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<p>Understanding the drying of droplets is of interest for processes such as spray drying, where particulate materials are produced by evaporating moisture. Even though spray-drying is a widely used method, there are still challenges, such as undesired agglomeration or controlling the morphology and size of the final dried product. This dissertation develops a physics based model that is used to examine the droplet dynamics and drying kinetics at large and small scales.  In addition, the model simulates the internal motion of insoluble particles and  is used to better understand particle formation during spray drying type processes.</p> <p><br></p> <p>The first part of this work examines the effect of droplet-droplet collisions on evaporation and the size distribution at a large scale. Droplet collision dynamics are implemented into an Eulerian-Lagrangian framework, where droplets are tracked in the Lagrangian frame, and the background gas is modeled as a continuum. The modeling framework includes fully coupled interphase heat and momentum transfer between the droplet and gas phases. Binary collision of droplets could result in coalescence, reduction in surface area, or separation of droplets, resulting in the generation of satellite droplets and an increase in total surface area. By capturing the change in size distribution due to the collision of particles, our results show a linear relationship between the Weber number and the evaporation rate at low droplet number densities. Further, it is shown that droplet number density is a critical factor influencing the evaporation rate. At high droplet number densities, the relationship between the evaporation rate and the Weber number becomes non-linear, and at extremely high droplet number densities, the evaporation rate decreases even at high Weber numbers.</p> <p><br></p> <p>In the next part of this dissertation, the drying of a single droplet containing insoluble solid particles is investigated. Using a volume-of-fluid framework coupled with the Lagrangian phase, we study the particle transport within a droplet, and how it is affected by airflow, phase properties (e.g., viscosity and density of each phase), surface tension, and evaporation. Unlike the traditional one-dimensional modeling approach, our multi-dimensional model can capture the generation of internal flow patterns due to shear flow and the accumulation of solid particles on the surface of the drying droplet. Our results show that the surface tension effect is more pronounced at larger droplet diameters and low airflow velocities. Our approach also provides a quantitative method for modeling crust growth and formation. </p> <p>Our results show that increasing solids mass fraction, and decreasing particle diameter, slow down the internal transport of solid particles, leading to a more quick accumulation near the surface of the droplet. Further, despite the droplet undergoing a constant-rate drying stage, the accumulation of solids near the surface is non-linear. In addition, the inclusion of solids within the droplet drastically reshapes the formation of internal vortices compared to the uncoupled case, which determines solids distribution.</p>

As autonomous ground vehicles grow in popularity,it is of interest to study how they could coordinate together andhow the technical systems can be implemented in a safe andeffective manner. The objective of this report is to examine howto autonomously move a formation of vehicles without collisionswith obstacles or other vehicles. This is done by considering threefundamental aspects: trajectory tracking, formation control andcollision avoidance. Firstly a trajectory tracking controller for anindividual vehicle is implemented, with the function of followinga desired trajectory. Secondly a displacement-based formationcontrol is explored for two models, the double-integrator modeland the nonholonomic model, with the objective of coordinatingmultiple vehicles to keep a certain formation. Lastly collisionavoidance is integrated in the formation control by adding arepulsive term to the formation controller. It is shown thatthe agents maintained formation while avoiding collision withobstacles and other agents. The implemented controllers wereverified through simulations in MATLAB. / Eftersom autonoma markfordon blir allt mer vanligt är det vikt att studera hur de kan samordna tillsammans och hur de tekniska systemen kan implementeras på ett säkert samt effektivt sätt. Syftet med denna rapport är att undersöka hur man autonomt kan flytta en formation av fordon utan kollisioner med hinder eller med andra fordon. Detta görs genom att tre grundläggande aspekter övervägs: projektilspårning, formationshållning och kollisionsundvikande. Först implementeras en regulator för projektilsspårning, där funktionen är att följa en önskad bana. Därefter undersöks två modeller inom förskjutningsbaserad formationshållning, med ambitionen att samordna alla fordon för att behålla formationen. Slutligen så integreras metoder för kollisionsundvikning med formationshållning genom att lägga till bortstötande teknik i regulatorn för formationshållning. Det visades att fordonen lyckades med att upprätthålla formationen samtidigt som kollisioner mellan hinder och andra fordon undveks. De implementerade regulatorerna verifierades genom simuleringar i MATLAB. / Kandidatexjobb i elektroteknik 2021, KTH, Stockholm

