4,652 research outputs found
Evidence of reverse and intermediate size segregation in dry granular flows down a rough incline
In a dry granular flow, size segregation behave differently for a mixture
containing a few large beads with a size ratio (S) above 5 (Thomas, Phys.Rev.E
62,96(2000)). For moderate large S, large beads migrate to an intermediate
depth in the bed: this is called intermediate segregation. For the largest S,
large beads migrate to the bottom: this is called reverse segregation (in
contrast with surface segregation). As the reversal and intermediate depth
values depend on the bead fraction, this numerical study mainly uses a single
large tracer. Small fractions are also computed showing the link between a
tracer behavior and segregation process. For half-filled rotating drum and for
rough incline, two and three (3D) dimensional cases are studied. In the
tumbler, trajectories of a large tracer show that it reaches a constant depth
during the flow. For large S, this depth is intermediate with a progressive
sinking when S increases. Largest S correspond to tracers at the bottom of the
flow. All 3D simulation are in quantitative agreement with the experiments. In
the flow down an incline, a large tracer reaches an equilibrium depth during
flow. For large S, its depth is intermediate, inside the bed. For the largest
S, its depth is reverse, near the bottom. Results are slightly different for
thin or thick flow. For 3D thick flows, the reversal between surface and bottom
positions occurs within a short range of S: no tracer stabilizes near
mid-height and two reachable intermediate depth layers exist, below the surface
and above the bottom. For 3D thin flows, all intermediate depths are reachable,
depending on S. The numerical study of larger tracer fractions (5-10%) shows
the 3 segregation patterns (surface, intermediate, reverse) corresponding to
the 3 types of equilibrium depth. The reversal is smoother than for a single
tracer. It happens around S=4.5, in agreement with experiments.Comment: 18 pages, 27 figure
Continuum theory of partially fluidized granular flows
A continuum theory of partially fluidized granular flows is developed. The
theory is based on a combination of the equations for the flow velocity and
shear stresses coupled with the order parameter equation which describes the
transition between flowing and static components of the granular system. We
apply this theory to several important granular problems: avalanche flow in
deep and shallow inclined layers, rotating drums and shear granular flows
between two plates. We carry out quantitative comparisons between the theory
and experiment.Comment: 28 pages, 23 figures, submitted to Phys. Rev.
Introduction to chaos and diffusion
This contribution is relative to the opening lectures of the ISSAOS 2001
summer school and it has the aim to provide the reader with some concepts and
techniques concerning chaotic dynamics and transport processes in fluids. Our
intention is twofold: to give a self-consistent introduction to chaos and
diffusion, and to offer a guide for the reading of the rest of this volume.Comment: 39 page
Turbulence lifetimes: what we can learn from the physics of glasses
In this note, we critically discuss the issue of the possible finiteness of
the turbulence lifetime in subcritical transition to turbulence in shear flows,
which attracted a lot of interest recently. We briefly review recent
experimental and numerical results, as well as theoretical proposals, and
compare the difficulties arising in assessing this issue in subcritical shear
flow with that encountered in the study of the glass transition. In order to go
beyond the purely methodological similarities, we further elaborate on this
analogy and propose a qualitative mapping between these two apparently
unrelated situations, which could possibly foster new directions of research in
subcritical shear flows.Comment: 10 pages, 4 figure
Numerical and experimental analysis of a thin liquid film on a rotating disk related to development of a spacecraft absorption cooling system
The numerical and experimental analysis of a thin liquid film on a rotating and a stationary disk related to the development of an absorber unit for a high capacity spacecraft absorption cooling system, is described. The creation of artificial gravity by the use of a centrifugal field was focused upon in this report. Areas covered include: (1) One-dimensional computation of thin liquid film flows; (2) Experimental measurement of film height and visualization of flow; (3) Two-dimensional computation of the free surface flow of a thin liquid film using a pressure optimization method; (4) Computation of heat transfer in two-dimensional thin film flow; (5) Development of a new computational methodology for the free surface flows using a permeable wall; (6) Analysis of fluid flow and heat transfer in a thin film in the presence and absence of gravity; and (7) Comparison of theoretical prediction and experimental data. The basic phenomena related to fluid flow and heat transfer on rotating systems reported here can also be applied to other areas of space systems
Nonaxisymmetric Neutral Modes in Rotating Relativistic Stars
We study nonaxisymmetric perturbations of rotating relativistic stars.
modeled as perfect-fluid equilibria. Instability to a mode with angular
dependence sets in when the frequency of the mode vanishes. The
locations of these zero-frequency modes along sequences of rotating stars are
computed in the framework of general relativity. We consider models of
uniformly rotating stars with polytropic equations of state, finding that the
relativistic models are unstable to nonaxisymmetric modes at significantly
smaller values of rotation than in the Newtonian limit. Most strikingly, the
m=2 bar mode can become unstable even for soft polytropes of index , while in Newtonian theory it becomes unstable only for stiff polytropes
of index . If rapidly rotating neutron stars are formed by the
accretion-induced collapse of white dwarfs, instability associated with these
nonaxisymmetric, gravitational-wave driven modes may set an upper limit on
neutron-star rotation. Consideration is restricted to perturbations that
correspond to polar perturbations of a spherical star. A study of axial
perturbations is in progress.Comment: 57 pages, 9 figure
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