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Non-spherical particles are present all around us, in biological, industrial and environmental processes. Making predictions of their impact on us and systems in our vicinity can make life better for everyone here on earth. For example, the ash particles from a volcano eruption are non-spherical and their spreading in the atmosphere can hugely impact the air traffic, as was also proven in 2010. Furthermore, the orientation of the wood fibres in a paper sheet influences the final properties of the paper, and the cause of a specific fibre orientation can be traced back to the fluid flows during the manufacturing process of the paper. In this thesis, experimental and numerical work is presented with the goal to understand and utilize the behavior of elongated particles in fluid flows. Two different experimental setups are used. The first one, a turbulent half channel flow, aims at increasing the understanding of how particles with non-zero inertia behave in turbulence. The second setup is an attempt to design a flow field with the purpose to align nanofibrils and create high performance cellulose filaments. Experiments were performed in a turbulent half channel flow at different flow set- tings with dilute suspensions of cellulose acetate fibres having three different aspect ratios (length to width ratio). The two main results were firstly that the fibres agglom- erated in streamwise streaks, believed to be due to the turbulent velocity structures in the flow. Secondly, the orientation of the fibres was observed to be determined by the aspect ratio and the mean shear, not the turbulence. Short fibres were oriented in the spanwise direction while long fibres were oriented in the streamwise direction. In order to utilize the impressive properties (stiffness comparable to Kevlar) of the cellulose nanofibril in a macroscopic material, the alignment of the fibrils must be controlled. Here, a flow focusing device (resulting in an extensional flow), designed to align the fibrils, is used to create a cellulose filament with aligned fibrils. The principle is based on a separation of the alignment and the assembly of the fibrils, i.e. first align the fibrils and then lock the aligned structure. With this process, continuous filaments were created, with properties similar to that of the wood fibre at the same fibril alignment. However, the highest alignment (lowest angle) of the fibrils in a filament created was only 31o from the filament axis, and the next step is to increase the alignment. This thesis includes modeling of the alignment process with the Smoluchowski equation and a rotary diffusion. Finding a model that correctly describes the alignment process should in the end make it possible to create a filament with fully aligned fibrils. / <p>QC 20140908</p>

Nous présentons dans cette thèse une utilisation de la structure dite de stries huileuses présente lors de la compétition entre deux ancrages antagonistes de part et d’autre d’un film sub-micrométrique de cristal liquide en phase smectique pour réaliser le contrôle à l’échelle individuelle de l’orientation d’émetteurs de photons unique ainsi que de l’auto-organisation ultérieure lorsque la concentration est augmentée. La morphologie interne de cette structure est étudiée à l’aide d’observations en microscopie optique polarisée, d’ellipsométrie et de diffraction de rayons X. L’analyse de ces résultats mène au modèle présenté en fin de chapitre 2 caractérisé par la formation d’un réseau de défauts topologiques orientés. Ces stries huileuses sont ensuite utilisées pour permettre l’alignement à l’échelle individuelle de particules semi-conductrices, des dots-in-rods de CdSe:CdS, suivant l’axe principal des défauts. Le chapitre 3 présente l’insertion des particules et la mesure de l’orientation de leur dipôle associé ainsi que celle du degré de polarisation. Le chapitre 4 discute du comportement des particules pour une concentration élevée, avec outre leur orientation leur auto-organisation au sein des stries huileuses. Nous faisons une analyse statistique de la densité des amas formés à la surface d’un échantillon et des informations résultant de la structure des amas sur l’organisation des défauts dans différentes situations. On observe un alignement des nanoparticules à l’échelle unique et en amas, ainsi qu’un maintien ou un renforcement du degré de polarisation mesuré, montrant une auto-organisation des amas favorisant l’interaction entre les dipôles des particules. / This thesis discusses the use of a liquid crystal structure, called oily streaks, formed by the competition of two antagonist anchoring on each side of a submicrometric liquid crystal layer, to achieve the self-alignment and, for higher concentration, the organization, of single photon emitters on an individual scale. We discuss the internal layout of these oily streaks through measurements done in polarized optical microscopy, ellipsometry and X-ray diffraction analysis. These data sets are correlated in chapter 2 into a description of the internal structure of the oily streaks, characterized by the formation of a pattern of oriented topological defects, and an analysis of their behaviour. These streaks are then used as a template to align single photon emitters, CdSe:CdS dot-in-rods, on an individual scale alongside the main axis of the defects. We measure the alignment of their dipoles as well as their degree of polarization in order to discuss the behaviour of the particles in the topological defects of the oily streaks. The last chapter presents a statistical analysis of the behaviour of nanoparticles in oily streaks once their concentration is increased, depending on their size in number of particles as well as the characteristics of the structure in which the single particles or the clusters are trapped. We use these elements to further our understanding of the layout of oily streaks in different thicknesses and configurations as we observe the alignment phenomenon both with single particles and clusters, as well as, for the clusters, a similar or superior degree of polarization compared to single particles, indicating an interaction between the particles’ dipoles.

