226 research outputs found
Stochastic analysis of ocean wave states with and without rogue waves
This work presents an analysis of ocean wave data including rogue waves. A
stochastic approach based on the theory of Markov processes is applied. With
this analysis we achieve a characterization of the scale dependent complexity
of ocean waves by means of a Fokker-Planck equation, providing stochastic
information of multi-scale processes. In particular we show evidence of Markov
properties for increment processes, which means that a three point closure for
the complexity of the wave structures seems to be valid. Furthermore we
estimate the parameters of the Fokker-Planck equation by parameter-free data
analysis. The resulting Fokker-Planck equations are verified by numerical
reconstruction. This work presents a new approach where the coherent structure
of rogue waves seems to be integrated into the fundamental statistics of
complex wave states.Comment: 18 pages, 13 figure
Orientational transition in nematic liquid crystals under oscillatory Poiseuille flow
We investigate the orientational behaviour of a homeotropically aligned
nematic liquid crystal subjected to an oscillatory plane Poiseuille flow
produced by an alternating pressure gradient. For small pressure amplitudes the
director oscillates within the flow plane around the initial homeotropic
position, whereas for higher amplitudes a spatially homogeneous transition to
out-of-plane director motion was observed for the first time. The orientational
transition was found to be supercritical and the measured frequency dependence
of the critical pressure amplitude in the range between 2 and 20 Hz was in
quantitative agreement with a recent theory.Comment: 11 pages, 4 figures, submitted to Europhys. Let
Improved estimation of Fokker-Planck equations through optimisation
An improved method for the description of hierarchical complex systems by
means of a Fokker-Planck equation is presented. In particular the
limited-memory Broyden-Fletcher-Goldfarb-Shanno algorithm for constraint
problems (L-BFGS-B) is used to minimize the distance between the numerical
solutions of the Fokker-Planck equation and the empirical probability density
functions and thus to estimate properly the drift and diffusion term of the
Fokker-Planck equation. The optimisation routine is applied to a time series of
velocity measurements obtained from a turbulent helium gas jet in order to
demonstrate the benefits and to quantify the improvements of this new
optimisation routine
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