bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of kefir grains grown in cows' milk and in soya milk
Authors: ABRAHAM ANALÍA G. , DE ANTONI GRACIELA L.
Published: July 2002

Kefir is a refreshing fermented milk with a slightly acidic taste obtained by
incubating milk with kefir grains (Saloff-Coste, 1996). Kefir grains are a complex
mixture of lactic acid bacteria and yeasts in a strong and specific association. They
are characterized by an irregular form, a folded and uneven surface and a white or
slightly yellow colour. They are tough and resilient and have a characteristic acid
taste (Bottazzi et al. 1994). The basic microflora contains lactococci, homofermentative
and heterofermentative lactobacilli, yeasts and acetic acid bacteria (Bottazzi et
al. 1994; Rea et al. 1996). Among the yeasts isolated from grains and identified
are Candida kefir, Saccharomyces cerevisiae, Sac.
delbrueckii, Cand. holmii, Sac. unisporus and Sac. lipolytica
(Angulo et al. 1993; Marshall, 1993; Garrote et al. 1997).
Lactobacillus brevis, Lb. viridescens, Lb. casei,
Lb. kefir, Lb. kefiranofaciens, Lb.
kefirgranum, Lb. parakefir, Leuconostoc spp. and
Lactococcus lactis are among the
lactic acid bacteria present in the grains (Marshall et al.
1984; Toba et al. 1991; Takizawa et al. 1994;
Garrote et al. 1997). The mixed microflora of yeasts and
bacteria is held together by a matrix containing (g/kg dry weight) protein 340,
polysaccharide 470 (Ottogalli et al. 1973).

The study of kefir grains in milk has been centred on the characteristics of the
polysaccharide produced by lactobacilli within the grain (Yokoi et al. 1991). This
polysaccharide, named kefiran, is composed of glucose and galactose (Yokoi et al.
1991). It has been suggested that proteins are incorporated from the growth media
(Bassette & Acosta 1988), but no details about structure and composition are
available.

Soyabeans are an important component of the diet in many countries and have
been used to obtain fermented products such as sogurt (Mann, 1991). Special
attention has been given to the growth of, and sugar utilization by, Bifidobacterium
spp., Lb. delbrueckii subsp. bulgaricus and Streptococcus
thermophilus in soya milk (Buono et al. 1990;
Murti et al. 1993; Ankenman Granata & Morr, 1996). To our
knowledge, there have been no attempts to ferment soya milk with kefir grains.

The aim of this study was to investigate the growth of kefir grains in soya milk
and the composition of these grains, focusing on the matrix proteins.
Process steps for the preparation of purified fractions of
α-lactalbumin and β-lactoglobulin from whey protein concentrates
Authors: GÉSAN-GUIZIOU GENEVIEVE , DAUFIN GEORGES , TIMMER MARTIN , ALLERSMA DURITA , VAN DER HORST CAROLINE
Published: July 2002

Fractions enriched with α-lactalbumin (α-la) and β-lactoglobulin
(β-lg) were produced by a process comprising the following successive steps:
clarification–defatting of whey protein concentrate, precipitation of α-lactalbumin,
separation of soluble β-lactoglobulin, washing the precipitate, solubilization of the
precipitate, concentration and purification of α-la. The present study evaluated the
performance of the process, firstly on a laboratory scale with acid whey and then on
a pilot scale with Gouda cheese whey. In both cases soluble β-lg was separated from
the precipitate using diafiltration or microfiltration and the
purities of α-la and β-lg were in the range 52–83 and 85–94%
respectively. The purity of the β-lg fraction was
higher using acid whey, which does not contain caseinomacropeptide, than using
sweet whey. With the pilot scale plant, the recoveries (6% for α-la; 51% for β-lg)
were disappointing, but ways of improving each step in the process are discussed.
Application of capillary electrophoresis to the characterization of processed cheeses
Authors: MIRALLES BEATRIZ , RAMOS MERCEDES , AMIGO LOURDES
Published: July 2002

Capillary electrophoresis using a hydrophilically coated capillary and a
low pH buffer containing urea has been used to characterize processed cheeses.
Different electrophoretic patterns were obtained depending on the ingredients used
in the blend such as acid casein, rennet casein, sodium and calcium caseinates and
skim milk powder. Isoelectric casein, and sodium and calcium caseinates were shown
to contain intact non-glycosylated κ-casein (κ-CN), while rennet casein contained
only trace amounts of κ-CN and mainly para-κ-CN. Therefore, the addition of casein
or caseinate to processed cheeses has been detected by analysing the intact non-
glycosylated κ-CN. Quantitation of intact non-glycosylated κ-CN in processed
cheeses of known and unknown composition was carried out using a regression curve
from standard mixtures of 150–550 g isoelectric casein/kg total rennet casein. This
capillary electrophoresis method successfully confirmed the addition of isoelectric
casein or caseinate to processed cheeses of known composition. The quantitative
determination range was 0·605–3·688 mg κ-CN/ml. This method cannot be used for
measuring additions of rennet casein or any caseinates that have been exposed to
chymosin.
Dissipation of shear-free turbulence near boundaries
Authors: TEIXEIRA M. A. C. , BELCHER S. E.
Published: July 2002

