98,271 research outputs found
Installation effects of long-duct pylon-mounted nacelles on a twin-jet transport model with swept supercritical wing
The installation interference effects of an underwing-mounted, long duct, turbofan nacelle were evaluated in the Langley 16-Foot Transonic Tunnel with two different pylon shapes installed on a twin engine transport model having a supercritical wing swept 30 deg. Wing, pylon, and nacelle pressures and overall model force data were obtained at Mach numbers from 0.70 to 0.83 and nominal angles of attack from -2 deg to 4 deg at an average unit Reynolds number of 11.9 x 1,000,000 per meter. The results show that adding the long duct nacelles to the supercritical wing, in the near sonic flow field, changed the magnitude and direction of flow velocities over the entire span, significantly reduced cruise lift, and caused large interference drag on the nacelle afterbody
Hardness of Graph Pricing through Generalized Max-Dicut
The Graph Pricing problem is among the fundamental problems whose
approximability is not well-understood. While there is a simple combinatorial
1/4-approximation algorithm, the best hardness result remains at 1/2 assuming
the Unique Games Conjecture (UGC). We show that it is NP-hard to approximate
within a factor better than 1/4 under the UGC, so that the simple combinatorial
algorithm might be the best possible. We also prove that for any , there exists such that the integrality gap of
-rounds of the Sherali-Adams hierarchy of linear programming for
Graph Pricing is at most 1/2 + .
This work is based on the effort to view the Graph Pricing problem as a
Constraint Satisfaction Problem (CSP) simpler than the standard and complicated
formulation. We propose the problem called Generalized Max-Dicut(), which
has a domain size for every . Generalized Max-Dicut(1) is
well-known Max-Dicut. There is an approximation-preserving reduction from
Generalized Max-Dicut on directed acyclic graphs (DAGs) to Graph Pricing, and
both our results are achieved through this reduction. Besides its connection to
Graph Pricing, the hardness of Generalized Max-Dicut is interesting in its own
right since in most arity two CSPs studied in the literature, SDP-based
algorithms perform better than LP-based or combinatorial algorithms --- for
this arity two CSP, a simple combinatorial algorithm does the best.Comment: 28 page
Pseudorandom Generators for Width-3 Branching Programs
We construct pseudorandom generators of seed length that -fool ordered read-once branching programs
(ROBPs) of width and length . For unordered ROBPs, we construct
pseudorandom generators with seed length . This is the first improvement for pseudorandom
generators fooling width ROBPs since the work of Nisan [Combinatorica,
1992].
Our constructions are based on the `iterated milder restrictions' approach of
Gopalan et al. [FOCS, 2012] (which further extends the Ajtai-Wigderson
framework [FOCS, 1985]), combined with the INW-generator [STOC, 1994] at the
last step (as analyzed by Braverman et al. [SICOMP, 2014]). For the unordered
case, we combine iterated milder restrictions with the generator of
Chattopadhyay et al. [CCC, 2018].
Two conceptual ideas that play an important role in our analysis are: (1) A
relabeling technique allowing us to analyze a relabeled version of the given
branching program, which turns out to be much easier. (2) Treating the number
of colliding layers in a branching program as a progress measure and showing
that it reduces significantly under pseudorandom restrictions.