Systems consisting of multiple robots are traditionallydifficult to optimize. This project considers such a systemin a simulated warehouse setting, where the robots are todeliver boxes while avoiding collisions. Adding such collisionconstraints complicates the problem. For dynamical multi-agentsystems as these, reinforcement learning algorithms are oftenappropriate. We explore and implement a reinforcement learningalgorithm, called multi-agent rollout, that allows for re-planningduring operation. The algorithm is paired with a base policyof following the shortest path. Simulation results with up to10 robots indicates that the algorithm is promising for largescalemulti-robot systems. We have also discussed the possibilityof using neural networks and partitioning to further increaseperformance. / System med flera robotar har traditionellt sett ansetts mycket svåra att optimera. I detta projekt undersöks ett sådant system i en simulerad lagerlokal, där robotarna skall förflytta lådor samtidigt som de undviker kollisioner. För dessa dynamiska system med flera robotar är förstärkande inlärning ofta lämpligt. Vi undersöker och implementerar en förstärkandeinlärningsalgoritm kallad ”multi-agent rollout” vilken möjliggör omdirigering under drift. Algoritmen används tillsammans med en så kallad ”base policy” som alltid väljer kortaste vägen. Baserat på simulationsresultaten med upp till tio robotar verkar algoritmen lovande för storskaliga flerrobotsystem. Det diskuteras även om möjligheten av att använda neurala nätverk och partitionering för att vidare öka prestandan. / Kandidatexjobb i elektroteknik 2021, KTH, Stockholm

Many satellites have an orbit of reference defined according to their mission. The satellites need therefore to navigate as close as possible to their reference orbit. However, due to external forces, the trajectory of a satellite is disturbed and actions need to be taken. For now, the trajectories of the satellites are monitored by the operations of satellites department which gives appropriate instructions of navigation to the satellites. These steps require a certain amount of time and involvement which could be used for other purposes. A solution could be to make the satellites autonomous. The satellites would take their own decisions depending on their trajectory. The navigation control would be therefore much more efficient, precise and quicker. Besides, the autonomous orbit control could be coupled with an avoidance collision risk management. The satellites would decide themselves if an avoidance maneuver needs to be considered. The alerts of collisions would be given by the ground segment. In order to advance in this progress, this internship enables to analyse the feasibility of the implementation of the two concepts by testing them on an experiments satellite. To do so, tests plans were defined, tests procedures were executed and post-treatment tools were developed for analysing the results of the tests. Critical computational cases were considered as well. The tests were executed in real operations conditions. / Många satelliter har en referensbana definierad enligt deras uppdrag. Satelliterna behöver därför navigera så nära deras referensbana som möjligt. På grund av externa krafter störs dock satellitbanan och åtgärder måste vidtas. För närvarande övervakas satellitbanorna av satellitavdelningar på marken vilka ger lämpliga instruktioner för navigering till satelliterna. Dessa steg kräver en tid och engagemang som skulle kunna användas för andra ändamål. En lösning är att göra satelliterna autonoma. Satelliterna skulle då kunna ta sina egna beslut beroende på deras bana. Navigeringskontrollen skulle därför vara mycket mer effektiv, exakt och snabbare. Dessutom kan den autonoma banregleringen kopplas till riskhantering för undvikande av kollision med rymdskrot och andra satelliter. Satelliterna skulle själva avgöra om en undvikande manöver måste övervägas. Varningar om kollisioner skulle ges av marksegmentet. För att gå vidare i denna utveckling analyserar detta arbete genomförbarheten av implementeringen av olika koncept för undanmanövrar genom att testa dem på en experimentsatellit. För att göra detta definierades testplaner, testprocedurer utfördes och efterbehandlingsverktyg utvecklades för analys av testresultaten. Kritiska beräkningsfall togs fram. Testerna utfördes under verkliga driftsförhållanden.