The rapid-distortion model of Hunt & Graham (1978) for the initial distortion of
turbulence by a flat boundary is extended to account fully for viscous processes. Two
types of boundary are considered: a solid wall and a free surface. The model is shown
to be formally valid provided two conditions are satisfied. The first condition is that
time is short compared with the decorrelation time of the energy-containing eddies,
so that nonlinear processes can be neglected. The second condition is that the viscous
layer near the boundary, where tangential motions adjust to the boundary condition,
is thin compared with the scales of the smallest eddies. The viscous layer can then
be treated using thin-boundary-layer methods. Given these conditions, the distorted
turbulence near the boundary is related to the undistorted turbulence, and thence
profiles of turbulence dissipation rate near the two types of boundary are calculated
and shown to agree extremely well with profiles obtained by Perot & Moin (1993) by
direct numerical simulation. The dissipation rates are higher near a solid wall than
in the bulk of the flow because the no-slip boundary condition leads to large velocity
gradients across the viscous layer. In contrast, the weaker constraint of no stress at a
free surface leads to the dissipation rate close to a free surface actually being smaller
than in the bulk of the flow. This explains why tangential velocity fluctuations parallel
to a free surface are so large. In addition we show that it is the adjustment of the
large energy-containing eddies across the viscous layer that controls the dissipation
rate, which explains why rapid-distortion theory can give quantitatively accurate
values for the dissipation rate. We also find that the dissipation rate obtained from
the model evaluated at the time when the model is expected to fail actually yields
useful estimates of the dissipation obtained from the direct numerical simulation at
times when the nonlinear processes are significant. We conclude that the main role of
nonlinear processes is to arrest growth by linear processes of the viscous layer after
about one large-eddy turnover time.
Theory of water waves derived from a Lagrangian. Part 1. Standing waves
Authors: LONGUET-HIGGINS MICHAEL S.
Published: July 2002

A new system of equations for calculating time-dependent motions of deep-water
gravity waves (Balk 1996) is here developed analytically and set in a form suitable
for practical applications. The method is fully nonlinear, and has the advantage of
essential simplicity. Both the potential and the kinetic energy involve polynomial
expressions of low degree in the Fourier coefficients Yn(t). This leads to equations of
motion of correspondingly low degree. Moreover the constants in the equations are
very simple. In this paper the equations of motion are specialized to standing waves,
where the coefficients Yn are all real.
Truncation of the series at low values of [mid ]n[mid ], say
n < N, leads to ‘partial waves’ with
solutions apparently periodic in the time t. For
physical applications N must however be large. The method will be applied to the
breaking of standing waves by the forming of sharp corners at the crests, and the
generation of vertical jets rising from the wave troughs.
The motion generated by a rising particle in a rotating fluid – numerical solutions.
Part 1. A short container
Authors: MINKOV E. , UNGARISH M. , ISRAELI M.
Published: July 2002

Numerical finite-difference results of the full axisymmetric incompressible
Navier–Stokes equations are presented for the problem of the slow axial motion of a disk
particle in an incompressible, rotating fluid in a cylindrical container. The governing
parameters are the Ekman number, E, the Rossby number, Ro, and the dimensionless
height of the container, H (with respect to the diameter of the particle). The study
concerns small values of E, Ro, and HE−1/2
and compares the numerical results with predictions of previous analytical (mostly
approximate) studies. Special attention is focused on the drag force. First, developed
(quasi-steady state) cases are considered. Excellent agreement with the exact linear
(Ro = 0) solution of Ungarish & Vedensky (1995) is obtained when the
computational Ro = 10−4. The effects of the nonlinear momentum
advection terms are analysed and shown to be proportional to
RoE−1/2. Next, the time-development for both (a)
impulsive start and (b) start under a constant axial force are considered, and good
qualitative agreement with previous analytical results (including the appearance of
oscillations in case (b)) is indicated.
Multi-layer hydraulic exchange flows
Authors: LANE-SERFF G. F. , SMEED D. A. , POSTLETHWAITE C. R.
Published: July 2002

Flows between ocean basins are often controlled by narrow channels and shallow sills.
A multi-layer hydraulic control theory is developed for exchange flow through such
constrictions. The theory is based on the inviscid shallow-water equations and extends
the functional approach introduced by Gill (1977) and developed by Dalziel (1991).
The flows considered are those in rectangular–cross-section channels connecting two
large reservoirs, with a single constriction (sill and/or narrows). The exchange flow
depends on the stratification in the two reservoirs, represented as a finite number of
immiscible layers of (different) uniform density. For most cases the flow is ‘controlled’
at the constriction and often at other points along the channel (virtual controls)
too. As with one- and two-layer hydraulics, controls are locations at which the flow
passes from one solution branch to another, and at which (at least) one internal
wave mode is stationary. The theory is applied to three-layer flows, which have two
internal wave modes, predicting interface heights and layer fluxes from the given
reservoir conditions. The theoretical results for three-layer flows are compared to a
comprehensive set of laboratory experiments and found to give good agreement. The
laboratory experiments also show other features of the flow, such as the formation of
waves on the interfaces. The implications of the results for oceanographic flows and
ocean modelling are discussed.
On the displacement of three-dimensional fluid droplets adhering to a plane wall in viscous pressure-driven flows
Authors: DIMITRAKOPOULOS P. , HIGDON J. J. L.
Published: July 2002

