14,866 research outputs found
Generation Efficiencies for Propagating Modes in a Supersolid
Using Andreev and Lifshitz's supersolid hydrodynamics, we obtain the
propagating longitudinal modes at non-zero applied pressure (necessary
for solid 4He), and their generation efficiencies by heaters and transducers.
For small , a solid develops an internal pressure . This
theory has stress contributions both from the lattice and an internal pressure
. Because both types of stress are included, the normal mode analysis
differs from previous works. Not surprisingly, transducers are significantly
more efficient at producing elastic waves and heaters are significantly more
efficient at producing fourth sound waves. We take the system to be isotropic,
which should apply to systems that are glassy or consist of many crystallites;
the results should also apply, at least qualitatively, to single-crystal hcp
4He.Comment: 10 pages. Accepted by Physical Review
Thermal Equilibration and Thermally-Induced Spin Currents in a Thin-Film Ferromagnet on a Substrate
Recent spin-Seebeck experiments on thin ferromagnetic films apply a
temperature difference along the length and measure a
(transverse) voltage difference along the width . The
connection between these effects is complex, involving: (1) thermal
equilibration between sample and substrate; (2) spin currents along the height
(or thickness) ; and (3) the measured voltage difference. The present work
studies in detail the first of these steps, and outlines the other two steps.
Thermal equilibration processes between the magnons and phonons in the sample,
as well as between the sample and the substrate leads to two surface modes,
with surface lengths , to provide for thermal equilibration.
Increasing the coupling between the two modes increases the longer mode length
and decreases the shorter mode length. The applied thermal gradient along
leads to a thermal gradient along that varies as ,
which can in turn produce fluxes of the carriers of up- and down- spins along
, and gradients of their associated \textit{magnetoelectrochemical
potentials} , which vary as
. By the inverse spin Hall effect, this spin current along
can produce a transverse (along ) voltage difference , which
also varies as .Comment: 14 pages, 7 figures, 1 tabl
Andreev-Lifshitz Hydrodynamics Applied to an Ordinary Solid under Pressure
We have applied the Andreev-Lifshitz hydrodynamic theory of supersolids to an
ordinary solid. This theory includes an internal pressure , distinct from
the applied pressure and the stress tensor . Under uniform
static , we have . For , Maxwell relations imply that . The theory also permits
vacancy diffusion but treats vacancies as conserved. It gives three sets of
propagating elastic modes; it also gives two diffusive modes, one largely of
entropy density and one largely of vacancy density (or, more generally, defect
density). For the vacancy diffusion mode (or, equivalently, the lattice
diffusion mode) the vacancies behave like a fluid within the solid, with the
deviations of internal pressure associated with density changes nearly
canceling the deviations of stress associated with strain. We briefly consider
pressurization experiments in solid He at low temperatures in light of this
lattice diffusion mode, which for small has diffusion constant . The general principles of the theory -- that both volume and
strain should be included as thermodynamic variables, with the result that both
and appear -- should apply to all solids under pressure,
especially near the solid-liquid transition. The lattice diffusion mode
provides an additional degree of freedom that may permit surfaces with
different surface treatments to generate different responses in the bulk.Comment: 10 pages. Accepted by Physical Review
Andreev-Lifshitz Supersolid Hydrodynamics Including the Diffusive Mode
We have re-examined the Andreev-Lifshitz theory of supersolids. This theory
implicitly neglects uniform bulk processes that change the vacancy number, and
assumes an internal pressure in addition to lattice stress .
