Astrophysikalische Jets sind stark kollimierte Materieströmungen hoher Geschwindigkeit. Sie stehen im Zusammenhang mit einer Fülle verschiedener astrophysikalischer Objekte wie jungen Sternen, stellaren schwarzen Löchern ('Mikro-Quasare'), Galaxien mit aktivem Kern (AGN) und wahrscheinlich auch mit dem beobachteten intensiven Aufblitzen von Gamma-Strahlung (Gamma Ray Bursts). Insbesondere hat sich gezeigt, dass die Jets der Mikro-Quasare wahrscheinlich als kleinskalige Version der Jets der AGN anzusehen sind. <br /> <br /> Neben den Beobachtungen haben vor allem auch theoretische Überlegungen gezeigt, dass Magnetfelder bei der Jetentstehung, -beschleunigung und -kollimation eine wichtige Rolle spielen. Weiterhin scheinen Jets systematisch verknüpft zu sein mit dem Vorhandensein einer Akkretionsscheibe um das zentrale Objekt. Insbesondere wenn ein schwarzes Loch den Zentralkörper darstellt, ist die umgebende Akkretionsscheibe der einzig mögliche Ort um Magnetfeld erzeugen zu können. <br /> <br /> Wir sind speziell interessiert am Entstehungsprozess hoch relativistischer Jets wie sie bei Mikro-Quasaren und AGN beobachtet werden. Insbesondere untersuchen wir die Region, in der der Jet kollimiert, eine Region, deren räumliche Ausdehnung extrem klein ist selbst im Vergleich zur Auflösung der Radioteleskope. Dies ist ein Grund, wieso zum heutigen Zeitpunkt für die meisten Quellen die theoretische Modellierung die einzige Möglichkeit darstellt, um Information über die physikalischen Prozesse in der innersten Region der Jetentstehung zu erhalten. <br /> <br /> Uns ist es zum ersten Mal gelungen, die globale zwei-dimensionale Magnetfeldstruktur stationärer, axialsymmetrischer, relativistischer und stark magnetisierter (kräfte-freier) Jets zu berechnen, die zum einen asymptotisch in einen zylindrischen Jet kollimieren, zum anderen aber in einer differential rotierenden Akkretionsscheibe verankert sind. Damit erlaubt dieser Ansatz eine physikalische Verkn&#168;upfung zwischen Akkretionsscheibe und dem asymptotischen Jet. Nimmt man also an, dass die Fußpunkte der Magnetfeldlinien mit Keplergeschwindigkeit rotieren, so kann man eine direkte Skalierung der Jetmagnetosphere mit der Größe des Zentralobjektes erhalten. Unsere Resultate zeigen eine gute Übereinstimmung zwischen unserem Modell und Beobachtungen des Jets von M87. <br /> <br /> Für das Beispiel eines relativistischen Mikroquasarjets haben wir die Röntgenemission im Bereich von 0.2-10.1 keV berechnet. Dafür haben wir in der Literatur aus den relativistischen magnetohydrodynamischen Gleichungen berechnete Jetgrößen (Dichte-, Geschwindigkeits-, und Temperaturprofil) verwendet und das Spektrum für jeden Punkt entlang der Jetströmung abgeleitet. Das theoretische thermische Röntgenspektrum des innersten, heißen Teils des Jets erhalten wir zusammengesetzt aus den spektralen Anteilen der einzelnen Volumenelemente entlang des Jets. Um relativistische Effekte wie Dopplerverschiebung und -verstärkung (boosting) aufgrund der Jetbewegung zu untersuchen, haben wir für verschiedene Inklinationswinkel des Jets zur Sichtlinie berechnet, wie die erhaltenen Spektren davon beeinflusst werden. <br /> <br /> Unsere Spektren zeigen deutlich die hochionisierten Eisen-Emissionslinien, die in den galaktischen Mikroquasaren GRS 1915+105 und XTE J1748-288 andeutungsweise beobachtet wurden.<br /> Eine Dopplerverschiebung dieser Linien ist in unseren Spektren deutlichzu sehen. Da die innerste, Röntgenstrahlung emittierende Region des magnetohydrodynamischen Jets allerdings noch unkollimiert ist, spielt Dopplerboosting in unseren Spektren, abhängig vom Sichtwinkel, keine große Rolle. Mit unseren Resultaten konnte zum ersten Mal ein Röntgenspektrum gewonnen werden, das auf der numerischen Lösung eines magnetohydrodynamischen Jets beruht. / Jets are highly collimated flows of matter. They are present in a large variety of astrophysical sources: young stars, stellar mass black holes (microquasars), galaxies with an active nucleus (AGN) and presumably also intense flashes of gamma-rays. In particular, the jets of microquasars, powered by accretion disks, are probably small-scale versions of the outflows from AGN. <br /> <br /> Beside observations of astrophysical jet sources, also theoretical considerations have shown that magnetic fields play an important role in jet formation, acceleration and collimation. Collimated jets seem to be systematically associated with the presence of an accretion disk around a star or a collapsed object. If the central object is a black hole, the surrounding accretion disk is the only possible location for a magnetic field generation. <br /> <br /> We are interested in the formation process of highly relativistic jets as observed from microquasars and AGN. We theoretically investigate the jet collimation region, whose physical dimensions are extremely tiny even compared to radio telescopes spatial resolution. Thus, for most of the jet sources, global theoretical models are, at the moment, the only possibility to gain information about the physical processes in the innermost jet region.