Pneumatic measurement and prediction method for full attack angle of wind turbine airfoil for wind tunnel experiment

Abstract

The invention discloses a wind turbine airfoil full-attack-angle pneumatic measurement and prediction method for a wind tunnel experiment in the field of wind tunnel experiments, and the method comprises the steps: 1) carrying out the modeling of a wind turbine airfoil, designing a wind turbine airfoil model of the wind tunnel experiment, and building a test platform; (2) checking the safety of equipment and an experimental environment; 3) opening a wind tunnel to obtain a set wind speed; 4) sequentially changing the attack angle, obtaining the pressure data of the wind turbine airfoil at the expected attack angle moment, and closing the wind tunnel after the measurement is finished; 5) adjusting the pitot tube to be right in front of the wind turbine airfoil, repeating the step 3), and collecting incoming flow pressure right in front of the wind turbine airfoil when the attack angles are 90 degrees and 270 degrees; (6) data processing is conducted, and correction coefficients related to the incoming flow wind speed are obtained; and 7) calculating a lift coefficient and a resistance coefficient, obtaining a resistance coefficient shrinkage ratio, and predicting the lift coefficient and the resistance coefficient of the wind turbine airfoil under the full-attack-angle working condition. The method can quickly predict the aerodynamic change condition of the wind turbine airfoil under the expected working condition.

Description

Technical Field

[0001] The present invention belongs to the technical field of wind tunnel tests and the aerodynamic characteristics modeling of wind turbines, and particularly relates to a method for measuring and predicting the aerodynamics of a wind turbine airfoil at all angles of attack for wind tunnel experiments. Background Art

[0002] With the continuous development trend of the large-scale of wind turbines, the field of research on the aerodynamic characteristics of wind turbine airfoils has put forward higher requirements for ultra-large blades of 20 MW and above in full operating conditions (including high turbulence and large yaw angles), and there is an urgent need for a refined aerodynamic database covering the entire angle-of-attack domain. The deficiencies of the traditional technical system in terms of data integrity, model generalization, and real-time performance have become the key obstacles restricting the research and development of the next-generation highly adaptable wind turbine airfoils. Aerodynamic measurement and performance prediction within the entire angle-of-attack range are the key basis for realizing blade optimization design and safety assessment.

[0003] However, traditional wind tunnel test methods usually rely on a step-by-step angle-of-attack adjustment mechanism, which not only has a long test period but also has the problem of incomplete extreme angle-of-attack data. Especially in the stall condition, it is difficult to accurately capture the aerodynamic characteristics. Traditional methods can generally meet the basic aerodynamic coefficient measurement requirements within an angle-of-attack range of 20°, but there are significant defects in extreme angle-of-attack conditions. The flow field interference caused by the support mechanism at high angles of attack is difficult to eliminate, resulting in a serious lack of aerodynamic data in the deep stall region. Although existing numerical simulation technologies can expand the calculation range of the angle of attack, their accuracy is limited by the accuracy of the turbulence model and the ability to simulate separated flows, and the prediction error in the deep stall region increases significantly. Currently, most technologies adopt the strategy of combining experimental data with CFD results, which may lead to an obvious mismatch between the angle of attack and the aerodynamic parameter curve, and there is a lack of a unified prediction model. In addition, existing prediction methods cannot meet the requirements of rapid evaluation of the real-time aerodynamic characteristics of airfoils. These technical bottlenecks seriously hinder the accurate prediction and optimization design of the aerodynamic performance of wind turbine airfoils under complex incoming flow conditions. Therefore, it has very important engineering significance to expand the aerodynamic characteristics through wind tunnel tests in combination with semi-empirical model formulas and explore the method for measuring and predicting the aerodynamics of wind turbine airfoils at all angles of attack. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for measuring and predicting the aerodynamics of a wind turbine airfoil at all angles of attack for wind tunnel experiments, which can overcome the above technical problems. The method has the characteristics of simple design, high reliability, and less calculation amount, can quickly predict the aerodynamic changes of the wind turbine airfoil under the desired working conditions, has good engineering practice value and economic benefits, and has important engineering significance for the evaluation of the existing aerodynamic efficiency of wind turbines.

[0005] To achieve the above object, the present invention adopts the following technical solutions: A method for measuring and predicting the aerodynamics of a wind turbine airfoil at all angles of attack for wind tunnel experiments, comprising the following steps:

[0006] Step 1) Use 3D modeling software to model the wind turbine airfoil and make the wind turbine airfoil from metal materials. Design a wind turbine airfoil model for wind tunnel experiments and build a test platform.

[0007] Step 2) Check the safety of the equipment and experimental environment, including: whether the connecting fasteners are loose; whether the airfoil is fixedly installed; whether the signal acquisition circuit is correctly connected and operating normally, and ensure that the inside of the wind tunnel is clean.

