A method for designing a ducted propeller

Abstract

The application discloses a design method of a ducted propeller, comprising the following steps: S1, clearly defining design conditions and initial design parameters; the design conditions comprise tension T generated by the duct and the propeller, and working speed n of the propeller; the initial design parameters comprise propeller diameter D, propeller-duct clearance δ and airfoil; S2, designing the propeller diameter D; S3, designing the propeller-duct clearance δ; S4, designing the airfoil; the prerequisite of parameterization of the propeller blade is to determine an airfoil family used by the propeller blade and a radial position, and to keep constant the chord lengths of a wing root, a middle part of the wing and a wing tip, and the duct; under the prerequisite, the airfoils of the wing root, the middle part of the wing and the wing tip section are determined through a CST type function; except the wing root and the wing tip, the negative torsion angles of 3-7 sections of the radial position are simultaneously taken as optimization variables, and a parameterization model of the propeller blade is generated. The method has the advantages that the iterative optimization design method is based on numerical simulation, can accurately evaluate and improve the aerodynamic efficiency of the ducted propeller.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ducted aircraft, in particular to a design method of ducted propeller. BACKGROUND

[0002] In recent years, civil unmanned aerial vehicles are a new industry in the field of aviation industry, and the use scenarios in the consumer field of personal entertainment application and the professional field of electric power, security, agriculture, forest fire prevention, police, etc. are increasing year by year. At present, civil unmanned aerial vehicles mainly adopt electric multi-rotor configuration, and a small amount of civil unmanned aerial vehicles are fixed-wing and helicopter configuration. However, there are still some bottleneck problems such as short flight time, low aerodynamic efficiency, poor safety and many environmental restrictions. The ducted aircraft places the propeller inside the duct body of the annular wing structure, which can solve the problem of too high propeller tip Mach number during forward flight at high speed, thereby having the advantages of high aerodynamic efficiency, compact structure layout, high safety and good environmental adaptability. However, so far, the application of ducted aircraft is not sufficient, and there is still a lack of complete optimization design method for propeller design. In the actual use process, how to quickly design a ducted propeller that meets the optimal aerodynamic efficiency or maximum lift coefficient is still a problem to be solved. SUMMARY

[0003] The technical problem solved by the present application is to iteratively optimize the design of the ducted propeller system based on the existing ducted propeller, improve the aerodynamic efficiency of the system, and better match the ducted propeller system with the power system of the ducted aircraft.

[0004] To solve the above technical problems, the technical scheme adopted by the present application is as follows:

[0005] The design method of the ducted propeller of the present application comprises

[0006] S1, the design conditions and initial design parameters are determined; the design conditions include the tension T generated by the duct and the propeller, and the working speed n of the propeller; the initial design parameters include the propeller diameter D, the propeller-duct gap δ and the airfoil;

[0007] S2, the propeller diameter D is designed; a plurality of gradually increasing propeller diameters are selected, and the propeller-duct gap is kept unchanged; the aerodynamic performance of the propeller disc at the speed n of different propeller diameters is obtained through CFD numerical simulation; and the most suitable propeller diameter D for the engine is selected by comparing with the engine output characteristic curve;

[0008] S3, the propeller-duct gap δ is designed; the aerodynamic performance of the ducted propeller with different propeller-duct gaps at the speed n is numerically simulated to determine the optimal value of the propeller-duct gap;

[0009] S4, design airfoil; the premise of the propeller blade parameterization is to determine the airfoil family used by the blade and the radial position, and keep the chord length of the wing root, middle and tip unchanged; under this premise, the wing root, middle and tip cross-section airfoils are determined by the CST type function; in addition to the wing root and tip, the negative torsion angle of 3-7 cross-sections in the radial position is taken as the optimization variable to generate the parameterized model of the blade.

[0010] Further, in S4, the negative torsion angle of the wing root, tip and five radial position cross-sections evenly selected in the middle of the wing is taken as the optimization variable, and the parameterized model of the blade is generated by the joint script file of Matlab and ICEM.

