Reversible airfoil optimization design method for tidal current energy water turbine blade

Abstract

The invention provides a tidal current energy water turbine blade reversible airfoil optimization design method, which is based on an initial reversible airfoil designed by a conventional airfoil, performs parametric modeling on the reversible airfoil, and simulates reversible airfoil hydrodynamic analysis in combination with a fluid mechanics simulation numerical value. A multi-island genetic algorithm is selected to carry out multi-objective optimization calculation on the lift coefficient and the lift-drag ratio of the reversible airfoil, and a reversible airfoil profile optimization design system with the hydrodynamic performance as the objective is built. Compared with an initial reversible airfoil, the lift coefficient and the lift-drag ratio of the obtained optimized reversible airfoil are increased, and the overall hydrodynamic performance is improved. The reversible airfoil profile designed by the invention is adopted to replace a conventional bidirectional water turbine blade airfoil profile, and high power generation efficiency and good economic value can be expected to be obtained.

Description

Technical Field

[0001] The present invention relates to the field of manufacturing of tidal current turbines, and particularly to an optimized design method for reversible airfoils of tidal current turbine blades. Background Art

[0002] As a new type of green and renewable energy, tidal current energy has many advantages such as environmental protection, sustainable use, and huge storage. In the prior art, a conventional way to utilize tidal current energy is to convert the water flow energy into mechanical energy through a hydraulic machine. A water turbine is a typical hydraulic machine used for tidal current energy conversion, and the water turbine blade is the core component that determines the efficiency and stability of tidal current energy acquisition. In the research on high-efficiency energy conversion of water turbines, how to effectively acquire tidal current energy from both forward and reverse flows is one of the difficult problems in the design of horizontal-axis tidal current turbines.

[0003] A two-way impeller is the main way for a reversible water turbine device to achieve two-way movement. As the basic unit in the design process of a reversible airfoil, its hydrodynamic performance plays an important role in the energy acquisition efficiency of the water turbine device. In the prior art, a combination of mathematical methods and empirical methods is usually used to reverse-derive the geometric curve of the reversible airfoil, which is rather cumbersome and the fitting effect of the airfoil profile is not satisfactory. Currently, the design method for the reversible airfoil of a tidal current turbine is mainly composed of cutting and splicing the geometric curves of the airfoil, and it is difficult to obtain a reversible airfoil shape with ideal hydrodynamic performance. Summary of the Invention

[0004] Object of the Invention: The object of the present invention is to provide an optimized design method for reversible airfoils of tidal current turbine blades that can automatically optimize the performance of reversible airfoils.

[0005] Technical Solution: An optimized design method for reversible airfoils of tidal current turbine blades includes the following steps:

[0006] S1. Intercept a given original airfoil according to a set chord length ratio, symmetrically splice the intercepted airfoil part to construct a reversible airfoil;

[0007] S2. Use a shape function and a type function to represent the coordinates of the reversible airfoil and generate a reversible airfoil coordinate data set;

[0008] S3. Import the reversible airfoil coordinate data set into a fluid mechanics simulation software to perform reversible airfoil modeling;

[0009] S4. Take the coefficients of the shape function as variables, perform multi-objective optimization processing based on a multi-island genetic algorithm, determine whether a global optimal solution is obtained, and if a global optimal solution is obtained, output the corresponding reversible airfoil modeling;

[0010] S5. If the global optimal solution is not obtained in step S4, update the coefficients of the shape function, and repeat steps S2 to S4 until the global optimal solution is obtained.

[0011] Specifically, in step S1, the reversible airfoil is a completely symmetric airfoil.

[0012] Specifically, in step S2, the shape function is:

[0013]

[0014] In the formula: S(ξ) is the shape function, A i is the coefficient of the shape function, S i is the Bernstein polynomial, n is the order of the Bernstein polynomial, and ξ is the dimensionless coordinate value of the reversible airfoil on the x-axis;

[0015] The category function is:

[0016] C(ξ) = ξ N1 ·(1 - ξ) N2

[0017] In the formula: C(ξ) is the category function, and the parameters N1 and N2 represent the geometric shape categories of the reversible airfoil.

[0018] Specifically, in step S2, the coordinate representation of the reversible airfoil is:

[0019]

[0020] In the formula: η is the dimensionless coordinate value of the reversible airfoil on the y-axis, and ξ is the dimensionless coordinate value of the reversible airfoil on the x-axis.

