1,639 research outputs found
Liquid meniscus friction on a wet plate: Bubbles, lamellae and foams
Many microfluidics devices, coating processes or diphasic flows involve the
motion of a liquid meniscus on a wet wall. This motion induces a specific
viscous force, that exhibits a non-linear dependency in the meniscus velocity.
We propose a review of the theoretical and experimental work made on this
viscous force, for simple interfacial properties. The interface is indeed
assumed either perfectly compressible (mobile interface) or perfectly
incompressible (rigid interface). We show that, in the second case, the viscous
force exerted by the wall on the meniscus is a combination of two power laws,
scaling like and , with the capillary number. We
provide a prediction for the stress exerted on a foam sliding on a wet solid
and compare it with experimental data, for the incompressible case
Accounting for convective effects in zero-Mach-number thermoacoustic models
This paper presents a methodology to account for some mean-flow effects on thermo-acoustic instabilities when using the zero-Mach-number assumption. It is shown that when a computational domain is represented under the M=0 assumption, a nonzero-Mach-number element can simply be taken into account by imposing a proper acoustic impedance at the boundaries so as to mimic the mean flow effects in the outer, not computed flow domain. A model that accounts for the coupling between acoustic and entropy waves is presented. It relies on a “delayed entropy coupled boundary condition” (DECBC) for the Helmholtz equation satisfied by the acoustic pressure. The model proves able to capture low-frequency entropic modes even without mean-flow terms in the fluctuating pressure equation
Brownian motors
In systems possessing a spatial or dynamical symmetry breaking thermal
Brownian motion combined with unbiased, non-equilibrium noise gives rise to a
channelling of chance that can be used to exercise control over systems at the
micro- and even on the nano-scale. This theme is known as ``Brownian motor''
concept. The constructive role of (the generally overdamped) Brownian motion is
exemplified for a noise-induced transport of particles within various set-ups.
We first present the working principles and characteristics with a
proof-of-principle device, a diffusive temperature Brownian motor. Next, we
consider very recent applications based on the phenomenon of signal mixing. The
latter is particularly simple to implement experimentally in order to optimize
and selectively control a rich variety of directed transport behaviors. The
subtleties and also the potential for Brownian motors operating in the quantum
regime are outlined and some state-of-the-art applications, together with
future roadways, are presented.Comment: 20 pages, 9 figures (slightly changed version
Numerical Ricci-flat metrics on K3
We develop numerical algorithms for solving the Einstein equation on
Calabi-Yau manifolds at arbitrary values of their complex structure and Kahler
parameters. We show that Kahler geometry can be exploited for significant gains
in computational efficiency. As a proof of principle, we apply our methods to a
one-parameter family of K3 surfaces constructed as blow-ups of the T^4/Z_2
orbifold with many discrete symmetries. High-resolution metrics may be obtained
on a time scale of days using a desktop computer. We compute various geometric
and spectral quantities from our numerical metrics. Using similar resources we
expect our methods to practically extend to Calabi-Yau three-folds with a high
degree of discrete symmetry, although we expect the general three-fold to
remain a challenge due to memory requirements.Comment: 38 pages, 10 figures; program code and animations of figures
downloadable from http://schwinger.harvard.edu/~wiseman/K3/ ; v2 minor
corrections, references adde
Solar Orbiter observations of the Kelvin-Helmholtz waves in the solar wind
Context. The Kelvin-HeImholtz (KH) instability is a nonlinear shear-driven instability that develops at the interface between shear flows in plasmas. KH waves have been inferred in various astrophysical plasmas, and have been observed in situ at the magnetospheric boundaries of solar-system planets and through remote sensing at the boundaries of coronal mass ejections. //
Aims. KH waves are also expected to develop at flow shear interfaces in the solar wind. While they were hypothesized to play an important role in the mixing of plasmas and in triggering solar wind fluctuations, their direct and unambiguous observation in the solar wind was still lacking. //
Methods. We report in-situ observations of quasi-periodic magnetic and velocity field variations plausibly associated with KH waves using Solar Orbiter during its cruise phase. They are found in a shear layer in the slow solar wind in the close vicinity of the Heliospheric Current Sheet. Analysis is performed to derive the local configuration of the waves. A 2-D MHD simulation is also set up with approximate empirical values to test the stability of the shear layer. In addition, magnetic spectra of the event are analyzed. Results. We find that the observed conditions satisfy the KH instability onset criterion from the linear theory analysis, and its de- velopment is further confirmed by the simulation. The current sheet geometry analyses are found to be consistent with KH wave development, albeit with some limitations likely owing to the complex 3D nature of the event and solar wind propagation. Addition- ally, we report observations of an ion jet consistent with magnetic reconnection at a compressed current sheet within the KH wave interval. The KH activity is found to excite magnetic and velocity fluctuations with power law scalings that approximately follow k−5/3 and k−2.8 in the inertial and dissipation ranges, respectively. Finally, we discuss reasons for the lack of in-situ KH wave detection in past data. //
Conclusions. These observations provide robust evidence of KH wave development in the solar wind. This sheds new light on the process of shear-driven turbulence as mediated by the KH waves with implications for the driving of solar wind fluctuations
Complete quantum-inspired framework for computational fluid dynamics
Computational fluid dynamics is both an active research field and a key tool
for industrial applications. The central challenge is to simulate turbulent
flows in complex geometries, a compute-power intensive task due to the large
vector dimensions required by discretized meshes. Here, we propose a full-stack
solver for incompressible fluids with memory and runtime scaling
polylogarithmically in the mesh size. Our framework is based on matrix-product
states, a powerful compressed representation of quantum states. It is complete
in that it solves for flows around immersed objects of diverse geometries, with
non-trivial boundary conditions, and can retrieve the solution directly from
the compressed encoding, i.e. without ever passing through the expensive
dense-vector representation. These developments provide a toolbox with
potential for radically more efficient simulations of real-life fluid problems
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