[0011] Further, in S4, the CST type function method is suitable for parameterization and control of airfoils, and each airfoil has 7 type function weights on the upper and lower surfaces; the type function weight of the shape function based on the type function family can be obtained by calculation, and the airfoil of the control cross-section can be changed by perturbing the weight:

[0012] ;

[0013] The change of the control cross-section airfoil is reflected in the change of the type function weight:

[0014] ;

[0015] Where δi is the change of the weight of each type function.

[0016] The CST type function method represents the airfoil by superimposing Bernstein polynomials, and any m-order Bernstein polynomial is composed of the following m terms:

[0017] .

[0018] Further, the gradient method is used to realize the iterative optimization of the design parameters of the ducted propeller, which can be realized by the function in the Matlab optimization tool package, and the corresponding optimization program is written in Matlab, which calls the ICEM script parameterization to generate the grid, calls the Fluent script to automatically solve the flow field, and outputs the file to feed back to Matlab, so as to constitute a complete optimization program to realize the rapid, real-time and high-precision three-dimensional shape optimization design.

[0019] Further, the gradient method is used to realize the iterative optimization of the design parameters of the ducted propeller, which calls the optimization function as follows:

[0020] ;

[0021] Where To optimize the initial value, x is the final optimization result, fun is the objective function, A, b, Aeq and beq are the coefficient matrixes of inequality constraint and equality constraint respectively, lb and ub define the value range of independent variable, and options defines the remaining optimization parameter settings;

[0022] The objective function of optimization is the system lift coefficient CL, and the constraint is the moment coefficient Cm; the optimization variable is a plurality of weight coefficients obtained after the geometric parameterization of the optimized shape; by applying a delta change to the optimization variable, the gradient values of the objective function CL and the constraint Cm can be obtained through difference.

[0023] Further, it further comprises S5, designing the duct shape; the duct body is regarded as a geometric body obtained by rotating a airfoil around an axis for one revolution, and the geometric parameterization problem of the duct body can be equivalent to the geometric parameterization problem of a two-dimensional airfoil; the geometric parameterization of the airfoil of the duct cross section is realized by using a non-uniform rational B-spline curve (Nurbs) combined with a feature parameter description method.

[0024] Compared with the prior art, the beneficial technical effects of the present application are as follows:

[0025] The advantage of the present application is that the iterative optimization design method is based on numerical simulation, which can improve the aerodynamic efficiency of the duct propeller through accurate evaluation; in addition, the iterative design is realized by calling the ICEM script and the Fluent script through the Matlab optimization tool package, and the model design and numerical simulation have the advantages of being fast, real-time and high-precision. BRIEF DESCRIPTION OF DRAWINGS

[0026] The present application will be further described below in combination with the description of the drawings:

[0027] Figure 1 It is a schematic diagram of the basic structure and related parameters of the duct propeller;

[0028] Figure 2 It is an image of the Bernstein polynomial when n=7;

[0029] Figure 3 It is a flow chart of the Matlab-ICEM-Fluent combined optimization algorithm;

[0030] Figure 4 It is a two-dimensional section view of the duct fan. DETAILED DESCRIPTION

[0031] The present application will be further described below in combination with the description of the drawings and the specific embodiments.

[0032] As Figure 1 shown, the present application provides a design method of a duct propeller, comprising

[0033] S1, defining design conditions and initial design parameters;

[0034] Propeller geometry parameters

[0035] (1) Diameter D

[0036] Diameter D is the most basic geometry parameter of propeller, which is defined as the diameter of the circular track formed by the tip of the propeller rotating one revolution;

[0037] (2) Section radius R

[0038] Section radius R is the distance from the section of the propeller blade to the center of the rotating shaft;

[0039] (3) Airfoil

[0040] Airfoil is the shape of the section of the propeller at any section radius. Important airfoil parameters include chord length, maximum thickness, etc.;

[0041] (4) Installation angle

[0042] The angle between the chord of the airfoil at any section radius and the plane of the propeller;

[0043] (5) Twist angle

[0044] The angle difference between the installation angle at any section radius and the installation angle at another section.

