bluuebunny/crossref_metadata_2025 huggingface dataset:
Modification of postmilking standing time by altering feed availability
Authors: TYLER JEFF W. , STEEVENS BARRY J. , HOLLE JULIE M. , CROUCHER SEAN M. , ANDERSON KERRI L.
Published: July 2002

Control of environmental mastitis remains a problem on many modern
dairy
farms. These infections are often transmitted between milkings, and milking
hygiene
will not prevent new infection. Consequently, the control of environmental
mastitis
has prompted dairy managers to develop new approaches that either limit
bacterial
contamination of teat ends between milkings or directly increase the cow's
resistance
to infection. Intervention strategies that may decrease the incidence of
environmental
mastitis include improved sanitation of housing areas, decreased water
use in udder preparation, optimal dietary concentrations of vitamin E and
selenium,
and use of R-mutant vaccines (Smith et al. 1984, 1985;
Weiss et al. 1990; Smith & Hogan, 1993; Tyler et al.
1993).

Dairy managers are encouraged to provide cows with clean, fresh, palatable
feed
immediately after milking. This practice is thought to provide cows with
an incentive
to remain standing for an extended interval, permit teat sphincters to
close, and limit
teat end contamination and new infections when cows lie down. In one recent
study,
cows that had access to feed remained standing for a significantly longer
time than
did cows that were denied access to feed (48 v.
21 min; Tyler et al. 1997). The purpose
of the present study was to substantiate that feed availability could be
used to
extend postmilking standing time in a larger population of cows maintained
under
different management conditions.
Formation of sand bars under surface waves
Authors: YU JIE , MEI CHIANG C.
Published: July 2002

A quantitative theory is described for the formation mechanism of sand bars under
surface water waves. By assuming that the slopes of waves and bars are comparably
gentle and sediment motion is dominated by the bedload, an approximate evolution
equation for bar height is derived. The wave field and the boundary layer structure
above the wavy bed are worked out to the accuracy needed for solving this evolution
equation. It is shown that the evolution of sand bars is a process of forced diffusion.
This is unlike that for sand ripples which is governed by an instability. The forcing is
directly caused by the non-uniformity of the wave envelope, hence of the wave-induced
bottom shear stress associated with wave reflection, while the effective diffusivity
is the consequence of gravity and modified by the local bed stress. During the
slow formation, bars and waves affect each other through the Bragg scattering
mechanism, which consists of two concurrent processes: energy transfer between
waves propagating in opposite directions and change of their wavelengths. Both
effects are found to be controlled locally by the position of bar crests relative to wave
nodes. Comparison with available laboratory experiments is discussed and theoretical
examples are studied to help understand the coupled evolution of bars and waves in
the field.
On the reflection and transmission of sound in a thick shear layer
Authors: CAMPOS L. M. B. C. , KOBAYASHI M. H.
Published: July 2002

The propagation of sound across a shear layer of finite thickness is studied using
exact solutions of the acoustic wave equation for a shear flow with hyperbolic-tangent velocity profile. The wave equation has up to four regular singularities: two
corresponding to the upper and lower free streams; one corresponding to a critical
layer, where the Doppler-shifted frequency vanishes if the free streams are supersonic;
and a fourth singularity which is always outside the physical region of interest. In
the absence of a critical layer the matching of the two solutions, around the upper
and lower free streams, specifies exactly the acoustic field across the shear layer. For
example, for a sound wave incident from below (i.e. upward propagation in the lower
free stream), the reflected wave (i.e. downward propagating in the lower free stream)
and the transmitted wave (i.e. upward propagating in upper free stream) are specified
by the continuity of acoustic pressure and vertical displacement. Thus the reflection
and transmission coefficients, which are generally complex, i.e. involve amplitude
and phase changes, are plotted versus angle of incidence for several values of free
stream Mach number, and ratio of thickness of the shear layer to the wavelength;
the vortex sheet is the particular case when the latter parameter is zero. The modulus
and phase of the total acoustic field are also plotted versus the coordinate transverse
to the shear flow, for several values of angle of incidence, Mach number and shear
layer thickness. The analysis and plots in the present paper demonstrate significant
differences between sound scattering by a shear layer of finite thickness, and the
limiting case of the vortex sheet.
The persistence of regular reflection during strong shock diffraction over rigid ramps
Authors: HENDERSON L. F. , TAKAYAMA K. , CRUTCHFIELD W. Y. , ITABASHI S.
Published: July 2002

We report on calculations and experiments with strong shocks diffracting over rigid
ramps in argon. The numerical results were obtained by integrating the conservation
equations that included the Navier–Stokes equations. The results predict that if the
ramp angle θ is less than the angle θe that corresponds to the detachment of a
shock, θ < θe, then the onset of Mach reflection (MR) will be delayed by the initial
appearance of a precursor regular reflection (PRR). The PRR is subsequently swept
away by an overtaking corner signal (cs) that forces the eruption of the MR which
then rapidly evolves into a self-similar state. An objective was to make an experimental
test of the predictions. These were confirmed by twice photographing the diffracting
shock as it travelled along the ramp. We could get a PRR with the first exposure
and an MR with the second. According to the von Neumann perfect gas theory,
a PRR does not exist when θ < θe. A viscous length scale
xint is a measure of the
position on the ramp where the dynamic transition PRR → MR takes place. It is
significantly larger in the experiments than in the calculations. This is attributed to
the fact that fluctuations from turbulence and surface roughness were not modelled
in the calculations. It was found that xint
→ ∞ as θ → θe. Experiments were done to find out how
xint depended on the initial shock tube pressure p0. The dependence was
strong but could be greatly reduced by forming a Reynolds number based on xint.
Finally by definition, regular reflection (RR) never interacts with a boundary layer,
while PRR always interacts; so they are different phenomena.
Gentle spilling breakers: crest flow-field evolution
Authors: QIAO HAIBING , DUNCAN JAMES H.
Published: July 2002

