Study On Load Distribution And Bearing Force Characteristics Of A Four-Screw Ship

The ship speed,tonnage and main engine power increase with the demands of large-scale and high-speed ships.A growing number of ships are equipped with the four-propeller propulsion system.The load distribution of the inside propellers and the outside propellers is one of the core problems in the study of the propulsion performance of four-propeller ships.On the one hand,the particularity of the four-propeller propulsion ship will lead to a significant load difference between the inside and outside propellers.This will not only affect the matching of the ship,engine and propeller but also is unconducive to the service life of the main engine and the efficient propulsion performance and economic benefits.On the other hand,the increase of the real ship dimensions leads to the larger scale ratio in the ship model test.This will lead to the bigger difference between the model scale and the real ship propulsion performance,which means the scale effect has a more significant impact.In this paper,the unbalanced load distribution and scale effect of the four-propeller ship are studied systematically through the research method of "theoretical analysis + model test + numerical simulation".The flow field test and self-propulsion test research of the four-propeller ship are carried out.The difference of wakefield between the inside and outside disks are analyzed,and the scale effect of the wakefield is investigated.Based on the interaction mechanism analysis between the blade and the flow field,the correlation between the flow field characteristics and the propeller load is established.Then the practical and feasible load balancing control measures for the inside and outside propeller are proposed.The main contents of this paper are as follows:PIV flow field test and self-propulsion test of the four-propeller ship with full appendages are carried out.The wakefield and propulsion performance of ship model are predicted.The numerical uncertainty level of each scale model flow field is controlled within15%,the maximum resistance error is 7%,and the propeller load error is controlled within5%.This section provides reliable numerical simulation data for the analysis of flow field characteristics,load distribution and scale effect of the four-propeller ship.The characteristics of the wake flow field of a four-propeller ship are studied.The difference of the wakefield between the inside and outside propeller is due to the difference of space position and the influence of viscous flow around the hull.Because the outside propeller is relatively forward,there will be high-speed upstream flow in the outer radius,which makes the axial velocity of the outside propeller larger than that of the inside propeller.Moreover,the flow of the outside propeller is the mixture of the boundary layer flow and turbulent wake flow of the hull,which makes the wakefield of the outside propeller more inhomogeneous;The scale effect of the flow field of the four-propeller ship is investigated.With the increase of Reynolds number,the velocity contour gradually shrinks to the bracket arm.Besides,the scale effect has a greater influence on the wakefield of the outside propeller.Based on the multi-scale numerical prediction results of the wakefield,a correction method of wake fraction suitable for four-propeller ship is proposed.When the wake field of ship with appendage is predicted or converted to real ship,the scale effect of appendage should be considered in particular.In addition,the basis for selecting the scale ratio of the large-scale ship model is proposed,which is to ensure that the appendage reaches the critical Reynolds number as far as possible.The bearing force and its scale effect of a four-propeller ship are studied.The differences of the mean and fluctuating values between the inside and outside propeller loads,as well as the differences between the model scale and the real ship bearing force,are analyzed.The load difference between inside and outside propeller increases from 4.2% of the model-scale ship to 10.63% of full-scale one.It is pointed out that for the four-propeller ship,the axial load fluctuation value of the outside propeller is greater than that of the inside one.When it comes to the lateral load fluctuation value,the opposite conclusion is drawn.In terms of scale effect,the axial fluctuating load of the full-scale ship is smaller than that of the model one,while the lateral fluctuating load of the full-scale ship is larger than the model one,which is related to the scale effect of the harmonic components of the wakefield.Based on the CFD flow field and self-propelled calculation results,considering the different working environment of the inside and outside propellers,in order to reduce the load difference between the inside and outside propellers of a four-propeller ship,the load balance control measures for the differential design of propeller adaptive flow and appendage optimization are proposed,and the feasibility of the two methods is verified by an example.