Study On The Rapidity Of High Speed Ship Fitted With Stern Flap And Its Energy Saving Mechanism

Rapidity is one of the most important properties of a ship.For military warships,the rapidity of warships is an important index to evaluate their combat capability.Under the condition of fixed ship form,the installation of stern flap,stern wedge and interceptor is a simple way to reduce ship resistance and improve ship combat speed.The research on stern appendages such as stern flap is mostly based on ship resistance test.Because the flow field around the ship can not be obtained completely,there is a lack of unified and systematic elaboration on the drag reduction and energy saving mechanism of stern flap,and there is no clear understanding of the propeller induced excitation force characteristics after the installation of stern flap.This doctoral dissertation is mainly based on overlapping grid and sliding grid technology to realize the two-degree-of-freedom motion of hull(Z-direction movement and rotation around Y-axis)and real propeller rotation,and ultimately realize the integrated coupling solution of ship-propeller-rudder-stern flap.Combining resistance test and PIV flow field test,the flow field around ship model can be captured in all directions,and the drag reduction mechanism,energy saving mechanism and the influence mechanism of propeller induced excitation force characteristics can be elaborate.The main contents are as follows:The turbulence model,discrete scheme,free surface capture method and mesh generation used in the study of ship-stern flap interaction mechanism were introduced.The numerical solution was based on the six-degree-of-freedom motion module of DFBI and overlapping grid technology,aiming at predicting the effect of stern flap on the ship attitude accurately.Based on standard k-ε,SST k-ω and DES turbulence models,the flow field around the ship model was solved,and the simulated results were compared with the experimental data of ship resistance,sailling attitude,free surface waveform and wake field to find a suitable turbulence model.By adjusting the reference size of the grid,three different sets of grids were generated in the same grid topology form,and the resistance,trim and sinkage of ships under specific conditions are solved.The irrelevance of the grid is verified and a set of computational strategies suitable for studying the interaction between ship and stern flap was obtaine.The overlapping grid technique was used to study the interaction mechanism between stwen flap and ship.The variable parameters of stern flap are studied by taking the span,chord length and down-reverse angle as variables.The influence of stern flap on the hydrodynamic performance of ship model was mainly studied from the changes of resistance,trim,sinkage,free surface waveform,wake flow field,velocity field and pressure field around stern flap.The drag reduction effect of stern flap on full scale ship was also predicted based on the 1+k method and the empirical formula of the U.S.Navy.By comparing and analyzing the results of sea trail,the characteristics of various methods for predicting stern flap perfoamance were expounded.The influence of ship on stern flap was studied from its resistance and lift.By adjusting the attitude of the ship fitted with stern flap,the role of the attitude change in the drag reduction of the stern flap was discussed.Based on the towing tank resistance test of ship model,the influence of stern flap on the resistance and attitude of ship model was analyzed,which was validated by numerical simulation.The mechanism of hydrodynamic change of ship model was further explained by analyzing the variation of stern flow field of transom stern ship.SPIV equipment was used to study the influence of stern flap on the wake field of the ship model.On the basis of verifying the accuracy of the experimental results,the velocity fields of different speed and cross-section conditions before and after the installation of the stern flap were analyzed,and the influence degree and regularity of thestern flap on the ship’s wake field were summarized.The open water performance of P4508 propeller was calculated based on sliding grid technology.The open water performance,vorticity field and pressure distribution characteristics of the propeller were analyzed and verified.On this basis,the numerical simulation of ship-propeller-rudder and ship-propeller-rudder-stern flap coupling was carried out successively.By comparing the shaft speed of self-propulsion point and the effective power of the ship,the accuracy of the numerical self propulsion calculation was verified.The effects of propeller on the resistance components,hull pressure distribution,free surface waveform and wake field of ship were analyzed and described.The variation of wake fraction,thrust reduction,hull efficiency,propulsion efficiency and power received by the propeller before and after the installation of stern flap were also analyzed.The reasons for the change of ship propulsion performance were analyzed from the changes of hull resistance component,free surface waveform,wake field and hull pressure distribution,and the energy saving mechanism of stern flap was concluded.In order to determine the mechanism of stern flap affecting the propeller induced excitation force characteristics,numerical prediction of propeller induced excitation force characteristics before and after installation of stern flap was carried out on the basis of ship-propeller-rudder-stern flap numerical self-propulsion.The influence of wave plate installation is analyzed from three aspects:the fluctuating pressure induced by propeller,the unsteady bearing force induced by propeller,and the unsteady rudder force.The contribution of two factors,wake change and speed reduction,to the propeller induced excitation force characteristics is analyzed by changing the propeller speed quantitatively.And the vibration reduction mechanism of the stern flap is summarized.