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

Queer animation: a creative project in constructing fantastical worlds of desire

A research report submitted to the Faculty of Humanities, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Art (Digital Art), September 2017The aim of the research report is to establish a link between the field/practice of queer theory and the field/practice of animation. I propose that the linking factor between these two bodies of theory and practice is fantasy. the purpose of exploring this link s to understand how formal elements of the animated medium can be used to articulate queer scenarios of desire: [Abbreviated Abstract. Open document to view full version]XL201

Analysis of electroencephalography signals collected in a magnetic resonance environment: characterisation of the ballistocardiographic artefact

L’acquisizione simultanea di segnali elettroencefalografici (EEG) e immagini di risonanza magnetica funzionale (fMRI) permette di investigare attivazioni cerebrali in modo non invasivo. La presenza del campo magnetico altera però in modo non trascurabile la qualità dei segnali EEG acquisiti. In particolare due artefatti sono stati individuati: l’artefatto da gradiente e l’artefatto da ballistocardiogramma (BCG). L’artefatto da BCG è legato all’attività cardiaca del soggetto, ed è caratterizzato da elevata variabilità tra un’occorrenza e l’altra in termini di ampiezza, forma d’onda e durata dell’artefatto. Differenti algoritmi sono stati implementati al fine di rimuoverlo, ma la rimozione completa rimane ancora un difficile obiettivo da raggiungere a causa della sua complessa natura. L’argomento della tesi riguarda l’analisi di segnali EEG acquisiti in ambiente di risonanza magnetica e la caratterizzazione dell’artefatto BCG. L’obiettivo è individuare ulteriori caratteristiche dell’artefatto che possano condurre al miglioramento dei precedenti metodi, o all’implementazione di nuovi. Con questa tesi abbiamo mostrato quali sono i motivi che causano la presenza di residui artefattuali nei segnali EEG processati con i metodi presenti in letteratura. Attraverso analisi statistica abbiamo riscontrato che occorrenze dell’artefatto BCG sono caratterizzate da un ritardo variabile rispetto al picco R sull’ECG, che nella nostra analisi rappresenta l’evento di riferimento nell’attività cardiaca. Abbiamo inoltre trovato che il ritardo R-BCG varia con la frequenza cardiaca. Le successive valutazioni riguardano i maggiori contributi all’artefatto BCG. Attraverso l’analisi alle componenti principali, sono stati individuati due contributi legati al fluire del sangue dal cuore verso il cervello e alla sua pulsatilità nei vasi principali dello scalpo

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

Short-range interaction vs long-range correlation in bird flocks

Bird flocks are a paradigmatic example of collective motion. One of the prominent experimental traits discovered about flocks is the presence of long range velocity correlations between individuals, which allow them to influence each other over the large scales, keeping a high level of group coordination. A crucial question is to understand what is the mutual interaction between birds generating such nontrivial correlations. Here we use the Maximum Entropy (ME) approach to infer from experimental data of natural flocks the effective interactions between birds. Compared to previous studies, we make a significant step forward as we retrieve the full functional dependence of the interaction on distance and find that it decays exponentially over a range of a few individuals. The fact that ME gives a short-range interaction even though its experimental input is the long-range correlation function, shows that the method is able to discriminate the relevant information encoded in such correlations and single out a minimal number of effective parameters. Finally, we show how the method can be used to capture the degree of anisotropy of mutual interactions.Comment: 21 pages, 7 figures, 1 tabl