The yield conditions for the displacement of three-dimensional fluid droplets adhering
to a plane solid boundary in pressure-driven flows are studied through a series of
numerical computations. The study considers low-Reynolds-number flows between
two parallel plates and includes interfacial forces with constant surface tension. A
comprehensive study is conducted, covering a wide range of viscosity ratio λ, capillary
number Ca, advancing and receding contact angles, θA
and θR, and dimensionless
plate separation H/h (where H is the plate spacing and h is the unperturbed droplet
height). This study seeks the optimal shape of the contact line which yields the
maximum flow rate (or Ca) for which a droplet can adhere to the surface. The
critical shear rates are presented as functions
Ca(λ, H/h, θA, Δθ)
where Δθ = θA − θR
is the contact angle hysteresis. The numerical solutions are based on an efficient,
three-dimensional Newton method for the determination of equilibrium free surfaces
and an optimization algorithm which is combined with the Newton iteration to
solve the nonlinear optimization problem. The critical shear rate is found to be
sensitive to viscosity ratio with qualitatively different results for viscous and inviscid
droplets. As the viscosity of a droplet increases, the critical flow rate decreases,
facilitating the displacement. This is consistent with our previous results for shear
flows (Dimitrakopoulos & Higdon 1997, 1998), which represent the limit of infinite
plate spacing. As the plate spacing is reduced, the critical flow rate increases until
a maximum value is reached. Further reduction in the plate spacing decreases the
critical flow rate. The effects of both viscosity ratio and plate separation are much more
pronounced for high contact angles. Inviscid droplets (or bubbles) show behaviour
dramatically different from that of viscous droplets. For these droplets, a significantly
higher flow rate is required for drop displacement, but this critical flow rate decreases
monotonically as the distance between the plates decreases. In the Appendix, we
clarify the necessary conditions for low-Reynolds-number flows past low viscosity
droplets or bubbles.
Lift-off of a single particle in Newtonian and viscoelastic fluids by direct numerical simulation
Authors: PATANKAR N. A. , HUANG P. Y. , KO T. , JOSEPH D. D.
Published: July 2002

In this paper we study the lift-off to equilibrium of a single circular particle in
Newtonian and viscoelastic fluids by direct numerical simulation. A particle heavier
than the fluid is driven forward on the bottom of a channel by a plane Poiseuille
flow. After a certain critical Reynolds number, the particle rises from the wall to an
equilibrium height at which the buoyant weight just balances the upward thrust from
the hydrodynamic force. The aim of the calculation is the determination of the critical
lift-off condition and the evolution of the height, velocity and angular velocity of the
particle as a function of the pressure gradient and material and geometric parameters.
The critical Reynolds number for lift-off is found to be larger for a heavier particle
whereas it is lower for a particle in a viscoelastic fluid. A correlation for the critical
shear Reynolds number for lift-off is obtained. The equilibrium height increases with
the Reynolds number, the fluid elasticity and the slip angular velocity of the particle.
Simulations of single particle lift-off at higher Reynolds numbers in a Newtonian
fluid by Choi & Joseph (2001) but reported here show multiple steady states and
hysteresis loops. This is shown here to be due to the presence of two turning points
of the equilibrium solution.
Effective pseudo-potentials of hydrodynamic origin
Authors: SQUIRES TODD M.
Published: July 2002

It is shown that low Reynolds number fluid flows can cause suspended particles to
respond as though they were in an equilibrium system with an effective potential.
This general result follows naturally from the fact that different methods of moving
particles in viscous fluids give rise to very different long-range flows. Two examples
are discussed: electrophoretic ‘levitation’ of a heavy charged sphere, for which
a hydrodynamic ‘pseudo-potential’ can be written in closed form, and quasi-two-dimensional
crystals of like-charged colloidal spheres which form near charged walls,
whose apparent attraction arises not from a force but from persistent fluid flows.
Stability of wall-bounded shear layers in the

presence of simulated distributed surface roughness
Authors: FLORYAN J. M.
Published: July 2002

Linear stability of wall-bounded shear layers modified by distributed

suction has been

considered. Wall suction was introduced in order to simulate distributed
surface

roughness. In all cases studied, i.e. Poiseuille and Couette flows and
Blasius

boundary layer, wall suction was able to induce a new type of instability

characterized by the

appearance of streamwise vortices. Results of calculations show that a

linear model of

suction-induced flow modifications provides a sufficiently accurate representation
of

the basic state. The effects of an arbitrary suction distribution can,
therefore, be

assessed by decomposing this distribution into Fourier series and carrying

out stability analysis on a mode-by-mode basis, i.e. once and for ever.
Effects of free-stream turbulence on surface
pressure fluctuations in a separation bubble
Authors: SAATHOFF P. J. , MELBOURNE W. H.
Published: July 2002

Wind-tunnel experiments were conducted to investigate the cause
of large
pressure
fluctuations near leading edges of sharp-edged bluff bodies. Measurements
obtained
with a blunt flat plate showed that very low pressures occur in a narrow
region
located
approximately 0.25XR from the leading edge,
where
XR defines the distance from the
leading edge to the mean reattachment location. This phenomenon occurs
in the
undisturbed flow as well as turbulent flow, although the magnitude of peak
pressure
fluctuations increases with both turbulence intensity,
σu/u,
and turbulence scale, LX.

