166 research outputs found
PMH14 HEALTH CARE EXPENDITURES OF PATIENTS WITH MAJOR DEPRESSIVE DISORDER AND POST TRAUMATIC STRESS DISORDER
A computer model is presented that describes soleus H-reflex recruitment as a function of electric stimulus intensity. The model consists of two coupled non-linear transfer functions. The first transfer function describes the activation of muscle spindle (Ia) afferent terminals as a function of the electric stimulus intensity; whereas the second describes the activation of a number of motoneurons as a function of the number of active Ia afferent terminals. The effect of change in these transfer functions on the H-reflex recruitment curve is simulated. In spastic patients, a higher average maximal H-response amplitude is observed in combination with a decreased H-reflex threshold. Vibration of the Achilles tendon reduces the H-reflex amplitude, presumably by reducing the excitatory afferent input. Vibratory inhibition is diminished in spasticity. In the model, the afferent-motoneuron transfer function was modified to represent the possible alterations occurring in spasticity. The simulations show that vibratory suppression of the H-reflex is determined only in part by the inhibition level of the afferent input. With a constant level of presynaptic inhibition, the suppression of reflexes of different sizes may vary. A lowering of the motoneuron activation thresholds in spastic patients will directly contribute to a decrease of vibratory inhibition in spasticit
Dissipative Particle Dynamics with energy conservation
Dissipative particle dynamics (DPD) does not conserve energy and this
precludes its use in the study of thermal processes in complex fluids. We
present here a generalization of DPD that incorporates an internal energy and a
temperature variable for each particle. The dissipation induced by the
dissipative forces between particles is invested in raising the internal energy
of the particles. Thermal conduction occurs by means of (inverse) temperature
differences. The model can be viewed as a simplified solver of the fluctuating
hydrodynamic equations and opens up the possibility of studying thermal
processes in complex fluids with a mesoscopic simulation technique.Comment: 5 page
Lattice Boltzmann Thermohydrodynamics
We introduce a lattice Boltzmann computational scheme capable of modeling
thermohydrodynamic flows of monatomic gases. The parallel nature of this
approach provides a numerically efficient alternative to traditional methods of
computational fluid dynamics. The scheme uses a small number of discrete
velocity states and a linear, single-time-relaxation collision operator.
Numerical simulations in two dimensions agree well with exact solutions for
adiabatic sound propagation and Couette flow with heat transfer.Comment: 11 pages, Physical Review E: Rapid Communications, in pres
Superfluid pairing in a polarized dipolar Fermi gas
We calculate the critical temperature of a superfluid phase transition in a
polarized Fermi gas of dipolar particles. In this case the order parameter is
anisotropic and has a nontrivial energy dependence. Cooper pairs do not have a
definite value of the angular momentum and are coherent superpositions of all
odd angular momenta. Our results describe prospects for achieving the
superfluid transition in single-component gases of fermionic polar molecules.Comment: 12 pages, 2 figure
Anisotropic thermally activated diffusion in percolation systems
We present a study of static and frequency-dependent diffusion with
anisotropic thermally activated transition rates in a two-dimensional bond
percolation system. The approach accounts for temperature effects on diffusion
coefficients in disordered anisotropic systems. Static diffusion shows an
Arrhenius behavior for low temperatures with an activation energy given by the
highest energy barrier of the system. From the frequency-dependent diffusion
coefficients we calculate a characteristic frequency ,
related to the time needed to overcome a characteristic barrier. We find
that follows an Arrhenius behavior with different activation
energies in each direction.Comment: 5 pages, 4 figure
A discretized integral hydrodynamics
Using an interpolant form for the gradient of a function of position, we
write an integral version of the conservation equations for a fluid. In the
appropriate limit, these become the usual conservation laws of mass, momentum
and energy. We also discuss the special cases of the Navier-Stokes equations
for viscous flow and the Fourier law for thermal conduction in the presence of
hydrodynamic fluctuations. By means of a discretization procedure, we show how
these equations can give rise to the so-called "particle dynamics" of Smoothed
Particle Hydrodynamics and Dissipative Particle Dynamics.Comment: 10 pages, RevTex, submitted to Phys. Rev.
Pauli Blocking of Collisions in a Quantum Degenerate Atomic Fermi Gas
We have produced an interacting quantum degenerate Fermi gas of atoms
composed of two spin-states of magnetically trapped K. The relative
Fermi energies are adjusted by controlling the population in each spin-state.
Measurements of the thermodynamics reveal the resulting imbalance in the mean
energy per particle between the two species, which is as large as a factor of
1.4 at our lowest temperature. This imbalance of energy comes from a
suppression of collisions between atoms in the gas due to the Pauli exclusion
principle. Through measurements of the thermal relaxation rate we have directly
observed this Pauli blocking as a factor of two reduction in the effective
collision cross-section in the quantum degenerate regime.Comment: 11 pages, 4 figure
Thermodynamically admissible form for discrete hydrodynamics
We construct a discrete model of fluid particles according to the GENERIC
formalism. The model has the form of Smoothed Particle Hydrodynamics including
correct thermal fluctuations. A slight variation of the model reproduces the
Dissipative Particle Dynamics model with any desired thermodynamic behavior.
The resulting algorithm has the following properties: mass, momentum and energy
are conserved, entropy is a non-decreasing function of time and the thermal
fluctuations produce the correct Einstein distribution function at equilibrium.Comment: 4 page
1S-2S Spectrum of a Hydrogen Bose-Einstein Condensate
We calculate the two-photon 1S-2S spectrum of an atomic hydrogen
Bose-Einstein condensate in the regime where the cold collision frequency shift
dominates the lineshape. WKB and static phase approximations are made to find
the intensities for transitions from the condensate to motional eigenstates for
2S atoms. The excited state wave functions are found using a mean field
potential which includes the effects of collisions with condensate atoms.
Results agree well with experimental data. This formalism can be used to find
condensate spectra for a wide range of excitation schemes.Comment: 13 pages, 4 figure
Observation of p-wave Threshold Law Using Evaporatively Cooled Fermionic Atoms
We have measured independently both s-wave and p-wave cross-dimensional
thermalization rates for ultracold potassium-40 atoms held in a magnetic trap.
These measurements reveal that this fermionic isotope has a large positive
s-wave triplet scattering length in addition to a low temperature p-wave shape
resonance. We have observed directly the p-wave threshold law which, combined
with the Fermi statistics, dramatically suppresses elastic collision rates at
low temperatures. In addition, we present initial evaporative cooling results
that make possible these collision measurements and are a precursor to
achieving quantum degeneracy in this neutral, low-density Fermi system.Comment: 5 pages, 3 figures, 1 tabl
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