[0008] Step 3) Start the wind tunnel to obtain the set wind speed.

[0009] Step 4) Change the angle of attack in sequence, obtain the pressure data of the wind turbine airfoil at the desired angle of attack moment, and close the wind tunnel after the measurement is completed.

[0010] Step 5) Adjust the pitot tube to the front of the wind turbine airfoil, repeat Step 3), collect the oncoming flow dynamic pressure in front of the wind turbine airfoil at the angle of attack of 90° and 270°, and close the wind tunnel after the measurement is completed.

[0011] Step 6) Process the data to obtain the correction coefficient related to the oncoming flow wind speed.

[0012] Step 7) Calculate the lift coefficient and drag coefficient, obtain the reduction ratio of the drag coefficient, and predict the lift coefficient and drag coefficient of the wind turbine airfoil under the full angle of attack condition.

[0013] The method of the present invention can cover any angle of attack state from 0° to 360° that the blade can experience under actual complex working conditions, overcoming the problem that the traditional limited angle of attack experiment measurement cannot accurately obtain the aerodynamic change of the airfoil at a larger angle of attack. On the other hand, the experimental workload of measuring the wind turbine airfoil at the full angle of attack is huge, resulting in an exponential increase in the data acquisition volume, and there are also difficulties in blocking effect correction, further increasing the complexity of data processing and verification. However, the method of the present invention has the characteristics of simple design, high reliability, and less calculation amount, and can predict the aerodynamic changes of the wind turbine airfoil at different angle of attack moments, providing the possibility for quickly predicting the wind energy utilization efficiency of the wind turbine in the real environment, having good engineering practice value and economic benefits, and having important engineering significance for the existing evaluation of the aerodynamic efficiency of wind turbines.

[0014] As a further improvement of the present invention, the wind turbine airfoil in Step 1) is processed by metal turning technology, the surface is polished many times, a fixing rod protruding outward is provided at the 1 / 4 chord length of the upper end face of the airfoil, the fixing rod is rotatably connected to the end plate through a bearing, a gap is left between the end plate and the upper end face of the airfoil, the gap is filled with sponge tape, grease is coated on the surface of the end plate corresponding to the sponge tape, an embedded hole is opened at the 1 / 4 chord length of the lower end face of the airfoil, and the embedded hole is fixedly connected to the turntable through a bolt.

[0015] As a further improvement of the present invention, the inside of the wind turbine airfoil in step 1) is hollow and provided with capillary metal tubes. A plurality of pressure measurement holes are distributed at intervals on both the upper surface and the lower surface of the airfoil. The projection angle between the connecting line of the plurality of upper surface pressure measurement holes and the chord length of the airfoil is 20°, and the projection angle between the connecting line of the plurality of lower surface pressure measurement holes and the chord length of the airfoil is 20°. One end of the capillary metal tube is inserted into the pressure measurement hole, and the other end is connected to a pressure measurement hose. The pressure measurement hose extends along the lower end surface of the airfoil and is connected to a pressure scanning valve.

[0016] As a further improvement of the present invention, in step 6), the ratio of the dynamic pressures obtained by measuring with the pitot tube located in front of the side of the airfoil in step 4) and the pitot tube located directly in front of the airfoil in step 5) is used as the correction coefficient γ, and then the pressure coefficient C after correction at 90° and 270° is calculated using Equation (1). pi_ :

[0017]

[0018] In the formula: p i is the static pressure of the i-th pressure measurement hole; p z is the total pressure obtained by measuring with the pitot tube in front of the side of the airfoil; p 0 is the static pressure obtained by measuring with the pitot tube in front of the side of the airfoil; γ is the ratio of the dynamic pressures obtained by measuring twice with the pitot tube in front of the side of the airfoil and after adjusting the pitot tube to directly in front of the airfoil.

[0019] In a wind tunnel experiment, due to the presence of the wall surface, it will interfere with the flow characteristics. This interference mainly comes from the influence of the wall surfaces at both ends of the airfoil on the flow. The boundary layers of the upper and lower wall surfaces are relatively thick. Under the action of the adverse pressure gradient at the leading edge of the airfoil, the wall boundary layer will separate prior to the airfoil boundary layer. The separation zone expands towards the middle of the airfoil at a certain angle, triggering spanwise flow. Especially when the aspect ratio is small, the influence of the above effects on its overall flow characteristics is more significant. In addition, the wind turbine airfoil and its wake will reduce the flow area, resulting in an increase in the air flow velocity. As the angle of attack increases, the air flow separation zone on the airfoil surface expands, further exacerbating the blockage effect. The method of the present invention involves the flow with a severe separation degree at a large angle of attack. As the angle of attack increases, the blockage ratio of the experiment increases correspondingly, which will cause a change in the incoming flow velocity. Therefore, a method of correcting the wind speed is adopted to solve the wall interference at a large angle of attack. The pitot tube originally located in front of the side of the airfoil is adjusted to directly in front of the airfoil, and the wind speed is measured again. Then, according to the ratio of the dynamic pressures measured before and after as the correction coefficient γ for adjustment, the pressure coefficient is recalculated.