Flow visualization experiments conducted with a high-speed cine-camera
reveal the
process that causes large pressure fluctuations in separation bubbles.
This
process is
initiated when a perturbation in the approaching flow causes a roll-up
of
the separated
shear layer, producing a strong vortex near the surface. Conditional sampling
of
pressure data was used to determine the spanwise length of the vortex.
A significant
increase in the spanwise correlation of pressure fluctuations occurs when
the shear
layer rolls up. Coherence measurements indicate that the spanwise length
of vortices
in the separation bubble is not directly related to longitudinal velocity

fluctuations in the free-stream.
Thickness and volume measurements of a

Richtmyer–Meshkov instability-induced mixing zone in a square shock
tube
Authors: JOURDAN G. , HOUAS L. , HAAS J.-F. , BEN-DOR G.
Published: July 2002

A simultaneous three-directional laser absorption technique for
the

study of a shock-induced Richtmyer–Meshkov instability mixing zone

is reported. It is an improvement

of a CO2 laser absorption technique, using three detectors

during the same run,

through three different directions of the test section, for the

simultaneous thickness

measurement of the mixing zone near the corner, near the wall and at the
centre of

a square-cross-section shock tube. The three-dimensional mean front and
rear shapes

of the mixing zone, its thickness and volume are deduced from the experimental

measurements. The cases when the shock wave passes from a heavy gas to
a light

one, from one gas to another of similar densities and from a light gas
to a heavy

one, are investigated before and after the mixing zone compression by the
reflected

shock, for different incident shock wave Mach numbers. It is shown that
the mixing

zone is strongly deformed by the wall boundary layer when it becomes turbulent.

Consequently, the thickness of the mixing zone is not constant along the
shock tube

cross-section, and the measurement of the mean volume of the mixing zone
appears

to be more appropriate than its mean thickness at the centre of the shock
tube.

The influence of the incident shock wave Mach number is also studied. When
the

Atwood number tends to zero, we observe a limit-like regime and the thickness,
or

the volume, of the mixing zone no longer varies with the incident shock
wave Mach

number. Furthermore, a series of experiments undertaken with an Atwood
number

close to zero enabled us to define a membrane-induced minimum mixing thickness,

L0, depending on the initial configuration of the experiments.

From the experimental

data, a hypothesis about the mixing zone thickness evolution law with

time is deduced

on the basis of L0. The results are found to follow
two very

different laws depending

on whether they are considered before or after the establishment of the
plenary

turbulent regime. However, no general trend can be determined to describe
the entire

phenomenon, i.e. from the initial conditions until the turbulent stage.
Mixing, structure and scaling of the jet in crossflow
Authors: SMITH S. H. , MUNGAL M. G.
Published: July 2002

The mixing of the round jet normal to a uniform crossflow is studied
for a range
of jet-to-crossflow velocity ratios, r, from 5 to 25.
Planar laser-induced fluorescence
(PLIF) of acetone vapour seeded into the jet is used to acquire quantitative
two-dimensional
images of the scalar concentration field. Emphasis is placed on r=10

and r=20 and a few select images are acquired up to r=200.

The Reynolds number based on the jet exit diameter, d,
and the exit velocity varies from 8400 to 41 500.
Images are acquired for conditions in which the product rd is

held constant, requiring decreasing d for increasing r.

Results from this experimental study concern structural events of the
vortex
interaction region, and mixing and mean centreline concentration decay
in the near and
far fields. The results cover all three regions of the transverse jet,
and suggest that the
jet scales with three length scales: d, rd and
r2d.

Events within the vortex interaction region display d-scaling,

including the crossflow boundary layer separation and roll-up. Over
the range of velocity ratios studied, the
vortex interaction region shows r-dependent variations in the

flow field, including the
emergence of jet fluid in the wake structures for r>10 and
a slower development of
the counter-rotating vortex pair (CVP) in higher-r jets.

The trajectory and physical dimension of the jet in both the near and
far field
display rd-scaling. The near field is characterized
by a centreline concentration decay
along the centreline coordinate s of s−1.3,
different
from the decay rate (s−1) of the free
jet. When normalized by rd, the decay of each velocity-ratio jet
branches away from
the s−1.3 decay, approaching a decay of
s−2/3, a rate predicted by modelling efforts.

The branch points represent a transition in the flow field from enhanced
mixing to
reduced mixing compared to the free jet. When normalized by
r2d, the branch points
occur at a uniform jet position, s/r2d=0.3,

which is viewed to be the division between
the near and far fields. Self-similarity is not seen in the near field,
but may be present
in the far field.

The view of the branch points as a place of transition in the flow is
supported by
the probability density function (p.d.f.) of concentration along the upper
edge of the
jet. Before the branch points, the p.d.f.s are non-marching in character,
and after the
branch points, they are tilted in character.

