15,354 research outputs found
On statistically stationary homogeneous shear turbulence
A statistically stationary turbulence with a mean shear gradient is realized
in a flow driven by suitable body forces. The flow domain is periodic in
downstream and spanwise directions and bounded by stress free surfaces in the
normal direction. Except for small layers near the surfaces the flow is
homogeneous. The fluctuations in turbulent energy are less violent than in the
simulations using remeshing, but the anisotropy on small scales as measured by
the skewness of derivatives is similar and decays weakly with increasing
Reynolds number.Comment: 4 pages, 5 figures (Figs. 3 and 4 as external JPG-Files
Antenna pointing compensation based on precision optical measurement techniques
The pointing control loops of the Deep Space Network 70 meter antennas extend only to the Intermediate Reference Structure (IRS). Thus, distortion of the structure forward of the IRS due to unpredictable environmental loads can result in uncompensated boresight shifts which degrade blind pointing accuracy. A system is described which can provide real time bias commands to the pointing control system to compensate for environmental effects on blind pointing performance. The bias commands are computed in real time based on optical ranging measurements of the structure from the IRS to a number of selected points on the primary and secondary reflectors
Moving to Extremal Graph Parameters
Which graphs, in the class of all graphs with given numbers n and m of edges
and vertices respectively, minimizes or maximizes the value of some graph
parameter? In this paper we develop a technique which provides answers for
several different parameters: the numbers of edges in the line graph, acyclic
orientations, cliques, and forests. (We minimize the first two and maximize the
third and fourth.)
Our technique involves two moves on the class of graphs. A compression move
converts any graph to a form we call fully compressed: the fully compressed
graphs are split graphs in which the neighbourhoods of points in the
independent set are nested. A second consolidation move takes each fully
compressed graph to one particular graph which we call H(n,m). We show
monotonicity of the parameters listed for these moves in many cases, which
enables us to obtain our results fairly simply.
The paper concludes with some open problems and future directions
Quantum channels with a finite memory
In this paper we study quantum communication channels with correlated noise
effects, i.e., quantum channels with memory. We derive a model for correlated
noise channels that includes a channel memory state. We examine the case where
the memory is finite, and derive bounds on the classical and quantum
capacities. For the entanglement-assisted and unassisted classical capacities
it is shown that these bounds are attainable for certain classes of channel.
Also, we show that the structure of any finite memory state is unimportant in
the asymptotic limit, and specifically, for a perfect finite-memory channel
where no nformation is lost to the environment, achieving the upper bound
implies that the channel is asymptotically noiseless.Comment: 7 Pages, RevTex, Jrnl versio
Indeterminate-length quantum coding
The quantum analogues of classical variable-length codes are
indeterminate-length quantum codes, in which codewords may exist in
superpositions of different lengths. This paper explores some of their
properties. The length observable for such codes is governed by a quantum
version of the Kraft-McMillan inequality. Indeterminate-length quantum codes
also provide an alternate approach to quantum data compression.Comment: 32 page
Antenna configurations provide polarization diversity
Compact back-to-back trapezoidal tooth log-periodic /TTLP/ antenna with frequency-independent characteristics is formed by reducing the angle between the two elements of a basic TTLP to zero. The back-to-back antenna, arranged in various configurations, provides monopulse operations in one or two planes and in various polarizations
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