Study On Characteristics Of Ventilated Cavitating Flows Around An Axisymmetric Body

It is well known that ventilated cavitating flow around an axisymmetric body is acomplex, multi-scale, multiphase phenomenon. The mechanism of the transientventilated cavitating flow is a core scientific issue, and related to the key technicalproblems of national security, such as aerospace and underwater weapons. Theventilated cavitating flow is investigated via combined physical and numericalanalysis.The experimental and numerical methods which are suitable for theinvestigation of ventilated cavitating flow are developed. Firstly an experimentalfacility for the research on ventilated cavitating flow with a free surface is designedand constructed, and the experiments are carried out in this facility and the watertunnel. A high-speed video camera is used to visualize the ventilated cavitatingphenomenon, and a particle image velocimetry (PIV) technique is used to process thevelocity and vorticity fields. Particularly, the complicated flow field inside theventilated cavity and the turbulent wake in the ventilated cavitating region are alsoinvestigated. In the numerical simulation, the Level Set multiphase model andFilter-based model(FBM), together with layering dynamic mesh method are appliedto predicate the unsteady ventilated cavitating flows.The flow pattern maps of the ventilated gas-liquid flow are proposed. Theexperimental results show that flow patterns of the ventilated cavitation varyconsiderably with various combination of the Euler number and ventilation rate.Three primary forms of ventilated cavitation can be described as: transparent cavityflow, water-gas mixture flow and translucent cavity flow. In the transparent cavityflow regime, the ventilated cavity is steady and it should be noted that the cavityfloats upward due to the gravity effect in the water tunnel. In the water-gas mixtureflow regime the mixture of gas and water could be observed in the ventilated cavityarea. In the translucent cavity flow regime, the transparent gas area appears near theforeside of the ventilated cavity, although water-gas mixtures are full of the rear part.The physical mechanism in different regions of the ventilated cavity isdemonstrated. In the fore of ventilated cavity, the entrained gas transports along thegas-water interface to the rear part of cavity. Some gas sheds and collapses from the domain of cavity, while some gas gets into the domain of cavity with the re-entrantflow. Compared to the water-gas mixture flow, in the translucent cavity flow regime,the range of low velocity area in the wake obviously expands，and velocities ofmeasuring point in various profile decrease. The upper and lower velocimetry ropesstretch backward and enlarge by degrees as the ventilated cavity region extendsrearward, becoming many dispersed velocimetry groups that gradually dissipatedownstream.The unsteady flow characteristics in ventilated cavitating wake are analyzed.The development of the re-entrant flow is of critical importance in the unsteadycharacteristics of ventilated cavitating flow. Due to the relatively high pressure in therear part of cavity and the low pressure in the attached cavity region, the adversepressure gradient emerges and the flow separation appears. The re-entrant flow withhigh water fraction is responsible for the cloud cavitation shedding, which has asignificant impact on the vorticiy field. In the water-gas mixture flow regime, there-entrant flow could reach the leading edge of the cavity and the mixture of waterand gas is intensfied. In the translucent cavity flow regime, it is difficult for re-entrantflow to reach the leading edge of cavity, resulting in the transparent gas region in thefore part, and the rear part is filled with cloud bubbles.The interaction between the free surface and ventilated cavity isdemonstrated. Both the experimental and numerical results show that when anaxisymmetric body passes through the free surface, the free surface is raised and aparabolic interface between water and gas is formed. The water layer between thecavity boundary and the free surface is elongated and thinned gradually, and the foreregion of the cavity is extruded continuously while the rear region was elongated.Finally the cavity collapses when it passing through the free surface.