[0020] As a further improvement of the present invention, in step 7), the pressure data obtained in step 4) is adopted, and in combination with the surface pressure integration method, the lift coefficient and drag coefficient of the wind turbine airfoil in the range of the angle of attack from 0° to 20° are obtained. The drag coefficients C d_90 and C d_270 corresponding to the angles of attack of 90° and 270° calculated by using the pressure coefficient corrected in step 6) are obtained, and the drag coefficient reduction ratio C rd is obtained through formula (2):

[0021]

[0022] In the formula: C rd is the drag coefficient reduction ratio.

[0023] Generally, the wind tunnel test data of the airfoil only includes the small angle of attack range, and it is difficult to obtain the aerodynamic performance parameters after stall and at larger angles. A reasonable prediction method regards the airfoil as a flat plate. Therefore, the maximum drag coefficient should exist at 90° and 270°. However, due to the symmetrical situation of the leading edge and trailing edge of the airfoil, there will inevitably be a difference in the drag coefficients at 90° and 270°. Therefore, this method introduces the drag coefficient reduction ratio to obtain the relationship between the two drag coefficient peaks.

[0024] As a further improvement of the present invention, in step 7), the drag coefficient of the wind turbine airfoil under the full angle of attack condition is obtained by piecewise calculation through formula (3):

[0025]

[0026] In the formula: α is the angle of attack; C d_α is the drag coefficient corresponding to the angle of attack α; C d_20 is the drag coefficient corresponding to the angle of attack of 20°; C d_90 is the drag coefficient calculated by using the corrected pressure coefficient at the angle of attack of 90°; C d_160 is the drag coefficient obtained by piecewise calculation through formula (3) at the angle of attack of 160°.

[0027] The drag in the stall area is mainly dominated by the pressure drag caused by flow separation and vortex dissipation. The drag coefficient of the wind turbine airfoil approximately varies sinusoidally within the full angle of attack range. Therefore, a sine-cosine function related to the angle of attack α is introduced to characterize the sudden increase effect of the drag in the deep stall area and quantify the non-linear growth of the drag after stall. For the deep stall area where the angle of attack exceeds 90°, the data range is extended by assuming the symmetry of the aerodynamic force. And in combination with the actual wind tunnel test results, the drag coefficient shows a symmetric distribution in the range of (90°, 160°]. And it is assumed that the drag coefficient at 180° is 0, and the drag coefficient in the range of (160°, 180°] is obtained by interpolation.

[0028] As a further improvement of the present invention, the drag coefficient within the range of the angle of attack α in (180°, 360°) in step 7) is obtained by mirroring the result obtained from Equation (3) along α = 180°, and then multiplying it by the drag coefficient reduction ratio C rd obtained

[0029] Due to the periodic characteristics of the flow around a blunt body in aerodynamics, when the angle of attack exceeds 180°, the flow field around the flat plate and the evolution of the drag coefficient will exhibit mirror symmetry characteristics with 180° as the axis. Based on this, it can be approximately considered that the pressure difference drag distribution of the wind turbine airfoil has a symmetric parabolic shape under positive and negative angles of attack. Therefore, by performing periodic extension and mirror mapping on the drag coefficient curve within the range of the angle of attack [0°, 180°] that has been obtained, the full angle of attack prediction result can be quickly generated without having to re-establish a complex flow field equation. This expansion strategy based on symmetric logic can reduce the computational complexity

[0030] As a further improvement of the present invention, the lift coefficient within the range of the angle of attack α in (20°, 360°) in step 7) is obtained by piecewise calculation according to Equation (4):

[0031]

[0032] where: α is the angle of attack; C l_α is the lift coefficient corresponding to the angle of attack α; C d_α is the drag coefficient corresponding to the angle of attack α; C d_(180-α) is the drag coefficient corresponding to the angle of attack 180° - α; C l_(α-180) is the lift coefficient corresponding to the angle of attack α - 180°, which is obtained by piecewise calculation according to Equation (4); C l_160 and C l_340 are the lift coefficients obtained by piecewise calculation according to Equation (4) at the angles of attack of 160° and 340° respectively; C l_0 is the lift coefficient obtained by the surface pressure integration method at the angle of attack of 0°

