9,320 research outputs found
Efficiency of Fish Propulsion
It is shown that the system efficiency of a self-propelled flexible body is
ill-defined unless one considers the concept of quasi-propulsive efficiency,
defined as the ratio of the power needed to tow a body in rigid-straight
condition over the power it needs for self-propulsion, both measured for the
same speed. Through examples we show that the quasi-propulsive efficiency is
the only rational non-dimensional metric of the propulsive fitness of fish and
fish-like mechanisms. Using two-dimensional viscous simulations and the concept
of quasi-propulsive efficiency, we discuss the efficiency two-dimensional
undulating foils. We show that low efficiencies, due to adverse body-propulsor
hydrodynamic interactions, cannot be accounted for by the increase in friction
drag
SAGE measurements of the stratospheric aerosol dispersion and loading from the Soufriere Volcano
Explosions of the Soufriere volcano on the Caribbean Island of St. Vincent reduced two major stratospheric plumes which the stratospheric aerosol and gas experiment (SAGE) satellite tracked to West Africa and the North Atlantic Ocean. The total mass of the stratospheric ejecta measured is less than 0.5% of the global stratospheric aerosol burden. No significant temperature or climate perturbation is expected. It is found that the movement and dispersion of the plumes agree with those deduced from high altitude meteorological data and dispersion theory. The stratospheric aerosol dispersion and loading from the Soufrier volcano was measured
Physics-Based Learning Models for Ship Hydrodynamics
We present the concepts of physics-based learning models (PBLM) and their relevance and application to the field of ship hydrodynamics. The utility of physics-based learning is motivated by contrasting generic learning models for regression predictions, which do not presume any knowledge of the system other than the training data provided with methods such as semi-empirical models, which incorporate physical insights along with data-fitting. PBLM provides a framework wherein intermediate models, which capture (some) physical aspects of the problem, are incorporated into modern generic learning tools to substantially improve the predictions of the latter, minimizing the reliance on costly experimental measurements or high-resolution high-fidelity numerical solutions. To illustrate the versatility and efficacy of PBLM, we present three wave-ship interaction problems: 1) at speed waterline profiles; 2) ship motions in head seas; and 3) three-dimensional breaking bow waves. PBLM is shown to be robust and produce error rates at or below the uncertainty in the generated data at a small fraction of the expense of high-resolution numerical predictions.United States. Office of Naval Researc
Structure optimization in an off-lattice protein model
We study an off-lattice protein toy model with two species of monomers
interacting through modified Lennard-Jones interactions. Low energy
configurations are optimized using the pruned-enriched-Rosenbluth method
(PERM), hitherto employed to native state searches only for off lattice models.
For 2 dimensions we found states with lower energy than previously proposed
putative ground states, for all chain lengths . This indicates that
PERM has the potential to produce native states also for more realistic protein
models. For , where no published ground states exist, we present some
putative lowest energy states for future comparison with other methods.Comment: 4 pages, 2 figure
A New Monte Carlo Algorithm for Protein Folding
We demonstrate that the recently proposed pruned-enriched Rosenbluth method
(P. Grassberger, Phys. Rev. E 56 (1997) 3682) leads to extremely efficient
algorithms for the folding of simple model proteins. We test them on several
models for lattice heteropolymers, and compare to published Monte Carlo
studies. In all cases our algorithms are faster than all previous ones, and in
several cases we find new minimal energy states. In addition to ground states,
our algorithms give estimates for the partition sum at finite temperatures.Comment: 4 pages, Latex incl. 3 eps-figs., submitted to Phys. Rev. Lett.,
revised version with changes in the tex
Phase Structures of Magnetic Impurity Models with Two-Body Hybridization
The most general model with a magnetic impurity coupled to hybridizing and
screening channels of a conduction band is considered. The partition function
of the system is asymptotically equivalent to that of the multi-component kink
plasma with a weak external field. The scaling properties of the models for
finite are sketched by using the Anderson-Yuval-Hamann-Cardy poor man's
scaling theory. We point out that it is proper to include a two-body
hybridization in order to obtain correct renormalization flows. The phase
structures are studied graphically for the general model and various reduced
models. A Fermi-non-Fermi liquid phase transition is found for all the models.
We also show all possible phases with different finite temperature behaviors
though they have the same Fermi liquid fixed point at low temperature. We also
discuss the fixed point behaviors in the mixed valence state regime.Comment: 18 pages, revtex, 3 figures in latex version, to be published in PR
Is Heteropolymer Freezing Well Described by the Random Energy Model?
It is widely held that the Random Energy Model (REM) describes the freezing
transition of a variety of types of heteropolymers. We demonstrate that the
hallmark property of REM, statistical independence of the energies of states
over disorder, is violated in different ways for models commonly employed in
heteropolymer freezing studies. The implications for proteins are also
discussed.Comment: 4 pages, 3 eps figures To appear in Physical Review Letters, May 199
Hydrodynamic Description of Granular Convection
We present a hydrodynamic model that captures the essence of granular
dynamics in a vibrating bed. We carry out the linear stability analysis and
uncover the instability mechanism that leads to the appearance of the
convective rolls via a supercritical bifurcation of a bouncing solution. We
also explicitly determine the onset of convection as a function of control
parameters and confirm our picture by numerical simulations of the continuum
equations.Comment: 14 pages, RevTex 11pages + 3 pages figures (Type csh
- …