<br /> <br /> For the first time, we determine the global two-dimensional field structure of stationary, axisymmetric, relativistic, strongly magnetized (force-free) jets collimating into an asymptotically cylindrical jet (taken as boundary condition) and anchored into a differentially rotating accretion disk. This approach allows for a direct connection between the accretion disk and the asymptotic collimated jet. Therefore, assuming that the foot points of the field lines are rotating with Keplerian speed, we are able to achieve a direct scaling of the jet magnetosphere in terms of the size of the central object. We find a close compatibility between the results of our model and radio observations of the M87 galaxy innermost jet.<br /> <br /> We also calculate the X-ray emission in the energy range 0.2--10.1,keV from a microquasar relativistic jet close to its source of 5 solar masses. In order to do it, we apply the jet flow parameters (densities, velocities, temperatures of each volume element along the collimating jet) derived in the literature from the relativistic magnetohydrodynamic equations. We obtain theoretical thermal X-ray spectra of the innermost jet as composition of the spectral contributions of the single volume elements along the jet. Since relativistic effects as Doppler shift and Doppler boosting due to the motion of jets toward us might be important, we investigate how the spectra are affected by them considering different inclinations of the line of sight to the jet axis. <br /> <br /> Emission lines of highly ionized iron are clearly visible in our spectra, probably also observed in the Galactic microquasars GRS 1915+105 and XTE J1748-288. The Doppler shift of the emission lines is always evident. Due to the chosen geometry of the magnetohydrodynamic jet, the inner X-ray emitting part is not yet collimated. Ergo, depending on the viewing angle, the Doppler boosting does not play a major role in the total spectra. This is the first time that X-ray spectra have been calculated from the numerical solution of a magnetohydrodynamic jet.

Our dynamic Sun manifests its activity by different phenomena: from the 11-year cyclic sunspot pattern to the unpredictable and violent explosions in the case of solar flares. During flares, a huge amount of the stored magnetic energy is suddenly released and a substantial part of this energy is carried by the energetic electrons, considered to be the source of the nonthermal radio and X-ray radiation. One of the most important and still open question in solar physics is how the electrons are accelerated up to high energies within (the observed in the radio emission) short time scales. Because the acceleration site is extremely small in spatial extent as well (compared to the solar radius), the electron acceleration is regarded as a local process. The search for localized wave structures in the solar corona that are able to accelerate electrons together with the theoretical and numerical description of the conditions and requirements for this process, is the aim of the dissertation. Two models of electron acceleration in the solar corona are proposed in the dissertation: I. Electron acceleration due to the solar jet interaction with the background coronal plasma (the jet--plasma interaction) A jet is formed when the newly reconnected and highly curved magnetic field lines are relaxed by shooting plasma away from the reconnection site. Such jets, as observed in soft X-rays with the Yohkoh satellite, are spatially and temporally associated with beams of nonthermal electrons (in terms of the so-called type III metric radio bursts) propagating through the corona. A model that attempts to give an explanation for such observational facts is developed here. Initially, the interaction of such jets with the background plasma leads to an (ion-acoustic) instability associated with growing of electrostatic fluctuations in time for certain range of the jet initial velocity. During this process, any test electron that happen to feel this electrostatic wave field is drawn to co-move with the wave, gaining energy from it. When the jet speed has a value greater or lower than the one, required by the instability range, such wave excitation cannot be sustained and the process of electron energization (acceleration and/or heating) ceases. Hence, the electrons can propagate further in the corona and be detected as type III radio burst, for example. II. Electron acceleration due to attached whistler waves in the upstream region of coronal shocks (the electron--whistler--shock interaction) Coronal shocks are also able to accelerate electrons, as observed by the so-called type II metric radio bursts (the radio signature of a shock wave in the corona). From in-situ observations in space, e.g., at shocks