35,311 research outputs found
Non-equilibrium thermodynamics in sheared hard-sphere materials
We combine the shear-transformation-zone (STZ) theory of amorphous plasticity
with Edwards' statistical theory of granular materials to describe shear flow
in a disordered system of thermalized hard spheres. The equations of motion for
this system are developed within a statistical thermodynamic framework
analogous to that which has been used in the analysis of molecular glasses. For
hard spheres, the system volume replaces the internal energy as a
function of entropy in conventional statistical mechanics. In place of the
effective temperature, the compactivity
characterizes the internal state of disorder. We derive the STZ equations of
motion for a granular material accordingly, and predict the strain rate as a
function of the ratio of the shear stress to the pressure for different values
of a dimensionless, temperature-like variable near a jamming transition. We use
a simplified version of our theory to interpret numerical simulations by
Haxton, Schmiedeberg and Liu, and in this way are able to obtain useful
insights about internal rate factors and relations between jamming and glass
transitions.Comment: 9 pages, 6 figure
Shear flow of angular grains: acoustic effects and non-monotonic rate dependence of volume
Naturally-occurring granular materials often consist of angular particles
whose shape and frictional characteristics may have important implications on
macroscopic flow rheology. In this paper, we provide a theoretical account for
the peculiar phenomenon of auto-acoustic compaction -- non-monotonic variation
of shear band volume with shear rate in angular particles -- recently observed
in experiments. Our approach is based on the notion that the volume of a
granular material is determined by an effective-disorder temperature known as
the compactivity. Noise sources in a driven granular material couple its
various degrees of freedom and the environment, causing the flow of entropy
between them. The grain-scale dynamics is described by the
shear-transformation-zone (STZ) theory of granular flow, which accounts for
irreversible plastic deformation in terms of localized flow defects whose
density is governed by the state of configurational disorder. To model the
effects of grain shape and frictional characteristics, we propose an Ising-like
internal variable to account for nearest-neighbor grain interlocking and
geometric frustration, and interpret the effect of friction as an acoustic
noise strength. We show quantitative agreement between experimental
measurements and theoretical predictions, and propose additional experiments
that provide stringent tests on the new theoretical elements.Comment: 12 pages, 3 figure
Computer simulation of preflight blood volume reduction as a countermeasure to fluid shifts in space flight
Fluid shifts in weightlessness may cause a central volume expansion, activating reflexes to reduce the blood volume. Computer simulation was used to test the hypothesis that preadaptation of the blood volume prior to exposure to weightlessness could counteract the central volume expansion due to fluid shifts and thereby attenuate the circulatory and renal responses resulting in large losses of fluid from body water compartments. The Guyton Model of Fluid, Electrolyte, and Circulatory Regulation was modified to simulate the six degree head down tilt that is frequently use as an experimental analog of weightlessness in bedrest studies. Simulation results show that preadaptation of the blood volume by a procedure resembling a blood donation immediately before head down bedrest is beneficial in damping the physiologic responses to fluid shifts and reducing body fluid losses. After ten hours of head down tilt, blood volume after preadaptation is higher than control for 20 to 30 days of bedrest. Preadaptation also produces potentially beneficial higher extracellular volume and total body water for 20 to 30 days of bedrest
High Momentum Dilepton Production from Jets in a Quark Gluon Plasma
We discuss the emission of high momentum lepton pairs in central Au+Au
collisions at RHIC (200 GeV) and Pb+Pb collisions at the LHC
(5500 GeV). Yields of dileptons produced through interactions
of jets with thermal partons have been calculated, with next-to-leading order
corrections through hard thermal loops (HTL) resummation. They are compared to
thermal dilepton emission and the Drell-Yan process. A complete leading order
treatment of jet energy loss has been included. Jet-plasma interactions are
found to dominate over thermal dilepton emission for all values of the
invariant mass . Drell-Yan is the dominant source of high momentum lepton
pairs for 3 GeV at RHIC, after the background from heavy quark decays is
subtracted. At LHC, the range GeV is dominated by jet-plasma
interactions. Effects from jet energy loss on jet-plasma interactions turn out
to be weak, but non-negligible, reducing the yield of low-mass dileptons by
about 30%.Comment: 20 pages, 12 figures, discussions adde
Stick-slip instabilities in sheared granular flow: the role of friction and acoustic vibrations
We propose a theory of shear flow in dense granular materials. A key
ingredient of the theory is an effective temperature that determines how the
material responds to external driving forces such as shear stresses and
vibrations. We show that, within our model, friction between grains produces
stick-slip behavior at intermediate shear rates, even if the material is
rate-strengthening at larger rates. In addition, externally generated acoustic
vibrations alter the stick-slip amplitude, or suppress stick-slip altogether,
depending on the pressure and shear rate. We construct a phase diagram that
indicates the parameter regimes for which stick-slip occurs in the presence and
absence of acoustic vibrations of a fixed amplitude and frequency. These
results connect the microscopic physics to macroscopic dynamics, and thus
produce useful information about a variety of granular phenomena including
rupture and slip along earthquake faults, the remote triggering of
instabilities, and the control of friction in material processing.Comment: 12 pages, 8 figure
Implementation of control point form of algebraic grid-generation technique
The control point form (CPF) provides explicit control of physical grid shape and grid spacing through the movement of the control points. The control point array, called a control net, is a space grid type arrangement of locations in physical space with an index for each direction. As an algebraic method CPF is efficient and works well with interactive computer graphics. A family of menu-driven, interactive grid-generation computer codes (TURBO) is being developed by using CPF. Key features of TurboI (a TURBO member) are discussed and typical results are presented. TurboI runs on any IRIS 4D series workstation
Space sickness predictors suggest fluid shift involvement and possible countermeasures
Preflight data from 64 first time Shuttle crew members were examined retrospectively to predict space sickness severity (NONE, MILD, MODERATE, or SEVERE) by discriminant analysis. From 9 input variables relating to fluid, electrolyte, and cardiovascular status, 8 variables were chosen by discriminant analysis that correctly predicted space sickness severity with 59 pct. success by one method of cross validation on the original sample and 67 pct. by another method. The 8 variables in order of their importance for predicting space sickness severity are sitting systolic blood pressure, serum uric acid, calculated blood volume, serum phosphate, urine osmolality, environmental temperature at the launch site, red cell count, and serum chloride. These results suggest the presence of predisposing physiologic factors to space sickness that implicate a fluid shift etiology. Addition of a 10th input variable, hours spent in the Weightless Environment Training Facility (WETF), improved the prediction of space sickness severity to 66 pct. success by the first method of cross validation on the original sample and to 71 pct. by the second method. The data suggest that WETF training may reduce space sickness severity
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