With the widespread use of portable digital devices, especially smartphones, learning a new language has become a popular trend in the age of globalization. Amongst mobile-assistant language-learning applications, Duolingo is widely used and employs persuasive design techniques to inspire and encourage users to acquire new linguistic skills. Several papers have been studied based on this application by exploring pedagogical issues and discussing the misuse of gamification design. From a holistic perspective, limited research exists on how Duolingo's persuasive design motivates and engages language learners. The study utilized a walkthrough and a focus group alongside semi-structured interviews with 11 Duolingo users to investigate how persuasive design elements are employed to engage users. The findings of this study suggest that the gamification design in Duolingo is enjoyable and enhances users’ motivation; streaks make users commit to their learning plan; rewards encourage users to learn constantly; notifications play a significant role as triggers for learning behavior to occur; notifications also remind users to return to the application after a suspension. Moreover, it is important to design notifications with both the attitude and content in mind, as they have the power to affect user experience.

Wall-bounded turbulent flows such as plane Couette flow, channel, pipe flows and boundary layer flows are fundamental problem of interest that we often meet in many scientific and engineering situations. The goal of the present thesis is to investigate the origin of large-scale streaky motions observed in the wall-bounded turbulent flows. Under a hypothesis that the large-scale streaky motions sustain with a process similar to the well-known near-wall self-sustaining cycle, the present thesis have pursued on four separate subjects: (i) non-modal amplification of streaks, (ii) the secondary instability of the finite amplitude streaks, (iii) existence of a self-sustaining process at large scale and (iv) turbulent skin friction reduction by forcing streaks. First, using a linear model with turbulent mean flow and the related eddy viscosity, it is shown that the streaks are largely amplified by harmonic and stochastic forcing. The largely amplified streaks undergo the secondary instability and it has been associated with the formation of the large-scale motions (bulge). The existence of a self-sustaining process involving the amplification and instability of streaks at large scale is proved by quenching the smaller-scale energy carrying eddies in the near-wall and logarithmic regions. Finally, it is shown that artificially forcing of large-scale streaks reduce the turbulent skin friction up to 10\% by attenuating the near-wall streamwise vortices.

<p>Transition from laminar to turbulent flow has beentraditionally studied in terms of exponentially growingeigensolutions to the linearized disturbance equations.However, experimental findings show that transition may occuralso for parameters combinations such that these eigensolutionsare damped. An alternative non-modal growth mechanism has beenrecently identified, also based on the linear approximation.This consists of the transient growth of streamwise elongateddisturbances, mainly in the streamwise velocity component,called streaks. If the streak amplitude reaches a thresholdvalue, secondary instabilities can take place and provoketransition. This scenario is most likely to occur in boundarylayer flows subject to high levels of free-stream turbulenceand is the object of this thesis. Different stages of theprocess are isolated and studied with different approaches,considering the boundary layer flow over a flat plate. Thereceptivity to free-stream disturbances has been studiedthrough a weakly non-linear model which allows to disentanglethe features involved in the generation of streaks. It is shownthat the non-linear interaction of oblique waves in thefree-stream is able to induce strong streamwise vortices insidethe boundary layer, which, in turn, generate streaks by thelift-up effect. The growth of steady streaks is followed bymeans of Direct Numerical Simulation. After the streaks havereached a finite amplitude, they saturate and a new laminarflow, characterized by a strong spanwise modulation isestablished. Using Floquet theory, the instability of thesestreaks is studied to determine the features of theirbreakdown. The streak critical amplitude, beyond which unstablewaves are excited, is 26% of the free-stream velocity. Theinstability appears as spanwise (sinuous-type) oscillations ofthe streak. The late stages of the transition, originating fromthis type of secondary instability, are also studied. We foundthat the main structures observed during the transition processconsist of elongated quasi-streamwise vortices located on theflanks of the low speed streak. Vortices of alternating signare overlapping in the streamwise direction in a staggeredpattern.</p><p><strong>Descriptors:</strong>Fluid mechanics, laminar-turbulenttransition, boundary layer flow, transient growth, streamwisestreaks, lift-up effect, receptivity, free-stream turbulence,nonlinear mechanism, streak instability, secondary instability,Direct Numerical Simulation.</p> / QC 20100518

