NCAD2008-73008 PLUNGE INSTABILITY OF A SQUARE BLUFF BODY IN CROSSFLOW

ABSTRACT Flow-induced vibration tests were performed on a beam with a square cross section. The test article was induced into a plunge mode self-excited vibration by forcing air across the beam while it was suspended by leaf springs. The natural frequency and the damping were varied during these tests to alter the variables that contribute the onset of flow-induced vibration. Additionally, cantilevered beams were tested in the same wind tunnel and results proved to be consistent with results from the leaf spring supported beam. The slope of the vertical force coefficient was determined from this testing; this coefficient was determined to be approximately unity, which was roughly 1/3 of the value found in other published data. INTRODUCTION Flow across bluff bodies can result in high amplitude vibration due to flow-induced instabilities. These galloping vibrations are considered to be in the class called self-excited vibrations (SEV). When the structure begins to move, the aerodynamic forces on the body change due to a change in the angle of attack of the fluid to the body. If the lift force, which is a function of the fluid velocity, become greater than the damping force, which is also a function of the velocity, then the net result is a negative damping term and the body becomes unstable. The resultant instability will cause the body to vibrate at its natural frequency at high amplitudes. Blevins shows an analytical method for predicting the onset of unstable bluff body behavior. [1] It should be noted that this type of vibration is not vortex-induced, but rather is induced due to an initial motion of the body and then sustained by extracting energy from the velocity of the fluid. This paper focuses on a square cross section excited by air flow. The response of a square bluff body as a function of flow, alon