NL2034625A

NL2034625A - Dynamic stall evaluation and prediction method for gurney flap wind turbine airfoil

Info

Publication number: NL2034625A
Application number: NL2034625A
Authority: NL
Inventors: Yang Hua; Li Nailu; Yang Junwei; Wang Xiangjun; Guo Kai
Original assignee: Guangling College Yangzhou Univ
Priority date: 2022-07-12
Filing date: 2023-04-19
Publication date: 2024-01-25
Also published as: NL2034625B1; CN115235724A

Abstract

The present disclosure discloses a dynamic stall evaluation and prediction method for Gurney ﬂap wind turbine airfoil, including: building a dynamic aerodynamic 5 characteristic test bench in a wind tunnel test section, and placing a airfoil section in a test section; setting acquisition parameters, and starting the wind tunnel, using a test device to collect the dynamic and static aerodynamic data of the test airfoil section under different working conditions, and equipping a Gurney ﬂap to repeat the above test, processing data, and evaluating the aerodynamic performance control effect of the 10 Gurney ﬂap wind turbine, substituting the empirical constants obtained from the test into a semi-empirical prediction model to predict the unsteady aerodynamics of the wind turbine airfoil equipped with the Gurney ﬂap.

Description

DYNAMIC STALL EVALUATION AND PREDICTION METHOD FOR

GURNEY FLAP WIND TURBINE AIRFOIL

TECHNICAL FIELD

[0001] The present disclosure relates to a field of wind tunnel test and aerodynamic characteristic test of airfoil, in particular to a dynamic stall evaluation and prediction method for Gurney flap wind turbine airfoil.

BACKGROUND

[0002] As a kind of renewable energy with rich reserves, wide distribution, huge development potential and environmental protection, the development and utilization of related technologies are in line with the vision of energy structure adjustment, reducing greenhouse gas emissions and mitigating environmental pollution. The main forms of wind energy utilization include wind power generation, wind water pumping, wind heating, etc. Because of the obvious advantages such as short infrastructure period, low operation and maintenance costs, and excellent environmental benefits, wind energy technology has developed rapidly in the past few decades and has been gradually become one of the new energy sources with the most development value. However, in the actual operation of wind turbines, incoming turbulence, gusts, yaw, and blade torsion will cause the angle of attack of each section of the blade to change continuously, and the airflow around the blade cannot be changed in real time due to the time delay effect, which will lead to the occurrence of dynamic stall and aerodynamic hysteresis, so the influence of the dynamic stall cannot be ignored in terms of accurately predicting the aerodynamic load on the blade. When the airfoil is undergoing unsteady motion, the aerodynamic force of the airfoil will hysteresis with the instantaneous change of the angle of attack, and the stall angle will be larger than that of the static state, this phenomenon is called dynamic stall.

[0003] According to the summary of previous studies, dynamic stall involves processes of fluid attachment, separation and reattachment on the airfoil surface. When the pitching angle of attack is small, the dynamic airfoil change is similar to the static airfoil, and the flow is in an attached state. When the angle of attack exceeds the static stall angle, a separation vortex appears at the leading edge of the airfoil, and reverse flow occurs at the trailing edge. As the angle of attack continues to increase, the leading edge vortex accumulates and moves chordwise, and the lift continues to increase. When the leading edge vortex surrounds the entire upper surface of the airfoil, the aerodynamic force reaches the maximum value. At this time, the dynamic stall occurs, and the leading edge stall vortex begins to fall off, and finally separates from the airfoil at the trailing edge. Until the angle of attack drops low enough, the airflow will reattach to the airfoil.