The flow fields at the crests of mechanically generated gentle spilling breakers are
studied experimentally with particle image velocimetry (PIV). During the initial
development of the breaker, a bulge forms on the forward face of the wave near the
crest. The present measurements show that the flow field in this bulge is wavelike in
nature and that the maximum horizontal component of the fluid velocity, μmax, is less
than the wave crest speed. No vorticity is found during this stage of the breaking
process. After a short while, the leading edge (called the toe) of the bulge begins to
move rapidly down the wave crest; μmax reaches the speed of the wave crest just as
the toe begins to move. Concurrent with the beginning of the toe motion, a vortical
region appears at the free surface just downstream of the toe. At about the point in
the wave development where the measurements end, the vortical region extends from
the toe to the crest.
Laminar mixed convection in two-dimensional far wakes above heated/cooled bodies: model and experiments
Authors: EHRHARD PETER
Published: July 2002

A heated or cooled body is positioned in a vertically rising forced flow. This develops
both a kinematic and a thermal wake, the latter adding buoyant effects to the
otherwise forced flow field. An asymptotic model is developed to treat this mixed
convection in both plane and axisymmetric geometry. The model holds for laminar
flow in the boundary layer approximation and uses a far-wake expansion for weak
buoyant forces. For plane geometry the model is validated against both experiments
in water and FEM simulations.

It is found for a heated wake that buoyant forces accelerate the fluid in the thermal
wake such that the vertical velocity deficit in the kinematic wake is reduced. For strong
heating this may even lead to vertical velocities larger than the forced flow amplitude.
In conjunction the entrainment is intensified in a heated wake. The effects in a cooled
wake are opposite in that the vertical velocity deficit is increased within the thermal
wake and the horizontal flow into the wake is weakened. In a strongly cooled wake
the horizontal flow may even invert, going from the wake centre into the ambient.
The Prandtl number controls the width of the thermal wake and, thus, the portion
of the kinematic wake which is affected by buoyant forces. Large Prandtl numbers
superimpose a narrow buoyant plume, small Prandtl numbers a wide buoyant plume,
onto the kinematic wake.
Spin-up of a two-layer rotating stratified fluid in a variable-depth container
Authors: HEWITT R. E. , FOSTER M. R. , DAVIES P. A.
Published: July 2002

We consider the spin-up of a two-layer, stably (density) stratified fluid in a rotating
container with an axisymmetric sloping base and cylindrical walls. Details of the spin-
up readjustment mechanisms are presented under the assumption of small impulsive
changes in the rotation rate of the container. It is shown that the relative positions
of the density interface and the discontinuity in wall slope determine the qualitative
large-time spin-up response of the fluid. The density interface leads to a spin-up
readjustment in each of the fluid layers that is essentially independent. However,
when the density interface is below the boundary-slope discontinuity, a sub-region
of the upper layer is predicted to readjust in an algebraic rather than exponential
manner. A detailed sequence of laboratory experiments have been performed to
confirm the predictions of the linear spin-up analysis.
Vortex merger in rotating stratified flows
Authors: DRITSCHEL DAVID G.
Published: July 2002

This paper describes the interaction of symmetric vortices in a three-dimensional

quasi-geostrophic fluid. The initial vortices are taken to be uniform-potential-vorticity

ellipsoids, of height 2h and width 2R, and with centres at (±d/2; 0, 0), embedded within

a background flow having constant background rotational and buoyancy frequencies,

f/2 and N respectively. This problem was previously studied by von Hardenburg et al.

(2000), who determined the dimensionless critical merger distance d/R as a function

of the height-to-width aspect ratio h/R (scaled by f/N). Their study, however, was

limited to small to moderate values of h/R, as it was anticipated that merger at large

h/R would reduce to that for two columnar two-dimensional vortices, i.e. d/R ≈ 3.31.

Here, it is shown that no such two-dimensional limit exists; merger is found to occur

at any aspect ratio, with d ∼ h for h/R [Gt ] 1.