In addition, we achieve nearly optimal seed-length
for the classes of: (1) read-once polynomials on
variables, (2) locally-monotone ROBPs of length and width
(generalizing read-once CNFs and DNFs), and (3) constant-width ROBPs of length
having a layer of width in every consecutive
layers.Comment: 51 page
When is an error not a prediction error? An electrophysiological investigation
A recent theory holds that the anterior cingulate cortex (ACC) uses reinforcement learning signals conveyed by the midbrain dopamine system to facilitate flexible action selection. According to this position, the impact of reward prediction error signals on ACC modulates the amplitude of a component of the event-related brain potential called the error-related negativity (ERN). The theory predicts that ERN amplitude is monotonically related to the expectedness of the event: It is larger for unexpected outcomes than for expected outcomes. However, a recent failure to confirm this prediction has called the theory into question. In the present article, we investigated this discrepancy in three trial-and-error learning experiments. All three experiments provided support for the theory, but the effect sizes were largest when an optimal response strategy could actually be learned. This observation suggests that ACC utilizes dopamine reward prediction error signals for adaptive decision making when the optimal behavior is, in fact, learnable
Prediction of force coefficients for labyrinth seals
The development of a linear model for the prediction of labyrinth seal forces and on its comparison to available stiffness data is presented. A discussion of the relevance of fluid damping forces and the preliminary stages of a program to obtain data on these forces are examined. Fluid-dynamic forces arising from nonuniform pressure patterns in labyrinth seal glands are known to be potentially destablizing in high power turbomachinery. A well documented case in point is that of the space Shuttle Main Engine turbopumps. Seal forces are also an important factor for the stability of shrouded turbines, acting in that case in conjunction with the effects of blade-tip clearance variations
Observation of Scalar Aharonov-Bohm Effect with Longitudinally Polarized Neutrons
We have carried out a neutron interferometry experiment using longitudinally polarized neutrons to observe the scalar Aharonov-Bohm effect. The neutrons inside the interferometer are polarized parallel to an applied pulsed magnetic field B(t). The pulsed B field is spatially uniform so it exerts no force on the neutrons. Its direction also precludes the presence of any classical torque to change the neutron polarization
Exact solutions for the Einstein-Gauss-Bonnet theory in five dimensions: Black holes, wormholes and spacetime horns
An exhaustive classification of certain class of static solutions for the
five-dimensional Einstein-Gauss-Bonnet theory in vacuum is presented. The class
of metrics under consideration is such that the spacelike section is a warped
product of the real line with a nontrivial base manifold. It is shown that for
generic values of the coupling constants the base manifold must be necessarily
of constant curvature, and the solution reduces to the topological extension of
the Boulware-Deser metric. It is also shown that the base manifold admits a
wider class of geometries for the special case when the Gauss-Bonnet coupling
is properly tuned in terms of the cosmological and Newton constants. This
freedom in the metric at the boundary, which determines the base manifold,
allows the existence of three main branches of geometries in the bulk. For
negative cosmological constant, if the boundary metric is such that the base
manifold is arbitrary, but fixed, the solution describes black holes whose
horizon geometry inherits the metric of the base manifold. If the base manifold
possesses a negative constant Ricci scalar, two different kinds of wormholes in
vacuum are obtained. For base manifolds with vanishing Ricci scalar, a
different class of solutions appears resembling "spacetime horns". There is
also a special case for which, if the base manifold is of constant curvature,
due to certain class of degeneration of the field equations, the metric admits
an arbitrary redshift function. For wormholes and spacetime horns, there are
regions for which the gravitational and centrifugal forces point towards the
same direction. All these solutions have finite Euclidean action, which reduces
to the free energy in the case of black holes, and vanishes in the other cases.
Their mass is also obtained from a surface integral.Comment: 31 pages, 1 figure, minor changes and references added. Final version
to be published in PR
Spin Hall effect due to intersubband-induced spin-orbit interaction in symmetric quantum wells
We investigate the intrinsic spin Hall effect in two-dimensional electron
gases in quantum wells with two subbands, where a new intersubband-induced
spin-orbit coupling is operative. The bulk spin Hall conductivity
is calculated in the ballistic limit within the standard Kubo
formalism in the presence of a magnetic field and is found to remain finite
in the B=0 limit, as long as only the lowest subband is occupied. Our
calculated exhibits a nonmonotonic behavior and can change its
sign as the Fermi energy (the carrier areal density ) is varied between
the subband edges. We determine the magnitude of for realistic
InSb quantum wells by performing a self-consistent calculation of the
intersubband-induced spin-orbit coupling.Comment: 7 pages, 3 figure
Scalar Aharonov-Bohm effect with longitudinally polarized neutrons
In the scalar Aharonov-Bohm effect, a charged particle (electron) interacts with the scalar electrostatic potential U in the field-free (i.e., force-free) region inside an electrostatic cylinder (Faraday cage). Using a perfect single-crystal neutron interferometer we have performed a “dual” scalar Aharonov-Bohm experiment by subjecting polarized thermal neutrons to a pulsed magnetic field. The pulsed magnetic field was spatially uniform, precluding any force on the neutrons. Aligning the direction of the pulsed magnetic field to the neutron magnetic moment also rules out any classical torque acting to change the neutron polarization. The observed phase shift is purely quantum mechanical in origin. A detailed description of the experiment, performed at the University of Missouri Research Reactor, and its interpretation is given in this paper
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