18,687 research outputs found
Low-Complexity Reduced-Rank Beamforming Algorithms
A reduced-rank framework with set-membership filtering (SMF) techniques is
presented for adaptive beamforming problems encountered in radar systems. We
develop and analyze stochastic gradient (SG) and recursive least squares
(RLS)-type adaptive algorithms, which achieve an enhanced convergence and
tracking performance with low computational cost as compared to existing
techniques. Simulations show that the proposed algorithms have a superior
performance to prior methods, while the complexity is lower.Comment: 7 figure
Grid-free compressive beamforming
The direction-of-arrival (DOA) estimation problem involves the localization
of a few sources from a limited number of observations on an array of sensors,
thus it can be formulated as a sparse signal reconstruction problem and solved
efficiently with compressive sensing (CS) to achieve high-resolution imaging.
On a discrete angular grid, the CS reconstruction degrades due to basis
mismatch when the DOAs do not coincide with the angular directions on the grid.
To overcome this limitation, a continuous formulation of the DOA problem is
employed and an optimization procedure is introduced, which promotes sparsity
on a continuous optimization variable. The DOA estimation problem with
infinitely many unknowns, i.e., source locations and amplitudes, is solved over
a few optimization variables with semidefinite programming. The grid-free CS
reconstruction provides high-resolution imaging even with non-uniform arrays,
single-snapshot data and under noisy conditions as demonstrated on experimental
towed array data.Comment: 14 pages, 8 figures, journal pape
Aspect ratio dependence of heat transfer and large-scale flow in turbulent convection
The heat transport and corresponding changes in the large-scale circulation
(LSC) in turbulent Rayleigh-B\'{e}nard convection are studied by means of
three-dimensional direct numerical simulations as a function of the aspect
ratio of a closed cylindrical cell and the Rayleigh number . For
small and moderate aspect ratios, the global heat transfer law shows a power law dependence of both fit coefficients and
on the aspect ratio. A minimum Nusselt number coincides with the point
where the LSC undergoes a transition from a single-roll to a double-roll
pattern. With increasing aspect ratio, we detect complex multi-roll LSC
configurations. The aspect ratio dependence of the turbulent heat transfer for
small and moderate is in line with a varying amount of energy
contained in the LSC, as quantified by the Proper Orthogonal Decomposition
analysis. For the heat transfer becomes independent of the
aspect ratio.Comment: 17 pages, 11 Postscript figures (in parts downscaled), accepted for
J. Fluid Mec
Compositional competitiveness for distributed algorithms
We define a measure of competitive performance for distributed algorithms
based on throughput, the number of tasks that an algorithm can carry out in a
fixed amount of work. This new measure complements the latency measure of Ajtai
et al., which measures how quickly an algorithm can finish tasks that start at
specified times. The novel feature of the throughput measure, which
distinguishes it from the latency measure, is that it is compositional: it
supports a notion of algorithms that are competitive relative to a class of
subroutines, with the property that an algorithm that is k-competitive relative
to a class of subroutines, combined with an l-competitive member of that class,
gives a combined algorithm that is kl-competitive.
In particular, we prove the throughput-competitiveness of a class of
algorithms for collect operations, in which each of a group of n processes
obtains all values stored in an array of n registers. Collects are a
fundamental building block of a wide variety of shared-memory distributed
algorithms, and we show that several such algorithms are competitive relative
to collects. Inserting a competitive collect in these algorithms gives the
first examples of competitive distributed algorithms obtained by composition
using a general construction.Comment: 33 pages, 2 figures; full version of STOC 96 paper titled "Modular
competitiveness for distributed algorithms.
Statistics of the Energy Dissipation Rate and Local Enstrophy in Turbulent Channel Flow
Using high-resolution direct numerical simulations, the height and Reynolds
number dependence of higher-order statistics of the energy dissipation rate and
local enstrophy are examined in incompressible, fully-developed turbulent
channel flow. The statistics are studied over a range of wall distances,
spanning the viscous sublayer to the channel flow centerline, for friction
Reynolds numbers and . The high resolution of
the simulations allows dissipation and enstrophy moments up to fourth order to
be calculated. These moments show a dependence on wall distance, and Reynolds
number effects are observed at the edge of the logarithmic layer. Conditional
analyses based on locations of intense rotation are also carried out in order
to determine the contribution of vortical structures to the dissipation and
enstrophy moments. Our analysis shows that, for the simulation at the larger
Reynolds number, small-scale fluctuations of both dissipation and enstrophy
become relatively constant for .Comment: Accepted by Physica
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