New results are also found for small to moderate values of h/R. In particular,

our numerical simulations reveal that asymmetric merger is predominant, despite

the initial conditions, if one includes a small amount of random noise. For small

to moderate h/R, decreasing the initial separation distance d first results in a weak

exchange of material, with one vortex growing at the expense of the other. As d

decreases further, this exchange increases and leads to two dominant but strongly

asymmetric vortices. Finally, for yet smaller d, rapid merger into a single dominant

vortex occurs – in effect the initial vortices exchange nearly all of their material with

one another in a nearly symmetrical fashion.
The thin liquid lining of a weakly curved cylindrical tube
Authors: JENSEN O. E.
Published: July 2002

A thin-film approximation is used to study the effects of
surface tension on a thin liquid layer lining the interior of a
cylindrical tube, where the tube has radius a and a
centreline with weak, uniform curvature δ/a.
Centreline curvature induces a pressure gradient in the fluid layer,
analogous to that due to a weak gravitational field, that drives
fluid from the inner to the outer wall of the tube, i.e. away from
the centre of centreline curvature. The resulting draining flow is
computed numerically under the assumption of axial uniformity, and
the large-time asymptotic draining regimes and flow structures are
identified. In the absence of destabilizing intermolecular
interactions, the inner wall remains wet, covered with a vanishingly
thin fluid layer, while a near-equilibrium lobe forms on the outer
wall. The stability of this quasi-static lobe to axial variations is
then investigated by using numerical and perturbation methods to
solve the linearized Young–Laplace equation, prescribing zero
contact angle at the lobe's free boundary. Conditions on
δ, the fluid volume a3V and the
tube length aL are identified separating axially uniform
lobes (which are stable for low V/(δL) or
small L), wavy lobes (some with a solitary structure) and
localized fluid droplets (which exist for sufficiently large
V/δ and L). Hysteresis is demonstrated
between multiple equilibria, the topology of which can change
dramatically as parameters are varied. The application of these
results to lung airways is discussed.
Three-dimensional ribbing instability in symmetric forward-roll film-coating processes
Authors: Castillo Mariano E. Gurfinkel , Patera Anthony T.
Published: July 2002

We propose a framework for interpreting the formation, evolution, and spatial persistence of ribbing in coating processes, and present companion ‘quantifying’ parallel spectral element simulations of fully nonlinear unsteady three-dimensional free-surface symmetric forward-roll film-coating fluid flows. The framework couples, by means of a transition region, two well-understood phenomena: the ‘viscous fingering’ instability of a splitting meniscus; and the levelling of viscous films under the effect of surface tension. The transition-region length, Lt, is on the order of the coating film thickness, while the downstream extent of the levelling region – the distance over which ribs persist – Lt, depends on the fluid properties, the flow conditions, and the wavenumber content of the nonlinear meniscus rib profile. Numerical results are presented for the evolution of the coating flow from perturbed unstable two-dimensional steady states to three-dimensional saturated ribbed states for several representative supercritical capillary numbers, Ca, and spanwise periodicity lengths, b. Nonlinear state selection is briefly discussed.
On the motion of gas bubbles in homogeneous isotropic turbulence
Authors: SPELT P. D. M. , BIESHEUVEL A.
Published: July 2002

This paper is concerned with the motion of small gas bubbles, equivalent

diameter about 1.0 mm, in isotropic turbulent flows. Data on the mean velocity
of

rise and the dispersion of the bubbles have been obtained numerically by
simulating

the turbulence as a sum of Fourier modes with random phases and amplitudes

determined by the Kraichnan and the von Kármán–Pao
energy-spectrum

functions, and by calculating

the bubble trajectories from a reasonably well-established equation of
motion. The

data cover the range β[les ]1, where β is the ratio between the

turbulence intensity and

the velocity of rise of the bubbles in still fluid. An approximate analysis
based

on the assumption that β is small yields results that compare favourably
with

the numerical data, and clarifies the important role played by the lift
forces

exerted by the fluid.
Oscillatory enhancement of the squeezing flow of
yield stress fluids: a novel experimental result
Authors: ZWICK K. J. , AYYASWAMY P. S. , COHEN I. M.
Published: July 2002

The extrusion of a yield stress fluid from the space between two
parallel plates is
investigated experimentally. Oscillating the magnitude of the squeezing force about
a mean value (F=f[1+αcos(ωt)])
was observed to significantly enhance the flow
rate of yield stress fluids, while having no effect on the flow rate of
Newtonian fluids.
This is a novel result. The enhancement depends on the magnitude of the force, the
oscillatory frequency and amplitude, the fluid being squeezed, and the thickness of
the fluid layer. Non-dimensional results for the various flow quantities have been
presented by using the flow predicted for the constant-force squeezing of a
Herschel–Bulkley yield stress fluid as the reference. In the limit of
constant-force squeezing, the
present experimental results compare very well with those of our earlier theoretical
model for this situation (Zwick, Ayyaswamy & Cohen 1996). The results presented
in this paper have significance, among many applications, for injection moulding, in
the adhesive bonding of microelectronic chips, and in surgical procedures employed
in health care.
Snakes and corkscrews in core–annular

down-flow of two fluids
Authors: RENARDY YURIKO Y.
Published: July 2002

Core–annular flow of two fluids is examined at the onset
of

a non-axisymmetric

instability. This is a pattern selection problem: the bifurcating solutions

are travelling

waves and standing waves. The former travel in the azimuthal direction
as well as the

axial direction and would be observed as corkscrew waves. The standing
waves travel

in the axial direction but not in the azimuthal direction and appear

as snakes. Weakly

nonlinear interactions are studied to see whether one of these

waves will be stable to

small-amplitude perturbations. Sample situations for down-flow are discussed.
The

corkscrews tend to be preferred when the annulus is narrow, while snakes
are more

likely when the annulus is wide.
Opposing mixed convection flow in a wall jet

over a horizontal plate
Authors: HIGUERA F. J.
Published: July 2002

The coupling of the temperature and velocity fields by

buoyancy in a laminar two-dimensional wall jet over a finite-length

horizontal plate is studied numerically and

analytically in the asymptotic limit of infinite Reynolds number. Two configurations

are considered leading to a cold layer of fluid over the plate, namely
an

ambient-temperature jet over a cooled plate and a cold jet over an

insulated plate. In both cases

buoyancy generates an adverse pressure gradient that may separate the flow
if the

