Study On Cavitating Flow And Structural Response During Vertical Water Entry Of Hyperelastic Spheres

Study on the water entry problem of sphere is the theoretical basis for the development of weapons such as cross medium aircraft and air drop torpedo.In the past,most of the studies focused on the rigid sphere.However,in recent years,hyperelastic materials have developed vigorously because of their unique mechanical properties,and have been widely used in the fields of aerospace and national defense.The water entry problem of hyperelastic spheres has gradually attracted close attention.Unlike the traditional water entry problem of rigid spheres,the water entry of hyperelastic spheres is a complex fluid-structure coupling problem involving deformations,the sphere deforms obviously during water entry and interacts with the evolution behaviors of the splash and cavity,thus forming the unique multiphase flow,motion and structural response characteristics.Therefore,it is of great theoretical significance and engineering value to carry out the researches on the water entry of hyperelastic spheres and grasp the characteristics of cavitation flows and structural responses of hyperelastic spheres during water entry.Based on the experimental and numerical methods,the fluid-solid interactions,multiphase flows and structural response characteristics of hyperelastic spheres during water entry are mainly explored in this paper.The main research contents and results are as follows:Based on the high-speed photography method,an experimental study on the splash and cavitating flows formed hyperelastic spheres during vertical water entry at low Reynolds number is carried out.The size variation in the three typical splash evolution characteristics with water-entry time is obtained,and grasp that the the sealing time of splash curtain generated by hyperelastic spheres with different diameters vary with dimensionless stress ratio.In additions,the variation laws of the cavity closure size and the sphere deformation coefficients with the Froude number and the dimensionless stress ratio are given.During water entry,the coupling mechanism of the structural response of the hyperelastic sphere with the splash and cavitating flows is revealed.By analyzing the fluctuations of the cavity wall and the degrees of sphere deformations,the cavities formed by the hyperelastic spheres during water entry are qualitatively classified.Based on the classification of the cavity shapes,the Froude number-dimensionless stress ratio map is established to distinguish the cavity region,in which the shape of cavity is predicted from the impact velocity and the material properties of spheres.Based on the numerical method,for two phases(impacting on the free surface and after water entry),the structural response characteristics of hyperelastic spheres during water entry are studied.By analyzing the impact effect of hyperelastic spheres with different materials on the free surface,the effects of the sphere shear moduli and densities on the stress of the sphere bottom and the pressure on the free surface are given,and the propagation law of internal elastic wave of spheres with different material properties is obtained.When the hyperelastic sphere moves in water,there are five typical deformation patterns of the hyperelastic sphere in a single deformation period,the law of the energy transmission in the deformation process of the hyperelastic sphere is revealed,and the differences in energy and motion characteristics in the transverse and longitudinal deformation stages of the hyperelastic sphere in water are evaluated.By studying the effects of material properties and impact velocity on the deformation amplitude and period of hyperelastic spheres,a prediction model of material properties on the deformation period of hyperelastic sphere in water is established.Compared with the stress distribution of rigid sphere in water,the hyperelastic spheres moving in water have experienced four typical stress distribution characteristics in a deformation cycle.The propagation characteristics of elastic wave in the sphere under the periodic deformation patterns are given,and obtain that the propagation characteristics and the laws affected by impact conditions and material properties.Based on the combination of experimental and numerical researches,the multiphase flow characteristics of deformable hyperelastic spheres during water entry are studied,it is found that there are three kinds of typical evolutionary cavities during the water entry of hyperelastic spheres,and the expansion characteristics of typical evolutionary cavities during the water entry are studied.Meanwhile,the influence of material properties and impact velocity on the cavitating separation characteristics of hyperelastic spheres is given,and the sensitivity analysis of the wet area of the sphere to its density ratio is carried out.Secondly,the periodicity,dimensional information and quantitative characteristics of the nested cavity phenomena during the water entry of hyperelastic spheres are studied and analyzed.Finally,based on the analytical model on the interaction between the deformations and hydrodynamic forces of the hyperelastic spheres during water entry,the change of the minimum impact velocity required for the formations of nested cavities is obtained with the different spheres properties,which provides a new method for determining the critical formation conditions of nested cavities.