Vortex-Induced Forces on Oscillating Bluff Cylinders

Abstract : Vortex-induced forces and consequent vibration of long cylindrical structures are important for a large number of engineering applications, while the complexity of the underlying physical mechanisms is such that this is one of the canonical problems of fluid mechanics. Vortex shedding force varies in frequency and magnitude along the length of the structure, causing the response at any point to be amplitude-modulated in space and time. The focus is on the measurement, via forced-oscillation experiments, of the vortex-induced lift and drag forces acting on circular cylinders undergoing sinusoidal and amplitude- modulated oscillations. Basic concepts on vortex formation and vortex-induced vibrations, a review of the existing literature, and details of the experimental apparatus and data processing methods are presented. Stationary and sinusoidal oscillation tests are presented. Several novel properties are described, among them the role of the lift force phase angle in causing the amplitude-limited nature of VIV, and use of the lift force excitation region in contrast with the often-quoted but quite different lift force lock-in region. Next, a comprehensive data error analysis, and a simple VIV prediction scheme are described. New data on amplitude-modulated oscillations are presented. The concept of control of the mean wake velocity profile via the control of the major vortical feature is explored, with possible applications being the reduction of the in-line wake velocity and alteration of the wake signature.