Cette étude porte sur l’étude et la caractérisation d’un plasma induit par laser produit par une configuration en impulsion unique et en double impulsion. Une étude bibliographique a en effet permis d’identifier que cette seconde configuration avait donné des résultats encourageants - augmentation de la probabilité d’allumage - dans le cadre d’un allumage d’un spray de kérosène à froid. Des mesures par imagerie intensifiée, strioscopie et spectroscopie ont permis de caractériser l’évolution spatiotemporelle des dimensions du plasma et l’onde de choc induite, ainsi que de mesurer l’évolution temporelle de la température et de la densité électronique dans les deux configurations d’impulsions laser considérées. Ce travail expérimental nous a permis d’identifier le délai interimpulsions comme l’un des paramètres essentiels déterminant l’efficacité du procédé en doubleimpulsion. Les observations ont également montré une physionomie du dépôt particulière, le claquage s’effectuant aux extrémités du plasma préexistant. Les observations et données obtenues lors de cette campagne expérimentale serviront de base de comparaison en vue de test d’allumages. Une étude préliminaire sur le claquage dans un spray de dodécane non-réactif a également été effectuée. / This study focuses on the characterization of laser-induced plasma with a configuration using one or two laser pulses. Indeed, a bibliographic review reported encouraging results - increase in ignition probability - for the ignition of a cold kerosene spray. Measurements were carried out, including emission imaging, schlieren imaging and emission spectroscopy, for a temporal and spatial characterization of the plasma and the shockwave. The emission spectra were used for the measurement of the temporal evolution of the temperature and electron density of the plasma, for both configuration of laser pulses. This experimental work highlighted the importance of the inter-pulses delay which is one of the key parameter for the efficiency of the dual pulse breakdown process. Observations also showed that a peculiar energy deposition process takes place when two pulses are used, the second breakdown occuring on both ends of the existing plasma. All data collected with this experimental campaign will be used for comparison in future ignition tests. A preliminary study of the breakdown process occuring in a dodecane spray has also been undertaken.

The ability of a propulsion system is very crucial for the capability of a missile or a rocket system. Unlike liquid propellant rocket motors, the only control mechanism of the thrust value is the propellant geometry in solid propellant rocket motors. When the operation of solid propellant rocket motor has started, it cannot be stopped anymore. For this main reason the advance of dual pulse motor technology has started. The aim of this study is to investigate the geometrical effects of design parameters on the flow behavior of a dual pulse solid propellant rocket motor by using commercial Computational Fluid Dynamics (CFD) methods. For the CFD analysis, a generic dual pulse rocket motor model is constituted. Within this model, initially four different geometry alternatives of Pulse Separation Device (PSD) are analyzed. To begin PSD analyses, mesh sensitivity analyses are performed on one PSD geometry alternative. By defined grid size, the analyses of PSD geometry alternatives are performed. Computed results were compared in terms of flow behavior (flow streamlines, velocity distribution, turbulent kinetic energy&hellip / etc.) with each other. With the selected PSD geometry alternative the effects of L/D ratio (Length/Diameter ratio) of first pulse chamber, Achamb/APSD ratio (Chamber area/PSD opening area) and APSD/Ath ratio (PSD opening area/Throat area) on the flow behavior is investigated. Flow analyses are performed by simulating the unsteady flow of second pulse operation. With the performed analyses, it is aimed to identify generic geometric definitions for a dual pulse rocket motor.

For low data rate ultra-wideband (UWB) communication systems employing non-coherent detection and autocorrelation detection schemes, timing of integration region significantly affects their error rate performance. Time-of-arrival (TOA) estimation of the first channel tap is also the foundation of the UWB based ranging applications. In this thesis, a data-aided, autocorrelation based timing and synchronization method is developed. First, estimation of the optimal integration region, i.e., the initial point and the length of the integration, using the new timing method is presented. It is shown that the proposed method enhances the error rate performance compared to non-optimal integration region-determining methods. After that, TOA estimation using the proposed timing method is studied for the dual pulse (DP) signal structure. The performance improvement of this approach over the conventional energy detection based method is demonstrated via simulation.

<p>Laser shock peening is a high strain rate plastic deformation process, and it has been widely used in automobile, aerospace, and nuclear industries for surface enhancement. Lots of new developments of the laser shock peeing process have been studied to expand its new applications such as cryogenic laser shock peeing, warm laser shock peening, laser shock peening without coating, laser shock peening without confinement. There are still some issues that has not been addressed: 1) interaction between laser shock wave and layer structured composite material has not been studied. 2) investigation on microstructure and mechanical properties of intermetallic phase strengthened composite material processed by warm laser shock peening is rare. 3) preheating method for warm laser shock peening needs improvement.</p> <p>In this study, thermal and temporal modulated laser shock peening process is developed to control microstructure and mechanical properties. 1) Laser shock peening and cryogenic laser shock peeing was applied to copper graphene heterostructure. Hardness, yield strength were measured and microstructures were characterized. Shock wave propagation and its interaction with monolayer graphene was studied by finite element analysis. Results showed that the yield strength of laser shock peeing and cryogenic laser shock peeing processed copper graphene samples increased by 40%, and 76% respectively. It was found that shock wave could pass through long-distance to generate dislocation transportation from one layer to another graphene with the shock wave interaction between graphene layers separated very far away. 2) Warm laser shock peening with different preheating temperature was performed on lightweight steel. Effect of temperature on mechanical properties, precipitates and dislocation distributions are investigated. A coupled phase field-dislocation dynamics model was developed to study the precipitates and dislocation generation mechanism. The yield strength of the lightweight steel after warm laser shock peeing reaches 2030Mpa, which is the highest for lightweight mid-carbon steel (70% Fe, 1%C). Experiment results have confirmed high density dislocations and precipitates are generated by warm laser shock peeing process. And we find a new mechanism, avalanche multiplication of dislocations and precipitates, during the warm laser shock peeing: I) Dislocations assist precipitates formation. II) Precipitates boost dislocation generation. 3) A novel dual pulse laser shock peening process was developed which combines preheating and laser shock peening process.The effect of modulating pulse width and pulse duration on processing temperature and material microstructures were studied. Results showed that single pulse laser processing could successfully remelted the second phase and had much smaller grain (500nm) due to fast cooling, and dual pulse with appropriate pulse duration resulted in high density nanosized (30nm) intermetallic phase. High hardness 59 HV and yield strength 547MPa could be achieved due to the combination of grain size refinement, hard second phase and dislocations.</p>