With the development of water transport, steel structures are more and more used in inland river port structures, deep water wharf substructures and shipborne components. According to statistics, the world's waste steel materials due to corrosion account for more than 1/5 of the annual output, and the direct economic losses caused by steel corrosion in developed countries account for 2% ~ 4% of GDP (Grundmeier, 2000; Hagen, 2019; Mahdavi, 2019). After corrosion, steel structure will not only wear out the material, but also affect the stress of the structure, thus reducing the safety of the structure, which may cause huge loss of life and property.

Due to the lack of specific codes and standards, the anticorrosion process of steel structures in mountainous rivers and other inland rivers in China mainly refers to the practices of seaport wharf and the relevant codes for coating protection of steel structures for anticorrosion, viscoelastic materials are used to protect steel structures (Chen, 2015). According to the field investigation, the dry season alternates with the wet season, and the steel structure coating in the water level fluctuation area is scoured by the sand flow and the sun's irradiation, resulting in partial overall peeling off. Different parts of the steel structure are exposed to rust, and the corrosion is increasingly aggravated, as shown in Fig.1. The main reason for this phenomenon is the serious erosion of anti-corrosion coating on steel structure by sand flow in inland river environment (Liu, 2018).

When the viscous fluid passes through the rotating cylinder, the relative velocity difference between the upper and lower sides of the cylinder will be generated due to the rotation of the cylinder. According to Bernoulli equation, the pressure difference between the upper and lower sides of the rotating cylinder will be generated, resulting in a force perpendicular to the flow direction. However, there are many cases of no flow in engineering that need Magnus force, such as the ship's zero speed anti rolling. At this time, Magnus force cannot be generated only by the rotation of the cylinder, and the cylinder needs to be swung to obtain the relative flow speed.At present, the researchers have not carried out relevant research on the swinging rotating cylinder. The main research focuses on the rotating cylinder. The first experiment of the rotating cylinder is Prandtl (1925). He found that there is a limit lift value of 4π in the laminar flow of the rotating cylinder. Due to the limitations of technology, it is impossible to measure and study the vortex shedding of a rotating cylinder. Kang (1999) studied the flow around a rotating cylinder under laminar flow. He found that a rotating cylinder based on viscous flow can improve the wake at certain speed ratios, suppress vortex shedding, and effectively reduce flow-induced vibration. Ingham (2014) studied the flow of rotating cylinders at low Reynolds numbers through numerical simulation. Chen (1993) studied the rotating cylinder at Re=200 through numerical simulation and found the vortex release process of multiple vortices in the wake at higher speeds. Kang (2006) found that the flow field will be relatively stable at certain relative speeds when the Reynolds number is small in subsequent studies. Mittal (2003) studied the time-domain variation of the lift resistance coefficient of a rotating cylinder at a Reynolds number of 200-1000. Dmitry A (2012) based on OpenFOAM used large eddy simulation to study the flow of a rotating cylinder with a Reynolds number of 3900. Karabelas (2012) used the LES method to calculate the flow around a rotating cylinder at a Reynolds number of 140,000. He found that as the speed ratio increased, the resistance of the cylinder decreased and the lift increased. In recent years, with the rapid development of computer technology, the speed of image processing has been rapidly improved. Many researchers have proposed new experiment methods for flow field measurement. Among them, the most widely used is particle image velocimetry..Lam (2002) studied the sinusoidal oscillatory motion of cylindrical turbulence through the PIV experiment. He analyzed the distribution and development of large-scale vortices in the flow. K.M. Lam (2009) subsequently analyzed the distribution and development of the vorticity field of the cylinder with low rotation speed in the range of 3600~5000 Reynolds number through the PIV experiment. Zhanqi Tang (2018) measured the cylindrical turbulent boundary layer by particle image velocimetry.Kumar studied the coupling characteristics of two parallel cylinders with the same rotation speed through hydrogen bubble flow display technology and particle image velocimetry. The maturity of PIV technology has further developed the research of rotating cylindrical flow field.

Flow around surface-piercing structures involves interactions between current, wave and body and is of importance for ship and ocean engineering. Wave-induced pressure gradient will affect the boundary layer around the submerged body and vice versa, boundary layer will affect the waves of first order forces and moments (Metcalf et al., 2006). On the other hand, the submerged structures are commonly with truncated free end at the bottom and the tip vortices shed from bottom will interact with free surface (Briggs et al., 2019).

Similar physical problems have been extensively studied. Stern et al. (1987) studied the effects of waves on the boundary layer of a surfacepiercing flat plate with an upstream horizontal foil with variable depth of submergence used for generation of Stokes waves in a towing tank for a range of wave steepness and average Re=1.64×10⁶. They observed wedge shaped, broken and turbulent separation region on the free surface.

Metcalf et al. (2006) experimentally investigated the unsteady freesurface wave-induced boundary-layer separation for a surface-piercing NACA 0024 foil in a towing tank at three different Froude numbers, 0.19, 0.37 and 0.55 and three Reynolds numbers, 0.822, 1.52 and 2.26×10⁶. They provided mean and unsteady far-field wave elevations, mean and unsteady foil-surface pressures and analyzed the frequency components of shear layer, Karman shedding, and flapping instabilities, respectively. However, no PIV measurement is conducted in their study.