AERODYNAMIC INSTABILITY OF CYLINDRICAL BLUFF-BODIES IN NONHOMOGENEOUSFLOW

The vibrations of bodies subject to wind action cause modifications in the flow conditions giving rise to aerodynamic forces which depend pr incipally on the velocities and displacements of the body in question. These forces can produce dynamic instability phenomena and should as a consequence be considered in the equations of motion of structures s usceptible to the effects of fluid-structure interaction, such as long -span bridges, overhead transmission lines and multiple tall towers an d stacks. In this paper the problem is formulated for prismatic sectio ns under two-dimensional flow conditions using linearized equations of particular interest in structural analysis, Three degrees of freedom of the body are considered, rotation, ''up and down'' and ''fore and a ft'' motions, thus extending the quasi-steady theory, well known for t wo degrees of freedom, namely rotation and ''up and down'' motions onl y. A new approach for obtaining the appropriate coefficients for arbit rary prismatic sections experimentally is also discussed. This method was employed in order to obtain the aerodynamic coefficients for an H- section beam in the Reynolds number range 10(4) to 10(5). The results obtained compare favourably with existing values given in the literatu re for this type of section.