Froude number is sufficiently small and always makes the solution everywhere
over

the plate dependent on the conditions at the downstream boundary. In the
limit of

very small Froude number separation occurs in a viscous–inviscid

interaction region near the origin of the jet, leading to a separation
bubble that

covers a fraction of the plate dependent on the Prandtl number. The scalings
of the

solution in this asymptotic

limit are obtained by order of magnitude estimations in the different regions
of the

bubble and in the buoyancy-dominated flow beyond the bubble, and the results
are

checked against the numerical solutions of the boundary layer equations.
A separate

analysis is carried out for very large Prandtl numbers showing

that the recirculation

bubble is then much shorter than the plate, also in agreement with the
numerical

results.
Direct simulations of turbulent flow in a pipe
rotating about its axis
Authors: ORLANDI P. , FATICA M.
Published: July 2002

Flow in a circular pipe rotating about its axis, at low Reynolds
number, is investigated.
The simulation is performed by a finite difference scheme, second-order accurate in
space and in time. A non-uniform grid in the radial direction
yields accurate solutions
with a reasonable number of grid points. The numerical method has been tested for
the non-rotating pipe in the limit ν→0 to prove the energy
conservation properties.
In the viscous case a grid refinement check has been performed and some conclusions
about drag reduction have been reached. The mean and turbulent quantities have
been compared with the numerical and experimental non-rotating pipe data of Eggels
et al. (1994a, b). When the pipe rotates, a
degree of drag reduction is achieved in the
numerical simulations just as in the experiments. Through the visualization of the
vorticity field the drag reduction has been related to the modification
of the vortical
structures near the wall. A comparison between the vorticity in the non-rotating
and in the high rotation case has shown a spiral motion leading to the transport of
streamwise vorticity far from the wall.
Direct simulation of the motion of solid particles in
Couette and Poiseuille flows of viscoelastic fluids
Authors: HUANG P. Y. , FENG J. , HU H. H. , JOSEPH D. D.
Published: July 2002

This paper reports the results of direct numerical simulation
of the motion of a two-dimensional circular cylinder in Couette flow
and in Poiseuille flow of an Oldroyd-B
fluid. Both neutrally buoyant and non-neutrally buoyant cylinders are considered. The
cylinder's motion and the mechanisms which cause the cylinders
to migrate are studied.
The stable equilibrium position of neutrally buoyant particles varies with inertia,
elasticity, shear thinning and the blockage ratio of the channel in both shear flows.
Shear thinning promotes the migration of the cylinder to the
wall while inertia causes
the cylinder to migrate away from the wall. The cylinder
moves closer to the wall in a
narrower channel. In a Poiseuille flow, the effect of elastic
normal stresses is manifested
by an attraction toward the nearby wall if the blockage is
strong. If the blockage is
weak, the normal stresses act through the curvature of the inflow
velocity profile and
generate a lateral force that points to the centreline. In both cases,
the migration of
particles is controlled by elastic normal stresses which in the limit
of slow flow in two
dimensions are compressive and proportional to the square of the shear rate on the
body. A slightly buoyant cylinder in Couette flow migrates
to an equilibrium position
nearer the centreline of the channel in a viscoelastic fluid
than in a Newtonian fluid. On
the other hand, the same slightly buoyant cylinder in Poiseuille
flow moves to a stable
position farther away from the centreline of the channel in a
viscoelastic fluid than in
a Newtonian fluid. Marked effects of shear thinning are documented and discussed.
Long-wavelength surface-tension-driven Bénard

convection: experiment and theory
Authors: VANHOOK STEPHEN J. , SCHATZ MICHAEL F. , SWIFT J. B. , MCCORMICK W. D. , SWINNEY HARRY L.
Published: July 2002

Surface-tension-driven Bénard (Marangoni) convection

in liquid layers heated from

below can exhibit a long-wavelength primary instability that differs from
the more

familiar hexagonal instability associated with Bénard. This

long-wavelength instability

is predicted to be significant in microgravity and for thin liquid

layers. The instability

is studied experimentally in terrestrial gravity for silicone oil layers

0.007 to 0.027 cm

thick on a conducting plate. For shallow liquid depths (<.017 cm for

0.102 cm2 s−1 viscosity liquid), the

system evolves to a strongly deformed long-wavelength

state which can take the form of a localized depression (‘dry spot’)

or a localized elevation (‘high spot’), depending on the thickness

and thermal conductivity of the

gas layer above the liquid. For slightly thicker liquid depths

(0.017–0.024 cm),

the formation of a dry spot induces the formation of hexagons. For even
thicker

liquid depths (>0.024 cm), the system forms only the hexagonal convection
cells.

A two-layer nonlinear theory is developed to account properly for the effect
of

deformation on the interface temperature profile. Experimental results
for the

long-wavelength instability are compared to our two-layer theory and to
a one-layer

theory that accounts for the upper gas layer solely with a heat transfer
coefficient.

