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16:15   Intermittency and Scaling 3 16:15 15 mins #353 Using persistent accelerations to disentangle Lagrangian turbulence Lukas Bentkamp, Cristian Lalescu, Michael Wilczek Abstract: see attached PDF 16:30 15 mins #554 Extension of Lagrangian multifractal formalism to inertial particle dynamics Bianca Viggiano, Romain Volk, Mickaël Bourgoin, Raúl Bayoán Cal, Laurent Chevillard Abstract: Interest in Lagrangian dynamics of turbulent flows has peaked in recent years due to advancements in experimental particle tracking techniques. Increased knowledge of the intermittent particle behavior in high Reynolds number flows leads to improved accuracy in simulating and modeling of particle trajectories. One such model, multifractal formalism, provides a description of the intermittent behavior of homogenous turbulent flows and accurately reproduces higher order velocity increment statistics. This is accomplished through assuming statistical properties in the inertial range and predicting them in the dissipative range. Several extensions of such a probabilistic description to the Lagrangian framework have been proposed in the literature (as reviewed in \cite{toschi2009lagrangian}) mostly for fluid particles, i.e. tracers. Further expansion to include the effects of an additional Stokes drag is proposed. Initial investigation is into the statistical analysis of such Lagrangian trajectories, in the spirit of and as they are provided in Bec et al. \cite{bec2010}. Figure~\ref{fig1} presents the second-order structure function, $S_{2} = \langle (\delta_{\tau} v )^2\rangle$, and the flatness, $F = \langle (\delta_{\tau} v)^4 \rangle / \langle (\delta_{\tau} v)^2 \rangle^2$, where the velocity increment is defined as $\delta_{\tau}v = v (t+\tau) -v(t)$. A range of Stokes numbers [0-2.6] are presented as a function of the time delay, $\tau$, normalized by the integral timescale, $T$. A dependence on the Stokes number can be observed for both sets of curves, particularly visible in the flatness. An extension of standard arguments pertaining to multifractal phenomenology is undertaken, mostly based on the formalism developed in \cite{chevillard2003}, in order to reproduce the effect of the Stokes drag. Despite the simplistic (linear) character of this modeling of inertia (as given by the Stokes drag), we report non trivial effects on higher order statistical quantities and in the dissipative range. Further predictions than those of structure functions, such as correlation of acceleration and probability density functions of velocity increments, are then systemically compared against numerical data. 16:45 15 mins #47 LAGRANGIAN VELOCITY AND POWER STRUCTURE FUNCTIONS FROM 4-D PARTICLE TRACKING VELOCIMETRY MEASUREMENTS OF A TURBULENT SWIRLING FLOW Valentina Valori, Paul Debue, Christophe Cuvier, Yasar Ostovan, Adam Cheminet, Tarek Chaabo, Cécile Wiertel, Vincent Padilla, Jean-Philippe Laval, Jean-Marc Foucaut, Bérengère Dubrulle, François Daviaud Abstract: We investigate intermittency and time irreversibility in a turbulent swirling flow, by computing the statistics of both Lagrangian velocity increments [2] and wavelet coefficients of the lagrangian power [3], [1]. For this aim, we perform time resolved, three dimensional, lagrangian velocity measurements of a turbulent von Karman flow. The flow is generated by two counterrotating impellers fitted with blades. The measurements volume is a parallelepipede of 45x40x6 mm^3 at the center of the domain. We use four high-speed cameras to perform 4D-PTV experiments, at four different Reynolds numbers: Re = 6.3 x 10^3; 3.2 x 10^4; 6.3 x 10^4; 1.6 x 10^6, by changing the frequency of the impeller, with water as working fluid. We compute the scaling of the Lagrangian velocity and Lagrangian power structure functions as a function of the Reynolds number. Comparison with corresponding Eulerian statistics is discussed. References [1] Massimo Cencini, Luca Biferale, Guido Boffetta, and Massimo De Pietro. Time irreversibility and multifractality of power along single particle trajectories in turbulence. Phys. Rev. Fluids, 2:104604, Oct 2017. [2] L. Chevillard, S. G. Roux, E. Levêque, N. Mordant, J.-F. Pinton, and A. Arneodo. Lagrangian velocity statistics in turbulent flows: Effects of dissipation. Phys. Rev. Lett., 91:214502, Nov 2003. [3] Jennifer Jucha, Haitao Xu, Alain Pumir, and Eberhard Bodenschatz. Time-reversal-symmetry breaking in turbulence. Phys. Rev. Lett., 113:054501, Jul 2014. 17:00 15 mins #350 Intermittency of inertial particle distribution in high Reynolds number turbulence Keigo Matsuda, Kai Schneider, Katsunori Yoshimatsu Abstract: The nonlinear dynamics of inertial particles in high Reynolds number turbulence, and in particular particle clustering, are important fundamental processes in atmospheric science, since they are related to the turbulent enhancement of droplet collision growth in convective clouds. The observed nonuniform particle distribution (Fig.