related to co-rotating interaction regions, it is known that nonthermal electrons are produced preferably at shocks with attached whistler wave packets in their upstream regions. Motivated by these observations and assuming that the physical processes at shocks are the same in the corona as in the interplanetary medium, a new model of electron acceleration at coronal shocks is presented in the dissertation, where the electrons are accelerated by their interaction with such whistlers. The protons inflowing toward the shock are reflected there by nearly conserving their magnetic moment, so that they get a substantial velocity gain in the case of a quasi-perpendicular shock geometry, i.e, the angle between the shock normal and the upstream magnetic field is in the range 50--80 degrees. The so-accelerated protons are able to excite whistler waves in a certain frequency range in the upstream region. When these whistlers (comprising the localized wave structure in this case) are formed, only the incoming electrons are now able to interact resonantly with them. But only a part of these electrons fulfill the the electron--whistler wave resonance condition. Due to such resonant interaction (i.e., of these electrons with the whistlers), the electrons are accelerated in the electric and magnetic wave field within just several whistler periods. While gaining energy from the whistler wave field, the electrons reach the shock front and, subsequently, a major part of them are reflected back into the upstream region, since the shock accompanied with a jump of the magnetic field acts as a magnetic mirror. Co-moving with the whistlers now, the reflected electrons are out of resonance and hence can propagate undisturbed into the far upstream region, where they are detected in terms of type II metric radio bursts. In summary, the kinetic energy of protons is transfered into electrons by the action of localized wave structures in both cases, i.e., at jets outflowing from the magnetic reconnection site and at shock waves in the corona. / Die Sonne ist ein aktiver Stern, was sich nicht nur in den allseits bekannten Sonnenflecken, sondern auch in Flares manifestiert. Während Flares wird eine große Menge gespeicherter, magnetischer Energie in einer kurzen Zeit von einigen Sekunden bis zu wenigen Stunden in der Sonnenkorona freigesetzt. Dabei werden u.a. energiereiche Elektronen erzeugt, die ihrerseits nichtthermische Radio- und Röntgenstrahlung, wie sie z.B. am Observatorium für solare Radioastronomie des Astrophysikalischen Instituts Potsdam (AIP) in Tremsdorf und durch den NASA-Satelliten RHESSI beobachtet werden, erzeugen. Da diese Elektronen einen beträchtlichen Anteil der beim Flare freigesetzten Energie tragen, ist die Frage, wie Elektronen in kurzer Zeit auf hohe Energien in der Sonnenkorona beschleunigt werden, von generellem astrophysikalischen Interesse, da solche Prozesse auch in anderen Sternatmosphären und kosmischen Objekten, wie z.B. Supernova-Überresten, stattfinden. In der vorliegenden Dissertation wird die Elektronenbeschleunigung an lokalen Wellenstrukturen im Plasma der Sonnenkorona untersucht. Solche Wellen treten in der Umgebung der magnetischen Rekonnektion, die als ein wichtiger Auslöser von Flares angesehen wird, und in der Nähe von Stoßwellen, die infolge von Flares erzeugt werden, auf. Generell werden die Elektronen als Testteilchen behandelt. Sie werden durch ihre Wechselwirkung mit den elektrischen und magnetischen Feldern, die mit den Plasmawellen verbunden sind, beschleunigt. Infolge der magnetischen Rekonnektion als Grundlage des Flares werden starke Plasmaströmungen (sogenannte Jets) erzeugt. Solche Jets werden im Licht der weichen Röntgenstrahlung, wie z.B. durch den japanischen Satelliten YOHKOH, beobachtet. Mit solchen Jets sind solare Typ III Radiobursts als Signaturen von energiereichen Elektronenstrahlen in der Sonnenkorona verbunden. Durch die Wechselwirkung eines Jets mit dem umgebenden Plasma werden lokal elektrische Felder erzeugt, die ihrerseits Elektronen beschleunigen können. Dieses hier vorgestellte Szenarium kann sehr gut die Röntgen- und Radiobeobachtungen von Jets und den damit verbundenen Elektronenstrahlen erklären. An koronalen Stoßwellen, die infolge Flares entstehen, werden Elektronen beschleunigt, deren Signatur man in der solaren Radiostrahlung in Form von sogenannten Typ II Bursts beobachten kann. Stoßwellen in kosmischen Plasmen können mit Whistlerwellen (ein spezieller Typ von Plasmawellen) verbunden sein. In der vorliegenden Arbeit wird ein Szenarium vorgestellt, das aufzeigt, wie solche Whistlerwellen an koronalen Stoßwellen erzeugt werden und durch ihre resonante Wechselwirkung mit den Elektronen dieselben beschleunigen. Dieser Prozess ist effizienter als bisher vorgeschlagene Mechanismen und kann deshalb auch auf andere Stoßwellen im Kosmos, wie z.B. an Supernova-Überresten, zur Erklärung der dort erzeugten Radio- und Röntgenstrahlung dienen.

