79,152 research outputs found
An analytical method to predict efficiency of aircraft gearboxes
A spur gear efficiency prediction method previously developed by the authors was extended to include power loss of planetary gearsets. A friction coefficient model was developed for MIL-L-7808 oil based on disc machine data. This combined with the recent capability of predicting losses in spur gears of nonstandard proportions allows the calculation of power loss for complete aircraft gearboxes that utilize spur gears. The method was applied to the T56/501 turboprop gearbox and compared with measured test data. Bearing losses were calculated with large scale computer programs. Breakdowns of the gearbox losses point out areas for possible improvement
Shock accelerated vortex ring
The interaction of a shock wave with a spherical density inhomogeneity leads
to the development of a vortex ring through the impulsive deposition of
baroclinic vorticity. The present fluid dynamics videos display this phenomenon
and were experimentally investigated at the Wisconsin Shock Tube Laboratory's
(WiSTL) 9.2 m, downward firing shock tube. The tube has a square internal
cross-section (0.25 m x 0.25 m) with multiple fused silica windows for optical
access. The spherical soap bubble is generated by means of a pneumatically
retracted injector and released into free-fall 200 ms prior to initial shock
acceleration. The downward moving, M = 2.07 shock wave impulsively accelerates
the bubble and reflects off the tube end wall. The reflected shock wave
re-accelerates the bubble (reshock), which has now developed into a vortex
ring, depositing additional vorticity. In the absence of any flow disturbances,
the flow behind the reflected shock wave is stationary. As a result, any
observed motion of the vortex ring is due to circulation. The shocked vortex
ring is imaged at 12,500 fps with planar Mie scattering.Comment: For Gallery of Fluid Motion 200
Nonlinear robust controller design for multi-robot systems with unknown payloads
This work is concerned with the control problem of a multi-robot system handling a payload with unknown mass properties. Force constraints at the grasp points are considered. Robust control schemes are proposed that cope with the model uncertainty and achieve asymptotic path tracking. To deal with the force constraints, a strategy for optimally sharing the task is suggested. This strategy basically consists of two steps. The first detects the robots that need help and the second arranges that help. It is shown that the overall system is not only robust to uncertain payload parameters, but also satisfies the force constraints
New interpretation of variational principles for gauge theories. I. Cyclic coordinate alternative to ADM split
I show how there is an ambiguity in how one treats auxiliary variables in
gauge theories including general relativity cast as 3 + 1 geometrodynamics.
Auxiliary variables may be treated pre-variationally as multiplier coordinates
or as the velocities corresponding to cyclic coordinates. The latter treatment
works through the physical meaninglessness of auxiliary variables' values
applying also to the end points (or end spatial hypersurfaces) of the
variation, so that these are free rather than fixed. [This is also known as
variation with natural boundary conditions.] Further principles of dynamics
workings such as Routhian reduction and the Dirac procedure are shown to have
parallel counterparts for this new formalism. One advantage of the new scheme
is that the corresponding actions are more manifestly relational. While the
electric potential is usually regarded as a multiplier coordinate and Arnowitt,
Deser and Misner have regarded the lapse and shift likewise, this paper's
scheme considers new {\it flux}, {\it instant} and {\it grid} variables whose
corresponding velocities are, respectively, the abovementioned previously used
variables. This paper's way of thinking about gauge theory furthermore admits
interesting generalizations, which shall be provided in a second paper.Comment: 11 page
Rhodium Pyrazolate Complexes as Potential CVD Precursors
Reaction of 3,5-(CF3)(2)PzLi with [Rh(mu-Cl)(eta(2)-C2H4)(2)](2) or [Rh(mu-Cl)(PMe3)(2)](2) in Et2O gave the dinuclear complexes [Rh(eta(2)-C2H4)(2)(mu-3,5-(CF3)(2)-Pz)](2) (1) and [Rh-2(mu-Cl)(mu-3,5-(CF3)(2)-Pz) (PMe3)(4)] (2) respectively (3,5-(CF3)(2)Pz = bis-trifluoromethyl pyrazolate). Reaction of PMe3 with [Rh(COD)(mu-3,5-(CF3)(2)-Pz)](2) in toluene gave [Rh(3,5-(CF3)(2)-Pz)(PMe3)(3)] (3). Reaction of 1 and 3 in toluene (1 : 4) gave moderate yields of the dinuclear complex [Rh(PMe3)(2)(mu-3,5-(CF3)(2)-Pz)](2) (4). Reaction of 3,5-(CF3)(2)PzLi with [Rh(PMe3)(4)]Cl in Et2O gave the ionic complex [Rh(PMe3)(4)][3,5-(CF3)(2)-Pz] (5). Two of the complexes, 1 and 3, were studied for use as CVD precursors. Polycrystalline thin films of rhodium (fcc-Rh) and metastable-amorphous films of rhodium phosphide (Rh2P) were grown from 1 and 3 respectively at 170 and 130 degrees C, 0.3 mmHg in a hot wall reactor using Ar as the carrier gas (5 cc min(-1)). Thin films of amorphous rhodium and rhodium phosphide (Rh2P) were grown from 1 and 3 at 170 and 130 degrees C respectively at 0.3 mmHg in a hot wall reactor using H-2 as the carrier gas (7 cc min(-1)).Welch Foundation F-816Petroleum Research Fund 47014-ACSNSF 0741973Chemistr
Size Gap for Zero Temperature Black Holes in Semiclassical Gravity
We show that a gap exists in the allowed sizes of all zero temperature static
spherically symmetric black holes in semiclassical gravity when only
conformally invariant fields are present. The result holds for both charged and
uncharged black holes. By size we mean the proper area of the event horizon.
The range of sizes that do not occur depends on the numbers and types of
quantized fields that are present. We also derive some general properties that
both zero and nonzero temperature black holes have in all classical and
semiclassical metric theories of gravity.Comment: 4 pages, ReVTeX, no figure
Localization fom conductance in few-channel disordered wires
We study localization in two- and three channel quasi-1D systems using
multichain tight-binding Anderson models with nearest-neighbour interchain
hopping. In the three chain case we discuss both the case of free- and that of
periodic boundary conditions between the chains. The finite disordered wires
are connected to ideal leads and the localization length is defined from the
Landauer conductance in terms of the transmission coefficients matrix. The
transmission- and reflection amplitudes in properly defined quantum channels
are obtained from S-matrices constructed from transfer matrices in Bloch wave
bases for the various quasi-1D systems. Our exact analytic expressions for
localization lengths for weak disorder reduce to the Thouless expression for 1D
systems in the limit of vanishing interchain hopping. For weak interchain
hopping the localization length decreases with respect to the 1D value in all
three cases. In the three-channel cases it increases with interchain hopping
over restricted domains of large hopping
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