Numerical And Experimental Study Of Wave Impact On Structures For Compressible Two-Phase Flow

In ocean engineering,when wave impacts on structures or the structure slamming on water,air is inevitably involved.In traditional engineering application or research,the impacting pro-cess was often simplified as a single-phase or incompressible two-phase flow problem.How-ever,with the shape changes of structures,the angle changes of incident wave or the types of wave breaking,when the air cushion layer is formed or air pocket is entrapped and small bub-bles are entrained during the impact,the impact pressure will be affected by the compressibility of air.Various changes of pressure will happen,such as the deviation,the weakening of the maximum peak pressure,and the violent oscillation of pressure.Therefore,it is necessary to discuss the effect of two-phase flow during wave impact process.Based on the above back-ground,the influence of two-phase flow in wave impact problems will be studied in this paper by numerical simulation and physical model experiments.The influence of the air compressi-bility on the impact loads to structures will be discussed and the mechanism of wave impact on structures is revealed.Firstly,a viscous compressible two-phase flow numerical model was proposed.In this model,continuity equation and Navier-Stokes equations are considered as governing equations,which are discretized by finite difference scheme.The convection term is solved by using Con-strained Interpolation Profile(CIP)method,which is third order accuracy in space;the free furface is captured by a VOF-typed method,Tangent of Hyperbola for Interface Capturing(THINC/SW)method;Immersed Boundary Method(IBM)is used to calculate the Fluid-Struc-ture Interaction(FSI).Secondly,the numerical model was verified.The accuracy for compressibility calculation is verified by the simulation of classical compressible two-phase flow problems.The ability of this model to calculate the deformation of free surface and its accuracy in calculating impact pressure verified by simulating dam-break experiments.Thirdly,the compressible two-phase flow numerical model is used to study the two-phase flow effect of wave impact in sloshing.On the basis of comparison with previous experimental results,the cause of pressure oscillation in air-entrapped wave impact is qualitatively explained.Then,water entry of a plate plate is studied by using implicit IBM method.The gas-liquid two-phase flow model is extended to the gas-liquid-solid three-phase interaction.Based on numer-ical results,the influence of air cushion on pressure under the plate is analyzed during the flate plate impact into water.The results show that the expansion process of the air cushion is the reason for the negative pressure.Due to the influence of air cushion,the distribution of pressure under the plate is unevenly.Based on this numerical model,a numerical wave tank(NWT)that can consider air com-pressibility is established.Using the numerical experiment of solitary wave impact on a vertical wall,the influence of wave development on impact pressure was studied.In addition to incident wave elements,whether the wave is broken or not is also an important factor affecting the impact pressure:from no breaking to well-developed plunging(WDP),and then from micro-breaking to broken,the impact pressure peak increases firstly,and then it decreases gradually.After WDP,air is entrapped.With the increase of the volume of air entrapped,the oscillation frequency of impact pressure decreases gradually,and its oscillation time increases accordingly.Through the quantitative analysis of the relationship between volume of air pocket and pressure oscillation frequency,this paper points out that the volume of air pocket entrapped and the change rate of pocket volume are the key factors affecting pressure oscillation frequency.Then,based on the NWT,wave impact on horizontal plate is simulated.Air cushion under the plate bottom has a cushioning effect on the middle and back end of the plate.However,with the rapid overflow of air cushion from the end,wave water slamms plate directly,which will cause a secondary impact pressure peak in the middle and end of the plate.Finally,a physical model experiment of wave impact on perforated caisson structure was carried out.In this experiment,three sizes of models are selected to study,and the characteris-tics of impact pressure affected by air are analyzed.Under the influence of enclosed air,the impact pressure inside the caisson is smooth,which is different from the process of wave im-pacting on a vertical wall or horizontal plate.And the impact time is prolonged.The relations between impact pressure inside perforated caisson and incident wave period,wave height and the clearance to the roof are given.Using physical model experiments and numerical simula-tions,the influence of caisson structure changes on its internal impact pressure was studied.This paper summarizes the influences of opening at the front,whether the roof is closed or not,and internal changes on wave impact load inside caisson.