In this analysis, the efficacy of cavitation jets for heat transfer enhancement was demonstrated. The cavitation jet was formed from a cluster of cavitation bubbles that are the result of a submerged piezoelectric diaphragms oscillating about a given velocity threshold Two different heaters operating in two different flow environments were examined. For each heater in each environment, the cavitation jet significantly increased the heat transfer

A numerical study of axisymmetric synthetic jet flow was conducted. The synthetic jet cavity was modeled as a rigid chamber with a piston-like moving diaphragm at its bottom. The Shear-Stress-Transportation (SST) k-omega and #61559; turbulence model was employed to simulate turbulence. Based on time-mean analysis, three flow regimes were identified for typical synthetic jet flows. Typical vortex dynamics and flow patterns were analyzed. The effects of changes of working frequency, cavity geometry (aspect ratio), and nozzle geometry were investigated. A control-volume model of synthetic jet cavity was proposed based on the numerical study, which consists of two first-order ODEs. With appropriately selected parameters, the model was able to predict the cavity pressure and average velocity through the nozzle within 10% errors compared with full simulations. The cavity model can be used to generate the boundary conditions for synthetic jet simulations and the agreement to the full simulation results was good. The saving of computational cost is significant. It was found that synthetic jet impingement heat transfer outperforms conventional jet impingement heat transfer with equivalent average jet velocity. Normal jet impingement heat transfer using synthetic jet was investigated numerically too. The effects of changes of design and working parameters on local heat transfer on the impingement plate were investigated. Key flow structures and heat transfer characteristics were identified. At last, a parametric study of an active heat sink employing synthetic jet technology was conducted using Large Eddy Simulation (LES). Optimal design parameters were recommended base on the parametric study.

Forward Jet Calibration for CMS detector is presented using Z boson + jet event samples in the pseudorapidity region of 0 &lt / |&eta / | &lt / 5. The results are based on proton-proton collision data at center of mass energy of &radic / s = 7 TeV corresponding to &sim / 1 fb&minus / 1 of data. Z bosons are reconstructed from opposite sign lepton pairs (&mu / +&mu / &minus / , e+e&minus / ) and the transverse momentum balance of the Z boson and the associated jet is used to derive the calibration coefficients. The coefficients are tested on jets from a WW Monte Carlo sample using the W-mass constraint. The W mass peak position is observed to be improved by 20 % without loss of resolution.

