Transient Instabilities in Fluid Structure Interaction Systems

This text concerns the transient mechanisms in fluid structure systems with coupled two degrees of freedom. Three different solid objects are studied in the order of increasingly streamlined cross section, namely a square cylinder, a streamlined bridge deck section and a symmetric airfoil. This work is divided into two parts. First part is reserved for the long term behavior of all the three fluid structure systems. The square cylinder is allowed to oscillate freely in a high mass ratio environment. Long term stability parameters for a bridge deck and a symmetric airfoil are measured and validated using simple theoretical tools detailed in Part-I. Part-II is dedicated to the transient behavior. Growth rate of oscillations amplitude for the square cylinder is measured. The bridge deck behavior is studied first, under the effect of a mechanical excitation and then under the effect of an excitation induced by an abrupt gust. The airfoil is studied in a linear and a non-linear structural environment subjected to an abrupt gust. A new combination of the quasi steady theory and the Kussner¿s aerodynamic admittance function is proposed to validate the results.