With the increasing number of satellites operating in orbit and the development of nanosatelliteconstellations, it has become more and more arduous for operators to keep track of every satellitestate, and perform corrective or avoidance manoeuvres. That is why CNES, the French space agency,is developing new algorithms, which aimed at making satellites more self-su cient. More especially,these algorithms are in charge of autonomous orbit control, collision risk calculations and satellitestatus monitoring. In this thesis, we present the architecture of these three algorithms and how theyinteract between them to deal with the autonomous control of a satellite. In addition, this paper studiestheir integration within the OPS-SAT nanosatellite, which is an in-orbit demonstrator developed bythe European Space Agency (ESA) and opened to worldwide experimenters. By analysing the dataused by the numerical propagators, the size of the input configuration files sent to the nanosatellitewas optimised. Thanks to this optimisation, the size of telecommands sent during each OPS-SATflyby above the ESOC ground station meets the requirements. Due to some issues encountered with the nanosatellite’s GPS, a solution was found to update thecurrent orbit on-board, and thus allow the proper algorithms’ operation. This thesis also introduceshow the tests were carried out in order to validate these algorithms, both on flat-sat and on the realsatellite. The results demonstrate that their integration on the OPS-SAT numerical environment issuccessful, meaning that the algorithms and their dependences are correctly packaged, sent and uploaded,and that they work as expected. Their execution time are of course longer due to the limitedcalculation capacity of the on-board computer, but are still compatible with real operations, except forthe collision risk computation, which can exceed the orbital period depending on the initial conditions.Finally, the thesis presents the process of real operations for one of the three algorithms developed byCNES, the di culties encountered and the solutions considered. / Med det ökande antalet satelliter i omloppsbana och utvecklingen av nanosatellitkonstellationer hardet blivit mer och mer krävande för operatörer att hålla reda på varje satellits tillstånd och utförakorrigerande eller undvikande manövrar. Det är därför som CNES, den franska rymdorganisationen,utvecklar nya algoritmer som syftar till att göra satelliter mer autonoma. Närmare bestämt ansvarardessa algoritmer för autonom omloppsbanereglering, kollisionsriskberäkningar och satellitstatusövervakning.I detta examensarbete presenterar vi arkitekturen för dessa tre algoritmer och hur de interagerarmellan sig för att hantera den autonoma styrningen av en satellit. Dessutom studeras deras integrationinom OPS-SAT-nanosatelliten, som är en demonstrator i omloppsbana som utvecklats av Europeiskarymdorganisationen (ESA) och öppnad för globala experiment. Genom att analysera de datasom används av de numeriska propagatorerna optimerades storleken på de ingångskonfigurationsfilersom skickades till nanosatelliten. Tack vare denna optimering uppfylls storlekskraven på telekommandonsom skickas under varje passage av OPS-SAT ovanför ESOC-markstationen. På grund av vissa problem med nanosatellitens GPS hittades en lösning för att uppdatera den aktuellaomloppsbanan ombord och därmed möjliggöra korrekt funktion av algoritmerna. Detta examensarbeteintroducerar också hur testerna genomfördes för att validera dessa algoritmer, både på en s.k. flat-satoch på den verkliga satelliten. Resultaten visar att deras integration i den numeriska miljön OPS-SATär framgångsrik, vilket innebär att algoritmerna och deras beroende är korrekt förpackade, skickade ochuppladdade och att de fungerar som förväntat. Deras exekveringstid är naturligtvis längre på grundav den inbyggda datorns begränsade beräkningskapacitet, men är fortfarande kompatibel med verkligaoperationer, förutom beräkningen av kollisionsrisk, som kan överstiga omloppsperioden beroende påde initiala förhållandena. Slutligen presenterar rapporten processen för verkliga operationer för en avde tre algoritmerna som utvecklats av CNES, svårigheterna och de lösningar som övervägs.

For the past couple of years researchers around theworld have tried to develop fully autonomous vehicles. One of theproblems that they have to solve is how to navigate in a dynamicworld with ever-changing variables. This project was initiated tolook into one scenario of the path planning problem; overtakinga human driven vehicle. Model Predictive Control (MPC) hashistorically been used in systems with slower dynamics but withadvancements in computation it can now be used in systems withfaster dynamics. In this project autonomous vehicles controlledby MPC were simulated in Python based on the kinematic bicyclemodel. Constraints were posed on the overtaking vehicle suchthat the two vehicles would not collide. Results show that anovertake, that keeps a proper distance to the other vehicle andfollows common traffic laws, is possible in certain scenarios. / Under de senaste åren har forskare världen över försökt utveckla fullt autonoma fordon. Ett av problemen som behöver lösas är hur man navigerar i en dynamisk värld med ständigt förändrande variabler. Detta projekt startades för att titta närmare på en aspekt av att planera en rutt; att köra om ett mänskligt styrt fordon. Model Predictive Control (MPC) har historiskt sett blivit använt i system med långsammare dynamik, men med framsteg inom datorers beräkningskraft kan det nu användas i system med snabbare dynamik. I detta projekt simulerades självkörande fordon, styrda av MPC, i Python. Fordonsmodellen som används var kinematic bicycle model. Begränsningar sattes på det omkörande fordonet så att de två fordonen inte kolliderar. Resultaten visar att en omkörning, som håller avstånd till det andra fordonet samt följer trafikregler, är möjligt i vissa scenarion. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm

