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Hydroelastic studies of bluff cylinders Dutta, Lila Kanta

Abstract

Uniform water flow past cylinders of various cross-sections produces oscillating lift and drag forces. In the tests made in this investigation the cylinders were elastically mounted on a cantilever of known stiffness. The experiments were carried out to obtain a series of two-dimensional vibration records which were then analysed to find: i) the variation of the types of lateral and longitudinal vibrations, ii) the variation of the amplitude of lateral and longitudinal vibrations with respect to Reynolds numbers, iii) the relation between frequency of lateral and longitudinal vibrations and the velocity of flow, iv) Strouhal number for both lateral and longitudinal vibrations and to relate them with the corresponding Reynolds numbers of the flow, v) Coefficient of lift and drag allowing for magnification factor. The range of investigation was within Re = 2.1 × 10³ and Re = 7.25 × 10⁴. The cantilever system including the model was 38 inches long, cylinders of various cross-sectional shapes, such as circular, truncated-conical, D-shaped, square and rectangular were tested. For the circular cylinders, the natural frequency of vibration was found to vary between 2.78 to 3.57 cycles/sec. and the damping coefficients varied between 0.068 to 0.12. For circular cylinders, the resonance of longitudinal vibration occured at [V] / [Vy res] = 0.42. The most interesting result, which has not been reported elsewhere, is that the longitudinal amplitude was re-excited by the lateral vibration and started rising again to a second maximum. For the circular cylinders, maximum values of coefficient of drag and of coefficient of lift were found to be 3.50 and 4.70 respectively even after allowing for magnification, Strouhal number variation in the lateral direction was found to be between 0.185 and 0.14. For circular cylinders, up to the point where lateral resonance occured, the frequency of the excitation in the longitudinal direction was found to be twice that in the lateral direction. Although the natural frequency of vibration was the same for both lateral and longitudinal direction, the Strouhal number was not constant, but varied within a narrow range. Thus, the velocity of resonance in the lateral direction was not twice that of the velocity of resonance in the longitudinal direction but was 2.3 times instead. For the D-section, the flat face normal to flow was found to be the most unstable of all orientations. The flat face parallel to flow gave the least coefficients of drag and lift out of all the various shapes investigated. The Strouhal number variation in the lateral direction for the circular cylinders, for the D-shaped cylinder with flat face parallel to the flow, and for rectangular cylinders with 2" face normal to the flow was the same. The maximum value of the coefficient of lift was higher for circular cylinders than that for any other shapes investigated.

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