[0033] Considering the anti-symmetric lift coefficient characteristic caused by the asymmetry of the airfoil's leading and trailing edges, when the angle of attack is small, the leading edge of the airfoil can increase the air flow contact area and the smoothness of the curvature, making the air flow accelerate more gently on the wing surface, delaying the flow separation and enhancing the leading edge suction effect, thereby increasing the lift coefficient. When the angle of attack exceeds the critical stall angle, the flow field around the airfoil undergoes a drastic separation. When the angle of attack is large, the airfoil enters a deep stall state. At this time, the air flow completely detaches from the airfoil surface, forming a large-scale turbulent wake, and the lift mainly depends on the instantaneous effect of the periodic shedding of vortices. Moreover, when the airfoil gradually enters an inverted attitude, the air flow direction acts in the opposite direction to the airfoil's geometric characteristics, generating a lift force opposite to the design direction. By empirically compensating for the lift loss caused by the periodic shedding and the generation of stall vortices in the flow field, the lift coefficient is scaled down in this method, and the coefficient is selected as 0.7. By scaling down the lift coefficient in the range of (90°, 160°] and then combining interpolation and mirroring to determine the lift coefficient in the range of (160°, 200°]. For the deep stall region, the lift characteristics within the obtained angle of attack range are extended to the full circumference by assuming aerodynamic symmetry.

[0034] The present invention adopts the above technical solutions. Compared with the prior art, the beneficial effects are as follows: The method of the present invention can realize the rapid prediction and verification of the aerodynamic characteristics of different airfoils under large angle of attack conditions. By using the aerodynamic data in the small angle of attack range and combining the prediction method, the wind tunnel experiment cost can be effectively reduced; The aerodynamic data of the wind turbine airfoil is an important basis and key reference for blade design and performance evaluation. The present invention can provide a high-precision and low-cost solution for airfoil design in fields such as wind power; The present invention has the characteristics of simple design, high reliability, and less calculation amount, and has important engineering application value and practical significance for the evaluation of the aerodynamic efficiency of existing wind turbines. Description of the Drawings

[0035] Figure 1 It is a flow chart of a method for measuring and predicting the aerodynamic characteristics of a wind turbine airfoil at all angles of attack for a wind tunnel experiment of the present invention.

[0036] Figure 2 It is a structural design diagram of a wind turbine airfoil model for a wind tunnel experiment of the present invention.

[0037] Figure 3 It is a top view of a wind turbine airfoil aerodynamic measurement test platform of the present invention.

[0038] Figure 4 It is a flow chart for calculating and predicting the lift coefficient and drag coefficient of a wind turbine airfoil under all angles of attack conditions of the present invention.

[0039] Figure 5 The ratio of the dynamic pressures measured twice before and after is used as the correction coefficient γ in the present invention.

[0040] Figure 6The pressure coefficient C after 90° and 270° correction in the present invention pi_ 。

[0041] Figure 7 In the present invention, the lift coefficient and drag coefficient of the wind turbine airfoil in the range of 0° to 20° of the measured angle of attack are obtained by using the surface pressure integration method.

[0042] Figure 8 Comparison of the predicted lift coefficient and drag coefficient with the wind tunnel test values in the present invention.

[0043] Figure 9 Comparison of the predicted lift coefficient and drag coefficient with the wind tunnel test values in the application of the DU180 wind turbine airfoil in the present invention.

[0044] Among them, 20 airfoil models, 21 pressure measurement holes, 22 fixed rods, 23 buried holes. Specific implementation mode

[0045] Such as Figure 1 shown, a full angle of attack aerodynamic measurement and prediction method for a wind turbine airfoil used in a wind tunnel experiment includes the following steps:

[0046] Step 1) Use 3D modeling software to model the wind turbine airfoil and make the wind turbine airfoil with metal materials, design a wind turbine airfoil model for wind tunnel experiments and build a test platform;

