18,325 research outputs found
The effect of small-amplitude time-dependent changes to the surface morphology of a sphere
Typical approaches to manipulation of flow separation employ passive means or active techniques such as blowing and suction or plasma acceleration. Here it is
demonstrated that the flow can be significantly altered by making small changes to the shape of the surface. A proof of concept experiment is performed using a very simple time-dependent perturbation to the surface of a sphere: a roughness element of 1% of the sphere diameter is moved azimuthally around a sphere surface upstream of the uncontrolled laminar separation point, with a rotational frequency as large as the vortex shedding frequency. A key finding is that the non-dimensional time to observe
a large effect on the lateral force due to the perturbation produced in the sphere boundary layers as the roughness moves along the surface is ˆt =tU_(∞)/D ≈4. This slow
development allows the moving element to produce a tripped boundary layer over an extended region. It is shown that a lateral force can be produced that is as large as the
drag. In addition, simultaneous particle image velocimetry and force measurements reveal that a pair of counter-rotating helical vortices are produced in the wake, which
have a significant effect on the forces and greatly increase the Reynolds stresses in the wake. The relatively large perturbation to the flow-field produced by the small
surface disturbance permits the construction of a phase-averaged, three-dimensional (two-velocity component) wake structure from measurements in the streamwise/radial
plane. The vortical structure arising due to the roughness element has implications for flow over a sphere with a nominally smooth surface or distributed roughness. In
addition, it is shown that oscillating the roughness element, or shaping its trajectory, can produce a mean lateral force
Trust and obfuscation principles for quality of information in emerging pervasive environments
Non peer reviewedPostprin
Predictions for Impurity-Induced Tc Suppression in the High-Temperature Superconductors
We address the question of whether anisotropic superconductivity is
compatible with the evidently weak sensitivity of the critical temperature Tc
to sample quality in the high-Tc copper oxides. We examine this issue
quantitatively by solving the strong-coupling Eliashberg equations numerically
as well as analytically for s-wave impurity scattering within the second Born
approximation. For pairing interactions with a characteristically low energy
scale, we find an approximately universal dependence of the d-wave
superconducting transition temperature on the planar residual resistivity which
is independent of the details of the microscopic pairing. These results, in
conjunction with future systematic experiments, should help elucidate the
symmetry of the order parameter in the cuprates.Comment: 13 pages, 4 figures upon request, revtex version
Sequentially evaporated thin Y-Ba-Cu-O superconductor films: Composition and processing effects
Thin films of YBa2Cu3O(7-beta) have been grown by sequential evaporation of Cu, Y, and BaF2 on SrTiO3 and MgO substrates. The onset temperatures were as high as 93 K while T sub c was 85 K. The Ba/Y ratio was varied from 1.9 to 4.0. The Cu/Y ratio was varied from 2.8 to 3.4. The films were then annealed at various times and temperatures. The times ranged from 15 min to 3 hr, while the annealing temperatures used ranged from 850 C to 900 C. A good correlation was found between transition temperature (T sub c) and the annealing conditions; the films annealed at 900 C on SrTiO3 had the best T sub c's. There was a weaker correlation between composition and T sub c. Barium poor films exhibitied semiconducting normal state resistance behavior while barium rich films were metallic. The films were analyzed by resistance versus temperature measurements and scanning electron microscopy. The analysis of the films and the correlations are reported
A Funny Thing Happened on the Way to PD: Revising PDS Leaders\u27 Roles in Support of Children\u27s Learning Directly
School leaders typically encounter district policies and procedures that make it difficult to facilitate school improvement efforts effectively (Bottoms & Fry, 2009; Bottoms & Schmidt-Davis, 2010). When the San Antonio Independent School District earned a state level distinction as a District of Innovation, the PDS principal and university faculty liaison took advantage of this opportunity to develop innovative curricular initiatives, including the implementation of a bi-weekly half-day of planning and professional development for elementary teachers while students take field trips and participate in Curiosity Corner, an 80-minute block for engineering challenges. When the PDS leaders\u27 plan to garner enough parent and community volunteers to sustain Curiosity Corner failed, they shifted their attention from directly supporting teachers\u27 learning during these half-day enrichment sessions, instead taking direct responsibility for planning and facilitating student learning. This unexpected turn of events brought with it unanticipated benefits to both students and PDS leaders
