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 Self-similar regimes in Unstably Stratified Homogeneous Turbulence Alan Burlot, Benoit-Joseph Grea, Jerome Griffond, Olivier Soulard Abstract: Unstably stratified homogeneous turbulence develops at late time a self-similar dynamics characterized by an exponential growth of turbulent quantities. It is believed from recent theoretical studies that different growth rates are possible, depending on the initial distribution of energy at large scales. In order to confirm these predictions, we run both highly resolved direct numerical simulations and a spectral model based on an eddy-damped quasi-normal closure. In addition to confirming the influence of initial conditions, our study sheds light on the anisotropic structures of the self-similar regimes. 10:45 15 mins Shear Capacity as prognostic of nocturnal boundary layer regimes Ivo van Hooijdonk, Judith Donda, Fred Bosveld, Arnold Moene, Herman Clercx, Bas van de Wiel Abstract: In some nights, the near-surface temperature can drop dramatically and turbulence in the stably stratified boundary layer becomes very weak, such that the lfow reaches a (quasi-) laminar state. In other cases, however, the atmosphere remains in a turbulent state and temperatures stay relatively high. Recently, the appearance of two distinct boundary layer regimes was explained by a new theoretical framework. This theory builds on the fact that the turbulent heat flux in stably stratified flow is limited to a maximum for given wind shear. This introduces a characteristic flux-based velocity scale, which can be used to predict the regimes. This hypothesis is consistent with field observations and numerical results. Also, the hypothesis is generalised to a dimensionless framework. 11:00 15 mins BRINGING CLOUDS INTO OUR LAB! - THE INFLUENCE OF TURBULENCE ON EARLY STAGE RAIN DROPLETS Mehmet Altug Yavuz, Rudie Kunnen, GertJan van Heijst, Herman Clercx Abstract: We are investigating a droplet-laden flow in an air-filled turbulence chamber, forced by speaker-driven air jets. The speakers are running in a random manner; yet they allow us to control and define the statistics of the turbulence. We study the motion of droplets with tunable size in a turbulent flow, mimicking the early stages of raindrop formation. 3D Particle Tracking Velocimetry (PTV) is chosen as the experimental method to track the droplets and collect data for statistical analysis. Thereby it is possible to study the spatial distribution of the droplets in turbulence using the so-called Radial Distribution Function (RDF), a statistical measure to quantify the clustering of particles. Additionally, this technique allows us to measure velocity statistics of the droplets and the influence of the turbulence on droplet trajectories, both individually and collectively. In this contribution, we will present velocity statistics of the droplets and quantify their clustering using the RDF for different turbulence conditions. 11:15 15 mins STABLY STRATIFIED SHEAR-PRODUCED TURBULENCE AND LARGE-SCALEWAVES IN A LID DRIVEN CAVITY Nimrod Cohen, Alexander Eidelman, Tov Elperin, Nathan Kleeorin, Igor Rogachevskii Abstract: We study experimentally stably stratified sheared turbulence and large-scale flows and waves in a lid driven cavity with a non-zero vertical mean temperature gradient. Geometrical properties of the large-scale vortex (e.g., its size and form) and the level of small-scale turbulence inside the vortex are controlled by the buoyancy (i.e., by the temperature stratification). The observed velocity fluctuations are produced by the shear of the large-scale vortex. At larger stratification obtained in our experiments, the strong turbulence region is located at the upper part of the cavity where the large scale vortex exists. In this region the Brunt-Väisälä frequency is small and increases in the direction outside the large-scale vortex. This is the reason of that the large-scale internal gravity waves are observed in the regions outside the large-scale vortex. We found these waves by analyzing the non instantaneous correlation functions of the temperature and velocity fields. The observed large-scale waves are nonlinear because the frequency of the waves determined from the temperature field measurements is two times smaller than that obtained from the velocity field measurements. The measured intensity of the waves is of the order of the level of the temperature turbulent fluctuations. 