[0021] Specifically, the coefficients of the shape function are solved by the following equation:

[0022]

[0023] In the formula: x i is the abscissa of the surface point of the reversible airfoil, y(x i ) is the ordinate of the surface point of the reversible airfoil corresponding to x i , A i is the coefficient of the shape function, and i = 1, 2, 3…, n.

[0024] Preferably, the order of the above Bernstein polynomial is 9.

[0025] Specifically, step S3 includes:

[0026] Perform numerical simulation settings in a fluid dynamics simulation software, including importing a reversible airfoil dataset, airfoil modeling, mesh generation, boundary condition settings, solution condition settings, lift coefficient calculation, drag coefficient calculation, lift-to-drag ratio calculation, and post-processing file output. Use script recording for numerical simulation settings and save the recorded script file.

[0027] Specifically, step S4 includes: taking the lift coefficient and lift-to-drag ratio of the reversible airfoil as the objective functions, taking the coefficients of the shape function as variables, and performing multi-objective optimization based on the multi-island genetic algorithm to solve the global optimal solution of the objective functions.

[0028] Specifically, step S5 includes: after updating the coefficients of the shape function, generating a reversible airfoil coordinate dataset, and then automatically performing numerical simulation settings by reading the recorded script file to obtain reversible airfoil modeling.

[0029] Preferably, the curvature of adjacent points on the upper surface curve of the reversible airfoil modeling is set between 0 and 2.38.

[0030] Beneficial effects: Compared with the prior art, the remarkable effect of the present invention is that based on the shape category function transformation method, combined with fluid dynamics simulation numerical simulation, taking the lift coefficient and lift-to-drag ratio of the reversible airfoil as the objective functions, and using the multi-island genetic algorithm for global optimization, an automatic optimization model of the reversible airfoil is constructed, solving the problem of difficult design of reversible airfoils by conventional methods, and providing a new idea for the design of reversible turbines. The reversible airfoil designed by this method has better hydrodynamic performance than the conventional reversible airfoil. Description of the Drawings

[0031] Figure 1 is the flowchart of the method of the present invention.

[0032] Figure 2 is the schematic diagram of the shapes of the NACA0012 airfoil and the initial reversible airfoil in Embodiment 1 of the present invention.

[0033] Figure 3 is the schematic diagram of the shapes of the initial reversible airfoil and the optimized reversible airfoil in Embodiment 1 of the present invention.

[0034] Figure 4 is the comparison diagram of the lift coefficients of the optimized and unoptimized reversible airfoils at different incoming flow angles of attack in Embodiment 1 of the present invention.

[0035] Figure 5 is the comparison diagram of the lift coefficients of the optimized and unoptimized reversible airfoils at different incoming flow angles of attack in Embodiment 1 of the present invention. Detailed Embodiments

[0036] The following further illustrates a preferred embodiment of the present invention with reference to the drawings.

[0037] Embodiment 1

[0038] Please refer to Figure 1 As shown, this embodiment provides a method for optimizing the reversible airfoil of a tidal current turbine blade, including the following steps:

[0039] S1. Intercept the given original airfoil according to the set chord length ratio, symmetrically splice the intercepted airfoil part, and construct a reversible airfoil.

[0040] In this embodiment, the symmetric airfoil NACA0012 with strong versatility, complete and open data is selected as the original airfoil. The airfoil is intercepted by a section perpendicular to the chord length, and the interception position is at the maximum thickness. Please refer to Figure 2 As shown, the position of the maximum thickness of the symmetric airfoil NACA0012 is at the red dotted line, that is, the airfoil part with the first 30% of its chord length is intercepted. The first 30% part of the airfoil is symmetrically spliced with this red dotted line as the axis of symmetry to form an initial reversible airfoil (i.e., Figure 2 the elliptical section obtained after symmetric splicing in

[0041] Please refer to Figure 2 As shown, the NACA0012 airfoil obtains the maximum thickness of 12% of the airfoil chord length at the position of the first 30% of its chord length, while the initial reversible airfoil is a completely symmetric airfoil and obtains the maximum thickness of 20.08% of the airfoil chord length at the 50% position.

[0042] S2. Use the shape function and type function to represent the coordinates of the reversible airfoil constructed in S1, and generate a reversible airfoil coordinate data set.

[0043] The type function is used to control the geometric shape category of the airfoil, and the shape function is used to control the curve change from the leading edge to the trailing edge of the airfoil. Taking the coefficient of the shape function as the design variable, determine the value of the design variable on the upper and lower surfaces of the reversible airfoil, and generate a parameterized reversible airfoil coordinate data set. In this embodiment, C++ software is used for parametric modeling, and the calculation process is as follows:

[0044] Coordinate representation of the airfoil:

[0045] η = C(ξ)·S(ξ)+ξ·Δz

[0046] In the formula: η = y / c, which is the dimensionless value of the airfoil on the x-axis; ξ = x / c, which is the dimensionless value of the airfoil on the x-axis, C(ξ) is the type function, S(ξ) is the shape function, and Δz is the thickness ratio of the trailing edge of the airfoil. In the reversible airfoil, Δz is 0.

