A Generalized In-Line Vortex-Induced Vibration Assessment Method

Abstract

ABSTRACT This paper identifies deficiencies and uncertainties in the present design procedures used for estimating inline (in the flow direction) vortex induced vibrations of circular cylinders in a near steady flow. A recently developed alternate design procedure which addresses some of these deficiencies, in particular non-uniform flow conditions, is discussed and compared with the existing methods. INTRODUCTION Vortex shedding from a bluff cylinder occurs in a wide range of flow conditions producing pressure fluctuations upon the cylinder. These fluctuations can excite structural elements with natural frequencies close to those of the pressure fluctuations and thus an assessment of this behavior is required in strength and fatigue evaluations during design. This paper concentrates upon estimating the response of elements exposed to possible vortex induced excitation in the inline (with the flow) direction in a near steady current. During design possible vortex induced cross flow vibrations in both steady and unsteady (wave and current) flows must also be considered as well as wave induced inline motions. Bronchi et al(1989)[ 1] indicates that inline vortex induced vibrations need only be considered when the peak wave induced velocity is less than 25% of the current velocity. Outside these conditions it is suggested that the unsteadiness of the flow inhibits the required vortex interaction from occurring and that the wave loading usually described by the Morison equation dominates the inline motions. INLINE VORTEX INDUCED VIBRATIONS For a detailed introduction to vortex shedding and its effects in near steady flows the reader is directed to King(I977)[2] and Griffin(I 981)[3]. The frequency of a vortex being shed from each side of a stationary cylinder in a Steady flow is described by the Strophes Number(St). This varies with Reynolds Number(Re) on a smooth stationary cylinder as presented in Figure I. The cross flow loading is dominated by this frequency while the inline loadings are around the frequency of shedding of individual vortices (? 2fv), In the zone 2xl05 < Re < 3xl06 the usually strong periodic vortex shedding is disrupted by the transition of the cylinder boundary layers from laminar to turbulent conditions. The location of this zone varies with surface roughness and turbulence levels. When a cylindrical structure vibrates in this zone, with surface roughness typical of marine structures, the usual narrow band vortex shedding is observed with a Strophes Number close to the lower bound of Figure I. For a particular structure, as St is nearly constant and (Vr)(St) = fv/fn then Vr describes the proximity of the system to a resonant condition. Inline vortex induced excitation occurs, for a particular element, at lower flow speeds than cross-flow excitation, and thus although it produces much lower response amplitudes it may dominate fatigue assessments.