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This work focuses on an investigation of multi-modality in typical aerodynamic shape optimization problems and development of optimization algorithms that can find a global optimum. First, a classification of problems based on the degree of multi-modality is introduced. Then, two optimization algorithms are described that can find a global optimum in a computationally efficient manner: a gradient-based multi-start Sobol algorithm, and a hybrid optimization algorithm. Two additional algorithms are considered as well: a gradient-based optimizer and a genetic algorithm. Finally, we consider a set of typical aerodynamic shape optimization problems. In each problem, the primary objectives are to classify the problem according to the degree of multi-modality, and to select the preferred optimization algorithm for the problem. We find that typical two-dimensional airfoil shape optimization problems are unimodal. Three-dimensional shape optimization problems may contain local optima. In these problems, the gradient-based multi-start Sobol algorithm is the most efficient algorithm.

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Note: / This thesis deals with flow-induced ovalling oscillation and the mechanical characteristics of cylindrical shells. The experiments were conducted with epoxy shells which were either clamped at both ends or clamped at one end and free at the other end. The response of the shells in cross flow was studied in a wind tunnel. A mini-shaker and a fotonic sensor were used to determine the mechanical characteristics (including the natural frequencies, modal damping, and the axial and circumferentia1 mode shapes) of the shells. An analytical method was developed, with the use of potential flow theory for the aerodynamic force, to calculate the oscillation frequencies as well as the negative aerodynamic damping values of clamped-clamped shells. Theory and experiments prove that flow-induced ovalling vibration of a cylindrical shell is not a three-dimensional effect, but rather an aeroelastic phenomenon. Vibration of the shell does not depend on flow periodicity in the wakes and an integral relationship (f/fvs) between the shell frequency (f) and the vortex shedding frequency (fvs) is not necessary at the onset of ovalling. / Cette these traite des oscillations induites par ecoulements, incluant les characteristiques mecaniques de coques cylindriques. Un mini-vibrateur et un capteur optique (Fotonic) ont ete utilises pour determiner les characteristiques mecaniques (inc1uant les frequences naturelles: l'amortissement, et les formes modales axiales et circonferentielles) des coques. Une methode analytique a ete developpee pour le calcul des frequences d'oscillations, ainsi que les valeurs negatives de l'amortissement aerodynamique des coques fixes, faisant usage de la theorie des ecoulements potentiels pour les forces aerodynamiques. La theorie et les essais sur ces coques demontrent que la vibration induite par ecoulements est un phenomene aeroelastique bi-dimensionnel, plutot que tri-dimensionnel. Les vibrations de la coque ne dependent pas de la periodicite de ecoulement dans le sillage, et un rapport integral (f/f ) vs entre la frequence de la coque et la frequence du detachement de tourbillons n'existe pas necessairement, au seuil des vibrations.

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