Dynamic stall can be considered to be related to the sharp change of unsteady flow, and the dynamic stall characteristics can be reflected by measuring the instantaneous aerodynamic force of airfoil during pitch oscillation in wind tunnel test. On the other hand, minor modifications to the blades and the addition of additional parts are one of the best ways to improve the aerodynamic performance of a wind turbine. It is necessary to study the aerodynamic changes of the airfoil when the dynamic stall interacts with the

Gurney flap. Therefore, the present disclosure designs a wind tunnel test method to reveal and evaluate the dynamic stall characteristics of the Gurney flap airfoil. And according to the aerodynamic changes of the Gurney flap, a high-precision dynamic stall semi-empirical model is designed to make it suitable for the aerodynamic prediction of wind turbine airfoil equipped with the Gurney flap. The obtained results are helpful to better understand and evaluate the change law of dynamic characteristics of wind turbine airfoil with the Gurney flap under high turbulence working condition.

[0004] In the existing research on the dynamic stall of wind turbine airfoils, the description of the dynamic stall phenomenon and the analysis of specific influencing factors are mainly involved, but the flow control evaluation of aerodynamic accessories and the prediction research of the engineering empirical model are rarely involved. The traditional method of evaluating the aerodynamic characteristics of aerodynamic attachments is relatively simple, and quantitative analysis is generally carried out by comparing the lift-drag characteristics of wind turbine airfoils with or without attachments. The existing airfoil dynamic stall prediction models need to assign values to semi-empirical constants in practical applications. However, due to the lack of public test data, the research on aerodynamic prediction of the L-B model 1s mainly verified by

NACA and S series airfoils. For the dynamic aerodynamic prediction of other types of airfoils, the semi-empirical models will have obvious shortcomings. In the existing semi- empirical models, there is no specific description of the effect of the Gurney flap on the dynamic stall, so it is necessary to design and establish a prediction method for the dynamic aerodynamic force of the Gurney flap airfoil.

SUMMARY

[0005] The purpose of the present disclosure is to overcome the defects of the prior art, provide a dynamic stall evaluation and prediction method for Gurney flap wind turbine airfoil to evaluate the dynamic aerodynamic performance control effect of the Gurney flap from multiple angles with higher accuracy and predict the dynamic stall characteristics, which has important engineering significance for the research of the aerodynamic characteristics of the wind turbine airfoil.

[0006] The purpose of the present disclosure is achieved as: a dynamic stall evaluation and prediction method for Gurney flap wind turbine airfoil, including the following steps:

[0007] (1) building a wind turbine airfoil dynamic aerodynamic stall test bench at a wind tunnel test section, vertically fixing a reference airfoil section used for test in the wind tunnel test section, placing a grid at an entrance of the wind tunnel test section, and modulating and simulating a grid turbulent field under a real wind working condition;

[0008] (2) adjusting parameters of a controller of a servo motor, controlling the servo motor to determine an angle of attack of an airfoil section; and setting a sampling frequency and sampling time of a pressure sensor;

[0009] (3) starting a wind tunnel, adjusting a frequency of a control cabinet to obtain a set wind speed, and collecting dynamic and static aerodynamic data of the airfoil section under different turbulence intensities and angles of attack;

[0010] (4) closing the wind tunnel, equipping a Gurney flap at a trailing edge of a pressure surface of a test airfoil section, repeating step (3), and closing the wind tunnel after the test is completed,

[0011] (5) processing data, calculating aerodynamic force of the airfoil under each working condition, and comparing and analyzing aerodynamic characteristics of the equipped Gurney flap rear airfoil;

[0012] (6) comprehensively evaluating an aerodynamic performance control effect of the Gurney flap wind turbine airfoil in a form of weighted average;

[0013] (7) performing a prediction of dynamic stall characteristics of the reference airfoil by modified empirical constant of a parametric semi-empirical model obtained from the wind tunnel test;

[0014] (8) completing a prediction of dynamic stall characteristics of the Gurney flap wind turbine airfoil by the modified empirical constant of the parametric semi-empirical model obtained from the wind tunnel test.

