8,868 research outputs found
Optimal Taylor-Couette flow: Radius ratio dependence
Taylor-Couette flow with independently rotating inner (i) and outer (o)
cylinders is explored numerically and experimentally to determine the effects
of the radius ratio {\eta} on the system response. Numerical simulations reach
Reynolds numbers of up to Re_i=9.5 x 10^3 and Re_o=5x10^3, corresponding to
Taylor numbers of up to Ta=10^8 for four different radius ratios {\eta}=r_i/r_o
between 0.5 and 0.909. The experiments, performed in the Twente Turbulent
Taylor-Couette (T^3C) setup, reach Reynolds numbers of up to Re_i=2x10^6$ and
Re_o=1.5x10^6, corresponding to Ta=5x10^{12} for {\eta}=0.714-0.909. Effective
scaling laws for the torque J^{\omega}(Ta) are found, which for sufficiently
large driving Ta are independent of the radius ratio {\eta}. As previously
reported for {\eta}=0.714, optimum transport at a non-zero Rossby number
Ro=r_i|{\omega}_i-{\omega}_o|/[2(r_o-r_i){\omega}_o] is found in both
experiments and numerics. Ro_opt is found to depend on the radius ratio and the
driving of the system. At a driving in the range between {Ta\sim3\cdot10^8} and
{Ta\sim10^{10}}, Ro_opt saturates to an asymptotic {\eta}-dependent value.
Theoretical predictions for the asymptotic value of Ro_{opt} are compared to
the experimental results, and found to differ notably. Furthermore, the local
angular velocity profiles from experiments and numerics are compared, and a
link between a flat bulk profile and optimum transport for all radius ratios is
reported.Comment: Submitted to JFM, 28 pages, 17 figure
Tailoring Anderson localization by disorder correlations in 1D speckle potentials
We study Anderson localization of single particles in continuous, correlated,
one-dimensional disordered potentials. We show that tailored correlations can
completely change the energy-dependence of the localization length. By
considering two suitable models of disorder, we explicitly show that disorder
correlations can lead to a nonmonotonic behavior of the localization length
versus energy. Numerical calculations performed within the transfer-matrix
approach and analytical calculations performed within the phase formalism up to
order three show excellent agreement and demonstrate the effect. We finally
show how the nonmonotonic behavior of the localization length with energy can
be observed using expanding ultracold-atom gases
Inductive Reasoning Games as Influenza Vaccination Models: Mean Field Analysis
We define and analyze an inductive reasoning game of voluntary yearly
vaccination in order to establish whether or not a population of individuals
acting in their own self-interest would be able to prevent influenza epidemics.
We find that epidemics are rarely prevented. We also find that severe epidemics
may occur without the introduction of pandemic strains. We further address the
situation where market incentives are introduced to help ameliorating
epidemics. Surprisingly, we find that vaccinating families exacerbates
epidemics. However, a public health program requesting prepayment of
vaccinations may significantly ameliorate influenza epidemics.Comment: 20 pages, 7 figure
Spin-orbit coupling and anisotropic exchange in two-electron double quantum dots
The influence of the spin-orbit interactions on the energy spectrum of
two-electron laterally coupled quantum dots is investigated. The effective
Hamiltonian for a spin qubit pair proposed in F. Baruffa et al., Phys. Rev.
