146 research outputs found

### Influence of friction on granular segregation

Vertical shaking of a mixture of small and large beads can lead to
segregation where the large beads either accumulate at the top of the sample,
the so called Brazil Nut effect (BNE), or at the bottom, the Reverse Brazil Nut
effect (RBNE). Here we demonstrate experimentally a sharp transition from the
RBNE to the BNE when the particle coefficient of friction increases due to
aging of the particles. This result can be explained by the two competing
mechanisms of buoyancy and sidewall-driven convection, where the latter is
assumed to grow in strength with increasing friction.Comment: 3 pages, 2 figure

### Observations of the stratorotational instability in rotating concentric cylinders

We study the stability of density stratified flow between co-rotating
vertical cylinders with rotation rates $\Omega_o < \Omega_i$ and radius ratio
$r_i/r_o=0.877$, where subscripts $o$ and $i$ refer to the outer and inner
cylinders. Just as in stellar and planetary accretion disks, the flow has
rotation, anticyclonic shear, and a stabilizing density gradient parallel to
the rotation axis. The primary instability of the laminar state leads not to
axisymmetric Taylor vortex flow but to the non-axisymmetric {\it
stratorotational instability} (SRI), so named by Shalybkov and R\"udiger
(2005). The present work extends the range of Reynolds numbers and buoyancy
frequencies ($N=\sqrt{(-g/\rho)(\partial \rho/\partial z)}$) examined in the
previous experiments by Boubnov and Hopfinger (1997) and Le Bars and Le Gal
(2007). Our observations reveal that the axial wavelength of the SRI
instability increases nearly linearly with Froude number, $Fr= \Omega_i/N$. For
small outer cylinder Reynolds number, the SRI occurs for inner inner Reynolds
number larger than for the axisymmetric Taylor vortex flow (i.e., the SRI is
more stable). For somewhat larger outer Reynolds numbers the SRI occurs for
smaller inner Reynolds numbers than Taylor vortex flow and even below the
Rayleigh stability line for an inviscid fluid. Shalybkov and R\"udiger (2005)
proposed that the laminar state of a stably stratified rotating shear flow
should be stable for $\Omega_o/ \Omega_i > r_i/r_o$, but we find that this
stability criterion is violated for $N$ sufficiently large; however, the
destabilizing effect of the density stratification diminishes as the Reynolds
number increases. At large Reynolds number the primary instability leads not to
the SRI but to a previously unreported nonperiodic state that mixes the fluid

### Propagating and evanescent internal waves in a deep ocean model

We present experimental and computational studies of the propagation of
internal waves in a stratified fluid with an exponential density profile that
models the deep ocean. The buoyancy frequency profile $N(z)$ (proportional to
the square root of the density gradient) varies smoothly by more than an order
of magnitude over the fluid depth, as is common in the deep ocean. The
nonuniform stratification is characterized by a turning depth $z_c$, where
$N(z_c)$ is equal to the wave frequency $\omega$ and $N(z < z_c) < \omega$.
Internal waves reflect from the turning depth and become evanescent below the
turning depth. The energy flux below the turning depth is shown to decay
exponentially with a decay constant given by $k_c$, which is the horizontal
wavenumber at the turning depth. The viscous decay of the vertical velocity
amplitude of the incoming and reflected waves above the turning depth agree
within a few percent with a previously untested theory for a fluid of arbitrary
stratification [Kistovich and Chashechkin, J. App. Mech. Tech. Phys. 39,
729-737 (1998)].Comment: 13 pages, 4 figures, 4 table

### Nucleation in sheared granular matter

We present an experiment on crystallization of packings of macroscopic
granular spheres. This system is often considered to be a model for thermally
driven atomic or colloidal systems. Cyclically shearing a packing of frictional
spheres, we observe a first order phase transition from a disordered to an
ordered state. The ordered state consists of crystallites of mixed FCC and HCP
symmetry that coexist with the amorphous bulk. The transition, initiated by
homogeneous nucleation, overcomes a barrier at 64.5% volume fraction.
Nucleation consists predominantly of the dissolving of small nuclei and the
growth of nuclei that have reached a critical size of about ten spheres

### Cracks in rubber under tension exceed the shear wave speed

The shear wave speed is an upper limit for the speed of cracks loaded in
tension in linear elastic solids. We have discovered that in a non-linear
material, cracks in tension (Mode I) exceed this sound speed, and travel in an
intersonic range between shear and longitudinal wave speeds. The experiments
are conducted in highly stretched sheets of rubber; intersonic cracks can be
produced simply by popping a balloon.Comment: 4 pages, 5 eps figure

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