[0047] In this embodiment, the relative thickness of the wind turbine airfoil is 21%, the trailing edge relative chord length thicknesses are 0.55% respectively, the maximum thickness of the wind turbine airfoil 20 is obtained at the position of 35% of the relative chord length, and the maximum camber is 3.08%. To facilitate the arrangement of pressure measurement holes 21 with an inner diameter of 0.5 mm on the surface, the wind turbine airfoil used in the experiment is cast on the upper and lower surfaces with metal respectively, the surface is polished many times, and then spliced after drilling. In order to avoid mutual interference between the measurement points, 22 pressure measurement holes are arranged at intervals on the upper and lower surfaces of the airfoil. The projection angle between the connection line of the 22 pressure measurement holes on the upper surface and the airfoil chord length is 20°, and the projection angle between the connection line of the 22 pressure measurement holes on the lower surface and the airfoil chord length is 20°. The joined metal surface is polished with high precision to meet the experimental requirements. Such as Figure 2As shown in the figure, the chord length of the wind turbine airfoil is 0.35 m, the span is 1 m, and a fixed rod 22 extending outward is provided at the 1 / 4 chord length position of the upper end face of the airfoil. Its diameter is 30 mm and its length is 25 mm. The fixed rod 22 is rotatably connected to the end plate through a bearing to fix the airfoil. An embedded hole 23 is opened at the 1 / 4 chord length position of the lower end face of the airfoil. Its diameter is 30 mm and its depth is 90 mm. The embedded hole 23 is fixedly connected to the turntable through bolts. The inside of the airfoil is hollow. One end of the capillary metal tube is inserted into the pressure measurement hole, and the other end is connected to the pressure measurement hose. The other end of the pressure measurement hose extends out from the reserved embedded hole in the middle position of the lower end face of the airfoil to facilitate access to the pressure scanning valve for the pressure measurement experiment, and ensure that there is no air leakage at the connection. In this experiment, a DTC Initium electronic pressure measurement system produced by PSI Company of the United States is used to obtain pressure information, and the measurement range is ±2500 Pa.

[0048] As Figure 3 shown, when installing the wind turbine airfoil, the aerodynamic center at the 1 / 4 chord length position of the wind turbine airfoil is aligned with the center of the turntable in the wind tunnel test section. There is a movable turntable at the center position of the bottom surface of the wind tunnel test section, and a servo motor precisely controlled by a DMC1000B control card is connected below, which can drive the turntable to rotate to achieve precise angle of attack setting. There is a reserved hole at the center of the turntable. Bolts are used to pass through the holes and then connected to the bolt holes at the bottom end of the airfoil to fix the wind turbine airfoil on the turntable. For airfoils with a limited span, due to the pressure difference between the suction surface and the pressure surface, the airflow will form wing tip vortices at the ends, inducing the downwash effect, resulting in a decrease in the lift coefficient and an increase in the drag coefficient. Therefore, installing end plates can significantly reduce the influence of flow around and downwash, ensuring that the flow approaches two-dimensional characteristics. In this embodiment, the cross-section of the wind tunnel test section is a rectangular structure of 3 m * 1.5 m, and a balance gap with a length of 20 cm is connected at the back. The end plates are arranged at a position 1.0 m away from the upper wall of the wind tunnel and are spliced by two semi-circular wooden boards with a radius of 0.7 m. There is a gap of about 2 mm between the end plates and the upper end face of the airfoil. The gap is filled with sponge tape, and the surface of the wooden board in the area swept by the tape and the rotating airfoil is coated with grease to reduce friction. The pitot tube 1 is located in front of the side of the airfoil, 60 cm above the ground, and 55 cm perpendicular to the side wall, which can avoid the interference of the wall boundary layer.

[0049] Step 2) Check the safety of the equipment and the experimental environment, including: whether the connecting bolts of the wind turbine airfoil are loose; whether the airfoil is fixedly installed; whether the pressure measurement hose is correctly connected; at the same time, check the installation firmness of the pitot tube and the pressure scanning valve measurement equipment to avoid falling off due to vibration or air flow impact, confirm the effectiveness of the locking device of the angle of attack adjustment mechanism to prevent accidental displacement during the experiment, and conduct a comprehensive inspection of the wind tunnel power system, measurement and control module, and data acquisition device to ensure stable power connection, effective sensor calibration, whether the signal acquisition circuit is correctly connected and operating normally, and on this basis, ensure that the inside of the wind tunnel is clean.

[0050] Step 3) Turn on the wind tunnel to obtain a set wind speed of 30.0 m / s;

[0051] Step 4) Successively change the angle of attack to obtain the pressure data of the wind turbine airfoil at intervals of 1° from 0° to 20°. The sampling time is 30 s and the sampling frequency is 331 Hz. After the measurement, turn off the wind tunnel; use formula (S-1) to obtain the pressure data measured from the pressure measurement holes on the airfoil surface from 0° to 20°, and express it in its dimensionless form:

[0052]

[0053] In the formula: C pi represents the pressure coefficient of the i-th pressure measurement hole on the airfoil surface; p z is the total pressure obtained by measuring with the pitot tube 1 in front of the side of the airfoil; p 0 is the static pressure obtained by measuring with the pitot tube 1 in front of the side of the airfoil; p i is the static pressure of the i-th pressure measurement hole.