[0047] The type function is:

[0048] C(ξ) = ξ N1 ·(1 - ξ) N2

[0049] In the formula: C(ξ) is the category function, and the parameters N1 and N2 represent the geometric shape categories of the reversible airfoil.

[0050] The shape function is:

[0051]

[0052] In the formula: S(ξ) is the shape function, A i is the coefficient of the shape function, S i is the Bernstein polynomial, n is the order of the Bernstein polynomial, and ξ is the dimensionless coordinate value of the reversible airfoil on the x-axis.

[0053] Derive the coordinate expression of the reversible airfoil from the above expressions:

[0054]

[0055] Select a set of control points [x i , y(x i )] on the upper surface of the airfoil, and establish a matrix equation system for solving the coefficients of the shape function through the above formula:

[0056]

[0057] The reversible airfoil in this embodiment is a completely symmetric airfoil, that is, the upper and lower surfaces of the airfoil are symmetric about the x-axis, so the two ordinates corresponding to the same abscissa are opposite to each other. From the above formula derivation, it can be seen that the coefficients of the upper and lower surfaces of the reversible airfoil are opposite to each other. To reduce the number of solutions of the airfoil design variables, the present invention only takes the coefficients of the shape function on the upper surface of the airfoil as the design variables. At the same time, to obtain a high fitting accuracy of the reversible airfoil, the value of the order n of the Bernstein polynomial is preferably 9.

[0058] Please refer to Table 1 below for the specific values of the design variables of the initial reversible airfoil in this embodiment.

[0059] Table 1

[0060] Design variable Value Design variable Value <![CDATA[A0]]> 0.21977 <![CDATA[A5]]> 3.12468 <![CDATA[A1]]> 0.27983 <![CDATA[A6]]> -2.85105 <![CDATA[A2]]> -0.06775 <![CDATA[A7]]> 3.09705 <![CDATA[A3]]> 1.16356 <![CDATA[A8]]> -1.36521 <![CDATA[A4]]> -1.64619 <![CDATA[A9]]> 1.96368

[0061] S3. Import the reversible airfoil coordinate data set into the fluid mechanics simulation software to perform reversible airfoil modeling.

[0062] In this embodiment, extract the reversible airfoil coordinate data set generated in S2 and import it into the CFD software to complete the reversible airfoil modeling, including the modeling sequence of each step of airfoil points, lines, surfaces, and volumes. Record the CFD numerical simulation process through the script function macro of the STAR-CCM+ software to generate a.java script file. In subsequent iterations, reading the script file recorded by the macro can automatically complete the CFD numerical simulation setup process.

[0063] The steps of CFD numerical simulation settings include: importing the reversible airfoil dataset, airfoil modeling, meshing, boundary condition setting, solution condition setting, lift coefficient calculation, drag coefficient calculation, lift-drag ratio calculation, post-processing file output, etc. The content recorded in the macro recording script file includes setting parameters such as mesh size, material properties, initial conditions, turbulence model, convergence residuals, number of iterations, and file read-in and write-out.

[0064] The CFD calculation settings of this embodiment are as follows: The computational domain consists of a semi-circle and a rectangle. The distance from the leading edge point of the airfoil to the front, upper, and lower boundaries is 30 times the chord length, and the distance to the rear boundary is 60 times the chord length. The front, upper, and lower boundary conditions are set as velocity inlets, and the rear boundary condition is set as a pressure outlet; The meshing is a C-type structured mesh, and the leading edge, trailing edge, and boundary layer of the airfoil are encrypted. The thickness of the first layer of the airfoil surface mesh is 5×10 -6 m, the thickness growth ratio is 1.1, and the corresponding y+ is 2. y+ is a dimensionless number used to describe the distance from the wall to the point where the fluid viscous effect becomes significant; The chord length of the airfoil is 7.62 cm, the water flow velocity is 9.63 m / s, the water flow angle of attack is 8°, the SST k-ω turbulence model is selected, the SIMPLE algorithm is used for calculation, the convergence residual is 1×10-5, and the number of iterations is 500 steps; Files for the lift coefficient, drag coefficient, and lift-drag ratio of the airfoil are established, and the lift coefficient and lift-drag ratio of the reversible airfoil in the file are extracted.

