ABSTRACT

We use high-order CFD methods and statistical analysis techniques to study the flow over a cylinder, approximating circular cross-section offshore structures, oscillating in a direction inclined to the horizontal direction. We vary the orientation angle from 0 to 180 degrees, where the 90-degree case corresponds to oscillation perpendicular to the incoming stream. We study (in the time-domain and through the angle-response curves) variations of the induced force properties, their symmetry or skew symmetry about the case of a 90-degree orientation angle, and the structure-induced damping of the flow. We identify the range of orientation angles where the entrainment phenomenon takes place and study the influence of the motion amplitude on this phenomenon. We propose a technique to derive reference (scaling) quantities for the oblique fluid force. Unlike the lift and drag, such reference quantities cannot be obtained from the stationary-structure case because such an oblique force is not defined then.

INTRODUCTION

Fixed platforms, tension-leg platforms, spars, offshore wind turbines, and chimneys share a rigid circular cross-section element. Several studies have investigated the flow over stationary and moving cylinders and cables, and the fluid-structure interaction (FSI) associated with the latter two cases. Different cross-sections have been considered, including circular, elliptic, and square ones. In addition, for two-dimensional moving-structure cases, different motion orientations have been also considered. These include 1 DOF perpendicular to the incoming flow stream (Carberry et al., 2001; Marzouk and Nayfeh, 2008), 1 DOF parallel to the incoming flow (Kim and Williams, 2006; Guilmineau and Queutey, 2004), and both 2 DOFs (Jauvtis and Williamson, 2003). There have been studies that even explored angular oscillations of the structure instead of the translational one (Baek and Lee, 2001). We here examine the influence of tilting an oscillating cylinder from the vertical direction (the incoming stream sets the horizontal direction).

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