Impact Dynamics Of Moving Objects Onto Two-phase Interface

Impact of moving objects onto two-phase interface problem widely exists in daily life,nature and engineering.Since the flow phenomena in the problem are complex and diverse,a large number of previous works have been done,however,there are still problems that have not been solved yet,for instance,the flow mechanism of the cavity formation in water entry and the whole physical picture of water exit.In this paper,two typical flow problems,water-entry problem and water-exit problem,are studied numerically and experimentally.Particular attention is given to the characteristic flow phenomena and the underlying flow mechanisms.The results and conclusions are briefly given as follows:(1)Water entry of spheres and cylinders is studied by a combination of experiments,simulations and theoretical analysis.The contact-line pinning and its relation with the subsequent cavity evolution are given particularly attention.It is found that the pinning of the contact line is related to the flow separation around the side wall of objects.The pinned contact lines were observed to be slightly above the equator of the sphere and the lower edge of the cylinder.We connect wetta-bility,inertia,geometry of the impact object,interface bending and contact-line positioin with the contact-line pinning by analysing the force balance at a pinned meniscus,and the result compares favourably with those from simulations and experiments.The theoretical model based on the Rayleigh-Besant problem ac-curately captures the radial expansion of the cavity after the pinning of the contact line.The vertical propagation of the ripples on the cavity wall is found to have a similar dispersion relation to the evolution of perturbations on a hollow jet and it is independent of the object geometry.In addition,the geometric effects and the liquid viscous effects on the cavity formation are investigated and we find that it is more difficult for a streamlined object to generate a cavity than an irregular object.Moreover,considering the viscous dissipation energy,a modified Bernoulli equation is derived to obtain the scaling relation between the liquid viscosity and the position of the pinned contact line.The theoretical scaling agrees with our numerical results,which indicates the high liquid viscosity will prevent the cavity formation.(2)Water exit of initially fully-submerged cylinders at a constant speed is studied nu-merically and theoretically.We mainly focus on the flow mechanisms underlying the dynamics of the free surface and the moving contact lines.We present a whole physical picture of water exit problem and identify two flow regimes according to the formation of contact lines,which,are described as the film rupture and object encapsulation.A parameter study demonstrates that the viscous effect is a key factor in determining whether the object can be encapsulated.In particular,the critical condition for the object encapsulation is determined by the Ohnesorge number,rather than the cylinder velocity.In object encapsulation regime,it is shown numerically that the liquid film on the sidewall of the cylinder tends to be quasi-static and the film thicknesses will eventually reach a certain value that depends on the Reynolds number of moving objects,which can be explained by a simple model based on the work-energy principle.An analysis of the pinch-off time of the liquid threads beneath the cylinder is proposed.As for the film rupture regime,we find that the velocity of moving contact lines relative to the cylinders depends on the Ohnesorge number and the wettabiity.Two distinct modes,the inertial and viscous modes,exist in appropriate Oh range.A force balance on the retracting rim is performed,which considered the combined effects of the wettabiity,inertia,surface tension and viscous force.Particularly,in the inertial mode,the relative velocity of contact lines is found to slightly depend on the cylinder speed at fixed the Ohnesorge number,while in the viscous mode regardless of the cylinder speed.Moreover,the geometric effects are considered and turn out to affect the flow regimes and thus affect the work done by the resistance.Generally,since the contact line may be easily formed on the north pole of the prolate spheroid,it endures the resistance much smaller than the other geometries,which provides a reasonable interpretation why aquatic lives always have the streamlined shape.