The usage of autonomous ground vehicles is growingextensively. Therefore, it is important to gain a better understand-ing for the fundamentals of their communication network. Inthis paper, three important areas will be considered: Trajectorytracking, formation control and collision avoidance. Firstly,trajectory tracking is implemented for unicycles to direct them toa reference path. Secondly, formation control is examined for twoapproaches: A method based on virtual structure with a presettrajectory for unicycles and a method based on displacement forpoint agents. Finally, collision avoidance is incorporated withthe displacement-based controller. For this case, agents keepformation without colliding within formation and with staticobstacles in the workspace. The proposed controllers are verifiedthrough simulations in MATLAB. / Användningen av autonoma markfordon har ökat kraftigt de senaste åren. Således är det viktigt att få en bättre förståelse för grunderna i deras kommunikationsnätverk. I detta projekt studeras tre essentiella områden: projektilspårning, formationshållning och undvikning av kollisioner. Först och främst implementeras projektilspårning för en enhjuling där den styrs mot en önskad projektilbana. Därefter betraktas formationshållning genom två metoder: Den första metoden handlar om virtuella strukturer med en förutbestämd bana för enhjulingar. Den andra metoden baseras på en förskjutningsbaserad regulator. Slutligen införs undvikning av kollisioner tillsammans med den förskjutningsbaserade regulatorn för att uppnå ett kollisionsundvikande robotsystem. Samtliga objektiv inom de tre områdena nås med varierande precision. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm

<p>In this thesis, we present theoretical and statistical techniques broadly related to systems of dynamically-interacting particles. We apply these techniques to observations of dense star clusters in order to study gravitational interactions between stars. These include both long- and short-range interactions, as well as encounters leading to direct collisions and mergers. The latter have long been suspected to be an important formation channel for several curious types of stars whose origins are unknown. The former drive the structural evolution of star clusters and, by leading to their eventual dissolution and the subsequent dispersal of their stars throughout the Milky Way Galaxy, have played an important role in shaping its history. Within the last few decades, theoretical work has painted a comprehensive picture for the evolution of star clusters. And yet, we are still lacking direct observational confirmation that many of the processes thought to be driving this evolution are actually occuring. The results presented in this thesis have connected several of these processes to real observations of star clusters, in many cases for the first time. This has allowed us to directly link the observed properties of several stellar populations to the physical processes responsible for their origins.</p> <p>We present a new method of quantifying the frequency of encounters involving single, binary and triple stars using an adaptation of the classical mean free path approximation. With this technique, we have shown that dynamical encounters involving triple stars occur commonly in star clusters, and that they are likely to be an important dynamical channel for stellar mergers to occur. This is a new result that has important implications for the origins of several peculiar types of stars (and binary stars), in particular blue stragglers. We further present several new statistical techniques that are broadly applicable to systems of dynamically-interacting particles composed of several different types of populations. These are applied to observations of star clusters in order to obtain quantitative constraints for the degree to which dynamical interactions affect the relative sizes and spatial distributions of their different stellar populations. To this end, we perform an extensive analysis of a large sample of colour-magnitude diagrams taken from the ACS Survey for Globular Clusters. The results of this analysis can be summarized as follows: (1) We have compiled a homogeneous catalogue of stellar populations, including main-sequence, main-sequence turn-off, red giant branch, horizontal branch and blue straggler stars. (2) With this catalogue, we have quantified the effects of the cluster dynamics in determining the relative sizes and spatial distributions of these stellar populations. (3) These results are particularly interesting for blue stragglers since they provide compelling evidence that they are descended from binary stars. (4) Our analysis of the main-sequence populations is consistent with a remarkably universal initial stellar mass function in old massive star clusters in the Milky Way. This is a new result with important implications for our understanding of star formation in the early Universe and, more generally, the history of our Galaxy. Finally, we describe how the techniques presented in this thesis are ideally suited for application to a number of other outstanding puzzles of modern astrophysics, including chemical reactions in the interstellar medium and mergers between galaxies in galaxy clusters and groups.</p> / Doctor of Philosophy (PhD)

This study assesses the viability of temporarily halting wind turbine operations as amitigation measure to protect bird populations during migration periods. Conducted in thenorthern Baltic, it examines the migration patterns, timings, and altitudes of various birdspecies, aiming to identify the most critical times for implementing turbine stoppages.Utilizing statistical analyses, including F-tests to evaluate migration intensity differencesamong species, the report proposes that strategic, short-duration shutdowns cansignificantly reduce avian collisions. The research emphasizes the importance of species-specific approaches and evaluates the cost-effectiveness of various bird collision reductiontechniques, such as blade painting and thermal detection for dynamic shutdowns. Thisapproach seeks to balance the ecological impact of wind turbines with the necessity forrenewable energy development, offering practical solutions that could enhance biodiversityconservation efforts without substantially compromising energy production efficiency.