228 research outputs found
Three-dimensional aspects of fluid flows in channels. II. Effects of Meniscus and Thin Film regimes on Viscous Fingers
We perform a three-dimensional study of steady state viscous fingers that
develop in linear channels. By means of a three-dimensional Lattice-Boltzmann
scheme that mimics the full macroscopic equations of motion of the fluid
momentum and order parameter, we study the effect of the thickness of the
channel in two cases. First, for total displacement of the fluids in the
channel thickness direction, we find that the steady state finger is
effectively two-dimensional and that previous two-dimensional results can be
recovered by taking into account the effect of a curved meniscus across the
channel thickness as a contribution to surface stresses. Secondly, when a thin
film develops in the channel thickness direction, the finger narrows with
increasing channel aspect ratio in agreement with experimental results. The
effect of the thin film renders the problem three-dimensional and results
deviate from the two-dimensional prediction.Comment: 9 pages, 10 figure
A classical explanation of quantization
In the context of our recently developed emergent quantum mechanics, and, in
particular, based on an assumed sub-quantum thermodynamics, the necessity of
energy quantization as originally postulated by Max Planck is explained by
means of purely classical physics. Moreover, under the same premises, also the
energy spectrum of the quantum mechanical harmonic oscillator is derived.
Essentially, Planck's constant h is shown to be indicative of a particle's
"zitterbewegung" and thus of a fundamental angular momentum. The latter is
identified with quantum mechanical spin, a residue of which is thus present
even in the non-relativistic Schroedinger theory.Comment: 20 pages; version accepted for publication in Foundations of Physic
A systematic correlation between two-dimensional flow topology and the abstract statistics of turbulence
Velocity differences in the direct enstrophy cascade of two-dimensional
turbulence are correlated with the underlying flow topology. The statistics of
the transverse and longitudinal velocity differences are found to be governed
by different structures. The wings of the transverse distribution are dominated
by strong vortex centers, whereas, the tails of the longitudinal differences
are dominated by saddles. Viewed in the framework of earlier theoretical work
this result suggests that the transfer of enstrophy to smaller scales is
accomplished in regions of the flow dominated by saddles.Comment: 4 pages, 4 figure
Velocity and Energy Profiles In Two- vs. Three-Dimensional Channels: Effects of Inverse vs. Direct Energy Cascade
In light of some recent experiments on quasi two-dimensional (2D) turbulent
channel flow we provide here a model of the ideal case, for the sake of
comparison. The ideal 2D channel flow differs from its 3D counterpart by having
a second quadratic conserved variable in addition to the energy, and the latter
has an inverse rather than a direct cascade. The resulting qualitative
differences in profiles of velocity, V, and energy, K, as a function of the
distance from the wall are highlighted and explained. The most glaring
difference is that the 2D channel is much more energetic, with K in wall units
increasing logarithmically with the Reynolds number \Ret instead of being
\Ret-independent in 3D channels.Comment: Theoretical; 4 pages, 3 figures (8 plots); Submitted to Physical
Review Letters on 16 February 200
Tunable bimodal explorations of space from memory-driven deterministic dynamics
We present a wave-memory-driven system that exhibits intermittent switching between two propulsion modes in free space. The model is based on a pointlike particle emitting periodically cylindrical standing waves. Submitted to a force related to the local wave-field gradient, the particle is propelled, while the wave field stores positional information on the particle trajectory. For long memory, the linear motion is unstable and we observe erratic switches between two propulsive modes: linear motion and diffusive motion. We show that the bimodal propulsion and the stochastic aspect of the dynamics at long time are generated by a Shil'nikov chaos. The memory of the system controls the fraction of time spent in each phase. The resulting bimodal dynamics shows analogies with intermittent search strategies usually observed in living systems of much higher complexity. © 2019 American Physical Society
