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Paul Umbanhowar

Publications and source records attributed to Paul Umbanhowar.

6 recordsLinked to original sources
PubMed2006

Dynamics of axial segregation in granular slurries: parallel experiments and influence of aspect ratio and periodic tilting.

An efficient technique for conducting rotating tumbler experiments in parallel is introduced and used to study the effect of tumbler length and periodic tilting of the tumbler on axial segregation. When rotated, bidisperse granular slurries segregate into what appear at the surface to be alternating bands of larger and smaller particles. The number of bands increases linearly with tumbler length while the fractional area occupied by each type of band is constant. Periodic tilting of the rotation axis induces a periodic axial flow of particles in the flowing layer. For the range of tilt angle amplitudes investigated (0 degrees -3.5 degrees), the number of bands decreases with increasing angle, but the rate of merging and the fractional area of bands rich in smaller particles are unaffected.

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PubMed2006

Enhanced Faraday pattern stability with three-frequency driving.

We report experimental observations of enhanced stability of quasipatterns and superlattice patterns in a vertically oscillated, deep viscous fluid layer with the addition of a third driving frequency. With two-frequency driving in the ratios 4:5 and 6:7, 12-fold quasipatterns and type-I superlattice patterns appear, respectively, as a secondary instability for a range of relative phases and amplitudes. Addition of a small third-frequency component at twice the difference frequency, i.e., 4:5:2 and 6:7:2, shifts the region of stability for these patterns closer to onset. For a range of parameter values the stabilized patterns become the primary instability. The degree of stabilization is sensitive to the amplitude and relative phase of the third-frequency term in qualitative agreement with a recent symmetry based analysis of resonant three-wave interactions.

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PubMed2006

Forcing function control of Faraday wave instabilities in viscous shallow fluids.

We investigate the relationship between the linear surface wave instabilities of a shallow viscous fluid layer and the shape of the periodic, parametric-forcing function (describing the vertical acceleration of the fluid container) that excites them. We find numerically that the envelope of the resonance tongues can only develop multiple minima when the forcing function has more than two local extrema per cycle. With this insight, we construct a multi-frequency forcing function that generates at onset a nontrivial harmonic instability which is distinct from a subharmonic response to any of its frequency components. We measure the corresponding surface patterns experimentally and verify that small changes in the forcing waveform cause a transition, through a bicritical point, from the predicted harmonic short-wavelength pattern to a much larger standard subharmonic pattern. Using a formulation valid in the lubrication regime (thin viscous fluid layer) and a Wentzel-Kramers-Brillouin (WKB) method to find its analytic solutions, we explore the origin of the observed relation between the forcing function shape and the resonance tongue structure. In particular, we show that for square and triangular forcing functions the envelope of these tongues has only one minimum, as in the usual sinusoidal case.

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PubMed2005

Force dynamics in weakly vibrated granular packings.

Variations in the oscillatory force F(b) on the bottom of a rigid, grain filled column, reveal rich granular dynamics when the column is vertically vibrated with an acceleration amplitude significantly less than the gravitational acceleration at the earth's surface. Large changes in F(b) occur even though the maximum relative motion of the container bottom with respect to the wall is less than 2 nm. For previously unshaken packings or high frequencies, F(b)'s dynamics are dominated by grain motion. For moderate driving conditions in already shaken samples, grain motion is virtually absent, but F(b) nevertheless exhibits strongly nonlinear and hysteretic behavior, evidencing a granular regime dominated by nontrivial force-network dynamics.

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PubMed2005

Creeping motion in granular flow.

The core of a quasi-two-dimensional rotating cylinder filled more than half full with glass beads rotates slightly faster than the cylinder itself and decreases in radius over time. Core precession depends linearly on the number of tumbler revolutions while core erosion varies logarithmically. Both processes serve to quantify the slow granular motion in the "fixed" bed and depend on the filling fraction and the tumbler rotation rate. A simple model, based on experimental observations of an exponential decrease in velocity parallel to the free surface, captures the primary features of the core dynamics.

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