[0065] S4. Taking the lift-drag ratio and lift coefficient of the reversible airfoil as the objective functions, and the coefficients of the shape function as variables, multi-objective optimization is carried out based on the multi-island genetic algorithm (MIGA) to determine whether the global optimal solution is obtained. If the global optimal solution is obtained, the corresponding reversible airfoil modeling is output.

[0066] S5. If the global optimal solution is not obtained in step S4, update the coefficients of the shape function, and repeat steps S2 to S4 until the global optimal solution is obtained.

[0067] The MIGA algorithm is a multi-objective genetic optimization algorithm, which is a distributed optimization technology improved based on the genetic algorithm. Compared with the traditional genetic optimization algorithm, it combines the ideas of genetic algorithm and islands, and has the ability to globally search for the optimal solution, accelerate the calculation speed, and evaluate the fitness function.

[0068] The optimization objective function of the reversible airfoil is:

[0069]

[0070] In the formula: C L is the lift coefficient of the airfoil, C D is the drag coefficient of the airfoil, and f is the lift-drag ratio.

[0071] In this embodiment, the coefficients of the reversible airfoil shape function are used as design variables, and the values of the design variables are changed through the MIGA algorithm to control the change of the reversible airfoil shape. To control the airfoil optimized subsequently to remain a reversible airfoil, the data points of the unit airfoil curve are set to be symmetric about the lines x = 0.5 and y = 0 in the C++ software. To prevent the occurrence of sunken and straight airfoil curves, the curvature values of adjacent points on the upper surface curve of the airfoil are set between 0 and 2.38; at the same time, to keep the airfoil curve smooth and continuous, the design variables are constrained to float within a small range of the initial values. Please refer to Table 2 below for the constraint ranges of the design variables of the reversible airfoil.

[0072] Table 2

[0073] Design variable Range of values Design variable Range of values <![CDATA[A0]]> [0.1,0.25] <![CDATA[A5]]> [2.5,3.2] <![CDATA[A1]]> [0.2,0.3] <![CDATA[A6]]> [-3,-2.3] <![CDATA[A2]]> [-0.15,-0.01] <![CDATA[A7]]> [2.2,3.2] <![CDATA[A3]]> [0.6,1.2] <![CDATA[A8]]> [-1.4,-1] <![CDATA[A4]]> [-1.7,-1.3] <![CDATA[A9]]> [1.2,2]

[0074] Please refer to Table 3 below for the numerical values of the design variables of the optimized reversible airfoil obtained according to the above method.

[0075] Table 3

[0076] Design variable Value Design variable Value <![CDATA[A0]]> 0.14876 <![CDATA[A5]]> 2.63158 <![CDATA[A1]]> 0.20590 <![CDATA[A6]]> -2.44661 <![CDATA[A2]]> -0.12400 <![CDATA[A7]]> 2.48105 <![CDATA[A3]]> 0.99582 <![CDATA[A8]]> -1.13892 <![CDATA[A4]]> -1.49936 <![CDATA[A9]]> 1.41137

[0077] Please refer to Figure 3 as shown. Compared with the initial reversible airfoil, the thickness of the leading edge and trailing edge parts of the optimized reversible airfoil has decreased, and it looks flatter as a whole. The maximum relative thickness has decreased from the original 20.08%c to 10.11%c, but the position of the maximum relative thickness of the airfoil is still at 50%c.

[0078] Please refer to Figure 4 as shown. The abscissa in the figure is the incoming flow angle of attack, and the ordinate is the lift coefficient; please refer to Figure 5 as shown. The abscissa in the figure is the incoming flow angle of attack, and the ordinate is the lift-to-drag ratio. Table 4 below shows the hydrodynamic data of the reversible airfoil before and after optimization at an incoming flow angle of attack of 8°. By comparison, it can be seen that at an incoming flow angle of attack of 8°, the lift coefficient of the optimized reversible airfoil has increased by 0.07030, while the drag coefficient has decreased by about 0.006, resulting in an increase in the lift-to-drag ratio of 11.76959, and the percentage increase is as high as 46.26%.