[0054] Step 5) Adjust the pitot tube to the front of the wind turbine airfoil. In this embodiment, it is placed at Figure 3 the position of pitot tube 2. Specifically, the height from the ground is 60 cm and the distance from the center of the turntable is 55 cm. Repeat step 3), collect the oncoming dynamic pressure in front of the wind turbine airfoil at the moments of the angle of attack of 90° and 270°. After the measurement, turn off the wind tunnel;

[0055] The specific process of calculating and predicting the lift coefficient and drag coefficient of the wind turbine airfoil under the full angle of attack condition is as Figure 4 shown. The pressure scanning valve obtains the instantaneous surface pressure distribution data, converts the data into the pressure coefficient distribution, calculates the normal and tangential components of the aerodynamic force based on the surface integral method, determines the lift coefficient and drag coefficient in combination with the flow direction parameters, and finally outputs the quantitative results for aerodynamic performance evaluation in combination with the semi-empirical formula. Therefore, the subsequent steps are processed based on the pressure data of the wind turbine airfoil.

[0056] Step 6) Data processing to obtain the correction coefficient related to the oncoming wind speed; use the ratio γ of the dynamic pressures obtained from the two measurements of the pitot tube in front of the side of the airfoil in step 4) and the pitot tube adjusted to the front of the wind turbine airfoil in step 5) as the correction coefficient. Specifically, the dynamic pressure is obtained by subtracting the measured static pressure from the total pressure, Figure 5 gives the ratio of the dynamic pressures measured before and after for the conditions of 90° and 270° as the correction coefficient γ. After processing, the correction coefficients γ at 90° and 270° are 1.237 and 1.258 respectively. Use the ratio γ of the obtained dynamic pressures as the correction coefficient to calculate the corrected pressure coefficient C pi_ .

[0057]

[0058] where: p i is the static pressure of the i-th pressure measuring hole; p z is the total pressure obtained by measuring with a pitot tube in front of the airfoil side; p 0 is the static pressure obtained by measuring with a pitot tube in front of the airfoil side; γ is the ratio of the dynamic pressures obtained by measuring twice at the pitot tube 1 position and at the pitot tube 2 position. The pressure coefficient C pi_ , as Figure 6 shown, the abscissa x / c in the figure represents the dimensionless relative chord length position. At this angle of attack position, the pressure distribution on the airfoil surface is dominated by stall separation. On the windward side of the airfoil, a positive pressure region is formed due to the direct impact of the airflow, and the pressure coefficients on the windward side at the two working conditions are relatively close. However, affected by the airfoil geometry, there are differences in the pressure distributions on the leeward sides at the 90° and 270° positions.

[0059] Step 7) Calculate the lift coefficient and drag coefficient, obtain the reduction ratio of the drag coefficient, and predict the lift coefficient and drag coefficient of the wind turbine airfoil at the full angle of attack condition; specifically, use the pressure data obtained in Step 4) and combine with the surface pressure integration method to obtain the lift coefficient and drag coefficient of the wind turbine airfoil every 1° within the range of the measured angle of attack from 0° to 20°.

[0060] First, integrate the pressure coefficient distribution in the directions perpendicular and parallel to the chord length to obtain the normal force coefficient C n and the tangential force coefficient C t , and the calculation formulas are as follows:

[0061]

[0062] where: x and y are the dimensionless coordinates of the surface points relative to the chord length when the angle of attack of the wind turbine airfoil is 0°; y umax and y lmax respectively represent the relative coordinate values corresponding to the maximum thickness positions on the upper and lower surfaces of the airfoil; the subscripts be and af of the pressure coefficient respectively indicate that the point is before and after the maximum thickness position of the airfoil.

[0063] The lift and drag are respectively perpendicular and parallel to the oncoming flow direction, and the lift coefficient and the pressure drag coefficient are obtained, and the formulas are as follows:

[0064]

[0065] where: α is the angle of attack.

[0066] As Figure 7As shown, the lift coefficient and drag coefficient of the wind turbine airfoil within the range of 0° to 20° of the angle of attack are obtained in the present invention. It can be seen from the figure that before stall, the lift coefficient increases with the increase of the angle of attack and shows a linear change, while the drag coefficient changes gently. When slightly stalled, the lift coefficient reaches the maximum value at the stall angle of attack (about 14°), and then decreases with the increase of the angle of attack. After the airfoil stalls, the lift coefficient decreases and the drag coefficient increases sharply.

[0067] Then, the pressure coefficient C corrected by step 6) is used. pi_ The drag coefficients C corresponding to the angles of attack of 90° and 270° calculated are d_90 and C d_270 , and the drag coefficient reduction ratio C is obtained through Equation (2). rd :

[0068]

[0069] In the formula: C rd is the drag coefficient reduction ratio, which is calculated through Equation (1), Equation (S-2), and Equation (S-3); in this embodiment, C d_90 is 1.8540, C d_270 is 1.9098, so C rd is calculated to be 0.97 through calculation.

[0070] Finally, using Equation (3), the drag coefficient of the wind turbine airfoil from 20° to 180° is obtained; the drag coefficient within the range of the angle of attack α (180°, 360°) is obtained by mirroring the result obtained from Equation (3) along α = 180° and then multiplying by the drag coefficient reduction ratio C rd obtained.

