published: July 2002

The partition of India is customarily described in surgical metaphors, as an operation, an amputation, a vivisection or a dismemberment. By extension, the new borders created in 1947 are often thought of as incision scars.
State and Criminal Tribes in Colonial Punjab: Surveillance, Control and Reclamation of the ‘Dangerous Classes’
Authors: Major Andrew J.
Published: July 2002

It is not always remembered that under British rule some 150,000 Punjabis were notified under the Criminal Tribes Act as belonging to tribes and castes whose hereditary occupation was deemed to be crime. More than any other class these criminal tribes felt the harsh impact of the colonial state, which sought to control, punish and reform them. This paper traces the evolution of a Punjab criminal tribes policy and argues that the British—assisted by the indigenous elite—achieved only partial success in assimilating these people into the wider community by 1947.
The Indian Hierarchy: Culture, Ideology and Consciousness in Bengali Village Politics
Authors: Ruud Arild Engelsen
Published: July 2002

The Subaltern studies project has been a major contribution towards rethinking the role of groups such as peasants, lower castes, labourers or women in forming the course of Indian history. The project has also brought the issues of culture, ideology and consciousness to the forefront of Indian history writing. Although the importance of non-elite action on the historical developments of the Indian independence movement has already been noted by more mainstream historians, the concertedness of the project has created a whole new situation in which the subalternist perspective has become a new paradigm for Indian history writing, indeed, the subalternist perspective has increasingly come to dominate the formation of perspectives and concepts. As Masselos points out, the Subaltern studies has become the establishment.
Mutation rates in the dihydrofolate reductase gene ofPlasmodium falciparum
Authors: PAGET-MCNICOL S. , SAUL A.
Published: July 2002

A new method has been established to define the limits on a spontaneous mutation rate for a gene inPlasmodium falciparum.The method combines mathematical modelling and large-scalein vitroculturing and calculates the difference in mutant frequencies at 2 separate time-points. We measured the mutation rate at 2 positions in the dihydrofolate reductase (DHFR) gene of 3D7, a pyrimethamine-sensitive line ofP. falciparum.This line was re-cloned and an effectively large population was treated with a selective pyrimethamine concentration of 40 nM. We detected point mutations at codon-46 (TTA to TCA) and codon-108 (AGC to AAC), resulting in serine replacing leucine and asparagine replacing serine respectively in the corresponding gene product. The substitutions caused a decrease in pyrimethamine sensitivity. By mathematical modelling we determined that the mutation rate at a given position in DHFR was low and occurred at less than 2.5×10−9mutations/DHFR gene/replication. This result has important implications forPlasmodiumgenetic diversity and anti-malarial drug therapy by demonstrating that even with low mutation rates anti-malarial resistance will inevitably arise when mutant alleles are selected under drug pressure.