The two-layer model better describes the onset of instability and also
predicts the

formation of localized elevations, which the one-layer model does not predict.
A

weakly nonlinear analysis shows that the bifurcation is subcritical.

Solving for steady states of the system shows that the subcritical pitchfork

bifurcation curve never turns

over to a stable branch. Numerical simulations also predict a subcritical
instability

and yield long-wavelength states that qualitatively agree with the experiments.
The

observations agree with the onset prediction of the two-layer model, except
for very

thin liquid layers; this deviation from theory may arise from small non-uniformities

in the experiment. Theoretical analysis shows that a small non-uniformity
in heating

produces a large steady-state deformation (seen in the experiment) that
becomes

more pronounced with increasing temperature difference across the liquid.
This

steady-state deformation becomes unstable to the long-wavelength instability
at a

smaller temperature difference than that at which the undeformed state
becomes

unstable in the absence of non-uniformity.
Influence of mean loading on noise generated by

the interaction of gusts with a flat-plate cascade: upstream radiation
Authors: PEAKE N. , KERSCHEN E. J.
Published: July 2002

The sound generated by the interaction between convected

vortical and entropic

disturbances and a blade row is a significant component of the total noise
emitted

by a modern aeroengine, and the blade geometry has an important effect
on this

process. As a first step in the development of a general prediction scheme,
we model

in this paper just the action of the blade mean loading by treating the
blades as

flat plates aligned at a non-zero incidence angle, δ,

to the oncoming stream, and

consider harmonic components of the incident field with reduced

frequency k. We then use asymptotic analysis in the realistic
limit

k[Gt ]1, δ[Lt ]1 with kδ=O(1)

to make a consistent asymptotic expansion of the compressible Euler equations.

The flow is seen to consist of inner regions around each leading edge,
in which

sound is generated by the local gust–airfoil and gust–flow

interactions, and an outer

region in which both the incident gust is distorted according to rapid
distortion

theory and the out-going sound is refracted by the non-uniform mean flow.
The

complicated multiple interactions between the sound and the cascade are
included to

the appropriate asymptotic order, and analytical expressions

for the forward radiation

are derived. It is seen that even a relatively small value of δ

can have a significant

effect, thanks to both the O(δk1/2)

change in the amplitudes and the O(kδ) change

in the phases of the various radiation components, corresponding to the
additional

source mechanisms associated with the flow distortion around each leading
edge and

the effects of propagation through the non-uniform flow, respectively.
Further work

will extend this analysis to include the effects of camber and thickness.
Properties of the mean recirculation region in the

wakes of two-dimensional bluff bodies
Authors: BALACHANDAR S. , MITTAL R. , NAJJAR F. M.
Published: July 2002

The properties of the time- and span-averaged mean wake recirculation

region are investigated in separated flows over several different two-dimensional

bluff bodies. Ten different cases are considered and they divide into two
groups:

cylindrical geometries

of circular, elliptic and square cross-sections and the normal plate. A
wide Reynolds

number range from 250 to 140000 is considered, but in all the cases the
attached

portion of the boundary layer remains laminar until separation. The lower
Reynolds

number data are from direct numerical simulations, while the data at the
higher

Reynolds number are obtained from large-eddy simulation and the experimental
work

of Cantwell & Coles (1983), Krothapalli (1996, personal communication),

Leder (1991) and Lyn et al. (1995). Unlike supersonic and subsonic

separations with a splitter plate

in the wake, in all the cases considered here there is strong interaction

between the shear layers resulting in Kármán vortex shedding.
The

impact of this fundamental

difference on the distribution of Reynolds stress components and pressure
in relation

to the mean wake recirculation region (wake bubble) is considered.

It is observed that

in all cases the contribution from Reynolds normal stress to the force

balance of the

wake bubble is significant. In fact, in the cylinder geometries this contribution
can

outweigh the net force from the shear stress, so that the net pressure

force tends to push

the bubble away from the body. In contrast, in the case of normal plate,

owing to the

longer wake, the net contribution from shear stress outweighs that from
the normal

stress. At higher Reynolds numbers, separation of the Reynolds stress components
into

incoherent contributions provides more insight. The behaviour of the coherent

contribution, arising from the dominant vortex shedding, is similar to
that at lower

Reynolds numbers. The incoherent contribution to Reynolds stress, arising
from

small-scale activity, is compared with that of a canonical free shear layer.
Based on

these observations a simple extension of the wake model (Sychev 1982; Roshko

1993a, b) is proposed.
On the chance of freak waves at sea
Authors: WHITE BENJAMIN S. , FORNBERG BENGT
Published: July 2002

When deep-water surface gravity waves traverse an
area with a curved or otherwise
variable current, the current can act analogously to an
optical lens, to focus wave
action into a caustic region. In this region, waves of
surprisingly large size, alternatively
called freak, rogue, or giant waves are produced. We show
how this mechanism
produces freak waves at random locations when ocean swell
traverses an area of
random current. When the current has a constant (possibly
zero) mean with small
random fluctuations, we show that the probability distribution
for the formation of a
freak wave is universal, that is, it does not depend on
the statistics of the current, but
only on a single distance scale parameter, provided that
this parameter is finite and
non-zero. Our numerical simulations show excellent agreement
with the theory, even
for current standard deviation as large as
1.0 m s−1. Since many of these results are
derived for arbitrary dispersion relations with certain
general properties, they include
as a special case previously published work on caustics
in geometrical optics.
Turbulence structures of wall-bounded shear flows found using DNS
data
Authors: CHONG M. S. , SORIA J. , PERRY A. E. , CHACIN J. , CANTWELL B. J. , NA Y.
Published: July 2002