11:30 15 mins Scaling analysis based on extremal point topology Lipo Wang, Yongxiang Huang Abstract: The interaction of different scales is among the most interesting and challenging features in turbulence research. Existing approaches used for scaling analysis such as structure-function and Fourier spectrum method have their respective limitations, for instance scale mixing, i.e. the so-called infrared and ultraviolet effects. To make improvement in this regard, a new method, multi-level segment analysis (MSA) based on the local extrema statistics, has been developed. Data test results show that MSA can successfully reveal different scaling regimes in complex systems such as Lagrangian and two-dimensional turbulence, which have been remaining controversial in turbulence research. In principle MSA can generally be applied for various analyses. 11:45 15 mins A Unified Shell model for Buoyancy-Driven Turbulence Abhishek Kumar, Mahendra K. Verma Abstract: We construct a unified shell model for stably stratified and convective turbulence. Shell model simulation of stably stratified flow in turbulent regime exhibit Bolgiano-Obukhbov (BO) scaling in which the kinetic energy spectrum varies as $k^{-11/5}$. However, simulation of convective turbulence shows Kolmogorov's spectrum. These results are consistent with the direct numerical simulations of Kumar {\em et al.} [Phys. Rev. E {\bf 90}, 023016 (2014)]. We also observe a dual scaling ($k^{-11/5}$ and $k^{-5/3}$) for a limited range of parameters in stably stratified flow. 12:00 15 mins Is isotropy restored at small scales in freely decaying strongly stratified turbulence ? ALEXANDRE DELACHE, Fabien S. Godeferd, Claude Cambon Abstract: We analyse the scale-dependent anisotropy of homogeneous stratified turbulence. The Ozmidov scale l_N (Ozmidov 1965) helps to compare the relative effects of inertia and of the buoyancy force, and thus to quantify the rise of anisotropy in different scale ranges: at large scales l >> l_N the anisotropy due to strong stratification is dominant, whereas at small scales l << l_N, universal 3D isotropic characteristic of turbulence appear to be restored. We investigate the corresponding dynamics using Direct Numerical Simulations (DNS) in freely decaying turbulence at different stratification rates. We confirm the return to isotropy of the small scales by analyzing the orientation-dependent power spectrum and poloidal/toroidal/density energy modes. To some extent, many characteristics of isotropic universality are restored at small scales but, surprisingly, the density spectrum (also potential energy spectrum) plays a particular role. 12:15 15 mins EXPERIMENTS WITH MICRO-MACHINED MULTI-ARRAY HOT-FILM PROBE TOWARDS FIELD EXPERIMENTS WITH SUB-KOLMOGOROV RESOLUTION Youry Borisenkov, Grigori Gulitski, Michael Kholmyansky, Slava Krylov, Alexander Liberzon, Arkady Tsinober Abstract: This is a report on implementation of the next stage of a project, motivated by the discovery of far more important role, played by the sub-Kolmogorov scales in high Reynolds number turbulence than commonly believed. The main results and issues that prompted the present work are in [1] – [3] and references therein. At this stage we report on the first successful experiments with a unique micro-machined hot-film multi-array probe. This includes design, assembly, implementation and testing of such probe in laboratory conditions. The probe is enabling to access, along with all three components of turbulent velocity fluctuations, also all nine components of the spatial velocity gradients tensor, including the option of obtaining the stream-wise velocity derivatives without employing the Taylor hypothesis, as well as velocity temporal derivatives. The key feature of the micro-machined multi-array probe (typically five arrays), with each array consisting of four micro-machined hot-film sensors, is that it is six times smaller than the conventional multi-array probe, based on arrays with four hot wires [4] – [5], used in the atmospheric surface layer at Taylor micro-scale Reynolds number up to Reλ ≈ 104. This part of work relates to Reynolds numbers Reλ < 500 and employs several laboratory flows.