13,237 research outputs found
Dark cloud cores and gravitational decoupling from turbulent flows
We test the hypothesis that the starless cores may be gravitationally bound
clouds supported largely by thermal pressure by comparing observed molecular
line spectra to theoretical spectra produced by a simulation that includes
hydrodynamics, radiative cooling, variable molecular abundance, and radiative
transfer in a simple one-dimensional model. The results suggest that the
starless cores can be divided into two categories: stable starless cores that
are in approximate equilibrium and will not evolve to form protostars, and
unstable pre-stellar cores that are proceeding toward gravitational collapse
and the formation of protostars. The starless cores might be formed from the
interstellar medium as objects at the lower end of the inertial cascade of
interstellar turbulence. Additionally, we identify a thermal instability in the
starless cores. Under par ticular conditions of density and mass, a core may be
unstable to expansion if the density is just above the critical density for the
collisional coupling of the gas and dust so that as the core expands the
gas-dust coupling that cools the gas is reduced and the gas warms, further
driving the expansion.Comment: Submitted to Ap
Wall slip and flow of concentrated hard-sphere colloidal suspensions
We present a comprehensive study of the slip and flow of concentrated
colloidal suspensions using cone-plate rheometry and simultaneous confocal
imaging. In the colloidal glass regime, for smooth, non-stick walls, the solid
nature of the suspension causes a transition in the rheology from
Herschel-Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip
behavior at low stress, which is suppressed for sufficient colloid-wall
attraction or colloid-scale wall roughness. Visualization shows how the
slip-shear transition depends on gap size and the boundary conditions at both
walls and that partial slip persist well above the yield stress. A
phenomenological model, incorporating the Bingham slip law and HB bulk flow,
fully accounts for the behavior. Microscopically, the Bingham law is related to
a thin (sub-colloidal) lubrication layer at the wall, giving rise to a
characteristic dependence of slip parameters on particle size and
concentration. We relate this to the suspension's osmotic pressure and yield
stress and also analyze the influence of van der Waals interaction. For the
largest concentrations, we observe non-uniform flow around the yield stress, in
line with recent work on bulk shear-banding of concentrated pastes. We also
describe residual slip in concentrated liquid suspensions, where the vanishing
yield stress causes coexistence of (weak) slip and bulk shear flow for all
measured rates
Preliminary catalog of pictures taken on the lunar surface during the Apollo 15 mission
Catalog of all pictures taken from lunar module or lunar surface during Apollo 15 missio
A Cut Finite Element Method for the Bernoulli Free Boundary Value Problem
We develop a cut finite element method for the Bernoulli free boundary problem. The free boundary, represented by an approximate signed distance function on a fixed background mesh, is allowed to intersect elements in an arbitrary fashion. This leads to so called cut elements in the vicinity of the boundary. To obtain a stable method, stabilization terms is added in the vicinity of the cut elements penalizing the gradient jumps across element sides. The stabilization also ensures good conditioning of the resulting discrete system. We develop a method for shape optimization based on moving the distance function along a velocity field which is computed as the Riesz representation of the shape derivative. We show that the velocity field is the solution to an interface problem and we prove an a priori error estimate of optimal order, given the limited regularity of the velocity field across the interface, for the the velocity field in the norm. Finally, we present illustrating numerical results
A Cut Finite Element Method for the Bernoulli Free Boundary Value Problem
We develop a cut finite element method for the Bernoulli free boundary problem. The free boundary, represented by an approximate signed distance function on a fixed background mesh, is allowed to intersect elements in an arbitrary fashion. This leads to so called cut elements in the vicinity of the boundary. To obtain a stable method, stabilization terms is added in the vicinity of the cut elements penalizing the gradient jumps across element sides. The stabilization also ensures good conditioning of the resulting discrete system. We develop a method for shape optimization based on moving the distance function along a velocity field which is computed as the Riesz representation of the shape derivative. We show that the velocity field is the solution to an interface problem and we prove an a priori error estimate of optimal order, given the limited regularity of the velocity field across the interface, for the the velocity field in the norm. Finally, we present illustrating numerical results
Mixing by polymers: experimental test of decay regime of mixing
By using high molecular weight fluorescent passive tracers with different
diffusion coefficients and by changing the fluid velocity we study dependence
of a characteristic mixing length on the Peclet number, , which controls
the mixing efficiency. The mixing length is found to be related to by a
power law, , and increases faster than
expected for an unbounded chaotic flow. Role of the boundaries in the mixing
length abnormal growth is clarified. The experimental findings are in a good
quantitative agreement with the recent theoretical predictions.Comment: 4 pages,5 figures. accepted for publication in PR
Motility of small nematodes in disordered wet granular media
The motility of the worm nematode \textit{Caenorhabditis elegans} is
investigated in shallow, wet granular media as a function of particle size
dispersity and area density (). Surprisingly, we find that the nematode's
propulsion speed is enhanced by the presence of particles in a fluid and is
nearly independent of area density. The undulation speed, often used to
differentiate locomotion gaits, is significantly affected by the bulk material
properties of wet mono- and polydisperse granular media for .
This difference is characterized by a change in the nematode's waveform from
swimming to crawling in dense polydisperse media \textit{only}. This change
highlights the organism's adaptability to subtle differences in local structure
and response between monodisperse and polydisperse media
Theory of nonlinear rheology and yielding of dense colloidal suspensions
A first principles approach to the nonlinear flow of dense suspensions is
presented which captures shear thinning of colloidal fluids and dynamical
yielding of colloidal glasses. The advection of density fluctuations plays a
central role, suppressing the caging of particles and speeding up structural
relaxation. A mode coupling approach is developed to explore these effects.Comment: 4 pages, 2 figures; slightly corrected version; Phys. Rev. Lett., to
be published (2002
- …