This work extends the study of the structure of wall-bounded flows
using the
topological properties of eddying motions as developed by Chong et
al. (1990),
Soria et al. (1992, 1994), and as recently extended by Blackburn

et al. (1996) and Chacin et al. (1996). In these works,

regions of flow which are focal in nature are identified by
being enclosed by an isosurface of a positive small value of the discriminant
of the
velocity gradient tensor. These regions resemble the attached vortex loops
suggested
first by Theodorsen (1955). Such loops are incorporated in the attached-eddy
model
versions of Perry & Chong (1982), Perry et al. (1986),
and Perry & Marusic (1995),
which are extensions of a model first formulated by Townsend (1976). The
direct
numerical simulation (DNS) data of wall-bounded flows studied here are
from the
zero-pressure-gradient flow of Spalart (1988) and the boundary layer with
separation
and reattachment of Na & Moin (1996). The flow structures are examined
from the
viewpoint of the attached eddy hypothesis.
Generalized Langevin equation for relative turbulent dispersion
Authors: HEPPE BURKHARD M. O.
Published: July 2002

The relative velocity of two fluid particles in homogeneous and
stationary turbulence
is considered. Looking for reduced dynamics of turbulent dispersion, we
apply the
nonlinear Mori–Zwanzig projector method to the Navier–Stokes
equations.
The projector method decomposes the Lagrangian acceleration into a conditionally
averaged
part and a random force. The result is an exact generalized Langevin equation
for
the Lagrangian velocity differences accounting for the exact equation of
the Eulerian
probability density. From the generalized Langevin equation, we obtain
a stochastic
model of relative dispersion by stochastic estimation of conditional averages
and by
assuming the random force to be Gaussian white noise. This new approach
to
dispersion modelling generalizes and unifies stochastic models based on
the well-mixed
condition and the moments approximation. Furthermore, we incorporate viscous

effects in a systematic way. At a moderate Reynolds number, the model agrees

qualitatively with direct numerical simulations showing highly non-Gaussian
separation
and velocity statistics for particle pairs initially close together. At
very large Reynolds
numbers, the mean-square separation obeys a Richardson law with coefficient
of the
order of 0.1.
Gentle spilling breakers: crest profile evolution
Authors: DUNCAN JAMES H. , QIAO HAIBING , PHILOMIN VASANTH , WENZ ALEXANDRA
Published: July 2002

The surface profile histories of gentle spilling breakers generated
mechanically with
a dispersive focusing technique are studied experimentally. Froude-scaled
generation
conditions are used to produce waves with three average frequencies:
f0=1.42, 1.26,
and 1.15 Hz. At each frequency, the strength of the breaker is varied by
varying the
overall amplitude of the wavemaker motion. It is found that in all cases
the beginning
of the breaking process is marked by the formation of a bulge in the profile
at the
crest on the forward face of the wave. The leading edge of this bulge is
called the toe.
As the breaking process continues, the bulge becomes more pronounced while
the toe
remains in nearly a fixed position relative to the crest. Capillary waves
form ahead of
the toe. At a time of about 0.1/f0
after the bulge first becomes visible, the toe begins
to move down the face of the wave and very quickly accelerates to a constant
velocity
which scales with the wave crest speed. During this phase of the breaker
evolution,
the surface profile between the toe and the crest develops ripples which
eventually
are left behind the wave crest. It is found that the height of the toe
above the mean
water level scales with the nominal wavelength
λ0=g/(2πf20)

of the breaker, while
the size and shape of the bulge and the length of the capillary waves ahead
of the
toe are independent of f0.
Two-layer thermal convection in miscible
viscous fluids
Authors: DAVAILLE ANNE
Published: July 2002

The influence of a viscosity stratification on the interaction
between thermal convection
and a stable density discontinuity is studied, using laboratory experiments.

Initially, two superposed isothermal layers of high-Prandtl-number miscible
fluids are
suddenly cooled from above and heated from below. By adjusting the concentrations

of salt and cellulose, Rayleigh numbers between 300 and 3×107

were achieved for density contrasts between 0.45 % and 5 % and
viscosity ratios between 1 and 6.4×104.
Heat and mass transfer through the interface were monitored.

