Study On Multiphase Flow Characteristics In Water-entry Compound Movement Of Cylinders

Cylinder water-entry compound movement is the multi-degree of freedom motion that a cylinder with a certain velocity and inclined angle enters the water domain.This is an extremely complex multiphase flow problem involving a suddenly changed medium states and strong coupling between gas,liquid and solid,which are occurring in a very short period.The caused water entry phenomena are quite different from that of classical vertical water entry.The cavity evolution,flow field structure,hydrodynamics and motion response have fundamentally changed in current study.The study of cylinder water-entry compound movement are widely applied in the ocean engineering and aerospace industry,such as ship slamming,designing ocean platform,launching air-to-sea project,and landing unpowered vehicle.Therefore,it is of great theoretical significance and engineering value to investigate multiphase flow characteristics of cylinder water entry.In this study,the experimental and numerical methods are employed to comprehensively and deeply investigate the multiphase flow of the cylinder water entry.The main research contents and achievement are as follows:Based on the high-speed camera technology,an experimental system of automatic control is established to execute given action,such as releasing cylinder at a certain position and triggering camera following the release.Systematic experiments carried out for the water entry of cylinder with different initial inclined angles and velocities.A digital image processing method based on gray gradient is proposed to extract the cylinder trajectory and angular displacement during water entry.In addition,combining discrete data processing technology of a quintic smoothing spline scheme,we obtain the response of cylinder velocity,acceleration and hydrodynamics.Based the governing equation of large eddy simulation,VOF multiphase models and dynamic overlapping grid method,a simulation method considering the conditions of static and wave water domain is established to capture the gas-liquid interface and predict the cylinder trajectory and angular displacement in water entry.The availability of numerical method is validated by comparing the numerical results with the experimental that.The experimental investigations on the multiphase flow characteristics of vertical water entry for the cylinder with different initial inclined angle are carried out.We obtain many flow evolution phenomena,such as,cavity created,developing,separating,pinchoff,and collapsing,and analyze the flow field structure of the cavity evolution and its inside mechanisms.According to the relation of impact jetting and initial inclined angle,we establish an empirical formula to predicting the impact jetting velocity for the cylinder with different initial inclined angle.The water entry for the different parameter cylinders are numerically studied.The pinch-off time linearly increase with increasing Froude number and the increase rate is equal to 1.63.In addition,when the cavity separating is occurred,the slamming load acting on the cylinder induced by the cavity pinching off greater than that of water impact under the special conditions.The experimental and numerical method is employed to investigate the multiphase flow characteristics of water entry of the complex moving cylinder under low Fr.The motion response of near free surface cavity wall are closely related to the cavity evolution.Based on that,a criterion for quantitative dividing the phase of cavity evolution is proposed.The effects of initial inclined angle and horizontal velocity on the asymmetric evolution of cavity,hydrodynamics and motion response are studied.The splash dome after surface seal collapses into the cavity from the middle dome under the action of pressure difference,and form a channel for suppling gas.There is serious effect of the upper cavity formed after cavity pinching off on the evolution of tail cavity.The average rate of lift coefficient increase linearly with increasing initial inclined angle.Moreover,the decline rate of this average rate before pinch-off is larger than that after pinch-off.The drag coefficient increase with increasing water entry time for the cylinder with different inclined angle.In addition,drag coefficient increase faster for the cylinder with a relative larger absolute angle of attack.The pinch-off time also increase linearly with increasing Fr.The relation between trajectory curvature direction and initial parameter is quantitatively described for the cylinder moving along non-circuitous trajectory after water entry.Multiphase flow characteristics are investigated for cylinder entering the water under different current flow and wave conditions.The effects of current flow velocity and wave parameter on impact cavity,hydrodynamic property and motion response are studied.The cavity evolution and hydrodynamic characteristics for different current flow velocities are very similar to that for different wave velocities.The pinch-off time piecewise linearly increases with increasing Fr.However,the depth of pinch-off point is almost equal.Changes in the hydrodynamic property and motion response are similar for cylinder entering the water from the same surface position of wave,which the ratio of wave lengthto-height is less than 20.In particular,the effect of the wave that the ratio of wave lengthto-height is more than 40 on the impact cavity of cylinder can be ignored.The lift and moment coefficients decrease rapidly during the period that the cylinder moves from the cylinder upstream sidewall slamming cavity wall to wave crest.Moreover,this decrease is more violent for greater wave velocity.