Efforts to improve diesel fuel sprays have led to a significant increase in fuel injection pressures and a reduction in nozzle-hole diameters. Under these conditions, the likelihood for the internal nozzle flow to cavitate is increased, which potentially affects spray breakup and atomisation, but also increases the risk of causing cavitation damage to the injector. This thesis describes the study of cavitating flow phenomena in various single and multi-hole optical nozzle geometries. It includes the design and development of a high-pressure optical fuel injector test facility with which the cavitating flows were observed. Experiments were undertaken using real-scale optical diesel injector nozzles at fuel injection pressures up to 2050 bar, observing for the first time the characteristics of the internal nozzle-flow under realistic fuel injection conditions. High-speed video and high resolution photography, using laser illumination sources, were used to capture the cavitating flow in the nozzle-holes and sac volume of the optical nozzles, which contained holes ranging in size from 110 micrometers to 300 micrometers. Geometric cavitation in the nozzle-holes and string cavitation formation in the nozzle-holes and sac volume were both observed using transient and steady-state injection conditions; injecting into gaseous and liquid back pressures up to 150 bar. Results obtained have shown that cavitation strings observed at realistic fuel injection pressures exhibit the same physical characteristics as those observed at lower pressures. The formation of string cavitation was observed in the 300 micrometers multi-hole nozzle geometries, exhibiting a mutual dependence on nozzle flow-rate and the geometry of the nozzle-holes. Pressure changes, caused by localised turbulent perturbations in the sac volume and transient fuel injection characteristics, independently affected the geometric and string cavitation formation in each of the holes. String cavitation formation of was shown to occur when free-stream vapour was entrained into the low pressure core of a sufficiently intense coherent vortex. Hole diameters less than or equal to 160 micrometers were found to suppress string cavitation formation, with this effect a result of the reduced nozzle flow rate and vortex intensity. Using different hole spacing geometries, it was demonstrated that the formation of cavitation strings in a particular geometry became independent of fuel injection and back pressure once a threshold pressure drop across the nozzle had been reached.

Le Morphisme Electroactif est un axe multidisciplinaire, associant l’aérodynamique, les matériaux innovants et la mécatronique. Ce concept consiste en l’amélioration des performances aérodynamiques par l’utilisation d’actionneurs déformant la surface portante d’un aéronef en temps réel. Soutenue par Airbus, la modélisation, conception et réalisation d’un démonstrateur petite échelle est une première étape. Basée sur un profil d’aile A320, il est équipé d’actionnements pour le morphisme électroactif hybride : de grandes déformations à faibles vitesses par des Alliages à Mémoire de Forme sont associés à l’intégration au bord de fuite d’actionneurs piézoélectriques permettant de hautes fréquences d’actionnement à amplitude moindre. Une seconde étape de la thèse est dédiés aux essais en soufflerie. La mesure de forces et la vélocimétrie d’images de particules permettent de comprendre la physique de l’écoulement et de la turbulence. L’étude de ce couplage fluide-structure-actionneurs présente les effets du morphisme par actionnement indépendant ; puis le couplage non linéaire de l’actionnement hybride. La troisième étape consiste au passage vers une échelle réaliste des actionneurs, par la conception d’un volet « électro-morphé ». Une approche de dimensionnement par optimisation est proposée. Basé sur des technologies nouvelles d’actionnement, un prototype d’un tel macroactionneur est alors conçu pour être testé. / Electroactive Morphing is a multidisciplinary axis, combining aerodynamics, innovative materials and mechatronics. This concept consists in improving the aerodynamic performance by the use of actuators deforming the airfoil of an aircraft in real time. Supported by Airbus, the modeling, design and implementation of a small scale demonstrator is a first step. Based on an A320 wing profile, it is equipped with actuators for hybrid electroactive morphing: large deformations at low speeds by Shape Memory Alloys are associated with the integration at the trailing edge of piezoelectric actuators allowing high operating frequencies at lower amplitude. A second step of the thesis is dedicated to wind tunnel tests. The measurement of forces and the Particle Image Velocimetries allow for the understanding of the flow and turbulence physics. The study of this fluid-structure-actuator coupling presents the effects of the morphism by independent actuation; then the nonlinear coupling of the hybrid actuation. The third step is the transition to a realistic scale of actuators, by designing an "electro-morphed" macro-actuator. An optimization sizing approach is proposed. Based on new actuation technologies, a prototype of such a macro-actuator is then designed to be tested.