Geometrical statistics of the vorticity vector and the strain rate tensor in rotating turbulence

We report results on the geometrical statistics of the vorticity vector obtained from experiments in electromagnetically forced rotating turbulence. A range of rotation rates $\Omega$ is considered, from non-rotating to rapidly rotating turbulence with a maximum background rotation rate of $\Omega=5$ rad/s (with Rossby number much smaller than unity). Typically, in our experiments ${\rm{Re}}_{\lambda}\approx 100$. The measurement volume is located in the centre of the fluid container above the bottom boundary layer, where the turbulent flow can be considered locally statistically isotropic and horizontally homogeneous for the non-rotating case, see van Bokhoven et al., Phys. Fluids 21, 096601 (2009). Based on the full set of velocity derivatives, measured in a Lagrangian way by 3D Particle Tracking Velocimetry, we have been able to quantify statistically the effect of system rotation on several flow properties. The experimental results show how the turbulence evolves from almost isotropic 3D turbulence ($\Omega\lesssim 0.2$ rad/s) to quasi-2D turbulence ($\Omega\approx 5.0$ rad/s) and how this is reflected by several statistical quantities. In particular, we have studied the orientation of the vorticity vector with respect to the three eigenvectors of the local strain rate tensor and with respect to the vortex stretching vector. Additionally, we have quantified the role of system rotation on the self-amplification terms of the enstrophy and strain rate equations and the direct contribution of the background rotation on these evolution equations. The main effect is the strong reduction of extreme events and related (strong) reduction of the skewness of PDFs of several quantities such as, for example, the intermediate eigenvalue of the strain rate tensor and the enstrophy self-amplification term.Comment: 17 pages, 6 figures, 3 table

Table-top rotating turbulence : an experimental insight through Particle Tracking

The influence of the Earth background rotation on oceanic and atmospheric currents, as well as the effects of a rapid rotation on the flow inside industrial machineries like mixers, turbines, and compressors, are only the most typical examples of fluid flows affected by rotation. Despite the Coriolis acceleration term appears in the Navier-Stokes equations with a straightforward transformation of coordinates from the inertial system to the rotating non-inertial one, the physical mechanisms of the Coriolis acceleration are subtle and not fully understood. Several fluid flows affected by rotation have been studied by means of numerical simulations and analytical models, but the experimental data available is scarce and purely of Eulerian nature. The present work addresses experimentally the topic, focusing on a class of fluid flows of utmost importance: confined and continuously forced rotating turbulence. Experiments of the same turbulent flow (maximum Re ¿¿ 110 for O = 0) subjected to different background rotation rates ( O ¿ {0; 0.2; 0.5; 1.0; 2.0; 5.0} rad/s) are performed, visualised by optical means, and measured quantitatively with Particle Tracking Velocimetry. The measurement system is designed and implemented around the experimental setup, using innovative solutions. The data collected is processed in the Lagrangian frame, where the trajectories are filtered and the 3D time-dependent signals of position, velocity, acceleration, temporal velocity derivatives, and full velocity gradient tensor are extracted. The data is further interpolated over a regular grid, in order to analyse it in the Eulerian frame. The background rotation is found to decrease the kinetic energy and the energy dissipation of the turbulent field, and to damp the coupling between large-scale flow and small-scale turbulence. Interesting large-scale features of the flow field are revealed: the increase of rotation rate induces vertical coherency of the fluid motion (in terms of velocity, velocity derivatives, Eulerian spatial and temporal auto-correlations of velocity), till at the maximum rotation rate of 5 rad/s a quasi-2D flow is measured, dominated by stable counter-rotating vertical tubes of vorticity. Exception is the 2 rad/s run, for which an anomalous behaviour of all the investigated flow features is observed: at this rotation rate, the vertical vortex tubes fluctuate in the measurement domain with much higher amplitude and on a longer time scale than for any other run. The estimated values for the critical Rossby number indicate that the stability of the large-scale anticyclonic vortices may be compromised for 1.0 <O <5.0 rad/s. No indications of resonant oscillations in the container, triggered by inertial waves, are instead recognised in the data. Further investigations are necessary to explain the anomaly measured for this run, but the present data suggest the possibility that anticyclone instabilities significantly alter the large-scale flow. The (non-)rotating turbulent flow is also investigated in terms of Eulerian spatial correlations of the velocity field, and – for the first time – of Lagrangian correlations of the velocity, acceleration, and vorticity vectors extracted along fluid particle trajectories. The increase of vertical (parallel to the rotation vector) and horizontal velocity correlations induced by rotation is measured in the Eulerian and the Lagrangian frames. Rotation is seen to strongly enhance the correlation of the vertical vorticity component, characteristic of a flow dominated by columnar vortex structures. It is also seen to enhance the longitudinal horizontal acceleration component, confirming the direct role played by the Coriolis acceleration in the amplification of the Lagrangian acceleration correlations in turbulence. In the same Lagrangian frame, the turbulent dispersion process at short times in the presence of rotation is investigated. The data permits to describe the initial ballistic dispersion regime, and the beginning of the inertial range regime. A more pronounced effect is observed on single-particle dispersion statistics, which are influenced by rotation in a non-monotonic way, strongly anisotropic only for the fastest rotating runs. Two-particle dispersion is monotonically reduced with increasing rotation rate, and the anisotropy is revealed only for maximum rotation rate. Some of the results presented in this thesis are completely new. Other results confirmed well-known features of rotating turbulent flows, further quantifying them on the basis of state-of-the-art Particle Tracking experimental data. Surely this work opened new questions. Concluding remarks give suggestions about possible future measurements in the same turbulence setup, as well as in view of the design of a new experimental setup specifically devoted to Lagrangian flow analysis and/or to the investigation of rotating steady turbulence. The results obtained in this study have been presented at international conferences and workshops, and will be submitted for publication to international journals

