Study On Two-phase Flow Behavior And Resistance Feature Of Air Lubrication Drag Reduction Beneath Flat Plate

For surface ships and underwater vehicles,the frictional resistance accounts for a large proportion of the total resistance during navigation.In order to reduce fuel consumption and exhaust emissions as well as to improve the speed and range of vehicles,drag reduction technology is widely used in the design of various water surface/underwater vehicles.As one of the most promising drag reduction methods,air lubrication drag reduction,which is an active flow control method to reduce wall resistance by injecting air into the liquid phase turbulent boundary layer,has become an important topic in high-tech ship research.The discrete bubbles and the continuous cavity induced by ventilation can effectively change the fluid property parameters and modulate the turbulent structure,thereby significantly reducing the wall frictional resistance.The air lubrication drag reduction has been proved to have good drag reduction performance on many occasions,and the scaling law of drag reduction has been widely studied.However,due to the lack of understanding in the drag reduction mechanism,some of the full-scale air lubrication drag reduction tests on boats have little effect on drag reduction.Therefore,in order to achieve high drag reduction efficiency during the application of air lubrication,this paper uses water tunnel experiments and numerical simulation method to study the two-phase flow as well as its drag reduction mechanism of air lubrication drag reduction.The main contributions of this thesis include:(1)Experimental methods of bubble drag reduction and ventilation partial cavity drag reduction have been established.Technologies for the generation of ventilation bubble/cavity,resistance measurement of the flat plate,and two-phase flow field measurement have been established.Three sets of experiment devices for air lubrication drag reduction over flat plates are built in water tunnel systems.In addition,for the purpose of revealing local flow structure and drag reduction mechanism of bubble drag reduction,a CFD-PBM coupled numerical simulation method is established.(2)The bubble flow characteristics,resistance features and the influence of flow parameters of ventilated bubble drag reduction on a flat plate are analyzed based on water tunnel experiment and CFD numerical simulation.Four bubble drag reduction regions with different resistance feature are proposed based on the distribution of local resistance and the characteristics of local bubbly flow.Moreover,the mechanism of bubble drag reduction based on the theory of wall shear stress is discussed.(3)Flow pattern visualization and internal flow PIV measurement are carried out in water tunnel experiments in order to study the two-phase flow pattern and flow field structure of ventilated cavity.After the classification and definition of four typical flow patterns of ventilated cavity,a flow pattern map is established map within the experimental ranges.Furthermore,the internal flow structure,closure mode,and air leakage mechanism are revealed.The demarcating curves for neighboring cavity regimes are captured.(4)Water tunnel experiments of ventilated partial cavity drag reduction across a wide range of parameters are carried out to study the resistance feature and its influencing factors.After measuring the resistance of the flat plate with ventilated cavity as well as the cavity length,the influence of flow parameters on the drag reduction of ventilated cavity is discussed.Also,the effects of structural changes such as the side baffle structure,the height of the backward facing steps and the angle of attack of the plate on drag reduction are discussed.This paper reveals the flow structure and the resistance feature of air lubrication drag reduction based on the experimental and numerical investigation.The distribution of resistance,as well as its influencing factors of bubble drag reduction on a flat plate,are obtained.The flow regimes,the regime map and the criterions for regime transitions of ventilated partial cavity are studied.These preliminarily results regarding the local two-phase flow and drag reduction mechanism of the air lubrication drag reduction can provide support for the optimal design of air lubrication drag reduction ship or underwater vehicle.