Two-layer convection is observed but a steady state is never obtained
since penetrative
convection occurs. A new interfacial instability is reported, owing to
the
nonlinear interaction of the unstable thermal and stable chemical density
gradients.
As a result, the temperature condition at the interface is highly inhomogeneous,

driving, on top of the classical small-scale thermal convection, a large-scale
flow in each
layer which produces cusps at the interface. Entrainment, driven by viscous
coupling
between the two layers, proceeds through those cusps. The pattern of entrainment
is
asymmetric: two-dimensional sheets are dragged into the more viscous layer,
while
three-dimensional conduits are produced in the less viscous layer. A simple
entrainment
model is proposed and scaling laws for the entrainment rate are derived;
they
explain the experimental data well.
The inertial lift on a spherical particle in a plane
Poiseuille flow at large channel Reynolds number
Authors: ASMOLOV EVGENY S.
Published: July 2002

The inertial migration of a small rigid sphere translating parallel
to the walls within
a channel flow at large channel Reynolds numbers is investigated. The method

of matched asymptotic expansions is used to solve the equations governing
the
disturbance flow past a particle at small particle Reynolds number and
to evaluate
the lift. Both neutrally and non-neutrally buoyant particles are considered.
The wall-induced inertia is significant in the thin layers near the walls where
the lift is close to
that calculated for linear shear flow, bounded by a single wall. In the
major portion
of the flow, excluding near-wall layers, the wall effect can be neglected,
and the outer
flow past a sphere can be treated as unbounded parabolic shear flow. The
effect of
the curvature of the unperturbed velocity profile is significant, and the
lift differs from
the values corresponding to a linear shear flow even at large Reynolds
numbers.
Insoluble surfactants on a drop in an extensional

flow: a generalization of the stagnated surface

limit to deforming interfaces
Authors: EGGLETON CHARLES D. , PAWAR YASHODHARA P. , STEBE KATHLEEN J.
Published: July 2002

A drop in an axisymmetric extensional ow is studied using boundary integral

methods to understand the effects of a monolayer-forming surfactant on a strongly

deforming interface. Surfactants occupy area, so there is an upper bound to the

surface concentration that can be adsorbed in a monolayer, Γ∞.

The surface tension is a highly nonlinear function of the surface concentration

Γ because of this upper

bound. As a result, the mechanical response of the system varies strongly with

Γ for realistic material parameters. In this work, an insoluble surfactant is

considered in the limit where the drop and external fluid viscosities are equal.

For Γ<Γ∞, surface convection sweeps

surfactant toward the drop poles. When surface diffusion is negligible, once the stable drop

shapes are attained, the interface

can be divided into stagnant caps near the drop poles, where

Γ is non-zero, and

tangentially mobile regions near the drop equator, where the surface concentration is

zero. This result is general for any axisymmetric fluid particle. For

Γ near Γ∞, the

stresses resisting accumulation are large in order to prevent the local concentration

from reaching the upper bound. As a result, the surface is highly stressed tangentially

while Γ departs only slightly from a uniform distribution. For this case,

Γ is never zero, so the tangential surface velocity is zero for

the steady drop shape.

This observation that Γ dilutes nearly uniformly for high surface

concentrations is used to derive a simplified form for the surface mass balance that applies in

the limit of high surface concentration. The balance requires that the tangential flux

should balance the local dilatation in order that the surface concentration profile will

remain spatially uniform. Throughout the drop evolution, this equation yields results

in agreement with the full solution for moderate deformations, and underscores the

dominant mechanism at high deformation. The simplified balance reduces to the

stagnant interface condition at steady state.

Drop deformations vary non-monotonically with concentration; for

Γ<Γ∞, the

reduction of the surface tension near the poles leads to higher deformations than the

clean interface case. For Γ near Γ∞,

however, Γ dilutes nearly uniformly, resulting in

higher mean surface tensions and smaller deformations. The drop contribution to the

volume averaged stress tensor is also calculated and shown to vary non-monotonically

with surface concentration.
Drag and lift forces on a rotating sphere in a linear shear flow
Authors: KUROSE RYOICHI , KOMORI SATORU
Published: July 2002

The drag and lift forces acting on a rotating rigid sphere in a
homogeneous linear shear
flow are numerically studied by means of a three-dimensional numerical
simulation.
The effects of both the fluid shear and rotational speed of the sphere
on the drag and
lift forces are estimated for particle Reynolds numbers of
1[les ]Rep[les ]500.

The results show that the drag forces both on a stationary sphere in
a linear shear
flow and on a rotating sphere in a uniform unsheared flow increase with
increasing
the fluid shear and rotational speed. The lift force on a stationary sphere
in a linear
shear flow acts from the low-fluid-velocity side to the high-fluid-velocity
side for low
particle Reynolds numbers of Rep<60,
whereas it acts from the high-velocity side to
the low-velocity side for high particle Reynolds numbers of
Rep>60. The change of
the direction of the lift force can be explained well by considering the
contributions of
pressure and viscous forces to the total lift in terms of flow separation.
The predicted
direction of the lift force for high particle Reynolds numbers is also
examined
through a visualization experiment of an iron particle falling in a linear
shear flow of
a glycerin solution. On the other hand, the lift force on a rotating sphere
in a uniform
unsheared flow acts in the same direction independent of particle Reynolds
numbers.
Approximate expressions for the drag and lift coefficients for a rotating
sphere in a
linear shear flow are proposed over the wide range of
1[les ]Rep[les ]500.
Dynamics of a passive tracer in a velocity field of
four identical point vortices
Authors: BOATTO STEFANELLA , PIERREHUMBERT RAYMOND T.
Published: July 2002