Η διδακτορική διατριβή αφορά την πειραματική μελέτη του ισόθερμου ροϊκού πεδίου που αναπτύσσεται λόγω της αλληλεπίδρασης μιας περιδινούμενης δέσμης με μια εξωτερική παράλληλη ροή. Η συγκεκριμένη μελέτη έχει άμεση αναφορά σε διατάξεις καύσης (στροβιλοκινητήρες, φούρνους υαλουργίας, καυστήρες ντίζελ σε πλοία) και ως στόχο έχει την εμβάθυνση της κατανόησης των φυσικών μηχανισμών που αναπτύσσονται στο μέσο και τυρβώδες ροϊκό πεδίο αλλά και στη βελτίωση της διαδικασίας μίξης μεταξύ καυσίμου και οξειδωτικού μέσω του χαρακτηρισμού του τρισδιάστατου διατμητικού στρώματος που δημιουργείται λόγω της αλληλεπίδρασης τους. Αναλυτικότερα, στα πλαίσια της παρούσης διδακτορικής διατριβής μελετώνται διαφορετικές συνθήκες αλληλεπίδρασης (λόγος παροχής μαζών) μεταξύ μιας εσωτερικής περιδινούμενης δέσμης εκροής και μια εξωτερικής ομοαξονικής ροής. Η δημιουργία της περιδίνησης βασίζεται στην εφαπτομενική έγχυση ρευστού, μια τεχνική της οποίας τα ιδιαίτερα χαρακτηριστικά έχουν μελετηθεί ελάχιστα στη μέχρι τώρα βιβλιογραφία. Το υπό μελέτη ροϊκό πεδίο παρουσιάζει ιδιαίτερο ενδιαφέρον τόσο ως προς την πολυπλοκότητα του όσο και ως προς τη διερεύνηση παραμέτρων που επηρεάζουν την απόδοση συστημάτων καύσης. Το κύριο σκέλος αποτελεσμάτων που παρουσιάζεται στην παρούσα διατριβή αφορά πειραματικές μετρήσεις οι οποίες πραγματοποιήθηκαν με τη μέθοδο Ταχυμετρίας Απεικόνισης Σωματιδίων (Digital Particle Image Velocimetry). Παρ’ όλα αυτά, στα πλαίσια του σχεδιασμού της πειραματικής διάταξης και της αρχικής αξιολόγησης του ροϊκού πεδίου, παρουσιάζονται αποτελέσματα τα οποία προέκυψαν με τη χρήση εμπορικού κώδικα Υπολογιστικής Ρευστοδυναμικής (CFD-ACE+) και με την πειραματική μέθοδο Ανεμομετρίας Θερμού Νήματος διάταξης Χ (HWA-X probe). Τα αποτελέσματα που προέκυψαν κατά πρώτα στάδια υλοποίησης της διδακτορικής διατριβής συσχετίζονται με αυτά της Ταχυμετρίας Απεικόνισης Σωματιδίων, επιτρέποντας είτε την αξιολόγηση των διάφορων τυρβωδών μοντέλων και διαφορικών σχημάτων των εξισώσεων κίνησης ή την επιβεβαίωση των συμπερασμάτων μέσω της σύγκρισης των αποτελεσμάτων των δύο πειραματικών μεθόδων. Στα πλαίσια της διδακτορικής διατριβής γίνεται αναφορά και σύγκριση με παρόμοιες διατάξεις ομοαξονικών ροών, ενώ προκύπτουν νέα στοιχεία σχετικά με τα κριτήρια ομοιότητας ομοαξονικών ροών με περιδίνηση και τους φυσικούς μηχανισμούς που αναπτύσσονται στο τρισδιάστατο στρώμα μίξης που διαμορφώνεται. Ιδιαίτερη έμφαση δίδεται στη σύνδεση της τοπολογίας του πεδίου ανακυκλοφορίας (φυσαλίδα ανακυκλοφορίας, στροβιλιζόμενος δακτύλιος) με τη δυναμική του μέσου και τυρβώδους ροϊκού πεδίου αλλά και στην επίδραση της μεταβολής του αριθμού Rossby στην ίδια τη φυσαλίδα ανακυκλοφορίας. Από την ανάλυση επίδρασης της φυσαλίδας ανακυκλοφορίας στο ροϊκό πεδίο προκύπτει η τοπολογία των ζωνών υψηλής μίξης μεταξύ των δύο ροών αλλά και αυτών που χαρακτηρίζονται από υψηλές τιμές τυρβώδους κινητικής ενέργειας όπως και από υψηλά επίπεδα ανακυκλοφορίας. Η μελέτη των χαρακτηριστικών του τρισδιάστατου διατμητικού στρώματος καταδεικνύει την ισχυρή αλληλεπίδραση του διαμήκους με το αζιμουθιακό διατμητικό στρώμα και παρέχει σημαντική πληροφορία ως προς την εξέλιξη της μίξης μεταξύ της περιδινούμενης δέσμης εκροής και της εξωτερικής ομοαξονικής ροής. Για την καλλίτερη κατανόηση της διαδικασίας μίξης εισαγάγεται ο συντελεστής διάχυσης στροφορμής (λ) ο οποίος παρέχει σημαντική πληροφορία ως προς την εξέλιξη της περιδινούμενης δέσμης εκροής σε σχέση με τις συνθήκες εισαγωγής του ροϊκού πεδίου. Τέλος, επιχειρείται η αναπαράσταση του τρισδιάστατου ροϊκού πεδίου από την επαλληλία των αποτελεσμάτων στο διαμήκες και εγκάρσιο επίπεδο μετρήσεων, όπου απεικονίζεται η τοπολογία της φυσαλίδας ανακυκλοφορίας. Από τη μελέτη των αποτελεσμάτων προκύπτει ότι η κλασική προσέγγιση ταξινόμησης ροών με περιδίνηση σύμφωνα με το βαθμό στροβιλισμού δεν επαρκεί για σύνθετες ροές όπως αυτή που εξετάζεται στην παρούσα διατριβή. Για αυτό το λόγο, προτείνεται ένας νέος αδιάστατος αριθμός (αριθμός Rossby) ο οποίος σχετίζει το πεδίο πιέσεων που δημιουργείται λόγω της συμπαράσυρσης της εσωτερικής δέσμης από την εξωτερική με αυτό που οφείλεται στην περιδίνηση της εσωτερικής δέσμης εκροής. Η εισαγωγή του τροποποιημένου αριθμού Rossby βασίστηκε στην ήδη υπάρχουσα βιβλιογραφία (σύγκριση δυνάμεων αδράνειας με τις δυνάμεις επιτάχυνσης Coriolis) ως προς την επιλογή των κλιμάκων ταχύτητας, παρ’ όλα αυτά διαφοροποιείται μιας και στην ήδη υπάρχουσα θεωρία δεν υπάρχουν αναφορές σε ομοαξονικές ροές. / In