Instantaneously, the CVP is asymmetric in shape and concentration. End
views
reveal extensive motion of the CVP and plan views show this motion can
occur in
both axisymmetric and sinusoidal motion. Ensemble-averaged images show
the jet
concentration is asymmetric about the centreline plane.
Stability of wall modes in a flexible tube
Authors: KUMARAN V.
Published: July 2002

The asymptotic results (Kumaran 1998b) obtained for Λ∼1

for the flow in a flexible tube are extended to the limit Λ[Lt ]1

using a numerical scheme, where Λ is the
dimensionless parameter Re1/3
(G/ρV2), Re=(ρVR/η)

is the Reynolds number, ρ and η are the density and viscosity of
the
fluid, R is the tube radius and G is the
shear modulus of the wall material. The results of this calculation indicate
that the
least-damped mode becomes unstable when Λ decreases below a transition
value at a
fixed Reynolds number, or when the Reynolds number increases beyond a transition

value at a fixed Λ. The Reynolds number at which there is a transition
from stable
to unstable perturbations for this mode is determined as a function of
the parameter
Σ=(ρGR2/η2), the scaled

wavenumber of the perturbations kR, the ratio of radii of
the wall and fluid H and the ratio of viscosities of the wall
material and the fluid
ηr. For ηr=0, the
Reynolds number at which there is a transition from stable to
unstable perturbations decreases proportional to Σ1/2
in the limit
Σ[Lt ]1, and the neutral stability curves have a rather complex behaviour

in the intermediate regime
with the possibility of turning points and isolated domains of instability.
In the limit
Σ[Gt ]1, the Reynolds number at which there is a transition from stable
to unstable
perturbations increases proportional to Σα, where
α
is between 0.7 and 0.75. An
increase in the ratio of viscosities ηr has a complex
effect
on the Reynolds number for
neutrally stable modes, and it is observed that there is a maximum ratio
of viscosities
at specified values of H at which neutrally stable modes exist;
when the ratio of
viscosities is greater than this maximum value, perturbations are always
stable.
Secondary instability in rotating channel flow
Authors: MATSUBARA M. , ALFREDSSON P. H.
Published: July 2002

Experiments on rotating channel flow, where both the primary (induced
by a Coriolis
instability) and the secondary instability are triggered independently,
are described,
focusing on the development of a secondary instability consisting of high-frequency

travelling waves and their subsequent breakdown. Detailed hot-wire velocity
measurements
of the secondary disturbance are made and the phase speed and growth
rate for various frequencies are determined. It is shown that the frequency
of highest
growth rate is close to that which is observed for naturally developing
flow. Some
information on the later stages in the transition process is obtained from
frequency
spectra, which show that interaction between various modes gives rise to
stochastic
low-frequency disturbances, which may play an important role in the transition

process. A theoretical model of the disturbance structure is described
which is used
to explain some of the measured results and also allows the determination
of the
disturbance cross-stream flow field from only streamwise velocity measurements.
Motion of inertial spheroidal particles in a shear
flow near a solid wall with special application to
aerosol transport in microgravity
Authors: GAVZE EHUD , SHAPIRO MICHAEL
Published: July 2002

Trajectories of inertial spheroidal particles moving in a shear

flow near a solid wall are
calculated numerically from the Stokes flow equations by computing the
hydrodynamic
forces and torques acting on the particles. Near the wall these interactions
cause
coupling between the particle's rotational and translational motions.
Due to this
coupling an inertial spheroid is shown to move along an oscillatory trajectory,
while
simultaneously drifting towards the wall. This phenomenon occurs in the
absence of
gravity as a combined effect of three factors: particle non-spherical shape,
its inertia
and particle–wall hydrodynamic interactions. This drift is absent
for inertialess
spheroids, and also for inertial spherical particles which move along flow
streamlines.

The drift velocity is calculated for various particle aspect ratios
γ and relaxation
times τ. An approximate solution, valid for small particle inertia
is developed, which
allows the contribution of various terms to the drift velocity to be elucidated.
It was
found that the maximum value of the drift velocity prevails for
N(γ)γ2τs∼4, where s

is the shear rate and N(γ) is a decreasing function of
γ, related to the particle–wall
hydrodynamic interactions. In the limiting cases of large and small inertia
and also of
very long and thin spheroids, the drift vanishes.

Possible applications of the results are discussed in the context of
transport of
micrometre particles in microgravity conditions. It is shown that the model
used is
applicable for analysis of the deposition of aerosol particles with sizes
above 10 μm
inhaled in the human respiratory tract in the absence of gravity.
The structure of wall-pressure fluctuations in
turbulent boundary layers with adverse pressure
gradient and separation
Authors: NA Y. , MOIN P.
Published: July 2002

Space–time correlations and frequency spectra
of wall-pressure fluctuations, obtained
from direct numerical simulation, are examined to reveal the effects of
pressure
gradient and separation on the characteristics of wall-pressure fluctuations.
In the
attached boundary layer subjected to adverse pressure gradient, contours
of constant
two-point spatial correlation of wall-pressure fluctuations are more elongated
in the
spanwise direction. Convection velocities of wall-pressure fluctuations
as a function
of spatial and temporal separations are reduced by the adverse pressure
gradient.
In the separated turbulent boundary layer, wall-pressure fluctuations are
reduced
inside the separation bubble, and enhanced downstream of the reattachment
region
where maximum Reynolds stresses occur. Inside the separation bubble, the
frequency
spectra of wall-pressure fluctuations normalized by the local maximum Reynolds
shear
stress correlate well compared to those normalized by free-stream dynamic
pressure,
indicating that local Reynolds shear stress has more direct influence on
the wall-pressure spectra. Contour plots of two-point correlation of wall-pressure
fluctuations
are highly elongated in the spanwise direction inside the separation bubble,
implying
the presence of large two-dimensional roller-type structures. The convection
velocity
determined from the space–time correlation of wall-pressure fluctuations
is as low
as 0.33U0 (U0 is the maximum inlet

velocity) in the separated zone, and increases downstream of reattachment.
Sound transmission in slowly varying circular
and annular lined ducts with flow
Authors: RIENSTRA S. W.
Published: July 2002