[0045] 1. Propeller motion parameters

[0046] n is the working speed of the propeller (rpm).

[0047] 2. Other parameters

[0048] T is the thrust generated by the duct and the propeller (N), M is the torque generated by the propeller rotation (N·m), P is the power of the propeller (W), and δ is the gap between the propeller and the duct (mm).

[0049] 3. Dimensionless parameters

[0050] Ducted fans belong to rotary-wing aircraft, whose lift direction is consistent with the incoming flow direction. Parameters such as thrust, torque, power, and efficiency are used to evaluate aerodynamic performance, and dimensionless forms are used to measure them respectively:

[0051] Thrust coefficient: ;

[0052] Torque coefficient: ;

[0053] Propeller power: ;

[0054] Power coefficient: ;

[0055] Efficiency:

[0056] Specifically, the design conditions include the pull force T generated by the duct and the propeller, the working speed n of the propeller, and the power system selection. On the basis of the existing ducted propeller design, the iterative optimization design of the ducted propeller is realized through the adjustment and evaluation of the initial design parameters. The given initial design parameters include the propeller diameter D, the propeller-duct clearance δ and the airfoil;

[0057] S2, the propeller diameter D is designed; wherein the propeller diameter has a great influence on the efficiency of the ducted fan. Under the condition that other parameters are consistent, the larger the propeller diameter is, the greater the pull force coefficient of the ducted fan is, and the higher the efficiency is. However, the efficiency improvement brought by the increase of the propeller diameter gradually weakens with the increase of the propeller diameter. A plurality of gradually increasing propeller diameters are selected, and the propeller-duct clearance is kept unchanged. The aerodynamic performance of the propeller disc with different propeller diameters at the speed n is obtained through CFD numerical simulation. By comparing with the engine output characteristic curve, the most suitable propeller diameter D for the engine is selected.

[0058] In one of the specific embodiments of designing the propeller diameter D, the propeller diameter is expanded by 10%, 20% and 50%, and the propeller-duct clearance is kept unchanged. The aerodynamic performance of the propeller disc with the propeller diameter of D, 1.1D, 1.2D and 1.5D at the speed n is obtained through CFD numerical simulation. By comparing with the engine output characteristic curve, the most suitable propeller diameter for the engine can be selected.

[0059] S3, the propeller-duct clearance δ is designed; the aerodynamic performance of the ducted propeller with different propeller-duct clearances at the speed n is numerically simulated to determine the optimal value of the propeller-duct clearance;

[0060] In one of the specific embodiments of designing the propeller-duct clearance δ, the aerodynamic force of the ducted fan system is closely related to the total pressure change before and after the propeller, and the propeller-duct clearance has a significant influence on the total pressure change. The propeller-duct clearance is optimized in this embodiment. On the basis of the optimized overall parameters, the aerodynamic performance of the ducted propeller with the propeller-duct clearances of 2mm, 3mm, 4mm and 5mm at the speed n is numerically simulated. Considering the propeller tip effect and the manufacturing process, the optimal value of the propeller-duct clearance is determined.

[0061] S4, the airfoil is designed; in order to quickly generate a geometric shape, the blade shape of the ducted propeller needs to be parameterized. The premise of parameterizing the propeller blade is to determine the airfoil family used by the propeller blade and the radial position, and to keep the chord length of the wing root, the middle part and the tip unchanged; under this premise, the wing root, the middle part and the tip cross section airfoil are determined by the CST type function; in addition to the wing root and the tip, the negative torsion angle at 3-7 radial positions is taken as an optimization variable to generate a parameterized model of the blade.