this work the isothermal flow field generated by the interaction of an internal swirling jet with an external parallel flow is experimentally investigated with the use of 2D Digital Particle Image Velocimetry. Swirl is produced through tangential injection of air. Parametric change of inlet flow rates (constant tangential injection with change of annular flow and vice versa) is being considered in order to study the mean and turbulent flow field. Coaxial swirling jets are widely used in combustion systems as they enhance fuel and oxidant mixing and flame stabilization. Amongst well known features of introducing swirl in jet flows (increase of jet growth, entrainment and decay), highly swirling jets have been studied in combustion configurations as they impose radial and axial pressure gradients generating an internal toroidal recirculation zone, a phenomenon known as “vortex breakdown”. The complex structure of vortex breakdown has been a challenging issue for experimentalists over the past few decades emphasizing on its effect on aerodynamic and mixing attributes of combustion flow fields. Focusing on the study of coaxial swirling jets, rather limited data has been presented up to now, regarding the topology and turbulent attributes of the flow field created by coaxial jets with inner and/or outer swirl. Following previous work on coaxial swirling jets with inner or outer swirl and coaxial jets without swirl which lead to recirculation, a sufficient need for a deeper understanding of the physical mechanisms developing in such complex flow fields, comes up. This Thesis stands as an attempt to present the main features of such a complex flow field, which results from the interaction of a typical swirling jet undergoing “vortex breakdown” with an outer annular flow with “back step flow” characteristics. An analysis of the mean and turbulent flow statistics is presented, correlating flow field mechanisms with the three dimensional shear layer characteristics and the topology of the recirculating flow field (recirculation bubble, vortex ring). Research on vortex breakdown phenomena has led to a parallel research on the critical parameters that could determine whether vortex breakdown will occur. The definition of non-dimensional parameters (Swirl/ Rossby number etc), mainly based on the correlation of axial and azimuthal velocities or momenta, has been an issue of scientific interest that has often led to different approaches and criteria for vortex breakdown prediction. Additionally, it is seen through literature review that predicting vortex breakdown is not by itself adequate to characterize the mean and turbulent features of the recirculating flow field. In the case of coaxial jets, with or without swirl, previous studies have shown that the flow field created is strongly affected not only by the velocity or mass flow ratio of the jets but also by the absolute values of the jets’ velocities or the velocity jump between the two streams. For the case of coaxial swirling jets it is apparent that the interaction between the shear layers (mainly azimuthal and axial) is the key to understand the features of such a complex flow field. Through the similarity study conducted within this Thesis, a modified Rossby number is proposed as a parameter sufficient to describe the flow field’s trends. The modified Rossby number correlates the pressure drop due to fluid entrainment to that due to the rotation of the inner swirling jet. Presentation of the experimental results breaks down into two main sections; the first one where the effect of inlet conditions on the recirculating flow field mean and turbulent characteristics is discussed and the second one dealing with the interaction between the azimuthal and longitudinal shear layer. Through the analysis of the recirculation bubble effect on flow field attributes, emphasis is given into the characterization of intense mixing and turbulence regions. Additionally, the interaction between the azimuthal and longitudinal shear layer is studied through a similarity approach, utilizing boundary layer non-dimensional scales. Finally, mixing between the two flows is studied in terms of angular momentum diffusion by introducing a non dimensional parameter (λ). Results show a global effect of the proposed Rossby number on the flow field attributes, such as the recirculation bubble length and flow characteristics and the mixing of the two flows.

