30,538 research outputs found
Experimental assessment of drag reduction by traveling waves in a turbulent pipe flow
We experimentally assess the capabilities of an active, open-loop technique
for drag reduction in turbulent wall flows recently introduced by Quadrio et
al. [J. Fluid Mech., v.627, 161, (2009)]. The technique consists in generating
streamwise-modulated waves of spanwise velocity at the wall, that travel in the
streamwise direction.
A proof-of-principle experiment has been devised to measure the reduction of
turbulent friction in a pipe flow, in which the wall is subdivided into thin
slabs that rotate independently in the azimuthal direction. Different speeds of
nearby slabs provide, although in a discrete setting, the desired streamwise
variation of transverse velocity.
Our experiment confirms the available DNS results, and in particular
demonstrates the possibility of achieving large reductions of friction in the
turbulent regime. Reductions up to 33% are obtained for slowly
forward-traveling waves; backward-traveling waves invariably yield drag
reduction, whereas a substantial drop of drag reduction occurs for waves
traveling forward with a phase speed comparable to the convection speed of
near-wall turbulent structures.
A Fourier analysis is employed to show that the first harmonics introduced by
the discrete spatial waveform that approximates the sinusoidal wave are
responsible for significant effects that are indeed observed in the
experimental measurements. Practical issues related to the physical
implementation of this control scheme and its energetic efficiency are briefly
discussed.Comment: Article accepted by Phys. Fluids. After it is published, it will be
found at http://pof.aip.or
Optimal control in ink-jet printing via instantaneous control
This paper concerns the optimal control of a free surface flow with moving
contact line, inspired by an application in ink-jet printing. Surface tension,
contact angle and wall friction are taken into account by means of the
generalized Navier boundary condition. The time-dependent differential system
is discretized by an arbitrary Lagrangian-Eulerian finite element method, and a
control problem is addressed by an instantaneous control approach, based on the
time discretization of the flow equations. The resulting control procedure is
computationally highly efficient and its assessment by numerical tests show its
effectiveness in deadening the natural oscillations that occur inside the
nozzle and reducing significantly the duration of the transient preceding the
attainment of the equilibrium configuration
Lattice Boltzmann Simulation of Non-Ideal Fluids
A lattice Boltzmann scheme able to model the hydrodynamics of phase
separation and two-phase flow is described. Thermodynamic consistency is
ensured by introducing a non-ideal pressure tensor directly into the collision
operator. We also show how an external chemical potential can be used to
supplement standard boundary conditions in order to investigate the effect of
wetting on phase separation and fluid flow in confined geometries. The approach
has the additional advantage of reducing many of the unphysical discretisation
problems common to previous lattice Boltzmann methods.Comment: 11 pages, revtex, 4 Postscript figures, uuencode
The origin of large amplitude oscillations of dust particles in a plasma sheath
Micron-size charged particles can be easily levitated in low-density plasma
environments. At low pressures, suspended particles have been observed to
spontaneously oscillate around an equilibrium position. In systems of many
particles, these oscillations can catalyze a variety of nonequilibrium,
collective behaviors. Here, we report spontaneous oscillations of single
particles that remain stable for minutes with striking regularity in amplitude
and frequency. The oscillation amplitude can also exceed 1 cm, nearly an order
of magnitude larger than previously observed. Using an integrated experimental
and numerical approach, we show how the motion of an individual particle can be
used to extract the electrostatic force and equilibrium charge variation in the
plasma sheath. Additionally, using a delayed-charging model, we are able to
accurately capture the nonlinear dynamics of the particle motion, and estimate
the particle's equilibrium charging time in the plasma environment
Ultra-high speed particle image velocimetry on drop-on-demand jetting
Se estudian los conceptos claves de fortuna, virtĂą y gloria, con elobjeto de buscar algunas aclaraciones sobre el problema de larelaciĂłn entre moral y polĂtica en Maquiavelo. La virtĂą maquiavelianaincluye, sin duda, mĂşltiples componentes de energĂa, talento,pero no está completamente exenta de elementos morales. Delconcepto de fortuna podemos extraer una serie de criterios parala acciĂłn polĂtica que forman parte de la virtĂą maquiaveliana, lacual, si bien se aleja de la moral cristiana vigente en su tiempo, lesitĂşa dentro de una moral republicana. Del examen del conceptode gloria descubrimos cĂłmo la gloria es recompensa de la virtĂą,pero no se reconoce a cualquier polĂtico que tiene Ă©xito en susempresas, sino solo a quien salva a la patria y lo hace con mediosbenignos.</jats:p
Primordial RNA Replication and Applications in PCR Technology
The emergence of self-replication and information transmission in life's
origin remains unexplained despite extensive research on the topic. A
hypothesis explaining the transition from a simple organic world to a complex
RNA world is offered here based on physical factors in hydrothermal vent
systems. An interdisciplinary approach is taken using techniques from
thermodynamics, fluid dynamics, oceanography, statistical mechanics, and
stochastic processes to examine nucleic acid dynamics and kinetics in a
hydrothermal vent from first principles. Analyses are carried out using both
analytic and computational methods and confirm the plausibility of a reaction
involving the PCR-like assembly of ribonucleotides. The proposal is put into
perspective with established theories on the origin of life and more generally
the onset of order and information transmission in prebiotic systems. A
biomimicry application of this hypothetical process to PCR technology is
suggested and its viability is evaluated in a rigorous logical analysis.