Le briofite della Grotta dell\u2019Orso (33-7VG, Carso triestino, NE Italia)

Vengono presentati i risultati di uno studio sulla flora briologica della Grotta dell\u2019Orso (33-7VG), situata sul Carso triestino. Sono state rinvenute 42 specie di briofite (33 specie di muschi e 9 epatiche); per ogni specie sono indicati l\u2019elemento corologico, la distribuzione nell\u2019area e note ecologico-stazionali. Aspetti floristici, biogeografici e conservazionistici della componente briologica dell\u2019area vengono discussi. Viene riportata una nuova specie per la Regione Friuli Venezia Giulia, l\u2019epatica Cololejeunea rossettiana; 2 altre specie di muschi sono segnalate come nuove per il Carso. La flora briologica aggiornata della cavit\ue0 comprende 46 specie

Collective behaviour without collective order in wild swarms of midges

Collective behaviour is a widespread phenomenon in biology, cutting through a huge span of scales, from cell colonies up to bird flocks and fish schools. The most prominent trait of collective behaviour is the emergence of global order: individuals synchronize their states, giving the stunning impression that the group behaves as one. In many biological systems, though, it is unclear whether global order is present. A paradigmatic case is that of insect swarms, whose erratic movements seem to suggest that group formation is a mere epiphenomenon of the independent interaction of each individual with an external landmark. In these cases, whether or not the group behaves truly collectively is debated. Here, we experimentally study swarms of midges in the field and measure how much the change of direction of one midge affects that of other individuals. We discover that, despite the lack of collective order, swarms display very strong correlations, totally incompatible with models of noninteracting particles. We find that correlation increases sharply with the swarm's density, indicating that the interaction between midges is based on a metric perception mechanism. By means of numerical simulations we demonstrate that such growing correlation is typical of a system close to an ordering transition. Our findings suggest that correlation, rather than order, is the true hallmark of collective behaviour in biological systems.Comment: The original version has been split into two parts. This first part focuses on order vs. correlation. The second part, about finite-size scaling, will be included in a separate paper. 15 pages, 6 figures, 1 table, 5 video

Finite-size scaling as a way to probe near-criticality in natural swarms

Collective behaviour in biological systems is often accompanied by strong correlations. The question has therefore arisen of whether correlation is amplified by the vicinity to some critical point in the parameters space. Biological systems, though, are typically quite far from the thermodynamic limit, so that the value of the control parameter at which correlation and susceptibility peak depend on size. Hence, a system would need to readjust its control parameter according to its size in order to be maximally correlated. This readjustment, though, has never been observed experimentally. By gathering three-dimensional data on swarms of midges in the field we find that swarms tune their control parameter and size so as to maintain a scaling behaviour of the correlation function. As a consequence, correlation length and susceptibility scale with the system's size and swarms exhibit a near-maximal degree of correlation at all sizes.Comment: Selected for Viewpoint in Physics; PRL Editor's Suggestio

High overtones of Dirac perturbations of a Schwarzschild black hole

Using the Frobenius method, we find high overtones of the Dirac quasinormal spectrum for the Schwarzschild black hole. At high overtones, the spacing for imaginary part of $\omega_{n}$ is equidistant and equals to $\Im{\omega_{n+1}}-\Im{\omega_{n}} =i/8M$, ($M$ is the black hole mass), which is twice less than that for fields of integer spin. At high overtones, the real part of $\omega_{n}$ goes to zero. This supports the suggestion that the expected correspondence between quasinormal modes and Barbero-Immirzi parameter in Loop Quantum Gravity is just a numerical coincidence.Comment: 5 pages, Latex, 3 figures, Physical Review D.,at pres

Expanding horizons: new roles for non-canonical RNA-binding proteins in cancer

Cancer development involves the stepwise accumulation of genetic lesions that overcome the normal regulatory pathways that prevent unconstrained cell division and tissue growth. Identification of the genetic changes that cause cancer has long been the subject of intensive study, leading to the identification of several RNA-binding proteins (RBPs) linked to cancer. Cross-reference of the complement of RBPs recently identified by RNA interactome capture with cancer-associated genes and biological processes led to the identification of a set of 411 proteins with potential implications in cancer biology. These involve a broad spectrum of cellular processes including response to stress, metabolism and cell adhesion. Future studies should aim to understand these proteins and their connection to cancer from an RNA-centred perspective, holding the promise of new mechanistic understanding of cancer formation and novel approaches to diagnosis and treatment