Investigation On Ventilated Partial Cavity Two-phase Flow And Its Effect On Drag Reduction And Stability Enhancement

Energy conservation,speed increase,and stability enhancement of ship equipment are important research topics in the field of ship transportation.The technology of drag reduction and stability enhancement by means of ventilation flow control on ship bottom is one of the important ways to improve the index of performance and energy consumption for the water surface ships.Among them,the ventilated partial cavity(VPC)with a certain scale formed by active ventilation can significantly affect the flow characteristics of the near-wall boundary layer fluid,which can effectively reduce the frictional drag and improve the overall longitudinal dynamic stability of ship transport equipment,and has important application value in the field of high-performance ships.This thesis focuses on the demand of performance improvement and stability enhancement for high-speed ships by means of VPC flow control.Based on the numerical simulation of two-phase flow,the plate ventilation experiment in the water tunnel,and the ship model ventilation experiment in the still-water towing tank,the issues of VPC flow,drag reduction,and ship dynamic stability are explored in order to understand the flow mechanism of the VPC two-phase flow and provide theoretical support for engineering applications.The main work of this thesis includes the following aspects:1.For the water tunnel experiment and static water towing experiment,a flat ventilation plate platform and ventilation model ship experimental apparatus were built,which included the key functions such as drag measurement and two-phase flow visualization system.Based on experimental conditions,the two-phase flow model and the numerical simulation method for ventilation plate and towed ship model were established.2.To understand the boundary layer characteristics and drag reduction mechanism of the ventilated cavity flows,it is found that the VPC formed in the incoming flow under medium to high Froude number range can be divided into three regions with obvious flow differences,namely continuous cavity,transition cavity,and mixed flow cavity regions.There are significant differences in flow state,density,viscosity,and wall shear stress among the three regions in flow direction.On this basis,the quantitative relationship between drag reduction and the eigenvalue of the VPC boundary layer state is established,and the semi-empirical model can predict the wall shear stress of flow direction in the continuous cavity and the mixed cavity regions well.3.To investigate the issue of ventilated cavity flows’ fluid dynamics,three kinds of cavities with stable geometry in the water tunnel experiment and seven kinds of cavities with different flow modes found in the towing tank experiment are sorted out;the formation and evolution characteristics,morphology(topology)distribution,and transformation mechanism of the ventilated cavity,as well as the closure and shedding law of the cavity are systematically analyzed.It is found that the momentum of gas jet and the pressure difference between gas and liquid phase are dominant in influencing the shape of cavity.4.In regard to the issue of drag reduction and stability enhancement of VPC flows,the net saving power estimation equation including additional energy consumption is established,and the relationship curve between net saved power and Froude number is obtained;the maximum drag reduction rate range of 10-30%’s net drag reduction effect for the model ship is obtained;The current study found and confirmed the inhibition effect on porpoiseing instability of high-speed planning vessel by VPC,and also reveals the mechanism of the cavity stabilization effect under medium/high Froude number range.It is found that the decrease of the trim value leads to the increase of hull’s displacement volume,which directly leads to the disappearance of the porpoising instability.Based on the existing theoretical and applied research findings of ventilation drag reduction,this thesis studies the fundamental flow mechanism of VPC flow,and finds the transformation mechanism of the VPC flow patterns,the regionalizational flow characteristics of the VPC boundary layer,and the inhibition effect on porpoiseing instability by the VPC.The current work clarifies the mechanisms of drag reduction and stability enhancement from VPC.The current results can provide support for the design of drag reduction,speed increase,and stability enhancement for high performance surface transport vechiles.