89 research outputs found
Orthogonal signed-distance coordinates and vector calculus near evolving curves and surfaces
We provide an elementary derivation of an orthogonal coordinate system for
boundary layers around evolving smooth surfaces and curves based on the
signed-distance function. We go beyond previous works on the signed-distance
function and collate useful vector calculus identities for these coordinates.
These results and provided code enable consistent accounting of geometric
effects in the derivation of boundary layer asymptotics for a wide range of
physical systems.Comment: 24 pages, 3 figures, Mathematica code available at
https://github.com/ericwhester/signed-distance-cod
On the stability of isothermal shocks in black hole accretion disks
Most black holes possess accretion disks. Models of such disks inform
observations and constrain the properties of the black holes and their
surrounding medium. Here, we study isothermal shocks in a thin black hole
accretion flow. Modelling infinitesimal molecular viscosity allows the use of
multiple-scales matched asymptotic methods. We thus derive the first explicit
calculations of isothermal shock stability. We find that the inner shock is
always unstable, and the outer shock is always stable. The growth/decay rates
of perturbations depend only on an effective potential and the
incoming--outgoing flow difference at the shock location. We give a
prescription of accretion regimes in terms of angular momentum and black hole
radius. Accounting for angular momentum dissipation implies unstable outer
shocks in much of parameter space, even for realistic viscous Reynolds numbers
of the order .Comment: 26 page
Bistability in Rayleigh-B\'enard convection with a melting boundary
A pure and incompressible material is confined between two plates such that
it is heated from below and cooled from above. When its melting temperature is
comprised between these two imposed temperatures, an interface separating
liquid and solid phases appears. Depending on the initial conditions, freezing
or melting occurs until the interface eventually converges towards a stationary
state. This evolution is studied numerically in a two-dimensional configuration
using a phase-field method coupled with the Navier-Stokes equations. Varying
the control parameters of the model, we exhibit two types of equilibria:
diffusive and convective. In the latter case, Rayleigh-B\'enard convection in
the liquid phase shapes the solid-liquid front, and a macroscopic topography is
observed. A simple way of predicting these equilibrium positions is discussed
and then compared with the numerical simulations. In some parameter regimes, we
show that multiple equilibria can coexist depending on the initial conditions.
We also demonstrate that, in this bi-stable regime, transitioning from the
diffusive to the convective equilibrium is inherently a nonlinear mechanism
involving finite-amplitude perturbations
Magneto-Stokes Flow in a Shallow Free-Surface Annulus
We analyse a magnetohydrodynamic flow inspired by the kinematic reversibility
of viscous Taylor-Couette flows. The system considered here shares the
cylindrical-annular geometry of the Taylor-Couette cell, but uses applied
electromagnetic forces to drive "magneto-Stokes" flow in a shallow,
free-surface layer of electrolyte. An analytical solution is presented and
validated with coupled laboratory and numerical experiments. The dominant
balance of Lorentz forcing and basal viscous drag reproduces the kinematic
reversibility observed by G.I. Taylor with precise electromagnetic control.
Induced fluid deformation may be undone by simply reversing the polarity of
electric current through the system. We illustrate this analogy with theory and
experiment, and we draw a further connection to potential flow using the
Hele-Shaw approximation. The stability and controllability of the
magneto-Stokes system make it an attractive tool for investigating shear flows
in a variety of settings from industrial to astrophysical. In addition, the
set-up's simplicity and robustness make magneto-Stokes flow a good candidate
for PIV calibration and for educational demonstrations of magnetohydrodynamics,
boundary layers, and flow transition
Fluid dynamics alters liquid-liquid phase separation in confined aqueous two-phase systems
Liquid-liquid phase separation is key to understanding aqueous two-phase
systems (ATPS) arising throughout cell biology, medical science, and the
pharmaceutical industry. Controlling the detailed morphology of
phase-separating compound droplets leads to new technologies for efficient
single-cell analysis, targeted drug delivery, and effective cell scaffolds for
wound healing. We present a computational model of liquid-liquid phase
separation relevant to recent laboratory experiments with gelatin-polyethylene
glycol mixtures. We include buoyancy and surface-tension-driven finite
viscosity fluid dynamics with thermally induced phase separation. We show that
the fluid dynamics greatly alters the evolution and equilibria of the phase
separation problem. Notably, buoyancy plays a critical role in driving the ATPS
to energy-minimizing crescent-shaped morphologies and shear flows can generate
a tenfold speedup in particle formation. Neglecting fluid dynamics produces
incorrect minimum-energy droplet shapes. The model allows for optimization of
current manufacturing procedures for structured microparticles and improves
understanding of ATPS evolution in confined and flowing settings important in
biology and biotechnology.Comment: 9 pages, 8 figures, 3 supplementary movies, to appear in Proceedings
of the National Academy of Sciences, accompanying code and parameters to
generate data available at
https://github.com/ericwhester/multiphase-fluids-cod
Pre-main sequence stars in the Cepheus flare region
We present results of optical spectroscopic and BVR_CI_C photometric
observations of 77 pre-main sequence (PMS) stars in the Cepheus flare region. A
total of 64 of these are newly confirmed PMS stars, originally selected from
various published candidate lists. We estimate effective temperatures and
luminosities for the PMS stars, and comparing the results with pre-main
sequence evolutionary models we estimate stellar masses of 0.2-2.4M_sun and
stellar ages of 0.1-15 Myr. Among the PMS stars, we identify 15 visual binaries
with separations of 2-10 arcsec. From archival IRAS, 2MASS, and Spitzer data,
we construct their spectral energy distributions and classify 5% of the stars
as Class I, 10% as Flat SED, 60% as Class II, and 3% as Class III young stellar
objects (YSOs). We identify 12 CTTS and 2 WTTS as members of NGC 7023, with
mean age of 1.6 Myr. The 13 PMS stars associated with L1228 belong to three
small aggregates: RNO 129, L1228A, and L1228S. The age distribution of the 17
PMS stars associated with L1251 suggests that star formation has propagated
with the expansion of the Cepheus flare shell. We detect sparse aggregates of
6-7 Myr old PMS stars around the dark clouds L1177 and L1219, at a distance of
400 pc. Three T Tauri stars appear to be associated with the Herbig Ae star SV
Cep at a distance of 600 pc. Our results confirm that the molecular complex in
the Cepheus flare region contains clouds of various distances and star forming
histories.Comment: 61 pages, 27 figures, 8 tables; accepted for publication by ApJ
The Uses of Stance in Media Production: Embodied Sociolinguistics and Beyond
While many conversation analysts, and scholars in related fields, have used video-recordings to study interaction, this study is one of a small but growing number that investigates video-recordings of the joint activities of media professionals working with, and on, video. It examines practices of media production that are, in their involvement with the visual and verbal qualities of video, both beyond talk and deeply shaped by talk. The article draws upon video recordings of the making of a feature-length documentary. In particular, it analyses a complex course of action where an editing team are reviewing their interview of the subject of the documentary, their footage is being intercut with existing reality TV footage of that same interviewee. The central contributions that the article makes are, firstly, to the sociolinguistics of mediatisation, through the identification of the workplace concerns of the members of the editing team, secondly showing how editing is accomplished, moment-by-moment, through the use of particular forms of embodied action and, finally, how the media themselves feature in the ordering of action. While this is professional work it sheds light on the video-mediated practices in contemporary culture, especially those found in social media where video makers carefully consider their editing of the perspective toward themselves and others
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