Sound transmission through straight circular ducts with a uniform
inviscid mean
flow and a constant acoustic lining (impedance wall) is classically described
by a
modal expansion. A natural extension for ducts with axially slowly varying
properties
(diameter and mean flow, wall impedance) is a multiple-scales solution.
It is shown
in the present paper that a consistent approximation of boundary condition
and
isentropic mean flow allows the multiple-scales problem to have an exact
solution.
Since the calculational complexities are no greater than for the classical
straight
duct model, the present solution provides an attractive alternative to
a full numerical
solution if diameter variation is relevant. A unique feature of the present
solution
is that it provides a systematic approximation to the hollow-to-annular
cylinder
transition problem in the turbofan engine inlet duct.
Turbulent gravitational convection from a point
source in a non-uniformly stratified environment
Authors: CAULFIELD C. P. , WOODS ANDREW W.
Published: July 2002

We examine the turbulent gravitational convection which develops
above a point
source of buoyant fluid in a stably stratified environment in which the
buoyancy
frequency varies with height according to
N2=N2s
(z/zs)β. This
generalizes the
classical model of turbulent buoyant plumes rising through uniform and
uniformly
stratified environments originally developed by Morton et al.
(1956). By analogy,
the height of rise of a plume with initial buoyancy flux Fs

has the form Hp=
Apεp−1/2Fs1/4Ns−3/4hp
(λ, β) where εp is the entrainment
constant
for plume motion, Ap
is an O(1) constant, and the non-dimensional plume height,
hp is a function of
&λ=Apεp−1/2Fs1/4Ns−3/4/zs

and β.

In the case β>0, the stratification becomes progressively stronger
with height, and
so plumes are always confined within a finite distance above the origin.
Furthermore,
the non-dimensional height of rise h decreases with λ.

In contrast, in the case β<0, the
stratification becomes progressively weaker with height, and so the non-dimensional

plume height increases monotonically with λ. For slowly decaying
stratification,
β>−8/3, the motion is confined within a finite distance
above the source.
However, for each value of β with β<−8/3, there
is a critical value
λc(β) such that for λ<λc

a plume is confined to a region near the source while for
λ[ges ]λc the motion
is unbounded. In the unbounded case, the motion asymptotes to the solution
for
a buoyant plume rising through a uniform environment, with asymptotic buoyancy

flux F∞(λ)<Fs.

We show that in the limiting case λ=λc,
dividing bounded and
unbounded motion, as z→∞ the plume asymptotes to a
new similarity solution
of the second kind which describes the motion of a plume in a non-uniformly

stratified environment. These similarity solutions are unstable in the
sense that small
perturbations to the initial conditions result in very different behaviour
far from the
source.

Analogous results for an instantaneous release of buoyant fluid from
a point
source, which forms a thermal, are also presented. The model is applied
to describe
the motion of plumes and thermals in the upper ocean and in naturally ventilated

buildings since in both cases the stratification is typically non-uniform.
Withdrawal from a two-layer inviscid fluid in a duct
Authors: FORBES LAWRENCE K. , HOCKING GRAEME C.
Published: July 2002

The steady simultaneous withdrawal of two inviscid fluids of different
densities in
a duct of finite height is considered. The flow is two-dimensional, and
the fluids
are removed by means of a line sink at some arbitrary position within the
duct.
It is assumed that the interface between the two fluids is drawn into the
sink, and
that the flow is uniform far upstream. A numerical method based on an integral

equation formulation yields accurate solutions to the problem, and it is
shown that
under normal operating conditions, there is a solution for each value of
the upstream
interface height. Numerical solutions suggest that limiting configurations
exist, in
which the interface is drawn vertically into the sink. The appropriate
hydraulic
Froude number is derived for this situation, and it is shown that a continuum
of
solutions exists that are supercritical with respect to this Froude number.
An isolated
branch of subcritical solutions is also presented.
Shear instability in shallow water
Authors: BALMFORTH N. J.
Published: July 2002

This study considers the linear stability of shear flows in shallow water. It explores
instabilities related to the classical incompressible (Rayleigh) instability, and those
caused by the over-reflection of surface gravity waves. Numerical solutions of the
linear stability problem are presented, together with analytical arguments elucidating
the role of finite potential vorticity gradients. The slow development of marginally
unstable modes is considered for almost inviscid flows. This is described by an
evolution equation for the amplitude of the unstable mode, coupled to a critical-layer
potential vorticity equation. This reduced system presents a compact description of
the linear stability problem and allows exploration of viscous effects.
Linear pulse structure and signalling in
a film flow on an inclined plane
Authors: BREVDO LEONID , LAURE PATRICE , DIAS FREDERIC , BRIDGES THOMAS J.
Published: July 2002

The film flow down an inclined plane has several features that make it an interesting
prototype for studying transition in a shear flow: the basic parallel state is an exact
explicit solution of the Navier–Stokes equations; the experimentally observed transition
of this flow shows many properties in common with boundary-layer transition;
and it has a free surface, leading to more than one class of modes. In this paper,
unstable wavepackets – associated with the full Navier–Stokes equations with viscous
free-surface boundary conditions – are analysed by using the formalism of absolute
and convective instabilities based on the exact Briggs collision criterion for multiple
k-roots of D(k, ω) = 0; where k is a wavenumber,
ω is a frequency and D(k, ω) is the
dispersion relation function.