The dynamics of a passive tracer in the velocity field of four identical point vortices,
moving under the influence of their self-induced advection, is investigated. Of interest
is the change in mixing and transport properties of the tracer for the three different
classes of vortex motion: periodic, quasi-periodic and chaotic. As a consequence of
conservation laws, the vortex motion is confined to a finite region of phase space;
therefore, the tracer phase space can be partitioned into an inner and an outer region.
We find that in the case of quasi-periodic vortex motion the tracer phase space
exhibits a well-defined barrier to transport between the central chaotic region and the
outer region, where the trajectories are regular. In the case of chaotic vortex motion
the barrier becomes permeable. The particle dynamics goes through an intermittent
behaviour, where forays into the central region alternate with trapping in outer
annular orbits. In the far field, an estimate of diffusion rates is made through a
multipole expansion of the tracer velocity field. We make use of a specific stochastic
model for the tracer velocity field which predicts no diffusion for the case of quasi-periodic vortex motion and, for the case of chaotic vortex motion, a diffusion rate
that goes to zero at large distances from the vortex cluster.
Three-dimensional large-amplitude drop
oscillations: experiments and
theoretical analysis
Authors: AZUMA HISAO , YOSHIHARA SHOICHI
Published: July 2002

Three-dimensional large-amplitude oscillations of a mercury drop were obtained by
electrical excitation in low gravity using a drop tower. Multi-lobed (from three to
six lobes) and polyhedral (including tetrahedral, hexahedral, octahedral and
dodecahedral) oscillations were obtained as well as axisymmetric oscillation patterns. The
relationship between the oscillation patterns and their frequencies was obtained, and
it was found that polyhedral oscillations are due to the nonlinear interaction of waves.

A mathematical model of three-dimensional forced oscillations of a liquid drop is
proposed and compared with experimental results. The equations of drop motion are
derived by applying the variation principle to the Lagrangian of the drop motion,
assuming moderate deformation. The model takes the form of a nonlinear Mathieu
equation, which expresses the relationships between deformation amplitude and the
driving force's magnitude and frequency.
A unified analysis of planar homogeneous turbulence using
single-point closure equations
Authors: JONGEN T. , GATSKI T. B.
Published: July 2002

A unified approach for assessing and characterizing both the non-equilibrium and
equilibrium states of planar homogeneous flows is analysed within the framework
of single-point turbulence closure equations. The underlying methodology is based
on the replacement of the modelled evolution equation for the Reynolds stress
anisotropy tensor by an equivalent set of three equations for characteristic scalar
invariants or state variables. For stress anisotropy evolution equations which use
modelled pressure–strain rate correlations that are quasi-linear, this equivalence then
leads to an analytic solution for the time evolution of the Reynolds stress anisotropy.
With this analysis, the transient system characteristics can be studied, including the
dependence on initial states, the occurrence of limit-cycle behaviour, and the system
global stability. In the fixed-point asymptotic limit, these results are consistent with
and unify previous equilibrium studies, and provide additional information allowing
the resolution of some questions that could not be answered in the framework of
previous developments. A new result on constraints applicable to the development of
realizable pressure–strain rate models is obtained from a re-examination of the stress
anisotropy invariant map. With the analytic solution for the transient behaviour, some
recent non-equilibrium models, which incorporate relaxation effects, are evaluated in
a variety of homogeneous flows in inertial and non-inertial frames.
Three-dimensional effects in wind tunnel studies of shock wave reflection
Authors: SKEWS B. W.
Published: July 2002

This paper concentrates on establishing the three-dimensional flow geometry
associated with studies of shock wave reflection between two symmetrical wedges in
supersonic flow. It considers the issue of hysteresis in such flows, and draws a distinction
between three different aspects of hysteresis, associated with: ideal two-dimensional
flow, flow with noise, and three-dimensional effects. The three-dimensional nature of
the flow field is elucidated by the use of oblique shadowgraph photography where
the optical axis of the shadowgraph system passes at an oblique angle, of as much as
55°, through the test section. The traces of the wave system reflecting off the tunnel
window are identified and are used to assist in identification of wave profiles. The
nature of the approach of the peripheral Mach reflections collapsing towards the
centre of the flow becomes evident, as does the mechanism of transition from Mach
reflection to regular reflection. Distinct evidence of the effects of flow perturbations
at the mechanical equilibrium transition point are presented, as are changes in the
rate of growth of the Mach stem near this point.

It is shown that three-dimensional effects can have a major effect on the wedge
angle for transition. In the present tests, at Mach 3.1 and a wedge aspect ratio of
0.5, this occurs at a wedge angle of about 5° higher than the theoretical maximum
for the corresponding two-dimensional flow, where the dual solution domain extends
over only two degrees.
Flow-pattern identification for two staggered circular cylinders in cross-flow
Authors: SUMNER D. , PRICE S. J. , PAÏDOUSSIS M. P.
Published: July 2002

The flow around two circular cylinders of equal diameter, arranged in a staggered
configuration, was investigated using flow visualization and particle image velocimetry
for centre-to-centre pitch ratio P/D = 1[ratio ]0 to 5.0 and angle of incidence. α = 0°
to 90°. Experiments were conducted within the low subcritical Reynolds number
regime, from Re = 850 to 1900. Nine flow patterns were identified, and processes of
shear layer reattachment, induced separation, vortex pairing and synchronization, and
vortex impingement, were observed. New insight was gained into previously published
Strouhal number data, by considering the flow patterns involved. The study revealed
that vortex shedding frequencies are more properly associated with individual shear
layers than with individual cylinders; more specifically, the two shear layers from the
downstream cylinder often shed vortices at different frequencies.