10,028 research outputs found
The stochastic dynamics of nanoscale mechanical oscillators immersed in a viscous fluid
The stochastic response of nanoscale oscillators of arbitrary geometry
immersed in a viscous fluid is studied. Using the fluctuation-dissipation
theorem it is shown that deterministic calculations of the governing fluid and
solid equations can be used in a straightforward manner to directly calculate
the stochastic response that would be measured in experiment. We use this
approach to investigate the fluid coupled motion of single and multiple
cantilevers with experimentally motivated geometries.Comment: 5 pages, 5 figure
Equation of state for hard sphere fluids with and without Kac tails
In this note, we propose a simple derivation of the one dimensional hard rod
equation of state, with and without a Kac tail (appended long range and weak
potential). The case of hard spheres in higher dimension is also addressed and
it is shown there that our arguments --which avoid any mathematical
complication-- allow to recover the virial form of the equation of state in a
direct way.Comment: pedagogical pape
Fluctuation-dissipation ratios in the dynamics of self-assembly
We consider two seemingly very different self-assembly processes: formation
of viral capsids, and crystallization of sticky discs. At low temperatures,
assembly is ineffective, since there are many metastable disordered states,
which are a source of kinetic frustration. We use fluctuation-dissipation
ratios to extract information about the degree of this frustration. We show
that our analysis is a useful indicator of the long term fate of the system,
based on the early stages of assembly.Comment: 8 pages, 6 figure
Unveiling Sources of Heating in the Vicinity of the Orion BN/KL Hot Core as Traced by Highly Excited Inversion Transitions of Ammonia
Using the Expanded Very Large Array, we have mapped the vicinity of the Orion
BN/KL Hot Core with sub-arcsecond angular resolution in seven metastable
inversion transitions of ammonia: (J,K)=(6,6) to (12,12). This emission comes
from levels up to 1500 K above the ground state, enabling identification of
source(s) responsible for heating the region. We used this multi-transition
dataset to produce images of the rotational/kinetic temperature and the column
density of ammonia for ortho and para species separately and on a
position-by-position basis. We find rotational temperature and column density
in the range 160-490 K and (1-4)x10^17 cm^-2, respectively. Our
spatially-resolved images show that the highest (column) density and hottest
gas is found in a northeast-southwest elongated ridge to the southeast of
Source I. We have also measured the ortho-para ratio of ammonia, estimated to
vary in the range 0.9-1.6. Enhancement of ortho with respect to para and the
offset of hot ammonia emission peaks from known (proto)stellar sources provide
evidence that the ammonia molecules have been released from dust grains into
the gas-phase through the passage of shocks and not by stellar radiation. We
propose that the combined effect of Source I's proper motion and its
low-velocity outflow impinging on a pre-existing dense medium is responsible
for the excitation of ammonia and the Orion Hot Core. Finally, we found for the
first time evidence of a slow (5 km/s) and compact (1000 AU) outflow towards
IRc7.Comment: To appear in Astrophysical Journal Letters Special Issue on the EVLA.
8 pages, 4 figure
Modal Test of the NASA Mobile Launcher at Kennedy Space Center
The NASA Mobile Launcher (ML), located at Kennedy Space Center (KSC), has recently been modified to support the launch of the new NASA Space Launch System (SLS). The ML is a massive structureconsisting of a 345-foot tall tower attached to a two-story base, weighing approximately 10.5 million poundsthat will secure the SLS vehicle as it rolls to the launch pad on a Crawler Transporter, as well as provide a launch platform at the pad. The ML will also provide the boundary condition for an upcoming SLS Integrated Modal Test (IMT). To help correlate the ML math models prior to this modal test, and allow focus to remain on updating SLS vehicle models during the IMT, a ML-only experimental modal test was performed in June 2019. Excitation of the tower and platform was provided by five uniquely-designed test fixtures, each enclosing a hydraulic shaker, capable of exerting thousands of pounds of force into the structure. For modes not that were not sufficiently excited by the test fixture shakers, a specially-designed mobile drop tower provided impact excitation at additional locations of interest. The response of the ML was measured with a total of 361 accelerometers. Following the random vibration, sine sweep vibration, and modal impact testing, frequency response functions were calculated and modes were extracted for three different configurations of the ML in 0 Hz to 12 Hz frequency range. This paper will provide a case study in performing modal tests on large structures by discussing the Mobile Launcher, the test strategy, an overview of the test results, and recommendations for meeting a tight test schedule for a large-scale modal test
Thermodynamics of Quantum Jump Trajectories
We apply the large-deviation method to study trajectories in dissipative
quantum systems. We show that in the long time limit the statistics of quantum
jumps can be understood from thermodynamic arguments by exploiting the analogy
between large-deviation and free-energy functions. This approach is
particularly useful for uncovering properties of rare dissipative trajectories.
We also prove, via an explicit quantum mapping, that rare trajectories of one
system can be realized as typical trajectories of an alternative system.Comment: 5 pages, 3 figure
Entropy and Temperature of a Static Granular Assembly
Granular matter is comprised of a large number of particles whose collective
behavior determines macroscopic properties such as flow and mechanical
strength. A comprehensive theory of the properties of granular matter,
therefore, requires a statistical framework. In molecular matter, equilibrium
statistical mechanics, which is founded on the principle of conservation of
energy, provides this framework. Grains, however, are small but macroscopic
objects whose interactions are dissipative since energy can be lost through
excitations of the internal degrees of freedom. In this work, we construct a
statistical framework for static, mechanically stable packings of grains, which
parallels that of equilibrium statistical mechanics but with conservation of
energy replaced by the conservation of a function related to the mechanical
stress tensor. Our analysis demonstrates the existence of a state function that
has all the attributes of entropy. In particular, maximizing this state
function leads to a well-defined granular temperature for these systems.
Predictions of the ensemble are verified against simulated packings of
frictionless, deformable disks. Our demonstration that a statistical ensemble
can be constructed through the identification of conserved quantities other
than energy is a new approach that is expected to open up avenues for
statistical descriptions of other non-equilibrium systems.Comment: 5 pages, 4 figure
Entropy and density of states from isoenergetic nonequilibrium processes
Two identities in statistical mechanics involving entropy differences (or
ratios of density of states) at constant energy are derived. The first provides
a nontrivial extension of the Jarzynski equality to the microcanonical ensemble
[C. Jarzynski, Phys. Rev. Lett. 78, 2690 (1997)], which can be seen as a
``fast-switching'' version of the adiabatic switching method for computing
entropies [M. Watanabe, W. P. Reinhardt, Phys. Rev. Lett. 65, 3301 (1990)]. The
second is a thermodynamic integration formula analogous to a well-known
expression for free energies, and follows after taking the quasistatic limit of
the first. Both identities can be conveniently used in conjunction with a
scaling relation (herein derived) that allows one to extrapolate measurements
taken at a single energy to a wide range of energy values. Practical aspects of
these identities in the context of numerical simulations are discussed.Comment: 5 pages, no figure
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