Process industries are in general depending, in one way or the other, on fluid mechanics.Specifically, paper manufacturing, which probably is the dominant processindustry in Sweden, is depending on the flow of cellulose fibres suspended in water.As a part of the process the suspension, consisting of fibres in water, is spread out onor between two moving permeable weaves, i.e. wires. The speed of this is usually 10–30 m/s and the suspension is spread out by a plane jet issuing from a headbox nozzle.It has been show that the conditions in the headbox and jet have a large influence onthe quality of the final paper sheet. Primarily, streaks in the paper sheet are believedto be the result of streamwise streaks in the headbox jet.The thesis is aimed at the flow phenomena which occur in the headbox jet. Theinvestigations have been made with numerical calculations, stability theory and modelexperiments using water, as well as experiments with a real paper machine headboxand fibre suspension. In the thesis an introduction to the hydrodynamics of planeliquid jets is presented together with a description of the paper forming process andthe fluid mechanics of headbox flow.The basic flow and stability of a two-dimensional plane liquid jet has been investigatedby numerical calculations, stability theory and experiments. The calculationsof the laminar basic flow is successfully compared to pitot-tube measurements of thestreamwise velocity profile. By visualisations of the flow it is found that wave disturbanceson the jet has a severe effect on the flow. These waves can be predicted bylinear stability theory, which shows the presence of five convectively unstable modes.These can be divided into three types and by comparison with the experiments thetype of the visible waves is determined. These waves seem to initiate a break-up ofthe jet, which leads to strong streamwise streaks inside the jet.By flow visualisation of headbox flow of an experimental paper machine, togetherwith analysis of the resulting paper structure using the wavelet method the correspondencebetween flow disturbances and paper quality was investigated. It was shownthat the wave instability, which is present on the low Reynolds number water jet, alsocan be found in the real the headbox jet. It is shown that these waves play an importantrole in the dynamics of the headbox jet and also have an influence on the final papersheet. / QC 20100825

Disturbances introduced in wall-bounded flows can grow and lead to transition from laminar to turbulent flow. In order to reduce losses or enhance mixing in energy systems, a fundamental understanding of the flow stability and transition mechanism is important. In the present thesis, the stability, transition mechanism and early turbulent evolution of wall-bounded flows are studied. The stability is investigated by means of linear stability equations and the transition mechanism and turbulence are studied using direct numerical simulations. Three base flows are considered, the Falkner-Skan boundary layer, boundary layers subjected to wall suction and the Blasius wall jet. The stability with respect to the exponential growth of waves and the algebraic growth of optimal streaks is studied for the Falkner-Skan boundary layer. For the algebraic growth, the optimal initial location, where the optimal disturbance is introduced in the boundary layer, is found to move downstream with decreased pressure gradient. A unified transition prediction method incorporating the influences of pressure gradient and free-stream turbulence is suggested. The algebraic growth of streaks in boundary layers subjected to wall suction is calculated. It is found that the spatial analysis gives larger optimal growth than temporal theory. Furthermore, it is found that the optimal growth is larger if the suction begins a distance downstream of the leading edge. Thresholds for transition of periodic and localized disturbances as well as the spreading of turbulent spots in the asymptotic suction boundary layer are investigated for Reynolds number Re=500, 800 and 1200 based on the displacement thickness and the free-stream velocity. It is found that the threshold amplitude scales like Re^-1.05 for transition initiated by streamwise vortices and random noise, like Re^-1.3 for oblique transition and like Re^-1.5 for the localized disturbance. The turbulent spot is found to take a bullet-shaped form that becomes more distinct and increases its spreading rate for higher Reynolds number. The Blasius wall jet is matched to the measured flow in an experimental wall-jet facility. Both the linear and nonlinear regime of introduced waves and streaks are investigated and compared to measurements. It is demonstrated that the streaks play an important role in the breakdown process where they suppress pairing and enhance breakdown to turbulence. Furthermore, statistics from the early turbulent regime are analyzed and reveal a reasonable self-similar behavior, which is most pronounced with inner scaling in the near-wall region. / QC 20101025

This work is concerned with the response of a Blasius boundary layer to dielectric-barrier-discharge (DBD) plasma actuators for the purpose of using these devices in bypass transition control. The plasma actuators consist of a spanwise-periodic array of high voltage electrodes, which are oriented to produce streamwise vortex pairs. The structure of actuator-induced streaks is measured using hot-wire anemometry over a streamwise distance of approximately 100 boundary layer thicknesses, and is decomposed into 4 spanwise Fourier modes. The modal content and corresponding streamwise growth characteristics are discussed for ten plasma actuator geometries over multiple excitation voltages and freestream velocities. Actuator power consumption was found to control the streak amplitude, whereas freestream velocity affected both amplitude and streamwise extent of the streaks. A common relationship between disturbance energy and power consumption was found among actuators of different dielectric thickness and similar electrode geometry.

This work is concerned with the response of a Blasius boundary layer to dielectric-barrier-discharge (DBD) plasma actuators for the purpose of using these devices in bypass transition control. The plasma actuators consist of a spanwise-periodic array of high voltage electrodes, which are oriented to produce streamwise vortex pairs. The structure of actuator-induced streaks is measured using hot-wire anemometry over a streamwise distance of approximately 100 boundary layer thicknesses, and is decomposed into 4 spanwise Fourier modes. The modal content and corresponding streamwise growth characteristics are discussed for ten plasma actuator geometries over multiple excitation voltages and freestream velocities. Actuator power consumption was found to control the streak amplitude, whereas freestream velocity affected both amplitude and streamwise extent of the streaks. A common relationship between disturbance energy and power consumption was found among actuators of different dielectric thickness and similar electrode geometry.