Flow-induced, in-line vibrations of a circular cylinder

• Main Author: Aguirre Romano, Jorge Enrique
• Published: Imperial College London 1978
• Subjects: 532
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.447000

An investigation has been conducted on the flow-induced, inline vibrations of circular cylinders. Two-dimensional cylinders immersed in a one-dimensional water stream were tested in the laboratory. Detailed observation of cylinder and flow behaviour revealed some novel characteristics of this type of excitation, such as a new type of wake, variations of the mean drag coefficient and frequency variations which reduced a variable mass coefficient to zero. A new "non-dimensional frequency parameter" was obtained which unlike the commonly used "reduced velocity", unified the results of previous researches and provided a precise definition of the instability regions; this allowed the avoidance of instability in engineering problems at the design stage. In contrast with aerodynamic practice, it was here concluded that density and damping should be considered separately. Density was found to determine the frequency response. Damping was divided into hydrodynamic (included in the total hydrodynamic force) and external (structural); the latter was represented by a modified "stability parameter" which is independent of cylinder density and which was found to determine the amplitude response. The identification and definition of the independent roles played by density and external damping, led to correlations which allowed the prediction of the amplitude and frequency response and of the instability regions, for any two-dimensional cylinder-flow arrangement. The hydrodynamic exciting, damping and added mass forces were analysed leading to a theoretical model which represents the excitation in terms of force coefficients and a phase angle; these parameters were found to represent the hydrodynamic processes. Frequency variations were attributed to a constant mass coefficient and variable drag forces; this led to a marked simplification of the theoretical model in the second instability region, and to the prediction of the force coefficients and the phase angle from knowledge of flow characteristics and cylinder motion and geometry. Good agreement was also found between the predictions of the model and the results of full-scale three-dimensional experiments.