The main results of this paper are threefold. First, we work with the full
Navier–Stokes equations with viscous free-surface boundary conditions, rather than a
model partial differential equation, and, guided by experiments, explore a large region of the
parameter space to see if absolute instability – as predicted by some model equations
– is possible. Secondly, our numerical results find only convective instability, in
complete agreement with experiments. Thirdly, we find a curious saddle-point bifurcation which
affects dramatically the interpretation of the convective instability. This is the first finding
of this type of bifurcation in a fluids problem and it may have implications for the
analysis of wavepackets in other flows, in particular for three-dimensional instabilities.
The numerical results of the wavepacket analysis compare well with the available experimental
data, confirming the importance of convective instability for this problem.

The numerical results on the position of a dominant saddle point obtained by
using the exact collision criterion are also compared to the results based on a
steepest-descent method coupled with a continuation procedure for tracking convective
instability that until now was considered as reliable. While for two-dimensional
instabilities a numerical implementation of the collision criterion is readily available,
the only existing numerical procedure for studying three-dimensional wavepackets
is based on the tracking technique. For the present flow, the comparison shows a
failure of the tracking treatment to recover a subinterval of the interval of unstable
ray velocities V whose length constitutes 29% of the length of the entire unstable
interval of V. The failure occurs due to a bifurcation of the saddle point, where V
is a bifurcation parameter. We argue that this bifurcation of unstable ray velocities
should be observable in experiments because of the abrupt increase by a factor of
about 5.3 of the wavelength across the wavepacket associated with the appearance of
the bifurcating branch. Further implications for experiments including the effect on
spatial amplification rate are also discussed.
The turbulent boundary layer over transverse square cavities
Authors: DJENIDI L. , ELAVARASAN R. , ANTONIA R. A.
Published: July 2002

Laser-induced uorescence (LIF) and laser Doppler velocimetry (LDV) are used
to explore the structure of a turbulent boundary layer over a wall made up of
two-dimensional square cavities placed transversely to the flow direction. There is
strong evidence of occurrence of outflows of fluid from the cavities as well as inflows
into the cavities. These events occur in a pseudo-random manner and are closely
associated with the passage of near-wall quasi-streamwise vortices. These vortices and
the associated low-speed streaks are similar to those found in a turbulent boundary
layer over a smooth wall. It is conjectured that outflows play an important role in
maintaining the level of turbulent energy in the layer and enhancing the approach
towards self-preservation. Relative to a smooth wall layer, there is a discernible
increase in the magnitudes of all the Reynolds stresses and a smaller streamwise
variation of the local skin friction coefficient. A local maximum in the Reynolds
shear stress is observed in the shear layers over the cavities.
Surface-tension-driven flow in fat fluid wedges and cones
Authors: BILLINGHAM JOHN
Published: July 2002

We consider the evolution under the action of surface tension of wedges and cones of
viscous fluid whose initial semi-angles are close to π/2. A short time after the fluid is
released from rest, there is an inner region, where surface tension and viscosity dominate,
and an outer region, where inertia and viscosity dominate. We also find that the
velocity of the tip of the wedge or cone is singular, of
O(log(1/t)), as time, t, tends to
zero. After a long time, the free surface asymptotes to a similarity form where deformations
are of O(t2/3), and capillary
waves propagate away from the tip. However, a
distance of O(t3/4) away from the tip,
viscosity acts to damp out the capillary waves.

We solve the linearized governing equations using double integral transforms, which
we calculate numerically, and use asymptotic techniques to approximate the solutions
for small and large times. We also compare the asymptotic solution for the inviscid
fat wedge with a numerical solution of the nonlinear inviscid problem for wedges of
arbitrary semi-angle.
On the influence of centrifugal buoyancy on rotating convection
Authors: HART J. E.
Published: July 2002

Centrifugal buoyancy forces in a container rotating about an axis aligned with
gravity lead to mean flows that interact with rotating convection. A model of the
interactions between these flows and the thermal instabilities that occur in weakly
nonlinear rapidly rotating convection is used to estimate when, in terms of external
parameters, centrifugal buoyancy has a substantial influence on thermal convective
instability. The significant physical effects include the direct action of centrifugal
buoyancy on the eddies themselves, the upwards advection of basic-state vertical
shear by the perturbation rolls, and the alteration of the mean thermal stratification
upon which eddies grow by the basic centrifugally induced circulation. It is shown
that the first effect is the most important for common laboratory settings, and can
lead to destabilization of the system at outer radii. Other manifestations of centrifugal
buoyancy include the generation of a positive offset of the mean temperature at the
centre of the cell, and a reduction of this offset by heat fluxes arising from the
centrifugally modified finite-amplitude convective eddies.
African business systems in a globalising world
Authors: Pedersen Poul Ove , McCormick Dorothy
Published: July 2002