Probing astrophysically important states in the ²⁶Mg nucleus to study neutron sources for the s process
Background: The ²²Ne(α,n) ²⁵Mg reaction is the dominant neutron source for the slow neutron capture process (s process) in massive stars, and contributes, together with C¹³(α,n)O¹⁶, to the production of neutrons for the s process in asymptotic giant branch (AGB) stars. However, the reaction is endothermic and competes directly with ²²Ne(α,γ)²⁶Mg radiative capture. The uncertainties for both reactions are large owing to the uncertainty in the level structure of ²⁶Mg near the α and neutron separation energies. These uncertainties affect the s-process nucleosynthesis calculations in theoretical stellar models. Purpose: Indirect studies in the past have been successful in determining the energies and the γ-ray and neutron widths of the Mg26 states in the energy region of interest. But, the high Coulomb barrier hinders a direct measurement of the resonance strengths, which are determined by the α widths for these states. The goal of the present experiments is to identify the critical resonance states and to precisely measure the α widths by α-transfer techniques. Methods: The α-inelastic scattering and α-transfer measurements were performed on a solid ²⁶Mg target and a ²²Ne gas target, respectively, using the Grand Raiden Spectrometer at the Research Center for Nuclear Physics in Osaka, Japan. The (α,α′) measurements were performed at 0.45°, 4.1°, 8.6°, and 11.1° and the (⁶Li,d) measurements at 0° and 10°. The scattered α particles and deuterons were detected by the focal plane detection system consisting of multiwire drift chambers and plastic scintillators. The focal plane energy calibration allowed the study of ²⁶Mg levels from Eₓ = 7.69–12.06 MeV in the (α,α′) measurement and Eₓ = 7.36–11.32 MeV in the (⁶Li,d) measurement. Results: Six levels (Eₓ = 10717, 10822, 10951, 11085, 11167, and 11317 keV) were observed above the α threshold in the region of interest (10.61–11.32 MeV). The α widths were calculated for these states from the experimental data. The results were used to determine the α-capture induced reaction rates. Conclusion: The energy range above the α threshold in ²⁶Mg was investigated using a high resolution spectrometer. A number of states were observed for the first time in α-scattering and α-transfer reactions. The excitation energies and spin-parities were determined. Good agreement is observed for previously known levels in ²⁶Mg. From the observed resonance levels the Eₓ = 10717 keV state has a negligible contribution to the α-induced reaction rates. The rates are dominated in both reaction channels by the resonance contributions of the states at Ex = 10951, 11167, and 11317 keV. The Eₓ = 11167 keV state has the most appreciable impact on the (α,γ) rate and therefore plays an important role in the prediction of the neutron production in s-process environments
Viscous shocks in Hele-Shaw flow and Stokes phenomena of the Painleve I transcendent
In Hele-Shaw flows at vanishing surface tension, the boundary of a viscous
fluid develops cusp-like singularities. In recent papers [1, 2] we have showed
that singularities trigger viscous shocks propagating through the viscous
fluid. Here we show that the weak solution of the Hele-Shaw problem describing
viscous shocks is equivalent to a semiclassical approximation of a special real
solution of the Painleve I equation. We argue that the Painleve I equation
provides an integrable deformation of the Hele-Shaw problem which describes
flow passing through singularities. In this interpretation shocks appear as
Stokes level-lines of the Painleve linear problem.Comment: A more detailed derivation is include
Hysteresis at low Reynolds number: the onset of 2D vortex shedding
Hysteresis has been observed in a study of the transition between laminar
flow and vortex shedding in a quasi-two dimensional system. The system is a
vertical, rapidly flowing soap film which is penetrated by a rod oriented
perpendicular to the film plane. Our experiments show that the transition from
laminar flow to a periodic K\'arm\'an vortex street can be hysteretic, i.e.
vortices can survive at velocities lower than the velocity needed to generate
them.Comment: RevTeX file 4 pages + 5 (encapsulated postscript) figures. to appear
in Phys.Rev.E, Rapid Communicatio
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