[0015] As a further improvement of the present disclosure, the reference airfoil section is manufactured by 3D printing, and three rows of pressure taps are arranged on upper and lower surfaces of the airfoil section, the pressure taps are connected to vent holes at a bottom of the airfoil through an internal channel of the airfoil, and a base of the reference airfoil section is arranged with grooves; the reference airfoil section is provided with a straight through hole at 1/4 of a leading edge; the Gurney flap adopts rectangular section and 3D printing processing, and grooves are set at upper and lower edges of the Gurney flap to fixedly connect with a trailing edge of the reference airfoil section. The design of the test model is reasonable and easy to operate. In an actual aerodynamic test comparison, it is only necessary to glue the Gurney flap to the trailing edge of the airfoil pressure surface without replacing the airfoil section model, which improves the test efficiency.

[0016] As a further improvement of the present disclosure, the step (5) of calculating aerodynamic force of the airfoil under each working condition, specifically includes: static lift-drag ratio, maximum lift coefficient in a pitching period, lift change in the pitching period, stability performance after stall in the pitching period, and safety and stability performance in the pitching period, wherein stability performance M after stall and the safety and stability performance of a pitching airfoil are calculated by formula (1) and formula (2);

M= a (E22 {Cy

[0017] ZN (1)

IC, da

[0018] : TA’ (2)

[0019] 1n the formulas, a is an actual angle of attack of the airfoil, Osu is a stall angle of attack of the airfoil, A is an amplitude of the airfoil oscillation, C:: is the lift coefficient in the pitching period, C's is a pitching moment coefficient in the pitching period, max{ C7} is the maximum lift coefficient in the pitching period, { is an aerodynamic damping coefficient used to quantify the safety and stability performance of a pitching dynamic airfoil.

[0020] As a further improvement of the present disclosure, the step (7) of the modified empirical constant specifically includes: for a correction of pitch-up stall, due to increase of vortex strength, the airfoil chord generates convection, causing additional overshoot of the normal force, and an overshoot value AC" x; of the normal force in a pitch-up stage is given by formula (3) and formula (4):

[0021] AC, =B (ff) 7. (3)

sin (ZL O<r<T, yo 27, 5

[0022] V, . [rte] ST, (

Ta

[0023] in the formulas, B is a relevant parameter of the overshoot value of the normal force, fi and fare a trailing edge separation point and a delay corrected final trailing edge separation point in a semi-empirical prediction model, and J; is a shape function 5 of a moving vortex influence on the normal force, T is a dimensionless time delay constant of the moving vortex, 7 is a decay time constant of the vortex, 71 is dimensionless time of the vortex propagation along a chord length, and subscript n is the n-th sampling time.

[0024] As a further improvement of the present disclosure, the step (7) of the modified empirical constant specifically includes: for a correction of a reattachment of the airfoil during a pitch-down stage, a secondary vortex is generated and convected during a reattachment process of airflow, and aerodynamic hysteresis leads to additional drop of the normal force, resulting in a low-key value AC"; as shown in formula (5) and formula (6);

[0025] AC, =B, (ff) Vs (5) sin’? (Zl O<zr, <T

[0026] V, = | 2 (6) les Tn 7, >,

[0027] 1n the formulas, B: is a relevant parameter of the low-key value of the normal force in a pitch-down stage, 7; 1s a dimensionless time delay constant of the reattachment process, Vy 1s a shape function of the influence on the normal force in an air reattachment stage, 7-is the dimensionless time of a vortex motion on a airfoil surface during the reattachment stage of the airfoil.

[0028] As a further improvement of the present disclosure, for the step (7) of performing a prediction of dynamic stall characteristics of the reference airfoil, a predicted value of a critical normal force coefficient is consistent with the normal force corresponding to the abrupt angle of attack of a static pitching moment of the airfoil.

[0029] As a further improvement of the present disclosure, the step (8) of completing a prediction of dynamic stall characteristics of the Gurney flap wind turbine airfoil, specifically includes: the lift increase caused by equipping flap in a pitch oscillation period 1s regarded as a result of a combined effect of an airfoil circulation and the moving vortex, as shown in formula (7) and formula (8);

[0030] Caen = Bs J" Cy gro (7)

CoB, (f+ 1) sin”

[0031] U, (8)

[0032] in the formula, Cir» 1s defined as an additional normal force coefficient of the circulation due to the equipped flap, B: is an additional normal force test parameter of the circulation, CN pro is defined as an additional normal force coefficient at a static zero angle of attack moment, Fis a shape function of the influence of the additional vortex motion on the normal force after equipping the Gurney flap, zis a dimensionless time parameter of the vortex motion caused by the Gurney flap, Bi is a normal force test parameter for the increase of the vortex strength caused by the equipped flap.

