Study On The Boundary Element-vortex Particle Couple Method For The Simulation Of Marine Propeller Flow

To improve the accuracy and simplicity of marine propeller analysis,a boundary elementvortex particle couple method for the analysis of marine propeller's performance and surrounding flow field is developed.The main conclusions and innovations are as follows:(1)For BEM(Boundary Element Method),a fast realization method for the pressure Kutta condition and a wake alignment method with adaptive variable velocity smoothing parameter are developed.Firstly,an analytical method to evaluate the Jacobian matrix for the pressure Kutta condition is built,which costs only around 1e-5 the time of the conventional method.Based on the analytical Jacobian matrix evaluation method,the fast realization method for the pressure Kutta condition is developed,and has not only reduced the computational time of BEM obviously,but also improved the convergence ability of the pressure Kutta condition.Besides,a model in which the velocity smoothing parameter is variable and correlated to the wake panel's age is developed to predict the wake surface's position more accurate.A formula to determine the range of the velocity smoothing parameter according to the propeller's attached circulation is also formulated.Finally,by comparing the open-water performances predicted by obtained BEM code with experimental data,the obtained BEM code is verified.(2)The two SGS(sub-grid scale)diffusion models in VPM(Vortex Particle Method)are studied,which provides a reference for the choice between them.The two models,i.e.Smagorinsky-Lilly SGS model and Cottet artificial vorticity model,are used in the VPM simulation of different vortex ring reconnection procedures.The variation of the vortex structure,maximum particle strength and three conserved quantities(the vorticity,impulse and angular impulse)during the simulations are analyzed.The results indicate that the Cottet artificial vorticity model costs more computational time,however performs better in the aspect of accuracy and stability.(3)A method to convert one low-order dipole boundary element into multi vortex particles is developed,and with the results obtained above for BEM and VPM,the boundary element-vortex particle couple method is built.Firstly,the 2D interpolation method is used to obtain continuous dipole distribution on the panels.With the continuous dipole distribution,the method to convert one boundary element to multi vortex particles is built,which makes the particle density free from the boundary element's size.Then,the coupling algorithm is described in detail,and the vortex particle redistribution procedure is modified for the couple method.Finally,with numerical simulations,the couple method is verified and the parameter sensibilities are analyzed.The conclusions state: all parameters have little influence on the obtained hydrodynamic performances(the differences are within 1%);The particle core size has an obvious influence on the vorticity strength,and a smaller core size leads to a larger vorticity strength.With analyses,it is advised the core size should be determined by the boundary layer thickness near the trailing edge.(4)The rotational periodic boundaries are utilized in the boundary element-vortex particle couple method for the computation of marine propeller's hydrodynamic performances and flow field,so that the computational efficiency is enhanced.To consider periodic boundary conditions,the way of vortex particles movement,FMM(Fast Multipole Method)and vortex particle redistribution procedure are modified and improved.Numerical simulations of different marine propellers are carried out and the results indicate: The obtained open-water performances and flow field details are all more accurate than the results obtained with BEM,and they correlate also very well with experimental results.