A Study On Stall Characteristics And Improvement Of Two-Element Wingsails For Sail-Assisted Ships

As a unpowered propulsion device,the two-element wingsail has the symmetrical blade and can obtain auxiliary thrust through wind from both sides of the ship.By adjusting the flap deflection angle,it can generate a small heeling moment with a large range of wind.Therefore,its auxiliary propulsion performance is much better than the traditional sails.However,the huge difference in Reynolds number make the difference in aerodynamic performance between the two-element wingsail and the wing.Otherwise,the existence of the atmospheric boundary layer make the difference in stall characteristic between the two-element wingsail and the wind turbine.Therefore,it is an important aspect of improving the load distribution of wingsails and ensuring the safety of wingsail by revealing the stall characteristics of the two-element wingsails under gradient wind conditions,exploring methods to improve the stall performance.And it has certain theoretical significance and application prospects.The thesis takes the two-element wingsail as the research object,with the goal of improving whose stall performance under gradient wind conditions.Through a combination of theoretical analysis,experiments,and numerical simulation,a systematic study was conducted on the aerodynamic characteristics of the two-dimensional and three-dimensional models of the two-element wingsail.The main work was completed as follows.（1）Study on the influence of flap structural parameters on the thrust performance of a two-element wingsail airfoilNumerical simulation of the aerodynamic performance of a two-element wingsail airfoil using a Transition SST turbulence model with transition correction.The numerical calculation domain and grid model is determined,and the independence of the grid is verified.The reliability of the numerical calculation method was verified through experimental data of NACA0018 airfoil.The influence of parameters such as flap rotation axis position,relative width of gaps,and flap chord length on the thrust performance of a two-element wingsail airfoil are studied.And the two-element wingsail model with the position of the flap rotation axis as90%c₁,the relative width of the gap as 2.4%c₁,and the chord length ratio of the main wing to the flap as 3:2 meets the aerodynamic performance requirements of wingsail-assisted vessels.（2）Study on the influence of different layout schemes on the aerodynamic interference performance of the two-element wingsail airfoilBased on the relative wind angle,various arrangements of the two-element wingsail were set up and the interstage interference performance are analized.A new optimization scheme for the angle of attack and flap deflection angle of the two-element wingsail is proposed to address the stall problem caused by multi-sail interference,and the interstage interference characteristics of the two-element wingsail are obtained.In a single row arrangement,the minimum transverse spacing was 1.5c when the relative wind angles were 30°,90°,and 120°.However,interstage interference at relative wind angles of 90°and 120°can cause the wingsail to stall.After optimizing the angle of attack and flap deflection angle,the auxiliary thrust coefficient can be increased by more than 5.2%,and the flow separation of the interstage wingsail disappears.In the double row arrangement,the downstream wingsail maybe stall interfered by the upstream wingsail wake.A two-element wingsail arrangement scheme with good propulsion performance was obtained through the lateral misalignment arrangement of downstream and upstream wingsails,as well as the optimization of attack angle and flap deflection angle.（3）Analysis of stall characteristics of two-element wingsail airfoilThe stall influence mechanism and performance of the two-element wingsail airfoil were studied by factors such as angle of attack,flap deflection,and Reynolds number,respectively.The lift/drag characteristics,pressure load distribution,flow separation bubble of the main wing and turbulence characteristics of airfoil are analyzed to reveal the transition process of boundary layer during the stall process.The numerical simulation results show that the lift coefficient of the main wing before stall is significantly increased.The main wing stall belongs to the leading-edge stall,and the lift coefficient decreases significantly in the early stages of stall.Flap stall belongs to thin wing stall,with relatively mild stall characteristics of lift.However,in the near stall angle area,the periodic increase in lift coefficient causes a fluctuating increase in thrust assist coefficient as the deflection angle of the flaps increases,causing wing load fluctuations.It provides a basis for improving the stall performance of the two-element wingsail.（4）Analysis of stall characteristics of two-element wingsails under gradient wind conditionsThe transition SST turbulence model is used to numerical simulation of the two-element wingsail model under gradient wind conditions.And the discretization error evaluation of the grid is carried out.The experimental results of AC72 wingsail are used to check the numerical calculation method.By analyzing gradient wind parameters such as roughness factor,power exponent function,and wind speed,the stall characteristics of two-element wingsails under gradient wind conditions was obtained.The stall process of the two-element wingsail was analyzed when the flap deflection angle was rotated in the near stall angle region under logarithmic gradient wind conditions.And the stall mechanism of two-element wingsails under gradient wind conditions was explored.The simulation results show that gradient wind can suppress the flow separation on the surface of the wingsail during the initial stage of stall,playing a role in delaying stall.However,the lift coefficient also decreases quickly after stall.At the same time,it is necessary to strictly control the average wind speed range of the wingsail to ensure the reliability of its wind load.（5）Study on the stall characteristics of a two-element wingsail with a modified main wingDrawing inspiration from the protruding structure of the leading-edge of the flippers of humpback whales,a leading-edge tubercle model of the wing is constructed by fitting the curve with a cubic B-spline.The effects of geometric parameters such as amplitude,wavelength,and Reynolds number on the aerodynamic performance of NACA0018 were investigated by numerical simulation,and the leading-edge protrusion configuration of the wing is determined.The effects of different protrusion structures on the stall performance of the two-element wingsail under logarithmic gradient wind conditions was studied,by establishing a two-element wingsail model with five types leading-edge protrusions of main wing.An improved scheme for non-uniform leading-edge tubercle modification of the main wing is proposed,while the lift/drag characteristics,pressure load,and flow field in the near stall angle region were analyzed.The simulation results indicate that the lift reduction at the initial stage of stall is only8.8%and lift coefficient is increased because of the increase in forward vortex intensity caused by larger protrusions and the attached flow on the side of small protrusions.Finally,experimental verification was conducted on the pressure load of the scheme at a flap deflection angle of 15°.It shows that the experimental results of the pressure coefficient distributions are basically consistent with the numerical simulation results,proving the reliability of the scheme.It provides a new approach for the design of wingsails for sail-assisted navigation vessels.