[0033] The present disclosure adopts the above technical scheme, compared with the prior art, the beneficial effects are as follows: the present disclosure evaluates and predicts the dynamic stall characteristics of the Gumey flap wind turbine airfoil, and provides theoretical and technical support for the design of wind turbine blades. As the most reliable way to verify the accuracy of the data, the wind tunnel test is used to evaluate and predict the dynamic stall of the wind turbine airfoil. Compared with the data obtained by numerical calculation and theoretical analysis, the test method is more direct and more accurate; and the empirical constants obtained through experiments can more accurately provide assistance for the subsequent determination of empirical constants.

[0034] The present disclosure comprehensively considers the aerodynamic change characteristics of the wind turbine airfoil in the whole range of pitching motion variation through the weighted average of various evaluation parameters. In addition to the comparison of the lift-drag coefficient, it also integrates flow development law and the vortex development change. A more overall and comprehensive evaluation is carried out by evaluating the stall smoothness characteristics, aerodynamic stability and lift force change rate, so as to evaluate the influence of airfoil aerodynamic characteristics.

[0035] In the existing semi-empirical models, the value of the critical normal force needs to be used to predict the occurrence of leading edge stall, that is, to judge the critical working conditions for the development of vortex motion. That is, the critical normal force coefficient is considered to be close to the normal force at the static stall position, and the pitching moment coefficient will also change suddenly at this time.

However, in the wind turbine wing with large thickness, the corresponding static stall angle and pitching moment coefficient sudden change angle are inconsistent, and the pitching moment abrupt angle of attack is obviously larger than the static stall angle. At this time, the critical normal force needs to find another suitable value. The value of the critical normal force in the present disclosure is consistent with the normal force corresponding to the abrupt angle of attack of the static pitching moment of the airfoil.

[0036] The existing airfoil dynamic stall prediction models need to assign values to the semi-empirical constant in practical application, but due to the lack of public test data, the research of L-B model aerodynamic prediction is mainly verified by NACA and S series airfoils. For the dynamic aerodynamic prediction of other types of airfoils, the semi-empirical models have obvious shortcomings. So that, the present disclosure obtains static parameters through wind tunnel tests, assigns values to parameters in semi- empirical formulas, further expands the applicability, and considers the correction of pitching stall and reattachment in pitching stage in the existing semi-empirical models, which provides reference for subsequent aerodynamic prediction of other airfoils.

[0037] The existing semi-empirical models lack a specific description of the effect of

Gurney flaps on dynamic stall, and it is necessary to design and establish a prediction method for the dynamic aerodynamic force of the Gurney flap. For this reason, according to different stages of dynamic stall development, the present disclosure regards the lift increase caused by equipping flaps in a pitch oscillation period as a result of a combined effect of an airfoil circulation and the moving vortex. So the semi- empirical prediction model is applicable to the wind turbine airfoil equipped with the

Gurney flap, and the accuracy of the semi-empirical prediction model designed by the present disclosure is verified in combination with wind tunnel test result, so that the model can be extended and applied to the unsteady aerodynamic prediction of the wind turbine airfoil equipped with the Gurney flap.