[0079] Table 4

[0080] Airfoil Lift coefficient Drag coefficient Lift-to-drag ratio Initial airfoil 0.65240 0.02564 25.44462 Optimized airfoil 0.72270 0.01942 37.21421

[0081] From Figure 4 and Figure 5 it can be seen that the lift coefficient of the optimized reversible airfoil at different incoming flow angles of attack is always greater than that of the initial reversible airfoil, and the relationship between the lift coefficients of the two and the change of the angle of attack is almost a linear curve. The overall lift-to-drag ratio of the optimized reversible airfoil is greater than that of the initial reversible airfoil. Compared with the initial reversible airfoil, the lift-to-drag ratio curve of the optimized reversible airfoil first rises and then falls, and reaches the maximum value at an incoming flow angle of attack of 8°.

[0082] According to the above analysis, it can be known that the lift coefficient of the optimized reversible airfoil increases, the lift-drag ratio is greatly improved, and the maximum thickness decreases. Therefore, the design method proposed by the present invention can solve the problem of difficult design of conventional reversible airfoils, and at the same time can meet the performance requirements of the reversible airfoils of the blades of a two-way tidal current turbine.

Claims

1. A method for optimizing the reversible airfoil of a tidal turbine blade, characterized in that: The following steps are involved: S1. Cutting a given original airfoil according to a set chord length ratio, symmetrically splicing the cut airfoil parts, and constructing a reversible airfoil; S2, using shape function and type function to represent the coordinates of the reversible airfoil, and generating a reversible airfoil coordinate data set; S3, importing the reversible airfoil coordinate data set into the fluid mechanics simulation software to perform reversible airfoil modeling; S4, taking the coefficient of the shape function as a variable, performing multi-objective optimization processing based on a multi-island genetic algorithm, and judging whether a global optimal solution is obtained. If a global optimal solution is obtained, outputting the corresponding reversible airfoil modeling; S5. If step S4 does not obtain a global optimal solution, update the coefficients of the shape function and repeat steps S2 to S4 until a global optimal solution is obtained.

2. The reversible airfoil optimization design method for tidal energy turbine blades according to claim 1 is characterized in that: In the step S1, the reversible airfoil is a completely symmetrical airfoil.

3. The reversible airfoil optimization design method for tidal energy turbine blades according to claim 1 is characterized in that: In step S2, the shape function is: Where: S(ξ) is the shape function, A i is the coefficient of the shape function, S i is the Bernstein polynomial, n is the order of the Bernstein polynomial, ξ is the dimensionless coordinate value of the reversible airfoil on the x-axis; The category function is: C(ξ)=ξ N1 ·(1-ξ) N2 Where: C(ξ) is the category function, and parameters N1 and N2 represent the geometric shape category of the reversible airfoil.

4. The reversible airfoil optimization design method for tidal energy turbine blades according to claim 3 is characterized in that: In step S2, the coordinates of the reversible airfoil are expressed as: Wherein: η is the dimensionless coordinate value of the reversible airfoil on the y-axis, ξ is the dimensionless coordinate value of the reversible airfoil on the x-axis.

5. The reversible airfoil optimization design method for tidal energy turbine blades according to claim 3 is characterized in that: The coefficients of the shape function are solved by the following equation: Where: x i is the horizontal coordinate of the reversible airfoil surface point, y(x i ) is the same as x i The ordinate of the corresponding reversible airfoil surface point, A i is the coefficient of the shape function, i=1,2,3…,n.

6. The method for optimizing the reversible airfoil of a tidal energy turbine blade according to claim 3, characterized in that: The order of the Bernstein polynomial is 9.

7. The reversible airfoil optimization design method for tidal energy turbine blades according to claim 1 is characterized in that: The step S3 comprises: Perform numerical simulation settings in fluid mechanics simulation software, including importing reversible airfoil data sets, airfoil modeling, meshing, boundary condition settings, solution condition settings, lift coefficient calculations, drag coefficient calculations, lift-to-drag ratio calculations, and post-processing file output. Use scripts to record numerical simulation settings and save recorded script files.

8. The method for optimizing the reversible airfoil of a tidal energy turbine blade according to claim 7, characterized in that: The step S4 includes: taking the lift coefficient and lift-to-drag ratio of the reversible airfoil as the objective function, taking the coefficient of the shape function as a variable, performing multi-objective optimization processing based on a multi-island genetic algorithm, and solving the global optimal solution of the objective function.

9. The method for optimizing the reversible airfoil of a tidal energy turbine blade according to claim 7, characterized in that: The step S5 comprises: after updating the coefficients of the shape function, generating a reversible airfoil coordinate data set, and then reading the recorded script file to automatically perform numerical simulation settings to obtain reversible airfoil modeling.

10. The reversible airfoil optimization design method for tidal turbine blades according to claim 1, characterized in that: The curvature of adjacent points of the surface curve on the reversible airfoil modeling was set between 0 and 2.38.

Images