[0062] The leading edge radius and trailing edge angle of the airfoil are described by the CST type function with the range of 1-m (m is the order of Bernstein polynomial), the weight values of the type function corresponding to the initial airfoil are fitted, and the iteration of the airfoil of the control section is realized by perturbing the weight values. Considering that the upper and lower surfaces constitute a complete airfoil, and the optimization of the airfoil of the wing root, the middle part of the wing and the wing tip, a total of 6m optimization variables are obtained, and the airfoil optimization design is realized by the iterative optimization algorithm.

[0063] In the embodiment, the negative torsion angles at the wing root, the wing tip and the five radial position sections uniformly selected in the middle part of the wing in step S4 are taken as the optimization variables, so that the parameterized model of the blade is generated, and the parameterized model of the blade is generated by the joint script file of Matlab and ICEM.

[0064] The CST type function method is suitable for parameterization and representation of the control airfoil, and each airfoil has 7 type function weights on the upper and lower surfaces. The type function weights of the shape function based on the type function family can be obtained by calculation, and the airfoil of the control section can be changed by perturbing the weights:

[0065] ;

[0066] The change of the airfoil of the control section is reflected by the change of the type function weight:

[0067] ;

[0068] Where δi is the change of the weight of each type function.

[0069] The CST type function method represents the airfoil by superimposing Bernstein polynomials, and any m-order Bernstein polynomial is composed of the following m terms:

[0070]

[0071] The gradient method is used to realize the iterative optimization of the design parameters of the ducted propeller, the function in the optimization tool package of Matlab is used to realize the iterative optimization of the design parameters of the ducted propeller, and the corresponding optimization program is written in Matlab, the grid is generated by calling the script parameterization of ICEM, the flow field is automatically solved by calling the script of Fluent, and the output file is fed back to Matlab, so as to constitute a complete optimization program, so as to realize the fast, real-time and high-precision three-dimensional shape optimization design. The gradient method is used to realize the iterative optimization of the design parameters of the ducted propeller, and the optimization function is called as follows:

[0072] ;

[0073] Where To optimize the initial value, x is the final optimization result, fun is the objective function, A, b, Aeq and beq are the coefficient matrixes of inequality constraints and equality constraints respectively, lb and ub define the value range of the independent variable, and options defines the remaining optimization parameter settings;

[0074] The objective function of the optimization is the system lift coefficient CL, and the constraint is the moment coefficient Cm; the optimization variable is a plurality of weight coefficients obtained after the optimization of the shape geometry parameterization; the gradient values of the objective function CL and the constraint Cm are obtained through the difference by applying a delta change to the optimization variable.

[0075] S5, design the duct shape; on the basis of the optimized propeller shape, the geometric shape of the duct is optimized by using the parameterization method. The duct body can be regarded as a geometric body obtained by rotating a airfoil around an axis one turn, and the geometric parameterization problem of the duct body can be equivalent to the geometric parameterization problem of a two-dimensional airfoil. However, for the duct body, the additional drag force generated by the propeller-duct gap has a great influence on the duct, and considering the difficulty in manufacturing process, the inner wall surface of the duct in the range of the propeller tip is usually a fixed curved surface, so the restriction on the geometric shape of the duct body is greater during optimization. The shape accuracy of CST is poor through the constraint function, so the non-uniform rational B-spline curve (Nurbs) is used in combination with the feature parameter description method to realize the geometric parameterization of the airfoil of the duct cross section.

[0076] As shown in Figure 4 , one of the specific embodiments of designing the duct shape, the non-uniform rational B-spline curve (Nurbs) is used in combination with the feature parameter description method to realize the geometric parameterization of the airfoil of the duct cross section. As shown in the figure, A point is taken as the trailing edge point of the airfoil, G point is taken as the leading edge point of the airfoil, AB, BC and KA are straight line segments, curve segment CGK is a non-uniform rational B-spline curve generated by 9 points from C point to K point, C, G and K are fixed points, the horizontal coordinates of A, D, E, F, H, I and J points are fixed values, and the vertical coordinate of B point is a fixed value; the horizontal coordinate of B point and the vertical coordinates of A, D, E, F, H, I and J are taken as design variables. In this way, there are 8 design variables, the vertical coordinate of A point is used to control the outlet radius of the duct, the horizontal coordinate of B point is used to control the length of the outlet expansion section of the duct, and the vertical coordinates of D, E, F, H, I and J are used to control the shape and radius of the leading edge of the duct.