Optimal temperature curves begin to be established using Monte Carlo
simulation, variational calculus, and Fourier analysis. The converse argument
is also made but qualitatively, asserting that the success of such a
modification to PCR would in turn reconfirm the biological theory.Comment: 20 pages, 7 figure
Coarsening dynamics of ternary amphiphilic fluids and the self-assembly of the gyroid and sponge mesophases: lattice-Boltzmann simulations
By means of a three-dimensional amphiphilic lattice-Boltzmann model with
short-range interactions for the description of ternary amphiphilic fluids, we
study how the phase separation kinetics of a symmetric binary immiscible fluid
is altered by the presence of the amphiphilic species. We find that a gradual
increase in amphiphile concentration slows down domain growth, initially from
algebraic, to logarithmic temporal dependence, and, at higher concentrations,
from logarithmic to stretched-exponential form. In growth-arrested
stretched-exponential regimes, at late times we observe the self-assembly of
sponge mesophases and gyroid liquid crystalline cubic mesophases, hence
confirming that (a) amphiphile-amphiphile interactions need not be long-ranged
in order for periodically modulated structures to arise in a dynamics of
competing interactions, and (b) a chemically-specific model of the amphiphile
is not required for the self-assembly of cubic mesophases, contradicting claims
in the literature. We also observe a structural order-disorder transition
between sponge and gyroid phases driven by amphiphile concentration alone or,
independently, by the amphiphile-amphiphile and the amphiphile-binary fluid
coupling parameters. For the growth-arrested mesophases, we also observe
temporal oscillations in the structure function at all length scales; most of
the wavenumbers show slow decay, and long-term stationarity or growth for the
others. We ascribe this behaviour to a combination of complex amphiphile
dynamics leading to Marangoni flows.Comment: 16 pages, 13 figures. Accepted for publication in Phys. Rev. E.
(Replaced for the latest version, in press.) Higher-quality figures can be
sent upon reques
State estimation of a solar direct steam generation mono-tube cavity receiver using a modified Extended Kalman Filtering scheme
State estimation plays a key role in the development of advanced control strategies for Concentrating Solar Thermal Power (CSP)
systems, by providing an estimate of process variables that are otherwise infeasible to measure. The present study proposes a state estimation
scheme for a once-through direct steam generation plant, the SG4 steam generation system at the Australian National University.
The state estimation scheme is a modified Extended Kalman Filter that computes an estimate of the internal variables of the mono-tube
cavity receiver in the SG4 system, from a dynamic non-linear model of the receiver. The proposed scheme augments the capabilities of a
Continuous-Direct Extended Kalman Filter to deal with the switched nature of the receiver, in order to produce estimates during system
start-up, cloud transients and operation of the plant. The estimation process runs at regular sample intervals and happens in two stages, a
prediction and a correction stage. The prediction stage uses the receiver model to calculate the evolution of the system and the correction
stage modifies the predicted estimate from measurements of the SG4 system. The resulting estimate is a set of internal variables describing
the current state of the receiver, termed the state vector. This paper presents a description of the modified Extended Kalman Filter
and an evaluation of the scheme using computer simulations and experimental runs in the SG4 system. Simulations and experimental
results in this paper show that the filtering scheme improves a receiver state vector estimation purely based on the receiver model
and provides estimates of a quality sufficient for closed loop control.This work has been supported by the Australian
Renewable Energy Agency (ARENA)
Discrete scale invariance and complex dimensions
We discuss the concept of discrete scale invariance and how it leads to
complex critical exponents (or dimensions), i.e. to the log-periodic
corrections to scaling. After their initial suggestion as formal solutions of
renormalization group equations in the seventies, complex exponents have been
studied in the eighties in relation to various problems of physics embedded in
hierarchical systems. Only recently has it been realized that discrete scale
invariance and its associated complex exponents may appear ``spontaneously'' in
euclidean systems, i.e. without the need for a pre-existing hierarchy. Examples
are diffusion-limited-aggregation clusters, rupture in heterogeneous systems,
earthquakes, animals (a generalization of percolation) among many other
systems. We review the known mechanisms for the spontaneous generation of
discrete scale invariance and provide an extensive list of situations where
complex exponents have been found. This is done in order to provide a basis for
a better fundamental understanding of discrete scale invariance. The main
motivation to study discrete scale invariance and its signatures is that it
provides new insights in the underlying mechanisms of scale invariance. It may
also be very interesting for prediction purposes.Comment: significantly extended version (Oct. 27, 1998) with new examples in
several domains of the review paper with the same title published in Physics
Reports 297, 239-270 (1998
Cosmology with Twisted Tori
We consider the cosmological role of the scalar fields generated by the
compactification of 11-dimensional Einstein gravity on a 7D elliptic twisted
torus, which has the attractive features of giving rise to a positive
semi-definite potential, and partially fixing the moduli. This compactification
is therefore relevant for low energy M-theory, 11D supergravity. We find that
slow-roll inflation with the moduli is not possible, but that there is a novel
scaling solution in Friedmann cosmologies in which the massive moduli oscillate
but maintain a constant energy density relative to the background barotropic
fluid
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