This work experimentally investigated the performance of hydrodynamic forces on a semi-submerged cylinder under an oscillatory flow. To generate the equivalent oscillatory flow, the semi-submerged cylinder is forced to oscillate in several combinations of different periods and amplitudes. The mean downward lift force was observed to be significant and the fluctuating lift forces show dominant frequency is twice that of oscillatory flow and amplitude that is the same as the mean lift force. Based on this main hydrodynamic feature, a novel empirical prediction formula for the lift forces on semi-submerged cylinder under oscillatory flow is proposed where the lift forces expression is proportional to the square of oscillatory flow velocity. This novel empirical formula directly assigns the fluctuating lift force with frequency twice of oscillatory flow and the amplitude that is the same as the mean lift force. This assignment of empirical lift force formula reduces parameters required to determine a dynamic lift force but is demonstrated to well predict the fluctuating lift force. The lift coefficient can reach 1.5, which is larger than the typical value 1.2 of the drag coefficient for a fully submerged cylinder with infinite depth. Moreover, relationships among hydrodynamic coefficients, Keulegan-Carpenter (KC) number, Stokes number and Froude number are studied. With the increase of KC number, the Froude number has a more significant influence on the distribution of hydrodynamic coefficients. As Froude number is increasing, the drag coefficient shows a nonlinear decay (KC < 20) but a linear increase (KC > 20), while the added mass coefficients show a nonlinear (KC < 20) and a linear (KC > 20) increase trend. The present work can provide useful references for design of the relevant marine structures and serve as the useful guideline for future research.

中文翻译：

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振荡流中半淹没圆柱体的流体动力

这项工作实验性地研究了在振荡流作用下半浸没圆柱体上的流体动力的性能。为了产生等效的振荡流，迫使半浸没的圆柱体以不同周期和振幅的几种组合进行振荡。观察到平均向下举升力是显着的，并且波动的举升力显示主导频率是振荡流的频率的两倍，并且振幅与平均举升力相同。基于这一主要的水动力特征，提出了一种新的经验推导公式，该公式对半潜缸在振荡流作用下的升力进行了计算，其中升力的表达式与振荡流速的平方成正比。这个新颖的经验公式直接为波动的升力分配了振荡流量两倍的频率，并且振幅与平均升力相同。经验升力公式的这种分配减少了确定动态升力所需的参数，但已证明可以很好地预测波动的升力。升力系数可以达到1.5，大于具有无限深度的完全浸没圆柱体的阻力系数的典型值1.2。此外，研究了水动力系数，Keulegan-Carpenter（KC）数，斯托克斯数和弗洛德数之间的关系。随着KC数的增加，弗洛德数对流体力学系数的分布影响更大。随着弗洛德数的增加，阻力系数显示出非线性衰减（KC < 20）但线性增加（KC> 20），而增加的质量系数显示非线性（KC <20）和线性（KC> 20）增加趋势。本工作可以为相关海洋结构的设计提供有用的参考，并为将来的研究提供有用的指导。