Each of and takes up a part of an external, or applied,
pressure (necessary for solid 4He). The theory gives four pairs of
propagating elastic modes, of which one pair corresponds to a fourth-sound
mode, and a single diffusive mode, which has not been analyzed previously. The
diffusive mode has three distinct velocities, with the superfluid velocity much
larger than the normal fluid velocity, which in turn is much larger than the
lattice velocity. The mode structure depends on the relative values of certain
kinetic coefficients and thermodynamic derivatives. We consider pressurization
experiments in solid 4He at low temperatures in light of this diffusion mode
and a previous analysis of modes in a normal solid with no superfluid
component.Comment: 8 pages. Accepted by Physical Review
General approach and scope
This paper describes a joint activity involving NASA and Army researchers at the NASA Langley Research Center to develop optimization procedures aimed at improving the rotor blade design process by integrating appropriate disciplines and accounting for all of the important interactions among the disciplines. The disciplines involved include rotor aerodynamics, rotor dynamics, rotor structures, airframe dynamics, and acoustics. The work is focused on combining these five key disciplines in an optimization procedure capable of designing a rotor system to satisfy multidisciplinary design requirements. Fundamental to the plan is a three-phased approach. In phase 1, the disciplines of blade dynamics, blade aerodynamics, and blade structure will be closely coupled, while acoustics and airframe dynamics will be decoupled and be accounted for as effective constraints on the design for the first three disciplines. In phase 2, acoustics is to be integrated with the first three disciplines. Finally, in phase 3, airframe dynamics will be fully integrated with the other four disciplines. This paper deals with details of the phase 1 approach and includes details of the optimization formulation, design variables, constraints, and objective function, as well as details of discipline interactions, analysis methods, and methods for validating the procedure
An initiative in multidisciplinary optimization of rotorcraft
Described is a joint NASA/Army initiative at the Langley Research Center to develop optimization procedures aimed at improving the rotor blade design process by integrating appropriate disciplines and accounting for important interactions among the disciplines. The activity is being guided by a Steering Committee made up of key NASA and Army researchers and managers. The committee, which has been named IRASC (Integrated Rotorcraft Analysis Steering Committee), has defined two principal foci for the activity: a white paper which sets forth the goals and plans of the effort; and a rotor design project which will validate the basic constituents, as well as the overall design methodology for multidisciplinary optimization. The optimization formulation is described in terms of the objective function, design variables, and constraints. Additionally, some of the analysis aspects are discussed and an initial attempt at defining the interdisciplinary couplings is summarized. At this writing, some significant progress has been made, principally in the areas of single discipline optimization. Results are given which represent accomplishments in rotor aerodynamic performance optimization for minimum hover horsepower, rotor dynamic optimization for vibration reduction, and rotor structural optimization for minimum weight
Integrated multidisciplinary design optimization of rotorcraft
The NASA/Army research plan for developing the logic elements for helicopter rotor design optimization by integrating appropriate disciplines and accounting for important interactions among the disciplines is discussed. The optimization formulation is described in terms of the objective function, design variables, and constraints. The analysis aspects are discussed, and an initial effort at defining the interdisciplinary coupling is summarized. Results are presented on the achievements made in the rotor dynamic optimization for vibration reduction, rotor structural optimization for minimum weight, and integrated aerodynamic load/dynamics optimization for minimum vibration and weight
Integrated multidisciplinary optimization of rotorcraft: A plan for development
This paper describes a joint NASA/Army initiative at the Langley Research Center to develop optimization procedures aimed at improving the rotor blade design process by integrating appropriate disciplines and accounting for important interactions among the disciplines. The paper describes the optimization formulation in terms of the objective function, design variables, and constraints. Additionally, some of the analysis aspects are discussed, validation strategies are described, and an initial attempt at defining the interdisciplinary couplings is summarized. At this writing, significant progress has been made, principally in the areas of single discipline optimization. Accomplishments are described in areas of rotor aerodynamic performance optimization for minimum hover horsepower, rotor dynamic optimization for vibration reduction, and rotor structural optimization for minimum weight
An initiative in multidisciplinary optimization of rotorcraft
Described is a joint NASA/Army initiative at the Langley Research Center to develop optimization procedures aimed at improving the rotor blade design process by integrating appropriate disciplines and accounting for important interactions among the disciplines. The activity is being guided by a Steering Committee made up of key NASA and Army researchers and managers. The committee, which has been named IRASC (Integrated Rotorcraft Analysis Steering Committee), has defined two principal foci for the activity: a white paper which sets forth the goals and plans of the effort; and a rotor design project which will validate the basic constituents, as well as the overall design methodology for multidisciplinary optimization. The paper describes the optimization formulation in terms of the objective function, design variables, and constraints. Additionally, some of the analysis aspects are discussed and an initial attempt at defining the interdisciplinary couplings is summarized. At this writing, some significant progress has been made, principally in the areas of single discipline optimization. Results are given which represent accomplishments in rotor aerodynamic performance optimization for minimum hover horsepower, rotor dynamic optimization for vibration reduction, and rotor structural optimization for minimum weight
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