[0071]

[0072] In the formula: α is the angle of attack; C d_α is the drag coefficient corresponding to the angle of attack α; C d_20 is the drag coefficient corresponding to the angle of attack of 20°; C d_90 is the drag coefficient calculated using the corrected pressure coefficient at the angle of attack of 90°; C d_160 is the drag coefficient calculated in segments by Equation (3) at the angle of attack of 160°.

[0073] The lift coefficient within the range of the angle of attack α (20°, 360°) is obtained by calculating in segments using Equation (4).

[0074]

[0075] In the formula: α is the angle of attack; C l_α is the lift coefficient corresponding to the angle of attack α; C d_αis the drag coefficient corresponding to the angle of attack α; C d_(180-α) is the drag coefficient corresponding to the angle of attack 180° - α; C l_(α-180) is the lift coefficient corresponding to the angle of attack α - 180°, which is obtained by piecewise calculation according to Equation (4); C l_160 and C l_340 are the lift coefficients obtained by piecewise calculation according to Equation (4) at the angles of attack of 160° and 340° respectively; C l_0 is the lift coefficient obtained by the surface pressure integration method at the angle of attack of 0°.

[0076] As Figure 8 shown, it is a comparison between the predicted lift coefficient and drag coefficient in the present invention and the wind tunnel experimental values. As can be seen from the figure, the variation laws of the lift coefficient and the drag coefficient are basically consistent with the trends of the experimental values. In the range from 20° to the deep stall section, a deviation begins to appear between the two, and the predicted lift coefficient is slightly lower than the experimental value. In the interval from 120° to 200°, the predicted lift coefficient is higher than the experimental value at most positions, while in the remaining intervals, the deviation amplitude between the predicted value and the experimental value is relatively small. For the drag coefficient, in the small angle of attack range, the trends of the predicted value and the experimental value are consistent; however, as the drag coefficient further increases, the deviation between the two gradually increases, and the predicted value is slightly lower than the experimental value at the peak position, and the drag coefficient peak in the experimental results shows a wider distribution characteristic. Generally speaking, the variation laws of the lift coefficient and the drag coefficient as a whole show a trend consistent with the experimental data, and they show consistency in the variation characteristics. As Figure 9 shown, when this improved method is applied to the DU180 airfoil of the open experimental data, a similar conclusion is also obtained. The variation laws of the lift coefficient and the drag coefficient also generally show significant consistency.

[0077] The present invention is applicable to the measurement and prediction of the aerodynamic characteristics of a wind turbine airfoil in the full angle of attack range in wind tunnel experiments. This method is simple in design, high in reliability and small in computational amount, and can accurately predict the aerodynamic changes of the wind turbine airfoil under the target working conditions. Through wind tunnel measurement, the aerodynamic data under some working conditions can be obtained, and combined with the prediction model, the aerodynamic characteristic changes of the wind turbine airfoil in the full angle of attack range can be obtained through a small amount of calculation. This provides convenience for estimating the aerodynamic performance of an actual full-scale wind turbine, has significant engineering application value and economic benefits, and has important engineering significance for the evaluation of the aerodynamic efficiency of modern wind turbines.

[0078] The present invention is not limited to the above embodiments. Based on the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and deformations to some technical features without creative labor according to the disclosed technical content, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A method for aerodynamic measurement and prediction of the full angle of attack of a wind turbine airfoil for wind tunnel experiments, characterized in that: The following steps are involved: Step 1) using three-dimensional modeling software to model a wind turbine airfoil and using metal materials to make the wind turbine airfoil, designing a wind turbine airfoil model for a wind tunnel experiment and building a test platform; Step 2) Check the safety of equipment and experimental environment, including: whether the connecting fasteners are loose; whether the airfoil is fixedly installed; whether the signal acquisition circuit is correctly connected and operating normally, and ensure that the inside of the wind tunnel is clean; Step 3) starting the wind tunnel to obtain a set wind speed; Step 4) changing the angle of attack in sequence, obtaining the pressure data of the wind turbine airfoil at the desired angle of attack, and closing the wind tunnel after the measurement is completed; Step 5) Adjust the pitot tube to the front of the wind turbine airfoil, repeat step 3), collect the incoming flow pressure in front of the wind turbine airfoil at the attack angle of 90° and 270°, and close the wind tunnel after the measurement is completed; Step 6) data processing to obtain the correction coefficient of the incoming wind speed; Step 7) Calculate the lift coefficient and the drag coefficient, obtain the drag coefficient ratio value, and predict the lift coefficient and the drag coefficient of the wind turbine airfoil at full angle of attack.

