ABSTRACT

A conceptual model-equation for the unsteady lift on a cylinder in a reversing planar Row is developed and verified with measurements. The model reflects a relationship between the transverse loads on the structure and vortex strength in an environment of periodic wake returns. A novel data sampling technique that discriminates the intermittent upwash and downwash modes of wake return is implemented to allow evaluation of nearfield Row parameters, such as Row circulation and added-mass, that carry theoretical ties with the lift. Although measurements and formulation deal primarily with lift on rigid cylinders in Keulegan-Carpenter number Rows up to 20 and Reynolds numbers up to 3*104, they should also provide a fundamental insight into the nearfield Row dynamics of typical offshore structures for wave force modelers.

INTRODUCTION

In addition to drag, a circular cylinder in steady translation may experience comparatively less fluctuating lift as a consequence of the continuous supply of kinetic energy to the trailing vortices as they grow, shed and convect downstream with some asymmetry to the centerline. When a body reverses motion periodically, say, with displacements ranging from 3 to 9 diameters, and frequencies giving Reynolds numbers smaller than about 3*105, the hydrodynamic lift fluctuates with magnitude comparable to drag and inertia combined in response to a sustained asymmetric nearfield vortex flow. The prediction of drag and inertia forces on cylinders has been extensively shown satisfactory by introducing undisturbed farflow velocity and acceleration into Morison's inline force equation (Morison et al, 1950). Here, an effort has been conducted to model the fluctuating lift using fundamental concepts of Fluid Dynamics which are implemented with suited measurements from the nearfield flow of a cylinder stationary in a planar harmonic flow. The scientific study of lift is associated with the names of Kutta and Joukowski, two pioneers of aeronautics (Batchelor, 1967).

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