Thesis (M. Phil.)--University of Hong Kong, 1998. / Includes bibliographical references (leaves 115-120).

Les interfaces entre deux fluides sont le siège de nombreuses instabilités de forme de l’interface si un champ électrique intense est appliqué : génération de gouttelettes, jets micrométriques, etc. Le contrôle de telles instabilités est indispensable pour une fabrication optimale de microsphères ou microfibres : taille, propriétés physico-chimiques, dispersion et structuration spatiale macroscopique d’un agrégat de tels objets. Cette diversité provient de la compétition entre la tension de surface et la gravité avec l’électrodynamique des fluides sous champ électrique induite par les charges électriques, les charges de polarisation, les décharges électriques et/ou le vent ionique. La thèse expérimentale s’articule autour de deux thèmes. Le premier, une compréhension intime des phénomènes spatio-temporels observables lorsqu’un injecteur métallique à la haute tension placé au-dessus d’un bain. Une instabilité originale menant à la formation d’une cloche fluide macroscopique connectant les deux électrodes a été mise en évidence et caractérisée non linéairement. La bifurcation est sous-critique et imparfaite. Le second thème propose une méthode originale de fabrication de microfibres modifiées en une étape par électrofilage au mouillé. Le polymère électrofilé choisi est le PSMA et celui permettant la modification, le PEGDA. Cette étude a été réalisée dans un contexte d’applications de type catalyse. Pour cela les fibres ont été fonctionnalisées à l’aide de la peroxydase (HRP) comme protéine modèle. Les résultats montrent notamment une meilleure stabilité temporelle avec la possibilité de réutilisation du matériau en comparaison à la catalyse utilisant des méthodes standards. / Interfaces between two fluids can lead to various interfacial shape instabilities if an electrical field is applied. Leading, for instance, to micrometric droplets or jets formation. Controlling those instabilities is much-needed for an optimal fabrication of microspheres or microfibers : size, physicochemical properties, dispersion and macroscopic spatial structuring of aggregates of those kind of objects. This diversity is based in the competition between surface tension and gravity forces with gravity during the electrodynamics of fluids under electric field induced by electrical charges, polarization charges, electrical discharges and ionic wind. The experimental thesis deals with two main topics. The first one is a precise understanding of spatiotemporals phenomena occurring in a configuration made of a metallic injector raised to high voltage placed above a liquid bath. We present the formation of an original instability leading to a macroscopic bell-shaped link between both electrodes and its non linear characterization. The bifurcation is subcritical and imperfect. The second topic, based on the experience gained with the first one, is an original method of fabrication of microfibers modified in only one step by wet electrospinning. The chosen electrospun polymer is PSMA and the one used for modification is PEGDA. This study has been realized with a catalyze application context. To do so, fibers has been functionalized with peroxydase (HRP) as the model protein. The results especially show a better temporal stability and possible reuse compared to catalysis with standard methods.

