Cette thèse a pour but de découvrir la faisabilité et le potentiel de la récupération d'énergie à partir de rafales atmosphériques pour les micro et mini véhicules aériens sans pilote. L'atmosphère sert de grande source d'énergie pouvant être récoltée afin d'accroître la performance des petits UAV sous la forme d'une autonomie et d'une autonomie étendues. Il est bien connu que de nombreuses espèces d'oiseaux utilisent diverses techniques de vol pour obtenir des performances de vol étonnantes. Compte tenu du fait que les véhicules susmentionnés partagent la taille et la vitesse de vol avec des dépliants naturels, cette thèse peut être considérée comme une application des techniques de vol bio-inspirées pour les véhicules construits par l’homme. Cette étude de trois ans visait à établir une dérivation théorique des équations qui décrivent la dynamique de vol d'un aéronef en présence d'un environnement en rafales. La première réalisation a été la démonstration du mécanisme de récupération d'énergie et des paramètres d'influence à travers des simulations décrivant le vol en modèle de masse ponctuelle d'aéronef avec un contrôle optimisé de l'ascenseur en présence d'un profil de vent sinusoïdal et stochastique. La réalisation suivante est liée à un système sensoriel inspiré par la biologie qui utilise des mesures de pression des ailes pour estimer l’angle d’attaque local. Ce système particulier a été utilisé dans l’estimation du champ de vent, en tant que mécanisme décisif et protection contre le décrochage. Enfin, les dernières contributions sont liées à l’expérience et aux résultats obtenus lors d’essais en vol visant à prouver l’augmentation de l’état énergétique de l’avion lors des manœuvres de récupération d’énergie. La première campagne d'essais en vol a été réalisée avec un mini-UAV disponible dans le commerce équipé de sondes à trous multiples et d'un contrôleur conçu sur mesure. Cette campagne a démontré l’augmentation de l’état d’énergie dans un fort gradient de vent horizontal. La deuxième campagne d'essais en vol a été réalisée avec une aile volante équipée d'un système de détection de pression pour l'estimation du champ de vent. Cette campagne a également impliqué des économies supplémentaires sur la consommation d'énergie électrique lors des vols de récupération d'énergie. / This thesis aims at discovering the feasibility and potential of energy-harvesting from atmospheric gusts for micro and mini unmanned aerial vehicles. The atmosphere serves as a great source of energy that can be harvested in order to increase performance of small UAVs in form of extended endurance and range. It is well known that many bird species use various flight techniques for achieving astonishing flight performances. Considering the fact that aforementioned vehicles share size and flight speed with natural flyers, this thesis can be considered as an application of bioinspired flight techniques for man made vehicles. This three-year study set out to establish a theoretical derivation of equations that describe flight dynamics of an aircraft in presence of gusty environment. The first achievement was demonstration of energy harvesting mechanism and influencing parameters through simulations that describe aircraft point mass model flight with optimized control of elevator in presence of sinusoidal and stochastic wind profile. The next achievement is related to a biologically inspired sensory system that uses wing pressure measurements for local angle of attack estimation. That particular system found purpose in wind field estimation, as decisive mechanism and stall protection. Finally, last contributions are related to experience and results gained from flight tests which aimed to prove increase in energy state of the aircraft while performing energy harvesting maneuvers. The first flight test campaign was performed with commercially available mini UAV equipped with multi-hole probes and custom designed controller. This campaign demonstrated the raise in energy state within strong horizontal wind gradient. The second flight test campaign was done with a flying wing equipped with pressure sensing system for wind field estimation. This campaign also involved additional insight savings in electrical power consumption during energy harvesting flights.

This thesis presents a study of stochastic models of continuous review of inventory systems of perishable and non-perishable products, as well as inventory systems operating in random environment. It contains five chapters. The first chapter is introductory in nature, containing the motivation for the study and the techniques required for the analysis of respective models described in the remaining chapters. Chapter 2 provides a model of perishable product inventory system operating in a random environment. For the sake of simplicity, the stochastic environment is considered to alternate randomly over time between two states 0 and 1 according to an alternating renewal process. When the environment is in state k, the items in inventory have a perishable rate ìk, the demand rate is ëk and the replenishment cost is CRk. The performance of various measures of the system evolution are obtained, assuming instantaneous replenishment at the epoch of the first demand after the stock-out and associating a Markov renewal process with the inventory level. In Chapter 3, a continuous review single product perishable inventory model is considered. Items deteriorate in two phases and then perish. Independent demands occur at constant rates for items in phase I and in phase II. Demand that occurs for an item in phase I during its stock-out period is satisfied by an item in phase II with some probability. However a demand for an item in phase II occurring during its stock-out period is lost. The reordering policy is an adjustable (S,s) policy with the lead-time following an arbitrary distribution. Identifying the stochastic process as a renewal process, the probability distribution of the inventory level at any arbitrary instant of time is obtained. The expressions for the mean stationary rates of demands lost, demands substituted, perished units and scrapped units are also derived. A numerical example is considered to highlight the results obtained. Chapter 4 is a study of a two-commodity inventory system under continuous review. The maximum storage capacity for the i-th item is Si (i=1, 2). The demand points for each commodity are assumed to form an independent Poisson process, with unit demand for one item and bulk demand for the other. The order level is fixed as si for the i-th commodity (i=1, 2) and the ordering policy is to place an order for Qi (= Si – si , i = 1,2) items for the i-the commodity when both the inventory levels are less than or equal to their respective reorder levels. The lead-time is assumed to be exponential. The joint probability distribution for both commodities is obtained in both transient and steady state cases. Various measures of systems performance and the total expected cost rate in the steady state are derived. The results are illustrated with numerical examples. Chapter 5 provides an analysis of a continuous review of two-product system with two types of demands and with individual (S,s) ordering policy. The lead-time distribution of product 1 is arbitrary and that of product 2 exponential. Two types of demands occur at constant rates either for both products or for product 2 alone. Expressions for the stationary distribution of the inventory level are obtained by identifying the underlying stochastic processes as a semi-regenerative process. The mean stationary rates of the lost demands, the demands that are satisfied and the number of reorders are obtained and these measures are used to provide an expression for the cost rate. The main objective of this thesis is to improve the state of art of continuous review inventory systems. The salient features of the thesis are summarized below: <ul>(a) Consideration of</ul> <ul>(i) The impact of the stochastic environment on inventory systems;</ul> <ul>(ii) The interactions existing among the products in multi-product systems;</ul> <ul>(iii) Individual and joint-ordering policies;</ul> <ul>(b) Discussion of inventory systems with perishable products; </ul> <ul>(c) Effective use of the regeneration point technique to derive expressions for various system measures;</ul> <ul>(d) Illustration of the various results by extensive numerical work;</ul> <ul>(e) Relevant optimization problems</ul> / Thesis (PhD (Industrial Engineering))--University of Pretoria, 2006. / Industrial and Systems Engineering / unrestricted