30,732 research outputs found
Dynamic stability and parametric resonance in cylindrical propellant tanks Final report
Dynamic stability and parametric resonance of longitudinally excited liquid propellant tank mode
Exhaust cloud rise and diffusion in the atmosphere
Analytical approach develops physical-mathematical model of rocket engine exhaust cloud rise, growth, and diffusion. Analytic derivations and resultant model apply to hot exhaust cloud study or industrial stack plumes, making work results applicable to air pollution. Model formulations apply to all exhaust cloud types and various atmospheric conditions
Perception of Lower Extremity Loads in Stroke Survivors
Objective: This study aimed to improve our understanding of static and dynamic lower extremity sensory perception and the impact of sensory impairments on the control of walking in stroke survivors.
Methods: Using a custom, real-time unloading system, we tested load perception at heel strike, mid stance and push off in 10 stroke survivors and compared their performance to 10 age-matched and 5 young adult control subjects. Dynamic load perception was based on a judgment of which leg was bearing more load, which was altered on a step by step basis. We also examined lower extremity static load perception, coordination, proprioception, balance, and gait symmetry.
Results: The stroke survivors performed significantly worse than the control subjects in dynamic load perception, coordination, proprioception, balance and gait symmetry. Gait symmetry correlated with static and dynamic load perception measures but not with age, proprioception, coordination, and balance.
Conclusions: Sensory deficits related to load detection in the impaired limb could result in an increased uncertainty of limb load and a gait strategy in which stroke survivors minimize loading of the impaired limb.
Significance: This new method of measuring lower extremity dynamic load perception provides a framework for understanding gait-related sensory impairments in stroke survivors
Rise and growth of space vehicle engine exhaust and associated diffusion models
Space vehicle plume rise and associated diffusion models at Cape Kennedy Launch Comple
Inflight estimation of gyro noise
A method is described and demonstrated for estimating single-axis gyro noise levels in terms of the Farrenkopf model parameters. This is accomplished for the Cosmic Background Explorer (COBE) by comparing gyro-propagated attitudes with less accurate single-frame solutions and fitting the squared differences to a third-order polynomial in time. Initial results are consistent with the gyro specifications, and these results are used to determine limits on the duration of batches used to determine attitude. Sources of error are discussed, and guidelines for a more elegant implementation, as part of a batch estimator or filter, are included for future work
Photoemission Spectroscopy of Magnetic and Non-magnetic Impurities on the Surface of the BiSe Topological Insulator
Dirac-like surface states on surfaces of topological insulators have a chiral
spin structure that suppresses back-scattering and protects the coherence of
these states in the presence of non-magnetic scatterers. In contrast, magnetic
scatterers should open the back- scattering channel via the spin-flip processes
and degrade the state's coherence. We present angle-resolved photoemission
spectroscopy studies of the electronic structure and the scattering rates upon
adsorption of various magnetic and non-magnetic impurities on the surface of
BiSe, a model topological insulator. We reveal a remarkable
insensitivity of the topological surface state to both non-magnetic and
magnetic impurities in the low impurity concentration regime. Scattering
channels open up with the emergence of hexagonal warping in the high-doping
regime, irrespective of the impurity's magnetic moment.Comment: 5 pages, 4 figure
Effective non-Markovian description of a system interacting with a bath
We study a harmonic system coupled to chain of first neighbor interacting
oscillators. After deriving the exact dynamics of the system, we prove that one
can effectively describe the exact dynamics by considering a suitable shorter
chain. We provide the explicit expression for such an effective dynamics and we
provide an upper bound on the error one makes considering it instead of the
dynamics of the full chain. We eventually prove how error, timescale and number
of modes in the truncated chain are related
Holography and Cosmological Singularities
Certain null singularities in ten dimensional supergravity have natural
holographic duals in terms of Matrix Theory and generalizations of the AdS/CFT
correspondence. In many situations the holographic duals appear to be well
defined in regions where the supergravity develops singularities. We describe
some recent progress in this area.Comment: Anomaly equation corrected. References adde
Expansion Aspect of Color Transparency on the Lattice
The opportunity to observe color transparency (CT) is determined by how
rapidly a small-sized hadronic wave packet expands. Here we use SU(2) lattice
gauge theory with Wilson fermions in the quenched approximation to investigate
the expansion. The wave packet is modeled by a point hadronic source, often
used as an interpolating field in lattice calculations. The procedure is to
determine the Euclidean time (t), pion channel, Bethe-Salpeter amplitude
, and then evaluate . This quantity represents the soft interaction of a small-sized
wave packet with a pion. The time dependence of is fit as a
superposition of three states, which is found sufficient to reproduce a reduced
size wave packet. Using this superposition allows us to make the analytic
continuation required to study the wave packet expansion in real time. We find
that the matrix elements of the soft interaction between the excited
and ground state decrease rapidly with the energy of the excited state.Comment: 19 pages, latex, 4 figure
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