Cette étude a pour objectif d'analyser le rôle des ondes de Rossby dans la variabilité des courants-jets troposphériques des moyennes latitudes à l'aide d'un modèle numérique idéalisé. Elle s'intéresse aux mécanismes dynamiques responsables de la persistance des principaux modes de variabilité : celui de déplacement méridien et celui de pulsation d'amplitude. Le premier est souvent le principal mode de variabilité du fait de sa grande persistance causée par une rétroaction positive des ondes de Rossby.Deux nouveaux types de rétroactions négatives ont été mis en évidence à une échelle de temps plus courte que cette rétroaction classique et dont le mécanisme dépend de la nature des ondes impliquées. Ces différents mécanismes sont aussi retrouvés et ainsi validés dans le contexte plus réaliste des données de réanalyse.Une réflexion sur les conséquences du changement climatique est aussi proposée via l'étude de sensibilité menée sur trois paramètres clés du modèle : le gradient méridien de température, conduisant à un mode de déplacement plus persistant dans le futur, la position moyenne du jet, conduisant à un mode moins persistant pour des jets plus proches du pôle, et enfin la friction dans les basses couches de l'atmosphère, aussi étudiée car étant un paramètre plutôt difficile à évaluer et présentant une forte disparité entre les modèles.Cette thèse a donc permis de mettre en évidence deux nouveaux mécanismes de rétroaction des ondes sur les courants-jets et de développer des diagnostics théoriques qui pourront être plus amplement testés et appliqués dans d'autres contextes, particulièrement des réanalyses et des simulations de climat réalistes. / This study investigate the impact of Rossby waves on the tropospheric midlatitude eddy-driven jets using an idealized numerical model. It focuses on the dynamical mechanisms driving the persistence of the main modes of variability: a shifting mode and a pulsing mode. The shifting mode is often found to be the leading mode of variability due to an enhanced persistence caused by a positive feedback of Rossby waves. Two new kinds of negative feedbacks have been found for a shorter time-scale than the more classical feedback which mechanism depends on wave properties. These new mechanisms have also been found in the more realistic set up of reanalysis. The impact of climate change is also investigated using the sensitivity analysis of the model to three key parameters : the meridional gradient of temperature, which leads to longer lasting phases of the shifting mode in the future, the jet mean position, which leads to less persistent shifting mode for poleward shifted jets, and frictional damping, because it is a parameter difficult to tune and which varies between numerical models. In conclusion, two new feedback mechanisms acting on eddy-driven jets variability have been found and theoretical diagnostics have been developed and could be used to probe more realistic data such as future climate simulations and reanalysis.

Magnetic flux emergence is a fundamental process in the Sun, during which magnetic fields emerge from the solar interior to the surface, to build up active regions and give onset to spectacular dynamic phenomena, such as eruptions and jets. In this thesis, we performed 3D, resistive MHD simulations to study the emergence and the associated magnetic activity of a quadrupolar region in the Sun. Our aim behind the setup of this initial condition (i.e. a quadrupolar region) was to study a magnetic field configuration, which has not been studied in detail before, although it has been repeatedly observed in the Sun and it has been shown that it can host intense magnetic activity (e.g. in the form of jets, flares and eruptions). The results of our experiments showed that the internal dynamics of such regions leads to the onset of eruptions in the form of twisted magnetic flux tubes (flux ropes). These eruptions are recurrent but they cannot escape the outermost field of the emerging flux (envelope field). They remain confined within the envelope field, as the downward tension of the outermost field lines overwhelms the upward Lorentz force of the erupting field. When we add an ambient magnetic field in the solar atmosphere, external reconnection between the emerging and the ambient field triggers the emission of (standard) reconnection jets. The external reconnection also releases the tension of the ambient field lines and, thus, the eruptions move in an ejective way towards the outer space. Namely, the confined eruptions become ejective eruptions, which escape from the numerical domain. These ejective eruptions drive a newly observed class of jets, the so called "blowout" jets. Our experiments reproduce some of the main observed characteristics of the "blowout" jets. We showed that "blowout" jets emit hot and cool plasma into the outer solar atmosphere simultaneously, and they undergo untwisting motion due to the relaxation of twist during their ejection. We found that the untwisting motion of the "blowout" jets is associated with the propagation of torsional Alfvén waves. Finally, we performed a parametric study to explore the effect of the ambient field strength on the onset and dynamics of the eruptive events. We found that one of the main effects is that the stronger ambient field suppresses the vertical expansion of the magnetic envelope of the quadrupolar region due to the higher magnetic pressure above it. This result has an effect on the emission of jets, which are emitted due to reconnection between the two fields. When the ambient field is relatively weak, it is pushed away from the strong emerging field and reconnection between them is not so persistent. On the other hand, when the ambient field is relatively strong, we find that more jets are ejected due to more efficient and more frequent reconnection between the two flux systems. As a consequence, we find that more mass and flux is being transferred into the solar corona by the reconnection jets. Also, we find that there are more eruptions when the ambient field is stronger. The study of the total energy flux carried by the jets showed that it is sufficient to provide the energy required to accelerate the high speed solar wind. This indicates that the "blowout" jets may play an important role in driving the solar wind.