Research On Verification And Validation Of Ship Propeller Cavitation Simulation And The Control Mechanism Of Hull Pressure Fluctuation

With the rapid increase of the deadweight and speed of modern ships,the problems of harmful propeller cavitation and hull pressure fluctuation often appear,which not only affect the operation safety of civil ships and the health of crews,but also limit the improvement of the speed and acoustic performance of naval ships.In order to further understand this problem and find useful solutions,it is necessary to study the phenomenon and mechanism of propeller cavitation induced hull pressure fluctuation,and to explore how to effectively reduce propeller cavitation and hull pressure fluctuation.In this paper,based on the recorded experimental data,the propeller cavitation and hull pressure fluctuation are numerically investigated with DDES method.Verification and validation(V&V)of ship propeller cavitation are carried out,and the influence of stern wake on propeller cavitation,the interaction between propeller cavitation and hull pressure fluctuation and control mechanism of vortex generator(VG)on propeller cavitation and hull pressure fluctuation are also studied.The mainly discussed and original contents of the manuscript are:(1)A method,suitable for cavitation simulation research,to carry out DDES V&V is proposed.·○1 DDES five-equation and three-equation methods for DDES V&V calculation in cavitation simulation are proposed by putting forward a dynamic error adjustment factor,so it can achieve quantitative calculation of DDES model error,numerical error and their uncertainties.On this basis,further combined with several evaluation indexes,such as the applicability in cavitation simulation research,Reliability value and Pearson correlation coefficient,this new proposed method to do DDES V&V is proved to be highly reliable.○2 By constructing 15 sets of structured grids,RANS,LES and DDES V&V studies of Clark-Y hydrofoil cavitation are carried out.The numerical results show that the DDES V&V method has a good adaptability in studying the cavitating flow,and it can reflect the influence of cavitating flow on the numerical accuracy.The existence of the unsteady cavitation will increase DDES error and uncertainty,and reasonable improved grid number and density distribution can improve simulation accuracy.○3 By combining the above DDES V&V method into the cavitation simulations of CP propeller and ship propeller cavitation,this paper has further studed the implementation strategy to improve the numerical accuracy of ship propeller cavitation simulation.From the Clark-Y hydrofoil to a separate CP propeller and the ship propeller,optimizations of reasonable distribution of structured grids are carried out,so a good simulation of ship propeller cavitation has been achieved with low DDES error and uncertainty considering the limited computing resources and ability of model.So it significantly improves the numerical accuracy of ship propeller cavitation in this paper.These studies can provide a powerful research basis for the in-depth analysis of the ship propeller cavitation flow and the high-precision simulation of other similar complex cavitation flows.(2)Based on the DDES simulation results of shio propeller cavitation,the influence mechanism of stern wake on the evolution of unsteady propeller cavitation is revealed,and the effects of propeller cavitation on the space-time distribution and frequency distribution characteristics of propeller pressure fluctuation are also revealed.○1 By analyzing the evolution of the unsteady cavitation of propeller and using the vorticity transport equation under cylindrical coordinate system,we find that periodic evolution of propeller cavitation can lead to complex re-entrant jet and blade tip vortex structures.The strong cavitation vortex structure and the shear layer distribution can continuously produce and change the local vorticity,which strengthen the interaction between different flow state of fluid,instability and three-dimensional features of propeller cavitation flow.○2 When the propeller runs to the high wake area,the wake degree increases obviously and the linear velocity of the upper middle part of the blade is higher,which results in the sharp increase of the attack angle and induces the generation of large-area cavitation.The distribution of the wake changes periodically due to the rotation of the propeller,so the propeller cavitation will undergo a periodic evolution process from inception to collapse.Further qualitative wake fraction and quantitative wake morphology analysis show that the wake distribution and its change will significantly affect the evolution of the propeller cavitation.The lower the wake degree is,the better the wake morphological characteristics is,and the weaker the cavitation degree and the gentler the propeller cavitation evolution are.The smaller the base width of wake and the larger the average wake slope are,and the faster the wake changes,which results in worser propeller cavitation,and the increase of cavitation variation and collapse intensity.○3The analysis of the spatial and temporal distributions of hull pressure shows that propeller cavitation can increase the pulsation range of hull instantaneous pressure.The initial and collapse process of sheet cavitation causes the main increase of hull pressure,and the dynamic change process of blade tip vortex cavitation has a few increase effect on hull pressure.The effect of cavitation on the spatial and temporal distribution of hull pressure is analyzed quantificationally by using Manhattan distance and similarity.Compared with the results of no cavitation,we find that the propeller cavitation can lead to a significant increase in Manhattan distance,decrease in similarity and increase in difference of time and space distribution of hull pressure.The influence degree and area on the hull pressure distribution has been a sharp increase due to propeller cavitation,and cavitation inception and collapse are the main factors causing the increase of the change degree of hull pressure distribution.○4 By comparing with the results of no cavitation,it is found that propeller cavitation will significantly increase each order blade frequency components of the hull pressure fluctuation,and the phase angle of each order blade frequency components shows the phenomenon of aggregation,resulting in the energy superposition and accumulation of the hull pressure fluctuation in the frequency domain.Further combining with wavelet analysis,different grids analysis and harmonic analysis methods,it is proved that the propeller cavitation inception,developing and the early stage of collaps have a great influence on the lower order blade frequency components of hull pressure fluctuation,and the late stage of propeller cavitation collaps and dynamic change process of the blade tip vortex cavitation have a larger influence on the higher order blade frequency components of hull pressure fluctuation.(3)A new type of ship VG,which can improve the stern wake distribution,is proposed in this paper,and its suppression mechanism on the the propeller cavitation and the hull pressure fluctuation is revealed.○1 The influences of key parameters of ship VG(height h,length l and angle of attack α)on the wake are studied.It is found that appropriate parameters of ship VG can increase the width of wake and reduce the average wake slope,so that the wake distribution can be significantly improved.With the increase of VG height,the improvement of wake increases firstly and then decreases,but in general,the improvement is more obvious when VG height is larger.The improvement of wake is significantly enhanced with the increase of VG length and angle of attack.However,when VG angle of attack is smaller,VG did not play a role in improving the wake distritbuion.○2 By comparing with no VG,this paper studies the suppression effect of VG on the propeller cavitation and hull pressure fluctuation.The results show that VG weakens the propeller cavitation and reduce the cavitation volume pulsation,which lead to the largely decrease of each order blade frequency components of hull pressure fluctuation.The results show that the first-order blade frequency component of hull pressure fluctuation is dereased by24.83%.○3 By conducting the investigation of VG control mechanism on propeller cavitation and hull pressure fluctuation,it is found that the streamwise vortex structure is induced by VG,which also leads to the upward movement in the boundary layer and downward movement in the upper high-speed fluid.It also forms positive and negative variations of vorticity distribution area,which strengthens the interaction between the high and low speed flows and increases the flow velocity in boundary layer.In addition,there is a general law for the development of the streamwise vortex,namely the longitudinal offset and the wall direction offset of the streamwise vortex core both show a linear relationship with the streamwise offset.In total,VG can improve the wake distribution by inducing streamwise vortex structure to suppress propeller cavitation and hull pressure fluctuation.