15th European Turbulence Conference 2015
August 25-28th, 2015, Delft, The Netherlands

## Invited speakers:

Prof. Marc Brachet. Ecole Normale Superieure, Paris, France

Prof. Peter G. Frick, Institute of Continuous Media Mechanics, Perm, Russia

Prof. Bettina Frohnapfel,  Karlsruher Institut fur Technology, Germany

Prof. Andrea Mazzino, Dipartimento di Fisica, University of Genova, Italy

Prof. Bernhard Mehlig. Department of Physics, University of Gothenburg, Sweden

Prof. Lex Smits, Mechanical and Aerospace Engineering, Princeton University, USA

Prof. Chao Sun Physics of Fluids, University of Twente, The Netherlands

Prof. Steve Tobias, Applied Mathematics, University of Leeds, United Kingdom

 10:30 15 mins High resolution simulations of random fields and implications on stochastic modelling of turbulence Rodrigo Pereira, Laurent Chevillard Abstract: We study through high resolution numerical simulations unknown features of a random velocity field based on the so-called multiplicative chaos, previously proposed in the literature to model turbulence. We investigate the influence of the intermittency parameter, observing the behaviour of the field as it is changed. In this way, we can determine whether corrections to the 4/5 law exist and pursue some properties that have not been analytically computed. We also study the effects on other realistic properties such as the preferential alignment of vorticity. 10:45 15 mins Small-Scale Properties of Two-Dimensional Rayleigh-Taylor Turbulence Quan ZHOU Abstract: We report a high-resolution numerical study of small-scale properties of two-dimensional (2D) miscible Rayleigh-Taylor (RT) incompressible turbulence with the Boussinesq approximation at small Atwood number and unit Prandtl number. Our results show that the buoyancy force balances the inertial force at all scales below the integral length scale and thus validate the basic force-balance assumption of the Bolgiano-Obukhov scenario in 2D RT turbulence. We further examine other small-scale properties of 2D RT turbulence, such as temporal evolution of energy and thermal dissipation rates, the emergence of intermittency and anomalous scaling for high order moments of velocity and temperature differences, distributions of local dissipation scales, and so on. 11:00 15 mins Log-stable law of energy dissipation rate for turbulence intermittency Hideaki Mouri Abstract: To describe the small-scale intermittency of turbulence, a self-similarity is imposed on the probability density function of a logarithm of the rate of energy dissipation smoothed over a length scale among those in the inertial range. The result is an extension of Kolmogorov's classical theory in 1941, i.e., a one-parameter framework where the logarithm obeys some stable distribution. We obtain the scaling laws for the dissipation rate and for the two-point velocity difference. 11:15 15 mins THE EFFECT OF LARGE-SCALE INHOMOGENEITIES ON SMALL-SCALE STRUCTURE IN A TURBULENT FLOW Hamed Sadeghi, Philippe Lavoie, Andrew Pollard Abstract: Kolmogorov’s equation, which indicates that the mean energy is transported by both turbulent advection and molecular diffusion at any scale of flow, cannot be balanced for flows encountered in the laboratory conditions at moderate range of Reynolds numbers. The main reason for this imbalance is inhomogeneous large-scales. In this work, a new generalized form of Kolmogorov’s equation is suggested for jet flows. The validity of this equation is investigated using hot-wire data obtained across the centreline of a round turbulent jet. In addition, the external intermittency and some other basic characteristics are studied. 11:30 15 mins Energy dissipation and flux laws for unsteady turbulence John Christos Vassilicos, Susumu Goto Abstract: Direct Numerical Simulations of spatially periodic unsteady turbulence show that the high Reynolds number scalings of the instantaneous energy dissipation rate and interscale energy flux at intermediate wavenumbers are qualitatively different from the well-known $u'(t)^{3}/L(t)$ cornerstone scalings of equilibrium turbulence where $u'(t)$ and $L(t)$ are time-dependent rms velocity and integral length-scales. Instead, they both scale as $U_{0}L_{0}\:u'(t)^2/L(t)^2$ where $L_0$ and $U_0$ are length and velocity scales characterizing initial/overall unsteady turbulence conditions. 11:45 15 mins Probing turbulence intermittency via Auto-Regressive Moving-Average models Davide Faranda, Flavio Pons, Francois Daviaud, Berengere Dubrulle Abstract: We suggest a new approach to probing intermittency corrections to the Kolmogorov law in turbulent flows based on the Auto-Regressive Moving-Average modeling of turbulent time series. We introduce an index Upsilon that measures the distance from a Kolmogorov-Obukhov model in the Auto-Regressive Moving-Average models space. Applying our analysis to Laser Doppler Velocimetry measurements in a von Karman swirling flow, we show that Upsilon is proportional to traditional intermittency corrections computed from structure functions. Therefore it provides the same information, using much shorter time series. We conclude that Upsilon is a suitable index to reconstruct intermittency in experimental turbulent fields. 12:00 15 mins Bulk statistics of stable and decaying Taylor-Couette turbulence Sander Huisman, Ruben Verschoof, Roeland van der Veen, Chao Sun, Detlef Lohse Abstract: In this talk we focus on the velocity fluctuations in highly turbulent Taylor-Couette flow for the case of stable flow (constant rotation) and for decaying flow. Turbulent flows are generally characterized by the range of scales of their fluctuations, and a statistical description of the flow is often done by calculating the correlations of velocity fluctuations. These correlations are found to behave like power-laws over a range of scales, and their exponents characterize a certain geometry of flow. Many systems have been investigated carefully: Pipe-flow, Von K\'arm\'an flow, Rayleigh B\'enard convection, \textit{et cetera}. There are, however, few reports \cite{lew99,she01} quantifying the turbulent properties in Taylor-Couette flow. In the presented work \cite{huisman2013b} we measure the longitudinal structure functions using laser Doppler anemometry, which is a non-intrusive technique and is able to measure the components of the velocity, and thus ideal for obtaining structure functions and the local velocity. We present the statistics of the turbulent velocity fluctuations for counter rotation for varying $a=-\omega_o/\omega_i$. 12:15 15 mins HYDRODYNAMICAL TURBULENCE BY FRACTAL FOURIER DECIMATION Alessandra Sabina Lanotte, Luca Biferale, Shiva Kumar Malapaka, Federico Toschi Abstract: We present a systematic numerical investigation of high-resolution 3D isotropic and homogeneous turbulence resolved on a decimated set of Fourier modes. Fractal decimation acts to decrease the effective dimensionality of the flow by allowing triadic interactions only in a set of Fourier modes N(k) proportional to k^DF for large k. While keeping the symmetries of the original 3D Navier-Stokes equations unchanged, a dramatic change in small-scale statistics is detected at decreasing the fractal dimension DF . Already at fractal dimension DF = 2.8, a global self-similar behaviour is observed in the inertial range of scales, the consequence of such transition are the restoration of the scaling symmetry and vorticity distribution that becomes close to Gaussian. We relate the results to the different roles of local vs non-local interactions in the energy transfer range.