Study On The Characteristics Of Vortex Flows Of Propeller Wake

The propeller wake dynamics is a fundamental but very complicated fluid mechanics problem.Its complexity comes from its sophisticated vortex system,which keeps evolving in high-speed shear layer flow.The mechanism of propeller wake behaviors such as the evolution from stable regime to unstable regime and the flow phenomenon in a complex operating environment have always been difficult and hot topics in the field of fluid mechanics.From the perspective of engineering applications,propeller wakes are directly related to the macroscopic characteristics of marine strcutures(like merging of tip vortices and vortex breakdown induced by short-and long wave instability will cause an increase in noise),a better understanding of the dynamic characteristic of the propeller wake under multiple operating conditions helps to improve the propulsion performance related to vibration,noise,and structure problems and has important practical significance for the design and optimization of next-generation propellers with good comprehensive performance.According to the propeller wake characteristics described above,the work that needs to be further studied and excavated is proposed based on the summarization of the current research foundations.Aiming at the problems of the propeller wake instability mechanism,the characteristics of the propeller wake under off-design conditions,and the interference of the propeller wake with structures,etc.,different turbulence simulation methods and PIV experiments were employed to analyzed the wakefield of the open propeller model,providing a basis for the CFD method to study the propeller wake instability from the perspective of quantitative analysis for the first time.A modified phase average algorithm was proposed and related program was established.The algorithm can automatically eliminate the statistics of the tip vortex that fails to be measured and find the location of tip vortex cores and effectively improves the authenticity and reliability of the phase-averaged results of propeller wake in PIV test and CFD calculation(especially in PIV test),which can effectively avoid the underestimation of the downstream tip vortex intensity and the overestimation of the turbulence dissipation rate caused by the meandering and instability motions of tip vortices in traditional phase average.In this paper,the uniform and non-uniform inflow conditions of the propeller under open water,the near free surface conditions of Suboff-propeller system with different shaft depths,and the wake dynamics analysis in the propeller-rudder system are detailed based on the CFD solver combined with the PIV experiments in cavitation tunnel and recirculating water channel.The non-uniform inflow conditions include the case with turbulent perturbation in the upstream and the case where the propeller own periodic oscillating motions.The dynamic behaviors of the propeller wake under multiple operating conditions were explored from different perspectives such as the average state and instantaneous state of each physical quantity,wake topologies,vortex kinematics and vortex-vortex interaction mechnisms.The trigger mechanism of the mutual induction effect is revealed.The triggering mechanism of the instability of the propeller wake in uniform inflow conditions is studied,and various factors that trigger the propeller wake from stable state to unstable state are determined for the first time.The evolution mechanism of propeller wake under non-uniform inflow conditions,propeller-rudder system,and near free surface was explained.Based on the evolution mechanism of the tip vortex in the uniform inflow,for the first time an evolution model of the tip vortices is proposed,which quantitively describes the mutual induction of tip vortices,the interaction between trailing edge vortices and tip vortices and the triggering of instability.The proposed model can accurately reproduce the evolution process of propeller tip vortex,explain the phenomena in tip vortex instability and predict the instant and position of tip vortex merging,which is of great significance to the prediction and control of propeller flow noise and the design of propellers with excellent performance.