The failure of structural adjustment programmes to promote industrialisation

in Africa may be at least partly explained by the fragmentation of African

business systems. In Africa, the parastatal, foreign-dominated formal and

indigenous informal sectors are poorly integrated, largely as a result
of the
institutional environment in which they have developed. The lack of supportive

financial, state and social institutions inhibits trust and accountability,
and
impedes the access to capital, labour market flexibility, and sub-contracting,

which are needed for modern industrial development. More research is
needed, both detailed studies of business systems in individual African

countries, and cross-country comparisons of the linkages between the economy

and the wider social and institutional environment.
The Boltzmann relation in electronegative plasmas: When is it permissible to use it?
Authors: FRANKLIN R. N. , SNELL J.
Published: July 2002

This paper reports the results of computations to obtain the spatial
distributions of the charged particles in a bounded active plasma dominated by
negative ions. Using the fluid model with a constant collision frequency for
electrons, positive ions and negative ions the cases of both detachment-dominated gases (such as oxygen) and recombination-dominated gases (such as
chlorine) are examined. It is concluded that it is valid to use a Boltzmann relation
ne = ne0exp(eV/kT)
for the electrons of density ne, where the
temperature T is approximately the electron temperature Te, and that the
density nn of the negative ions at low pressures obeys
nn = nn0exp(eV/kTn),
where Tn is the negative-ion temperature. However, at high pressure in
detachment-dominated gases where the ratio of negative-ion density to electron
density is constant and greater than unity, and when the attachment rate is
larger than the ionization rate, the negative ions are distributed with the same
effective temperature as the electrons. In all other cases there is no simple
relationship. Thus to put nn/ne = const,
nn = ne0exp(eV/kTe) and
nn = nn0exp(eV/kTn)
simultaneously is mathematically inconsistent and physically
unsound. Accordingly, expressions deduced for ambipolar diffusion coefficients
based on these assumptions have no validity. The correct expressions for the
situation where nn/ne = const
are obtained without invoking a Boltzmann relation for the negative ions.
Generation of Bernstein modes in space and
laboratory plasmas by injection of an electron beam
Authors: LIZUNOV GEORGYI V.
Published: July 2002

The theory of electron-beam interaction with Bernstein plasma modes
is
developed for conditions of active space plasma experiments as well as
experiments in large vacuum chambers. Translational movement of the beam
along a magnetic field and Larmor rotation of the beam are considered as
sources of free energy for wave build-up. Part of the energy of translational
movement is lost to the excitation of Langmuir plasma oscillations. Bernstein
modes then slowly grow owing to a ‘fan’-type instability.
In this case the
emission spectrum consists of lines near the half-harmonics of the electron
gyrofrequency, ≈(n+½)ωc. Larmor
gyration of the beam is not destroyed by
the excitation of Langmuir turbulence, and thus there is strong hydrodynamic
interaction of the rotated beam with the Bernstein modes. Integer harmonics
of gyrofrequency nωc are generated in
this
case, with the spectral maximum near the upper-hybrid plasma frequency.
Plasma dynamics with electron-inertia effects
Authors: SHIVAMOGGI BHIMSEN K.
Published: July 2002

Some aspects of plasma dynamics with electron-inertia effects are
considered.
The behaviour of (i) one-dimensional steady current sheets, (ii) two-dimensional
steady reconnection models, (iii) one-dimensional current-sheet evolution,
(iv)
linear tearing-mode evolution and (v) nonlinear magnetic-island evolution
in
this regime is investigated. Stability of the two-dimensional plasma in
this
regime is then discussed, and necessary conditions for linear
stability in the Liapunov sense for steady states are then given.
Behaviour of phase-space holes in 2D simulations
Authors: FIJALKOW ERIC
Published: July 2002

This work shows creation of phase-space holes when an initially
perturbed plasma
evolves in time, in the absence of external magnetic fields. When such
a field is
applied to the plasma ab initio, these structures are not created. If such
hole
structures are created with the magnetic field turned off, the holes are
destroyed under
the influence of a magnetic field when one is applied to the plasma.
Reflection and transmission at the boundary between two counterstreaming
MHD plasmas – active boundaries or negative-energy waves?
Authors: WALKER A. D. M.
Published: July 2002

A magnetohydrodynamic (MHD) wave, incident on the boundary between
two MHD media in relative motion, may be amplified or attenuated by exchanging
energy with the kinetic energy of the background flow. The conventional treatment
of this problem uses a definition of the wave energy such that energy is conserved
at the boundary. An amplified reflected wave then leads to the requirement of a
transmitted wave carrying negative energy. Such an approach, while producing
correct results, obscures the nature and location of the energy interchange. In this
paper, the proper definitions of energy density and flux in a moving plasma are
discussed, and the relationship of the group velocity and the energy flow is clarified.
The mechanism by which energy is exchanged between streaming plasma and wave
is through the work done by the Maxwell and Reynolds stresses on the gradient of
velocity at the boundary. The location of the energy exchange is identified as the
active boundary, with no need to invoke ideas of negative energy. The relationship
between the two approaches is critically discussed.
The first evidence of offshore spawning in the squid species Loligo forbesi
Authors: Lordan C. , Casey J.
Published: July 2002

Records of the squid (Loligo forbesi) egg masses which were incidentally caught in demersal trawls at three offshore sites are presented. These sites were at water depths of 135, 302 and 507 m. This represents the deepest ever record of spawning in a loliginid squid species. There has been speculation in the literature about offshore spawning in L. forbesi but it has not previously been recorded and may have important implications for the life cycle.