The effect of yaw angle (α) on hydrodynamic forces of a yawed square cylinder oscillating in still water is studied experimentally. The independent principle (IP) was examined by evaluating the hydrodynamic coefficients that derived from the measured in-line and lift forces using a 3-component force link at a Keulegan-Carpenter (KC) number range of 3~30 and a Stokes parameter (β) range of 500~1600. Significant differences are observed in normalized drag and inertia coefficients for the KC ranges of 8~20 and 10~18, respectively, indicating the invalidity of the IP. The peak on the spectra of the lift force becomes pronounced as α becomes larger, which implies that the yaw angle has an intensified effect on the vortex shedding process around a square cylinder in oscillatory flow.
Oscillatory flow around a cylindrical structure is of importance due to its intrinsic interest in theoretical hydrodynamic research and its relevance to practical engineering applications. Knowledge of the hydrodynamics is essential for both the design and operation of offshore structures, such as semisubmersibles and tension leg platforms. For some cylindrical structures, owning to their directionality, the sea waves and currents can approach the structures obliquely. In these cases, the fluid velocity in the axial direction of the structure is not negligible, which may have profound effect on the vortex instability and force characteristics of the structure.