2. The method for aerodynamic measurement and prediction of the total angle of attack of a wind turbine airfoil for wind tunnel experiments according to claim 1, characterized in that: The wind turbine airfoil in step 1) is manufactured by metal turning technology, and its surface is polished for multiple times; a fixing rod extending outward is provided at 1 / 4 of the chord length of the upper end face of the airfoil; the fixing rod is rotatably connected to the end plate through a bearing; a gap is left between the end plate and the upper end face of the airfoil; the gap is filled with sponge tape; grease is coated on the surface of the end plate corresponding to the sponge tape; an embedded hole is provided at 1 / 4 of the chord length of the lower end face of the airfoil; the embedded hole is fixedly connected to the turntable through bolts.

3. The method for aerodynamic measurement and prediction of the total angle of attack of a wind turbine airfoil for wind tunnel experiments according to claim 1, characterized in that: The wind turbine airfoil in step 1) is hollow inside and is provided with a capillary metal tube. The upper and lower surfaces of the airfoil are provided with a plurality of spaced pressure measuring holes. The projected angle between the connecting line of the plurality of upper surface pressure measuring holes and the chord length of the airfoil is 20°, and the projected angle between the connecting line of the plurality of lower surface pressure measuring holes and the chord length of the airfoil is 20°. One end of the capillary metal tube is inserted into the pressure measuring hole, and the other end is connected to a pressure measuring hose. The pressure measuring hose extends along the lower end surface of the airfoil and is connected to a pressure scanning valve.

4. The method for aerodynamic measurement and prediction of the total angle of attack of a wind turbine airfoil for wind tunnel experiment according to claim 1, characterized in that: In step 6), the ratio of the dynamic pressure obtained by measuring the pitot tube located in front of the airfoil in step 4) and the pitot tube located directly in front of the airfoil in step 5) is used as the correction coefficient γ, and then the pressure coefficient C after 90° and 270° correction is calculated using formula (1): pi_ : Where: p i is the static pressure of the ith pressure measuring hole; p z is the total pressure measured by the Pitot tube in front of the airfoil; p0 is the static pressure measured by the Pitot tube in front of the airfoil; γ is the ratio of the dynamic pressure measured by the Pitot tube in front of the airfoil and the Pitot tube after adjusting the Pitot tube to the front of the airfoil.

5. The method for aerodynamic measurement and prediction of the total angle of attack of a wind turbine airfoil for wind tunnel experiment according to claim 1, characterized in that: In step 7), the pressure data obtained in step 4) is used, and the surface pressure integration method is combined to obtain the lift coefficient and drag coefficient of the wind turbine airfoil in the range of the measured attack angle of 0° to 20°, and the drag coefficient C corresponding to the attack angle of 90° and 270° is calculated using the pressure coefficient corrected in step 6). d_90 and C d_270 , the drag coefficient ratio C is obtained by formula (2) rd : Where: C rd is the drag coefficient scaling value.

6. The method for aerodynamic measurement and prediction of the total angle of attack of a wind turbine airfoil for wind tunnel experiment according to claim 1, characterized in that: In step 7), the drag coefficient of the wind turbine airfoil at full angle of attack is obtained by segmented calculation using formula (3): Where: α is the angle of attack; C d_α is the drag coefficient corresponding to the attack angle α; C d_20 is the drag coefficient corresponding to the attack angle of 20°; C d_90 When the attack angle is 90°, the drag coefficient is calculated using the corrected pressure coefficient; C d_160 When the angle of attack is 160°, the drag coefficient is calculated piecewise using formula (3).

7. The method for aerodynamic measurement and prediction of the total angle of attack of a wind turbine airfoil for wind tunnel experiments according to claim 6, characterized in that: In step 7), the drag coefficient at the angle of attack α in the range of (180°, 360°) is obtained by mirroring the result obtained by formula (3) along α=180° and then multiplying it by the drag coefficient reduction value C rd get.

8. The method for aerodynamic measurement and prediction of the total angle of attack of a wind turbine airfoil for wind tunnel experiment according to claim 1, characterized in that: In step 7), the lift coefficient of the angle of attack α in the range of (20°, 360°) is obtained by segmented calculation using formula (4): Where: α is the angle of attack; C l_α is the lift coefficient corresponding to the angle of attack α; C d_α is the drag coefficient corresponding to the attack angle α; C d_(180-α) is the drag coefficient corresponding to the attack angle of 180°-α; C l_(α-180) is the lift coefficient corresponding to the attack angle of α-180°, which is obtained by segmented calculation of formula (4); C l_160 and C l_340 The lift coefficient calculated by the formula (4) at the attack angle of 160° and 340° respectively; C l_0 The lift coefficient obtained by the surface pressure integration method at an angle of attack of 0°.

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