Lett. 104, 126401 (2010) is confronted with exact numerical results in single
and double quantum dots in zero and finite magnetic field. The anisotropic
exchange Hamiltonian is found quantitatively reliable in double dots in
general. There are two findings of particular practical importance: i) The
model stays valid even for maximal possible interdot coupling (a single dot),
due to the absence of a coupling to the nearest excited level, a fact following
from the dot symmetry. ii) In a weak coupling regime, the Heitler-London
approximation gives quantitatively correct anisotropic exchange parameters even
in a finite magnetic field, although this method is known to fail for the
isotropic exchange. The small discrepancy between the analytical model (which
employes the linear Dresselhaus and Bychkov-Rashba spin-orbit terms) and the
numerical data for GaAs quantum dots is found to be mostly due to the cubic
Dresselhaus term.Comment: 15 pages, 11 figure
A Lattice Boltzmann study of the effects of viscoelasticity on droplet formation in microfluidic cross-junctions
Based on mesoscale lattice Boltzmann (LB) numerical simulations, we
investigate the effects of viscoelasticity on the break-up of liquid threads in
microfluidic cross-junctions, where droplets are formed by focusing a liquid
thread of a dispersed (d) phase into another co-flowing continuous (c)
immiscible phase. Working at small Capillary numbers, we investigate the
effects of non-Newtonian phases in the transition from droplet formation at the
cross-junction (DCJ) to droplet formation downstream of the cross-junction (DC)
(Liu Zhang, , 082101 (2011)). We will
analyze cases with (DV), where viscoelastic
properties are confined in the dispersed phase, as well as cases with (MV), where viscoelastic properties are confined in
the continuous phase. Moderate flow-rate ratios of the
two phases are considered in the present study. Overall, we find that the
effects are more pronounced in the case with MV, where viscoelasticity is found
to influence the break-up point of the threads, which moves closer to the
cross-junction and stabilizes. This is attributed to an increase of the polymer
feedback stress forming in the corner flows, where the side channels of the
device meet the main channel. Quantitative predictions on the break-up point of
the threads are provided as a function of the Deborah number, i.e. the
dimensionless number measuring the importance of viscoelasticity with respect
to Capillary forces.Comment: 15 pages, 14 figures. This Work applies the Numerical Methodology
described in arXiv:1406.2686 to the Problem of Droplet Generation in
Microfluidic Cross Junctions. arXiv admin note: substantial text overlap with
arXiv:1508.0014
Dynamics of elastocapillary rise
We present the results of a combined experimental and theoretical
investigation of the surface-tension-driven coalescence of flexible structures.
Specifically, we consider the dynamics of the rise of a wetting liquid between
flexible sheets that are clamped at their upper ends. As the elasticity of the
sheets is progressively increased, we observe a systematic deviation from the
classical diffusive-like behaviour: the time to reach equilibrium increases
dramatically and the departure from classical rise occurs sooner, trends that
we elucidate via scaling analyses. Three distinct temporal regimes are
identified and subsequently explored by developing a theoretical model based on
lubrication theory and the linear theory of plates. The resulting free-boundary
problem is solved numerically and good agreement is obtained with experiments
Relaxation and breakup of an initially extended drop in an otherwise quiescent fluid
In this paper we examine some general features of the time-dependent dynamics of drop deformation and breakup at low Reynolds numbers. The first aspect of our study is a detailed numerical investigation of the âend-pinchingâ behaviour reported in a previous experimental study. The numerics illustrate the effects of viscosity ratio and initial drop shape on the relaxation and/or breakup of highly elongated droplets in an otherwise quiescent fluid. In addition, the numerical procedure is used to study the simultaneous development of capillary-wave instabilities at the fluid-fluid interface of a very long, cylindrically shaped droplet with bulbous ends. Initially small disturbances evolve to finite amplitude and produce very regular drop breakup. The formation of satellite droplets, a nonlinear phenomenon, is also observed
Experimental evidence for three universality classes for reaction fronts in disordered flows
Self-sustained reaction fronts in a disordered medium subject to an external
flow display self-affine roughening, pinning and depinning transitions. We
measure spatial and temporal fluctuations of the front in dimensions,
controlled by a single parameter, the mean flow velocity. Three distinct
universality classes are observed, consistent with the Kardar-Parisi-Zhang
(KPZ) class for fast advancing or receding fronts, the quenched KPZ class
(positive-qKPZ) when the mean flow approximately cancels the reaction rate, and
the negative-qKPZ class for slowly receding fronts. Both quenched KPZ classes
exhibit distinct depinning transitions, in agreement with the theory
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