[0038] For the prediction of the unsteady aerodynamics of the wind turbine airfoil equipped with the Gurney flap, the present disclosure uses the existing empirical model to define the trailing edge separation point reflecting the separation working condition of the airfoil, and predicts the additional aerodynamic force of the Gurney flap in attached flow in combination with the additionally increased normal force coefficient at the time of static zero angle of attack. At the moment when the angle of attack is large, the vortex motion intensity function representing the airfoil surface and the trailing edge separation point reflecting the airfoil separation working condition are defined, that is,

the additional normal force generated by the vortices in the flap airfoil is positively correlated with magnitude of the aerodynamic lag of the vortex, so it is not necessary to add additional reference variables in the semi-empirical model, that is, on the basis of obtaining empirical constants through wind tunnel tests, the aerodynamic coefticient of the wind turbine airfoil with the Gurney flap can be more simply and accurately predicted, which is convenient for practical engineering design and application.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Fig. 1 is a schematic diagram of an overall process of the present disclosure;

[0040] Fig. 2 is a schematic diagram of a wind turbine airfoil dynamic aerodynamic stall test bench of the present disclosure;

[0041] Fig. 3 is a schematic diagram of the reference airfoil section used in the test of the present disclosure;

[0042] Fig. 4 is a schematic diagram of the Gurney flap of the present disclosure;

[0043] Fig. 5isa schematic diagram of the connection between the Gurney flap and the reference airfoil section of the present disclosure;

[0044] Fig. 6 is a comparison result of the safety and stability performance of pitching airfoil under different turbulence working conditions of the present disclosure;

[0045] Fig. 7 is a comparison result between a predicted value and a test value of the reference airfoil lift in a pitch period in the present disclosure;

[0046] Fig. 8 is a comparison result between a predicted value and a test value of the additional lift of the Gurney flap in a pitch period of the present disclosure;

[0047] Fig. 9 is a lift predicted value of the Gurney flap airfoil in a pitch period in the present disclosure.

[0048] Wherein, 1 reference airfoil section, 1-1 pressure tap, 1-2 through hole, 1-3 vent hole, 2 Gurney flap, 2-1 groove.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] As shown in Fig. 1, a dynamic stall evaluation and prediction method for Gurney flap wind turbine airfoil includes the following steps:

[0050] (1) building a wind turbine airfoil dynamic aerodynamic stall test bench at a wind tunnel test section, vertically fixing a reference airfoil section used for test in the wind tunnel test section, placing a grid at an entrance of the wind tunnel test section, and modulating and simulating a grid turbulent field under a real wind working condition.

The specific arrangement is shown in Fig. 2. A length, width and height of the arranged test section are 1m, 0.4m and 0.4m respectively. The airfoil section used in the test is

DTU-LN221. A design chord length of the airfoil section is 0.15m and a span length is 0.395m. The grid is placed at the entrance of the test section, and two pitot tubes are placed at the same section of the leading edge of the airfoil section, that is 0.42m away from the rear of the grid, which is used to record the incoming wind speed. The airfoil section is also placed 0.42m away from the rear of the grid.

[0051] The airfoil section used in an aerodynamic characteristic test is shown in Fig. 3.

The reference airfoil section 1 adopts the method of surface pressure measurement for aerodynamic analysis, and the reference airfoil section 1 is manufactured by 3D printing, and three rows of pressure taps 1-1 are arranged on upper and lower surfaces of the airfoil section, the pressure taps 1-1 are connected to vent holes at a bottom of the airfoil through an internal channel of the airfoil, and a base of the reference airfoil section is arranged with grooves. The reference airfoil section is provided with a straight through holel-2 at 1/4 of a leading edge. From the leading edge point to 90% of the relative chord length, 38 pressure taps 1-1 are arranged on a suction surface and a pressure surface respectively, a total of 61 pressure taps extends to the vent holes 1-3 at the bottom surface in the internal passage. The form of uniform arrangement is used to reduce the mutual interference between adjacent vent holes 1-3. The measured pressure is closely connected with the pressure scanning valve through a plastic hose with an inner diameter of 1.lmm connected to the vent hole, to ensure that there is no air leakage in each plastic hose during the measurement process.

[0052] The Gurney flap used in the test is shown in Fig. 4. The Gurney flap 2 adopts a rectangular cross-section, the flap is in a rectangular shape, and it is manufactured by 3D printing, a height is 1.5% of the relative chord length (2.25mm), a thickness is 0.75% of the relative chord length (1.1125mm), and a length of 25cm, processed by 3D printing.

