Study On The Mechanism Of Ventilation Cavitation Drag Reduction And Hydrophobic Surface Drag Reduction Of Underwater Vehicle

The work done by underwater vehicle to overcome the navigation resistance during navigation accounts for almost 80%of the total work,which leads to a large amount of energy consumption,and also reduces the speed and range of the underwater vehicle.In order to improve the working efficiency of the underwater vehicle,it is necessary to reduce its navigation resistance.Both ventilated cavitation and hydrophobic surface drag reduction methods have been shown to have significant drag reduction potential,and their applications are becoming more sophisticated as computational fluid dynamics(CFD)methods have been investigated in both of these drag reduction methods.However,related studies have shown that the drag reduction effects of both ventilated cavitation drag reduction and hydrophobic surface drag reduction methods are subject to complex changes under various influencing factors.Therefore,the formation mechanism of ventilated cavitation and the drag reduction mechanism of hydrophobic surface need to be further investigated to make them better applied to underwater underwater vehicle.In this paper,the filter-based model(FBM model)and partially averaged Navier-Stokes model(PANS model)for turbulence field calculation and the slip boundary condition for flow around a hydrophobic geometry were embedded in ANSYS CFX software,and the prediction accuracy of the FBM model and the PANS model were compared.Then,the complex variation and the formation mechanism of the ventilation cavitation were investigated on the basis of the PANS model;the flow characteristics and drag reduction mechanism of the hydrophobic surface were investigated on the basis of the PANS model and the slip boundary condition.which provided some references for the application of the ventilation cavitation drag reduction method and the hydrophobic surface drag reduction method on underwater vehicle.Firstly,the natural cavitation flow,ventilated cavitation flow and the flow around a square were simulated using the k-ε model,FBM model and PANS model,respectively.The effects of the filter scale of the FBM model and the resolution of the PANS model on the calculation results were investigated,and the analytical methods of the optimal filter scale and resolution were given.On this basis,the prediction results of different turbulence models were compared with the experimental results,and the effects of these three turbulence models in single-phase and multiphase flows were evaluated comprehensively from several aspects,and the reasons for the different prediction performances exhibited by different turbulence models were explained.The results show that the hybrid turbulence model performs significantly better than the model,and the PANS model performs slightly better than the FBM model.Then,for the gas jet cavitator whose cavitation morphology changes continuously with the external conditions,the formation mechanism of the ventilating cavitation was investigated by using the PANS model.The formation mechanism of the ventilating cavitation was further elaborated by studying the effects of the gas ventilation coefficient,vent hole diameter and Froude number on the flow characteristics of the ventilating cavitation pattern,re-entrant jet and vortex structure.The results show that the k-ε model cannot predict the ventilating cavitation flow,and the PANS model predicts significantly better than the FBM model.Ventilation cavitation morphology is directly related to vortex structure type,and vortex structure type is determined by the re-entrant jet flow caused by the inverse pressure gradient.Finally,flow characteristics and drag reduction mechanism of hydrophobic surfaces were studied using the PANS model(or laminar model)and slip boundary condition.The effect of Reynolds number,slip length and hydrophobic position on shear stress,recirculation characteristics and vortex structure of hydrophobic sphere was studied to explain the drag reduction mechanism of hydrophobic surface in single-phase flow.The application of hydrophobic surface on underwater vehicle and its drag reduction mechanism in two-phase flow were described through the flow characteristics of hydrophobic slender body under different cavitation states,such as cavitation shape,pressure distribution and re-entrant jet.The results show that the slip velocity generated by the hydrophobic surface decreases the velocity gradient near the wall.which reduces the wall shear stress and the pressure difference between the front and back of the sphere,weakening the strength of the vortex structure near the wall.and ultimately leading to a reduction in drag.When a cavity is formed on the surface of the slender body,the application of the hydrophobic surface increases the length of the cavity,decreases the turbulent kinetic energy of the flow field,and leads to the weakening or disappearance of the vortex structure.