5,954 research outputs found
Nonlinear optimal guidance algorithms Interim report
Nonlinear optimal guidance algorithms for space mission
Approximations of thermoelastic and viscoelastic control systems
Well-posed models and computational algorithms are developed and analyzed for control of a class of partial differential equations that describe the motions of thermo-viscoelastic structures. An abstract (state space) framework and a general well-posedness result are presented that can be applied to a large class of thermo-elastic and thermo-viscoelastic models. This state space framework is used in the development of a computational scheme to be used in the solution of a linear quadratic regulator (LQR) control problem. A detailed convergence proof is provided for the viscoelastic model and several numerical results are presented to illustrate the theory and to analyze problems for which the theory is incomplete
Results on Transversal and Axial Motions of a System of Two Beams Coupled to a Joint through Two Legs
In recent years there has been renewed interest in inflatable-rigidizable space structures because of the efficiency they offer in packaging during boost-to-orbit. However, much research is still needed to better understand dynamic response characteristics, including inherent damping, of truss structures fabricated with these advanced material systems. We present results of an ongoing research related to a model consisting of an assembly of two beams with Kelvin-Voight damping, coupled to a simple joint through two legs. The beams are clamped at one end but at the other end they satisfy a boundary condition given in terms of an ODE coupling boundary terms of both beams, which reflects geometric compatibility conditions. The system is then written as a second order differential equation in an appropriate Hilbert space in which well-posedness, exponential stability as well as other regularity properties of the solutions can be obtained. Two different finite dimensional approximation schemes for the solutions of the system are presented. Numerical results are presented and comparisons are made.Fil: Burns, J. A.. Interdisciplinary Center for Applied Mathematics; Estados UnidosFil: Cliff, E. M.. Interdisciplinary Center for Applied Mathematics; Estados UnidosFil: Liu, Z.. University of Minnesota at Duluth; Estados UnidosFil: Spies, Ruben Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Matemática Aplicada del Litoral. Universidad Nacional del Litoral. Instituto de Matemática Aplicada del Litoral; Argentin
Internal stress wave measurements in solids subjected to lithotripter pulses
Semiconductor strain gauges were used to measure the internal strain along the axes of spherical and disk plaster specimens when subjected to lithotripter shock pulses. The pulses were produced by one of two lithotripters. The first source generates spherically diverging shock waves of peak pressure approximately 1 MPa at the surface of the specimen. For this source, the incident and first reflected pressure (P) waves in both sphere and disk specimens were identified. In addition, waves reflected by the disk circumference were found to contribute significantly to the strain fields along the disk axis. Experimental results compared favorably to a ray theory analysis of a spherically diverging shock wave striking either concretion. For the sphere, pressure contours for the incident P wave and caustic lines were determined theoretically for an incident spherical shock wave. These caustic lines indicate the location of the highest stresses within the sphere and therefore the areas where damage may occur. Results were also presented for a second source that uses an ellipsoidal reflector to generate a 30-MPa focused shock wave, more closely approximating the wave fields of a clinical extracorporeal lithotripter
Molecular interpretation of the Pc(4440) and Pc(4457) states
A molecular model of the and LHCb states is proposed.