~1, a, b) in turbulent flows is due to the centrifugal force in vortices; i.e., particles are concentrated in low-vorticity regions. Here we analyze particle data from high-resolution three-dimensional direct numerical simulations of particle-laden homogeneous isotropic turbulence at high Reynolds number, up to $Re_\lambda = 531$ and with up to $10^9$ particles (Matsuda et al. 2014; Matsuda & Onishi 2019). Wavelet analysis of particle-laden turbulence has been proposed in Bassenne et al. (2017) and Bassenne et al. (2018). In the current work, we apply orthogonal wavelet decomposition to the particle density fields. Computing scale-dependent flatness (Yoshimatsu et al. 2009), we quantify the intermittency of the density fields, which can characterize the clustering, and investigate the influence of the Reynolds number. We also show that the number of particles has some impact on high-order statistics reflecting the intermittency, especially at small scales. The results in Fig. 1(c) for unitary Stokes number show that the scale-dependent flatness $F(n_j)$ increases with decreasing scale in all cases dramatically. For the random distribution (i.e. a Poisson noise) shown as reference, we also find some increase, however much less pronounced. At intermediate scales we observe that the flatness does increase with $Re_\lambda$, while at small scales no significant influence can be found. Increasing the number of particles from $N_p = 1.5 \times 10^7$ to $1.07 \times 10^9$ in the case of $Re_\lambda=204$, we find a big difference in the small-scale flatness values, of about a factor 3.3. Hence we can conclude that a sufficiently large value of $N_p$ is important to catch correctly the intermittency of the particle density. In the final paper, we will furthermore assess the influence of the Stokes number on the intermittency of the particle density, and show different multiscale statistics, e.g. the scale-dependent skewness. 17:15 15 mins #6 TOPOLOGY OF QUASI-SINGULARITIES IN AN EXPERIMENTAL TURBULENT SWIRLING FLOW Paul Debue, Valentina Valori, Ostovan Yasar, Christophe Cuvier, Jean-Philippe Laval, Jean-Marc Foucaut, Bérengère Dubrulle, Cécile Wiertel-Gasquet, Vincent Padilla, François Daviaud Abstract: Even though they are more than 150 years old, the incompressible 3D Navier-Stokes equations remain an open mathe- matical problem [1] : the existence of a solution was proven by Leray [4] but it is still unknown whether such a solution is unique and regular. The quest for singularities in Navier-Stokes equations is of fundamental interest but may also have more applied consequences, regarding the relevance of numerical simulation and small scale modelling for instance. In this work we aim at providing insight on what such singularities may resemble and how they may form by studying the distribution and topology of extreme events of energy transfer in a real turbulent flow. Indeed, a singularity is charac- terized by a refinement of scales and may result in a non-zero or diverging inter-scale transfer with decreasing scale, as suggested by Duchon and Robert [3]. Local inter-scale energy transfer terms are computed from 3D-3C velocity fields, experimentally obtained by to particle image velocimetry (TPIV) implemented in a compact set-up involving 5 cameras. Extreme events ar following the methodology described in a previous work of the group [5], with the improvement that we are n measure the gradients in the three directions and compute the full transfer terms. This also allows to study th of such events following the classification provided in [2] and based on the velocity gradient tensor invariants. 17:30 15 mins #143 Instanton Calculus for the Onset of Turbulent Intermittency Luca Moriconi, Gabriel Brito Apolinário, Rodrigo Miranda Pereira Abstract: We provide a detailed study of the onset of turbulent intermittency in stochastic hydrodynamics, as signalized by the statistical behavior of velocity gradient fluctuations. Our attention is focused on two distinct, but phenomenologically paradigmatic, contexts: Lagrangian Navier-Stokes turbulence and one-dimensional Burgers hydrodynamics. A unified approach is designed upon the response functional formalism, where specific velocity configurations - the viscous instantons - are assumed to play a dominant role in modeling the non-gaussian tails of velocity gradient probability distribution functions (vgPDFs). We find, as expected on general grounds, that the field theoretical approach becomes meaningful in practice only if the effects of fluctuations around instantons are taken into account. Working with a systematic cumulant expansion, it turns out that the integration of fluctuations yields, in leading perturbative order, to an effective description of stochastic dynamics given by the renormalization of its associated heat kernel propagator and the external force-force correlation function. 17:45 15 mins #403 Turbulent dissipative anomaly and Lagrangian irreversibility Jeremie Bec, Simon Thalabard Abstract: We present results on possible links between Eulerian dissipative events and Lagrangian irreversible events. We find that the time-reversal symmetry is, to a large extent, broken by instantaneous extreme events during which pairs of tracers violently approach each other and possibly remain trapped within a small distance. We quantify the contribution of such events and relate them to turbulent dissipative structures.