Investigation On Sidewall Effects On Near-trapping Phenomenon Of A Four-cylinder Structure

In recent years,the exploitation of offshore resources has gradually developed into deep sea areas.Traditional jacket and jack-up offshore platforms cannot meet the engineering needs of deep-sea resources development.Floating offshore platforms,including tension legs and semi-submersible platforms,has attracted more and more attention.The supporting structure of these offshore platforms are often composed of multiple columns.These supporting columns are relatively close to each other and have a strong hydrodynamic influence on each other.Under certain wave conditions,near-trapping phenomenon occurs between the columns.When the near-trapping phenomenon occurs,large amounts of incident waves remain around the structure,the fluid inside the cylinder group undergoes resonant motion,and under the action of multiple reflections of the waves,the wave elevation near the platform will increase significantly,which will have a negative effect on the air gap performance of the platform.Numerical simulation and model experiment are usually used in the study of the hydrodynamic response of Marine structures.The model experiment is usually carried out in the middle of the wave flume,and the experimental results will be affected no matter how wide the wave tank is.In response to the above problems,this paper uses computational fluid dynamics method to investigate the near-trapping phenomenon of the four-cylinder structure.First,based on the potential flow theory of the ideal fluid hypothesis and using the constant element method,the influence of the cylinder radius on the near-trapping phenomenon of the four-cylinder structure is analyzed with the help of the computational fluid dynamics software HydroStar.The study found that the increase of the radius of the cylinder can move the natural frequency of the near-trapped mode to the high-frequency direction,and has a significant impact on the high-order near-trapped mode.The change in the radius of the cylinder also leads to the change of the wave surface distribution characteristics.The key factor affecting the occurrence of the near-trapping phenomenon is actually the distance between the sidewalls of the cylinders.As the distance between the sidewalls of the cylinders decreases,more wave energy is concentrated on the inner side of the structure,leading to the significant lift of local wave elevation.Furthermore,the influence of the sidewall effect of the flume on the near-trapping phenomenon is investigated.The source image method is used to introduce the Green's function of the wave tank to eliminate the integral of the side wall of the wave tank.The research results show that the reflection of the side wall in the flume leads to the generation of the horizontal standing wave natural vibration mode of the wave tank.There is a coupling effect between the natural frequency of horizontal stand wave of the wave tank and the near-trapping frequency.The two resonance modes are excited at the same time,resulting in a significant increase in the peak value of the wave elevation near the corresponding frequency.More importantly,after the wave acts on the cylinder,the diffracted wave is reflected by the side wall of the wave tank,which causes the wave energy to be unable to radiate to the far field,and the tendency of wave energy to concentrate to the inside of the cylinder is more obvious,which also leads to the increase of the maximum wave elevation near the structure.On the basis of the above research,the gas-liquid two-phase flow model is adopted and the finite volume method is used for discretization,the free water surface is captured based on the volume fraction function of the fluid,and the pressure and velocity fields are solved separately by the PISO method.The computational fluid dynamics software OpenFOAM is used to numerically simulate and analyze the influence of fluid viscosity and energy dissipation on the wave load on the four-cylinder structure and the nearby free water surface changes.Numerical results show that the existence of the side wall of the wave tank will lead to an overestimation of the wave elevation,especially at the near-trapping frequency.In the potential flow model The peak values of dimensionless wave run-ups measured in 7m and 14 m wide flume are about 2.2times and 1.47 times of those in open sea area,while they are about 1.77 times and 1.32 times in the viscous flow model.Compared with the potential flow model,the viscous damping and energy dissipation can suppress the wave climb and wave load near the structure,and further reduce the wave loads on each cylinder.