The model relies on channels coupled by long range pion-exchange dynamics with
features that depend crucially on the novel addition of the
channel. A striking prediction of the model is the
unusual combination of quantum numbers and . Unlike in other models, a simultaneous description of both states is
achieved without introducing additional short-range interactions. The model
also gives a natural explanation for the relative widths of the states. We show
that the usual molecular scenarios cannot explain the production rate of
states in decays, and that this can be resolved by including
and related channels. Experimental tests and other
states are discussed in the conclusions.Comment: 15 pages, 6 figures. Some new material in the conclusions, and a typo
corrected in Table I
A Unified Approach for the Enantioselective Synthesis of the Brominated Chamigrene Sesquiterpenes
The brominated chamigrene sesquiterpenes constitute a large subclass of bromocyclohexane‐containing natural products, yet no general enantioselective strategy for the synthesis of these small molecules exists. Herein we report a general strategy for accessing this family of secondary metabolites, including the enantioselective synthesis of (−)‐α‐ and (−)‐ent‐β‐bromochamigrene, (−)‐dactylone, and (+)‐aplydactone. Access to these molecules is enabled by a stereospecific bromopolyene cyclization initiated by the solvolysis of an enantiomerically enriched vicinal bromochloride.Dihalides light the way: A stereospecific bromopolyene cyclization of an enantiomerically enriched bromochloride was developed as a highly general approach to the brominated chamigrene sesquiterpenes (see scheme). The total synthesis of (+)‐aplydactone was completed by an intramolecular [2+2] cycloaddition.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137372/1/anie201605722.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137372/2/anie201605722_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137372/3/anie201605722-sup-0001-misc_information.pd
Feather growth rate and mass in nearctic passerines with variablemigratory behavior and molt pattern
Bird species vary greatly in the duration of their annual complete feather molt. However, such variation is not well documented
in birds from many biogeographic areas, which restricts our understanding of the diversification of molt strategies. Recent research has revealed
that molt duration can be estimated in passerines from ptilochronology-based measurements of the growth rate of their tail feathers. We used
this approach to explore how molt duration varied in 98 Nearctic species that have different migratory strategies and molt patterns. As previously
documented for Palearctic species, migration was associated with a shortening of molt duration among species that molted during summer on
their breeding range. However, molts of winter-molting migratory species were as long as those of summer-molting sedentary species, which
suggests that winter molt also allows Nearctic migrants to avoid the temporal constraints experienced during summer. Our results also suggest
that migratory species that undergo a stopover molt within the Mexican monsoon region have the shortest molt duration among all Nearctic
passerines. Interestingly, and contrary to expectations from a potential tradeoff between molt duration and feather quality, observed variation
in feather growth rate was positively correlated with differences in tail feather mass, which may be caused by differences among groups in the
availability of resources for molting. We encourage the use of similar approaches to study the variation in molt duration in other geographic areas
where knowledge of the evolution of molt is limited.
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Copula/Auxiliary Comparisons in African American and Impaired Standard American English
The valid identification and description of language impairment in children who speak African American English (AAE) has been a major clinical challenge for over 30 years. This challenge centers on the issue of deficit versus difference for language features that contrast with Standard American English. The distinction between deficit and difference in identifying language disorders in child African American English speakers is the key to valid language assessment in AAE. Most syntactic targets in SAE are presumably invariable while many syntactic targets in AAE are variable. For example, the SAE target syntactic form for the copula is would be represented by He is bad . Whereas that same production in AAE might yield either He is bad or He_bad . Our research focuses on how one determines if a child AAE speaker who uses He_bad does so as a function of dialect, not impairment
Soft Mode Dynamics Above and Below the Burns Temperature in the Relaxor Pb(Mg_1/3Nb_2/3)O_3
We report neutron inelastic scattering measurements of the lowest-energy
transverse optic (TO) phonon branch in the relaxor Pb(Mg_1/3Nb_2/3)O_3 from 400
to 1100 K. Far above the Burns temperature T_d ~ 620 K we observe well-defined
propagating TO modes at all wave vectors q, and a zone center TO mode that
softens in a manner consistent with that of a ferroelectric soft mode. Below
T_d the zone center TO mode is overdamped. This damping extends up to, but not
above, the waterfall wave vector q_wf, which is a measure of the average size
of the PNR.Comment: 4 pages, 4 figures; modified discussion of Fig. 3, shortened
captions, added reference, corrected typos, accepted by Phys. Rev. Let
Elicitation of US and Chinese expert judgments show consistent views on solar geoengineering
Expert judgments on solar geoengineering (SG) inform policy decisions and influence public opinions. We performed face-to-face interviews using formal expert elicitation methods with 13 US and 13 Chinese climate experts randomly selected from IPCC authors or supplemented by snowball sampling. We compare their judgments on climate change, SG research, governance, and deployment. In contrast to existing literature that often stress factors that might differentiate China from western democracies on SG, we found few significant differences between quantitative judgments of US and Chinese experts. US and Chinese experts differed on topics, such as desired climate scenario and the preferred venue for international regulation of SG, providing some insight into divergent judgments that might shape future negotiations about SG policy. We also gathered closed-form survey results from 19 experts with >10 publications on SG. Both expert groups supported greatly increased research, recommending SG research funding of ~5% on average (10th–90th percentile range was 1–10%) of climate science budgets compared to actual budgets of <0.3% in 2018. Climate experts chose far less SG deployment in future climate policies than did SG experts
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