[0077] The above-described embodiments only describe the preferred modes of the present application, and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope defined by the claims of the present application.

Claims

1. A method of designing a ducted propeller, characterized in that: Comprising S1, clear design conditions and initial design parameters; design conditions include the pull force T generated by the duct and the propeller, the working speed n of the propeller; the initial design parameters include the propeller diameter D, the propeller-duct gap δ and the airfoil; S2, design the propeller diameter D; Select a plurality of gradually increasing propeller diameters, and keep the propeller-duct gap unchanged, and obtain the aerodynamic performance of the propeller disc at the speed n of different propeller diameters through CFD numerical simulation; by comparing with the engine output characteristic curve, the most suitable propeller diameter D for the engine is selected; S3, design the propeller-duct gap δ; the aerodynamic performance of the ducted propeller at the speed n of different propeller-duct gaps is numerically simulated to determine the optimal value of the propeller-duct gap; S4, design the airfoil; the premise of parameterization of propeller blades is to determine the airfoil family used by the blades and the radial position, and keep the chord lengths of the wing root, the middle part of the wing and the wing tip unchanged; under this premise, the wing root, the middle part of the wing and the wing tip section airfoil are determined by the CST function; the negative torsion angles of the wing root, the wing tip and five radial positions selected uniformly in the middle part of the wing are taken as optimization variables, and the parameterized model of the blades is generated by the joint script file of Matlab and ICEM; In S4, the CST function method has strong applicability and is used to parameterize the control airfoil. Each airfoil has 7 function weights on the upper and lower surfaces. The function weights of the shape function based on the function family are calculated, and the airfoil of the control section can be changed by perturbing the weights: ； The change of the control section airfoil is reflected by the change of the function weights: ； Where δi is the change of the function weights of each function; The CST function method represents the airfoil by superimposing Bernstein polynomials. Any m-order Bernstein polynomial is composed of the following m terms: ； The gradient method is used to realize the iterative optimization of the design parameters of the ducted propeller. The optimization function in the Matlab optimization tool package can be used to write the corresponding optimization program in Matlab, call the ICEM script parameterization to generate the grid, call the Fluent script to automatically solve the flow field, and output the file to feed back to Matlab, so as to form a complete optimization program to realize the rapid, real-time and high-precision three-dimensional shape optimization design; The optimization function is as follows: ； wherein x = fmincon(x0, fun, A, b, Aeq, beq, lb, ub, options) for optimizing the initial value, x is the final optimization result, fun is the objective function, A, b, Aeq and beq are the coefficient matrices of inequality constraints and equality constraints respectively, lb and ub define the value range of the independent variable, and options defines the remaining optimization parameter settings. The objective function of the optimization is the system lift coefficient CL, and the constraint is the moment coefficient Cm. The optimization variables are the weight coefficients obtained after the geometric parameterization of the optimized shape. By applying δ change to the optimization variables, the gradient values of the objective function CL and the constraint Cm can be obtained by difference.

2. The method of designing a ducted propeller according to claim 1, wherein: It also includes S5, design the duct shape; the duct body is regarded as a geometric body obtained by rotating an airfoil around an axis, and the geometric parameterization problem of the duct body can be equivalent to the geometric parameterization problem of a two-dimensional airfoil; the non-uniform rational B-spline curve (Nurbs) is used to realize the geometric parameterization of the duct cross-section airfoil in combination with the feature parameter description method.

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