15:00
Lagrangian aspects of turbulence 3
Chair: Markus Holzner
15:00
15 mins
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INVESTIGATION OF LAGRANGIAN COHERENT STRUCTURES IN A WAKE-INDUCED BOUNDARY LAYER TRANSITION
Guosheng He, Chong Pan, Qi Gao, Lihao Feng, Jinjun Wang
Abstract: The evolution of coherent structures in a flat plate boundary layer transition induced by the cylinder wake is investigated using the particle image velocimetry (PIV) technique. The finite-time Lyapunov exponent (FTLE), which characterizes the amount of stretching about the flow trajectory, is used to extract the Lagrangian coherent structures. It is revealed that secondary vortex is induced by the cylinder wake vortices in the near wall region,which would evolve into hairpin vortex as it convects downstream. The subsequent evolvement of the hairpin vortex, characterized by the regeneration of offspring hairpin vortex upstream of it, leads to the appearance of the hairpin packet and the boundary layer finally reaches a turbulent state.
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15:15
15 mins
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Angular statistics of lagrangian trajectories
Wouter Bos, Benjamin Kadoch, Kai Schneider
Abstract: The angle between subsequent particle displacement increments is evaluated as a function of the timelag in isotropic turbulence. It is shown that the evolution of the average of this angle contains two well-defined power-laws, reflecting the multi-scale dynamics of high-Reynolds number turbulence.
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15:30
15 mins
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Direction change of fluid particles in confined two-dimensional turbulence
Benjamin Kadoch, Wouter Bos, Kai Schneider
Abstract: The directional change of a fluid particle can be measured by the angle between two subsequent particle displacement increments. At small values of the time-increment the so-defined angle is proportional to the curvature of the trajectory. At large values this coarse-grained curvature should be affected by the presence of solid no-slip walls around the flow domain. We compare homogeneous and confined two-dimensional turbulent flows and show that the PDF of the angle is indeed strongly modified by the presence of walls.
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15:45
15 mins
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Excess statistics with applications to turbulence in marine environments
Hans Pecseli, Jan Trulsen
Abstract: Studies of excess statistics are standard in signal analysis, addressing basic questions like average frequencies of level crossings in a random signal, and average time durations between an up- and a down-crossing. The results can be applied to estimating the noise experienced by aquatic microorganisms in a turbulent environment. This problem has particular interest in relations to predators and prey that rely solely on signals transmitted through the surrounding turbulent flow. It is possible to give estimates for how often a predator can mistake turbulent induced noise for a signal from prey in given turbulence conditions. Such mistakes can be observed as unmotivated attacks. Similarly, prey can mistake noise for presence of a predator, giving rise to seemingly unmotivated observable escape responses.
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16:00
15 mins
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Dynamic of large particles embedded in shear flows
Miguel López-Caballero, Nathanaël Machicoane, Lionel Fiabane, Jean-Françoise Pinton, Mickael Bourgoin, Romain Volk
Abstract: Large particles ($D \gg \eta$) immersed in a closed turbulent flow tend to explore in a non-uniformly way the cavity in which they are placed.
Here we study the slow dynamics of large particles (with various size) advected in closed turbulent flows at different Reynolds numbers. We investigate the spatial sampling experienced by large particles in two fully turbulent closed flows generated between counter-rotating disks (so called von Karman flow), focusing in the slow frequency's ($f_{slow} < \Omega$, where $\Omega$ is the rotation rate of the driving impellers) and characterize the power spectrum of the slow fluctuations of particles position. Both considered flows share a common feature : the presence of a shear region dividing two mean re-circulation regions ; however the spatial symmetries and the temporal behaviors of both setups are very different. The principal result in this research is that despite these differences both flows exhibit a well defined slow dynamical behavior that can be identified in Fourier space. We report on the universal characteristics of such slow motion.
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16:15
15 mins
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LAGRANGIAN RAYLAIGH-BÉNARD CONVECTION
Sergio Chibbaro, Francesco Zonta
Abstract: Using passive tracers as sensors, we obtain Lagrangian measurements of tracers position, velocity and temperature in
Rayleigh-Bénard convection at Ra=10^7-10^9. We report on statistics of temperature, velocity, and heat transport (Nusselt number).
We observe that the Nusselt number is characterized by a largely intermittent behavior, likely due to the interaction of temperature with
turbulent velocity fluctuations.
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16:30
15 mins
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Triangular Constellations in Flows
Michael Wilkinson, John Grant
Abstract: Particles advected on the surface of a fluid can exhibit fractal clustering.
The local structure of a fractal set is described by its dimension $D$, which is the exponent of a power-law relating the mass ${\cal N}$ in a ball to its radius $\varepsilon$: ${\cal N}\sim \varepsilon^D$. It is desirable to characterise the {\em shapes} of constellations of points sampling a fractal measure, as well as their masses. The simplest example is the distribution of shapes of triangles formed by triplets of points, which we investigate for
fractals generated by chaotic dynamical systems. The most significant parameter describing the triangle shape is the ratio $z$ of its area to the radius of gyration squared. We show that the probability density of $z$ has a phase transition: $P(z)$ is independent of $\varepsilon$ and approximately uniform below a critical flow compressibility $\beta_{\rm c}$, which we estimate. For $\beta>\beta_{\rm c}$ the distribution appears to be described by two power laws: $P(z)\sim z^{\alpha_1}$ when $1\gg z\gg z_{\rm c}(\varepsilon)$, and $P(z)\sim z^{\alpha_2}$ when $z\ll z_{\rm c}(\varepsilon)$.
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