## 1 Introduction
(1995) [14] conducted numerical simulation of cylinders near a free surface under steady flow and studied corresponding motion behavior. The jet state and metastable behavior were noted by these researchers, at the free surface under certain gap ratio and Froude (Fr) number. The configuration of a partially submerged cylinder under flow is an unavoidable and actually widely observed problem for float collars of the fish farming structures and numerous studies were conducted in the literature of the flow past float collars. For example, [PERSON] and [PERSON] (1989) [15] studied the stability of a semi-submerged cylinder under flow, where results are influenced by the Fr number. At all downstream locations, convective instabilityof the wake is also observed based on the computation of potential flow theory. However, potential flow theory precludes the effect of the free surface and fluid viscosity, which cannot be ignored for a partially submerged cylinder. As a result, it is questionable whether traditional diffraction and radiation theories will provide a complete answer for this problem. [PERSON] and [PERSON] (2008) [16] and [PERSON] (2010) [17] experimentally measured the sway and heave motion of a semi-submerged circular cylinder under action of waves. [PERSON] et al. (2013) [18] experimentally investigated hydrodynamic loads of semi-submerged cylinders that are employed in fish farming systems. It is reported that the effects of free surface are critically important for the hydrodynamic coefficients including drag coefficients and added mass coefficients in the IL direction; the direction parallel to the oscillatory flow. However, hydrodynamic forces and coefficients in the cross-flow (CF) direction have not yet been revealed and considered into models and this relationship between hydrodynamic loads and Froude number has not been studied. Recently, [PERSON] et al. (2019) [19] conducted experiments of the partially submerged cylinder with different submerged depths in steady flow. Surprisingly, the mean downward transverse force is witnessed to be significant, which catches more and more attentions from both academia and industry. The lift coefficient was found to be greatly affected by Froude number. However, the understanding of hydrodynamic load in the CF direction on the semi-submerged cylinder under the oscillatory flow is still blank and potential influence parameters were not studied comprehensively. Therefore, it is urgent to investigate the hydrodynamic features of the semi-submerged cylinder in an oscillatory flow, especially for the lift coefficient and providing a further understanding for relevant structural design. In this study, model tests of semi-submerged cylinders in an oscillatory flow were carried out in a towing tank. The rigid cylinder was enforced to oscillate harmonically under numerous combinations of periods and amplitudes to generate an equivalent oscillatory flow. The KC number was designed to range from 6.3 to 37.7. The Stokes number and corresponding Froude number varied from 5434.7 to 11,363.6 and from 0.12 to 1.55, respectively. The hydrodynamic force in both IL and CF directions and the displacement in the IL direction were measured. We also establish the method for identifying the drag and added mass coefficients in the IL direction. Based on the observations of the lift force, a novel empirical prediction method of lift force is first proposed. The features of hydrodynamic loads, the relationships between hydrodynamic coefficients and influential parameters are further studied and discussed. The remainder of this paper is organized as what follows. Section 2 describes the experimental set-up, test arrangement and definition of the key parameters such as Froude number, Stokes number and KC number. In Section 3, the identification of the drag and added mass coefficient are presented.