3,047 research outputs found
Spectra of turbulence in dilute polymer solutions
We investigate turbulence in dilute polymer solutions when polymers are
strongly stretched by the flow. We establish power-law spectrum of velocity,
which is not associated with a flux of a conserved quantity, in two cases. The
first case is the elastic waves range of high Reynolds number turbulence of
polymer solutions above the coil-stretch transition. The second case is the
elastic turbulence, where chaotic flow is excited due to elastic instabilities
at small Reynolds numbers.Comment: 14 pages, RevTe
Stochastic Structural Stability Theory applied to roll/streak formation in boundary layer shear flow
Stochastic Structural Stability Theory (SSST) provides an autonomous,
deterministic, nonlinear dynamical system for evolving the statistical mean
state of a turbulent system. In this work SSST is applied to the problem of
understanding the formation of the roll/streak structures that arise from
free-stream turbulence (FST) and are associated with bypass transition in
boundary layers. Roll structures in the cross-stream/spanwise plane and
associated streamwise streaks are shown to arise as a linear instability of
interaction between the FST and the mean flow. In this interaction incoherent
Reynolds stresses arising from FST are organized by perturbation streamwise
streaks to coherently force perturbation rolls giving rise to an amplification
of the streamwise streak perturbation and through this feedback to an
instability of the combined roll/streak/turbulence complex. The dominant
turbulent perturbation structures involved in supporting the
roll/streak/turbulence complex instability are non-normal optimal perturbations
with the form of oblique waves. The cooperative linear instability giving rise
to the roll/streak structure arises at a bifurcation in the parameter of STM
excitation parameter. This structural instability eventually equilibrates
nonlinearly at finite amplitude and although the resulting statistical
equilibrium streamwise streaks are inflectional the associated flows are
stable. Formation and equilibration of the roll/streak structure by this
mechanism can be traced to the non-normality which underlies interaction
between perturbations and mean flows in modally stable systems.Comment: 16 pages, 24 figures, has been submitted for publication to Physics
of Fluid
Manifestations of Drag Reduction by Polymer Additives in Decaying, Homogeneous, Isotropic Turbulence
The existence of drag reduction by polymer additives, well established for
wall-bounded turbulent flows, is controversial in homogeneous, isotropic
turbulence. To settle this controversy we carry out a high-resolution direct
numerical simulation (DNS) of decaying, homogeneous, isotropic turbulence with
polymer additives. Our study reveals clear manifestations of
drag-reduction-type phenomena: On the addition of polymers to the turbulent
fluid we obtain a reduction in the energy dissipation rate, a significant
modification of the fluid energy spectrum especially in the deep-dissipation
range, a suppression of small-scale intermittency, and a decrease in
small-scale vorticity filaments.Comment: 4 pages, 3 figure
Birth then and now
Halfway through the 20th century, views on pain in labour encompassed almost everything from women's self-blame to blaming nurses, midwives, doctors or partners for 'bad experiences'. Soon after that, giving birth came to be seen – in some settings and by some caregivers – as a 'natural' and thus benign event which women could 'master'. In their recent systematic review of women's expectations and experiences of pain relief in labour, Joanne Lally, Madeleine Murtagh, Sheila Macphail, and Richard Thomson show that there is wide variation in women's expectations and knowledge about the first birth. This systematic review provides us with a strong basis for reflection and action
Single polymer dynamics: coil-stretch transition in a random flow
By quantitative studies of statistics of polymer stretching in a random flow
and of a flow field we demonstrate that the stretching of polymer molecules in
a 3D random flow occurs rather sharply via the coil-stretch transition at the
value of the criterion close to theoretically predicted.Comment: 4 pages, 5 figure
Post-copulatory opportunities for sperm competition and cryptic female choice provide no offspring fitness benefits in externally fertilizing salmon
There is increasing evidence that females can somehow improve their offspring fitness by mating with multiple males, but we understand little about the exact stage(s) at which such benefits are gained. Here, we measure whether offspring fitness is influenced by mechanisms operating solely between sperm and egg. Using externally-fertilising and polyandrous Atlantic salmon (Salmo salar), we employed split-clutch and split-ejaculate in vitro fertilisation experiments to generate offspring using designs that either denied or applied opportunities for sperm competition and cryptic female choice. Following fertilisations, we measured 140 days of offspring fitness after hatch, through growth and survival in hatchery and near-natural conditions. Despite an average composite mortality of 61%, offspring fitness at every life stage was near-identical between groups fertilised under the absence versus presence of opportunities for sperm competition and cryptic female choice. Of the 21,551 and 21,771 eggs from 24 females fertilised under monandrous versus polyandrous conditions, 68% versus 67.8% survived to the 100-day juvenile stage; sub-samples showed similar hatching success (73.1% versus 74.3%), had similar survival over 40 days in near-natural streams (57.3% versus 56.2%), and grew at similar rates throughout. We therefore found no evidence that gamete-specific interactions allow offspring fitness benefits when polyandrous fertilisation conditions provide opportunities for sperm competition and cryptic female choice
Toward a structural understanding of turbulent drag reduction: nonlinear coherent states in viscoelastic shear flows
Nontrivial steady flows have recently been found that capture the main
structures of the turbulent buffer layer. We study the effects of polymer
addition on these "exact coherent states" (ECS) in plane Couette flow. Despite
the simplicity of the ECS flows, these effects closely mirror those observed
experimentally: Structures shift to larger length scales, wall-normal
fluctuations are suppressed while streamwise ones are enhanced, and drag is
reduced. The mechanism underlying these effects is elucidated. These results
suggest that the ECS are closely related to buffer layer turbulence.Comment: 5 pages, 3 figures, published version, Phys. Rev. Lett. 89, 208301
(2002
Evaluating Invariances in Document Layout Functions
With the development of variable-data-driven digital presses - where each document printed is potentially unique - there is a need for pre-press optimization to identify material that is invariant from document to document. In this way rasterisation can be confined solely to those areas which change between successive documents thereby alleviating a potential performance bottleneck.
Given a template document specified in terms of layout functions, where actual data is bound at the last possible moment before printing, we look at deriving and exploiting the invariant properties of layout functions from their formal specifications. We propose future work on generic extraction of invariance from such properties for certain classes of layout functions
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