4,136 research outputs found
Stress accumulation versus shape flattening in frustrated, warped-jigsaw particle assemblies
Geometrically frustrated assembly has emerged as an attractive paradigm for
understanding and engineering assemblies with self-limiting, finite equilibrium
dimensions. We propose and study a novel 2D particle based on a so-called
"warped jigsaw" (WJ) shape design: directional bonds in a tapered particle
favor curvature along multi-particle rows that frustrate 2D lattice order. We
investigate how large-scale intra-assembly stress gradients emerge from the
microscopic properties of the particles using a combination of numerical
simulation and continuum elasticity. WJ particles can favor anisotropic ribbon
assemblies, whose lateral width may be self-limiting depending on the relative
strength of cohesive to elastic forces in the assembly, which we show to be
controlled by the range of interactions and degree of shape misfit. The upper
limits of self-limited size are controlled by the crossover between two elastic
modes in assembly: the accumulation of shear with increasing width at small
widths giving way to unbending of preferred row curvature, permitting assembly
to grow to unlimited sizes. We show that the stiffness controlling distinct
elastic modes is governed by combination and placement of repulsive and
attractive binding regions, providing a means to extend the range of
accumulating stress to sizes that are far in excess of the single particle
size, which we corroborate via numerical studies of discrete particles of
variable interactions. Lastly, we relate the ground-state energetics of the
model to lower and upper limits on equilibrium assembly size control set by the
fluctuations of width along the ribbon boundary.Comment: 18 pages, 9 figures, 2 appendice
Integrating aerodynamics and structures in the minimum weight design of a supersonic transport wing
An approach is presented for determining the minimum weight design of aircraft wing models which takes into consideration aerodynamics-structure coupling when calculating both zeroth order information needed for analysis and first order information needed for optimization. When performing sensitivity analysis, coupling is accounted for by using a generalized sensitivity formulation. The results presented show that the aeroelastic effects are calculated properly and noticeably reduce constraint approximation errors. However, for the particular example selected, the error introduced by ignoring aeroelastic effects are not sufficient to significantly affect the convergence of the optimization process. Trade studies are reported that consider different structural materials, internal spar layouts, and panel buckling lengths. For the formulation, model and materials used in this study, an advanced aluminum material produced the lightest design while satisfying the problem constraints. Also, shorter panel buckling lengths resulted in lower weights by permitting smaller panel thicknesses and generally, by unloading the wing skins and loading the spar caps. Finally, straight spars required slightly lower wing weights than angled spars
Application of multidisciplinary optimization methods to the design of a supersonic transport
An optimization design method is discussed. This method is based on integrating existing disciplinary analysis and sensitivity analysis techniques by means of generalized sensitivity equations. A generic design system implementing this method is described. The system is being used to design the configuration and internal structure of a supersonic transport wing for optimum performance. This problem combines the disciplines of linear aerodynamics, structures, and performance. Initial results which include the disciplines of aerodynamics and structures in a conventional minimum weight design under static aeroelastic constraints are presented
The Sun-like activity of the solar twin 18 Scorpii
We present the results of 10 yr of complementary spectroscopic and
photometric observations of the solar twin 18 Scorpii. We show that over the
course of its ~7 year chromospheric activity cycle, 18 Sco's brightness varies
in the same manner as the Sun's and with a likely brightness variation of
0.09%, similar to the 0.1% decadal variation in the total solar irradiance
Limits of economy and fidelity for programmable assembly of size-controlled triply-periodic polyhedra
We propose and investigate an extension of the Caspar-Klug symmetry
principles for viral capsid assembly to the programmable assembly of
size-controlled triply-periodic polyhedra, discrete variants of the Primitive,
Diamond, and Gyroid cubic minimal surfaces. Inspired by a recent class of
programmable DNA origami colloids, we demonstrate that the economy of design in
these crystalline assemblies -- in terms of the growth of the number of
distinct particle species required with the increased size-scale (e.g.
periodicity) -- is comparable to viral shells. We further test the role of
geometric specificity in these assemblies via dynamical assembly simulations,
which show that conditions for simultaneously efficient and high-fidelity
assembly require an intermediate degree of flexibility of local angles and
lengths in programmed assembly. Off-target misassembly occurs via incorporation
of a variant of disclination defects, generalized to the case of hyperbolic
crystals. The possibility of these topological defects is a direct consequence
of the very same symmetry principles that underlie the economical design,
exposing a basic tradeoff between design economy and fidelity of programmable,
size controlled assembly.Comment: 15 pages, 5 figures, 6 supporting movies (linked), Supporting
Appendi
Magnetic White Dwarfs from the SDSS II. The Second and Third Data Releases
Fifty-two magnetic white dwarfs have been identified in spectroscopic
observations from the Sloan Digital Sky Survey (SDSS) obtained between mid-2002
and the end of 2004, including Data Releases 2 and 3. Though not as numerous
nor as diverse as the discoveries from the first Data Release, the collection
exhibits polar field strengths ranging from 1.5MG to ~1000MG, and includes two
new unusual atomic DQA examples, a molecular DQ, and five stars that show
hydrogen in fields above 500MG. The highest-field example, SDSSJ2346+3853, may
be the most strongly magnetic white dwarf yet discovered. Analysis of the
photometric data indicates that the magnetic sample spans the same temperature
range as for nonmagnetic white dwarfs from the SDSS, and support is found for
previous claims that magnetic white dwarfs tend to have larger masses than
their nonmagnetic counterparts. A glaring exception to this trend is the
apparently low-gravity object SDSSJ0933+1022, which may have a history
involving a close binary companion.Comment: 20 pages, 4 figures Accepted for publication in the Astronomical
Journa
- …