311 research outputs found
Experimental investigation of vertical turbulent transport of a passive scalar in a boundary layer: Statistics and visibility graph analysis
The dynamics of a passive scalar plume in a turbulent boundary layer is
experimentally investigated via vertical turbulent transport time-series. Data
are acquired in a rough-wall turbulent boundary layer that develops in a
recirculating wind tunnel set-up. Two source sizes in an elevated position are
considered in order to investigate the influence of the emission conditions on
the plume dynamics. The analysis is focused on the effects of the meandering
motion and the relative dispersion. First, classical statistics are
investigated. We found that (in accordance with previous studies) the
meandering motion is the main responsible for differences in the variance and
intermittency, as well as the kurtosis and power spectral density, between the
two source sizes. On the contrary, the mean and the skewness are slightly
affected by the emission conditions. To characterize the temporal structure of
the turbulent transport series, the visibility algorithm is exploited to carry
out a complex network-based analysis. Two network metrics -- the average peak
occurrence and the assortativity coefficient -- are analysed, as they can
capture the temporal occurrence of extreme events and their relative intensity
in the series. The effects of the meandering motion and the relative dispersion
of the plume are discussed in the view of the network metrics, revealing that a
stronger meandering motion is associated with higher values of both the average
peak occurrence and the assortativity coefficient. The network-based analysis
advances the level of information of classical statistics, by characterizing
the impact of the emission conditions on the temporal structure of the signals
in terms of extreme events and their relative intensity. In this way, complex
networks provide -- through the evaluation of network metrics -- an effective
tool for time-series analysis of experimental data
Gravitational Instantons from Gauge Theory
A gauge theory can be formulated on a noncommutative (NC) spacetime. This NC
gauge theory has an equivalent dual description through the so-called
Seiberg-Witten (SW) map in terms of an ordinary gauge theory on a commutative
spacetime. We show that all NC U(1) instantons of Nekrasov-Schwarz type are
mapped to ALE gravitational instantons by the exact SW map and that the NC
gauge theory of U(1) instantons is equivalent to the theory of hyper-Kaehler
geometries. It implies the remarkable consequence that ALE gravitational
instantons can emerge from local condensates of purely NC photons.Comment: 4 pages with two columns; comments and references added, to appear in
Phys. Rev. Let
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