Advertising communication gets fashionable: branded content and fashion films

Los cambios vividos por la industria comunicativa en los últimos años, entre otros motivos debido al auge de las redes sociales, han convertido a los contenidos, informativos o de entretenimiento, en la piedra angular para hacer llegar el mensaje publicitario al destinatario. El esfuerzo por parte de los anunciantes de tener una estrategia de contenidos definida para su presencia en medios sociales o nuevos formatos comunicativos como el branded content, son ejemplo de que el advertainment está actualmente de moda. Este artículo se centra en el branded content y en otro ejemplo de esta tendencia: los fashion films, un nuevo género publicitario que ha irrumpido especialmente en los sectores de la moda y los productos de lujo, con el fin, entre otros, de ofrecer una nueva visión de éstos basada en la originalidad, contando historias que conecten con su público objetivo.Changes experienced by communication industry in recent years, among other reasons due to the rise of social networks, have turned contents, information or entertainment, into the touchstone for advertising message to reach its audience. The effort made by advertisers to have a defined content strategy for their presence in social media or the new communicative formats such as branded content are an example of advertainment being popular nowadays. This article focuses on branded content as well as another example of this trend: the fashion films, a new advertising genre that has erupted especially in the sectors of fashion and luxury products, with the goals of showing products in an original way and telling stories which are able to connect with your target audience

Advisors, brand and tweets: influence marketing

La figura del prescriptor ha sido uno de los recursos más utilizados en publicidad a lo largo de toda su historia. Con la popularización de las redes sociales, el poder de recomendación de productos y marcas se ha extendido prácticamente a cualquier individuo, en tanto en cuanto interactúa con otros usuarios a través de estas plataformas, conversando también a propósito de sus experiencias como comprador y consumidor. Además, blogs y espacios como Twitter o Instagram han motivado la aparición de un nuevo perfil de prescriptores, líderes de opinión a los que las marcas recurren para multiplicar el alcance de sus acciones comunicacionales. En este artículo se analiza en qué consiste el marketing de influencia así como se describen las normas deontológicas y legales que anunciantes y agencias deben tener en cuenta a la hora de poner en marcha acciones publicitarias con influencers a través de redes sociales.Advertiser figure has been one of the resources most widely used in advertising throughout its history. However, with the popularization of social networks, power of recommendation of products and brands has spread practically to anybody, insofar individual interacts with other users through these platforms, also talking concerning his/her experiences as buyer and consumer. In addition to this, blogs and platforms such as Twitter or Instagram have motivated the emergence of a new profile of prescribers, opinion leaders who brands resort to with the aim of multiplying the reach of their communication actions. This article focuses on what influence marketing is and which are its characteristics as well as describes the legal and ethical rules advertisers and agencies must take into account to perform advertising campaigns with influencers via social networks

Analysis of branded content on television: Spanish case studies

El branded content se ha popularizado como formato de comunicación corporativa y publicitaria alternativa a las acciones comunicacionales tradicionales. El presente trabajo analiza el uso del branded content por parte de los anunciantes españoles en el medio televisión, con el fin de identificar las características de la aplicación de esta técnica al principal medio en inversión publicitaria en España. La metodología empleada se basa en el estudio de casos, analizando para cada uno de ellos una serie de variables previamente definidas. Los resultados muestran el carácter de utilidad y servicio que, en su mayoría, tienen las acciones de branded content difundidas en el medio televisión, ya que ofrecen información de interés para el ciudadano, como consejos y tutoriales, con un mensaje centrado en el destinatario, y transmitiendo a través de los contenidos los valores de la marca. Como conclusión, podemos destacar la transmedialidad que caracteriza a las acciones de branded content televisivas, puesto que la conversación entre marca y usuario sobre la temática tratada en estos espacios continúa en los medios sociales, a través de la presencia de los programas en plataformas como Twitter.Branded content has become popular as an alternative corporate and advertising communication format with respect to traditional communication actions. This study analyzes the use of branded content by Spanish advertisers on television, in order to identify the characteristics of the application of this communicative technique to the main medium in advertising investment in Spain. The methodology used is based on the case study, analyzing a number of predefined variables for each of the selected sample. The results show the character of utility and service which most of branded content actions broadcast on television have, since they offer information of interest to the citizen, such as tips and tutorials, with a message focused on the audience and conveying the brand values through the contents. As a conclusion, we can highlight the transmediality that characterizes the actions of branded content on television, given that the conversation between brand and user about the programmes subjects continues in social media, thanks to presence of these programmes on platforms such as Twitter