Implementing a Business Process Management System Using ADEPT: A Real-World Case Study
This article describes how the agent-based design of ADEPT (advanced decision environment for processed tasks) and implementation philosophy was used to prototype a business process management system for a real-world application. The application illustrated is based on the British Telecom (BT) business process of providing a quote to a customer for installing a network to deliver a specified type of telecommunication service. Particular emphasis is placed upon the techniques developed for specifying services, allowing heterogeneous information models to interoperate, allowing rich and flexible interagent negotiation to occur, and on the issues related to interfacing agent-based systems and humans. This article builds upon the companion article (Applied Artificial Intelligence Vol.14, no 2, pgs. 145-189) that provides details of the rationale and design of the ADEPT technology deployed in this application
Shear-flow transition: the basin boundary
The structure of the basin of attraction of a stable equilibrium point is
investigated for a dynamical system (W97) often used to model transition to
turbulence in shear flows. The basin boundary contains not only an equilibrium
point Xlb but also a periodic orbit P, and it is the latter that mediates the
transition. Orbits starting near Xlb relaminarize. We offer evidence that this
is due to the extreme narrowness of the region complementary to basin of
attraction in that part of phase space near Xlb. This leads to a proposal for
interpreting the 'edge of chaos' in terms of more familiar invariant sets.Comment: 11 pages; submitted for publication in Nonlinearit
Forces on a Sphere in the Presence of Static and Dynamic Roughness Elements
Though the effect of distributed roughness on
flow over a sphere has been examined in detail, there have been few observations as to the effect of an isolated roughness element on the forces induced on a sphere that is in uniform flow. In this experimental study, we
examine how the forces are altered due to both a stationary and dynamic three-dimensional roughness element in the Reynolds number range of 5 x 104 to 5 x 105. It is found that even a small change to the geometry of the sphere, by adding a cylindrical roughness element with a width and height of 1% the sphere diameter, dramatically alters the drag and lateral forces over a wide range of Reynolds numbers. Of particular interest is that the mean of
the lateral force magnitude can be increased by a factor of about seven, compared with a stationary stud, by moving the isolated roughness at a constant angular velocity about
the sphere. These results can be applied to tripping a laminar boundary layer, steering a bluff body, and increasing the mixing of two fluids, using a minimal amount of energy input. This research is a first step towards understanding the interaction between time dependent surface motion and the subsequent alteration of the location of the boundary layer separation line and wake development
Dynamic nuclear polarization from current-induced electron spin polarization
Current-induced electron spin polarization is shown to produce nuclear
hyperpolarization through dynamic nuclear polarization. Saturated fields of
several millitesla are generated upon the application of electric field over a
timescale of a hundred seconds in InGaAs epilayers and measured using optical
Larmor magnetometry. The dependence on temperature, external magnetic field,
and applied voltage is investigated. We find an asymmetry in which the
saturation nuclear field depends on the relative alignment of the electrically
generated spin polarization and the external magnetic field, which we attribute
to an interplay between various electron spin dynamical processes.Comment: 5 pages, 4 figure
About the relation between the quasiparticle Green's function in cuprates obtained from ARPES data and the magnetic susceptibility
Angle resolved photoemission spectroscopy (ARPES) provides a detailed view of
the renormalized band structure in cuprates and, consequently, is a key to the
self-energy and the quasiparticle Green's function. Such information gives a
clue to the comparison of ARPES with scanning tunneling microscopy, inelastic
neutron scattering (INS), and Raman scattering data. Here we touch on a
potential possibility of such a comparison with the dynamical magnetic
susceptibility measured in INS experiments. Calculations based on the
experimentally measured quasiparticle self-energies in cuprates lead to the
estimated magnetic susceptibility response with many-body effects taken into
account.Comment: Will be presented at the M2S-HTSC-VIII conference in Dresde
- …