Shear flow induced vibrations of long slender cylinders with a wake oscillator model

Abstract

A time domain model is presented to study the vibrations of long slender cylinders placed in shear flow. Long slender cylinders such as risers and tension legs are widely used in the field of ocean engineering. They are subjected to vortex-induced vibrations (VIV) when placed within a transverse incident flow. A three dimensional model coupled with wake oscillators is formulated to describe the response of the slender cylinder in cross-flow and in-line directions. The wake oscillators are distributed along the cylinder and the vortex-shedding frequency is derived from the local current velocity. A non-linear fluid force model is accounted for the coupled effect between cross-flow and in-line vibrations. The comparisons with the published experimental data show that the dynamic features of VIV of long slender cylinder placed in shear flow can be obtained by the proposed model, such as the spanwise average displacement, vibration frequency, dominant mode and the combination of standing and traveling waves. The simulation in a uniform flow is also conducted and the result is compared with the case of nonuniform flow. It is concluded that the flow shear characteristic has significantly changed the cylinder vibration behavior.

Study on Vortex-Induced Vibration of Deep-Water Marine Drilling Risers in Linearly Sheared Flows in consideration of Changing Added Mass

“…e geometric nonlinearity has little effect on the dynamic response of the risers. us, the motion equations of marine drilling risers can be expressed as follows [18]:…”

“…where m r , m f , m a , and m d are the mass of per-unit-length riser, internal fluid, external added seawater, and external displaced seawater, respectively; g is the gravity acceleration; ρ r , ρ f , and ρ w are the densities of the riser, internal fluid, and external seawater, respectively; D and d are the outer and inner diameters of the riser, respectively; and C a is the added mass coefficient. e formulas of the equivalent damping coefficients can be expressed as follows [6,18]:…”

“…where C D0 is the mean drag force coefficient of stationary risers and y is the cross-flow VIV amplitude of vibrating risers. e vortex shedding angular frequency can be expressed as follows [18]:…”

“…According to previous studies [6,18], the in-line and cross-flow hydrodynamic forces acting on risers can be expressed as follows:…”

“…en, wake oscillator models have been continuously improved by some researchers [4][5][6][7][8][9][10][11][12][13]. Based on wake oscillator models, some studies have analyzed the coupled in-line and cross-flow VIV response of long slender flexible cylinders in uniform flows [14][15][16], and some scholars have also focused their attention on the study of coupled in-line and cross-flow VIV response of long slender flexible cylinders in sheared flows [17][18][19][20][21][22][23]. Leclercq and de Langre [24] also studied the VIV response of cylinders with large in-line bending deformation.…”