At the same time, in order to increase the contact area between the Gurney flap and the trailing edge of the airfoil, the upper and lower edges of the Gurney flap 2 is provided with a groove 2-1 fixedly connected with the trailing edge of the reference airfoil section 1, as shown in Fig. 5.

[0053] (2) adjusting parameters of a controller of a servo motor, controlling the servo motor to determine an angle of attack of an airfoil section; and setting a sampling frequency as 333.3Hz and sampling time of a pressure sensor as 30s.

[0054] (3) starting a wind tunnel, adjusting a frequency of a control cabinet to obtain a set wind speed of 15m/s, and test the dynamic and static aerodynamic data of the airfoil section when the turbulence degree is 0.5%, 6.35% and 10.18% by changing the grid.

Specifically, a static working condition: 0° to 28° (interval 2°); a dynamic working condition: oscillation amplitude A=10°, average angle of attack Omean=10°, reduced frequency K=0.0167.

[0055] (4) closing the wind tunnel, equipping a Gurney flap at a trailing edge of a pressure surface of a test airfoil section, repeating step (3), and closing the wind tunnel after the test is completed.

[0056] (5) processing data and calculating aerodynamic force of the airfoil under each working condition, specifically including: static lift-drag ratio, maximum lift coefficient in a pitching period, lift change in the pitching period, stability performance after stall in the pitching period, and safety and stability performance in the pitching period, and comparing and analyzing aerodynamic characteristics of the equipped Gurney flap rear airfoil.

[0057] wherein stability performance M after stall and the safety and stability performance of a pitching airfoil are calculated by formula (1) and formula (2);

M = max | (C-max iC) PD

[0058] | oo (1) [ica (00597 = 7A’ 2)

[0060] in the formulas, a is an actual angle of attack of the airfoil, sau 1s a stall angle of attack of the airfoil, A is an amplitude of the airfoil oscillation, Cis the lift coefficient in the pitching period, Cy 1s a pitching moment coefficient in the pitching period, max{ C:} is the maximum lift coefficient in the pitching period, { is an aerodynamic damping coefficient used to quantify the safety and stability performance of a pitching dynamic airfoil.

[0061] The results of stability M after stall are shown in Table 1. Table 1 lists the stability performance of lift coefficient after stall under each working condition. It can be seen from the table that, as a flow control element installed at the tail edge of the airfoil, the effect of Gurney flap on the stability of the airfoil after separation stall is small, and does not show the advantages. Installing Gurney flap will reduce the stability performance of lift coefficient after stall, with the increase of turbulence intensity and pitching motion amplitude, the values representing the stability performance tend to increase, but at higher turbulence intensity, the difference between the Gini flap airfoil and the reference airfoil will decrease.

[0062] Table 1 Stability performance after pitching period stall

[0063] (A=10, Omean=10, K=0.0167) (A=12.5, Omean=12.5, K=0.0167) reference airfoil gurney reference airfoil gurney flap flap uniform 0.0149 0.0398 (167.11%) 0.0324 0.0448 (38.27%) flow

T.J. =6.35% 0.019% 0.0539 (170.85%) 0.0558 0.0766 (37.28%)

T1.=10.18% 0.0676 0.0849(25.59%) 0.0712 0.0724 (1.69%)

[0064] For the safety and stability of the airfoil, the present disclosure uses { in the formula (2) to represent the work done by the pitching airfoil to the surrounding flow field during the motion period, and the area formed by the pitching moment moving in the counterclockwise direction will produce positive aerodynamic damping, otherwise negative aerodynamic damping is formed. When C is negative, the airfoil is considered unstable. And when C is positive, the pitching airfoil is considered safe and stable. Fig. 6 is a comparison chart of aerodynamic damping coefficients at three different turbulence intensities. In all cases, { will deflect positively with the increase of turbulence. When the turbulence intensity is 6.35% and 10.35%, the installation of the

Gurney flap will make the aerodynamic damping coefficient of the DTU-LN221 airfoil positively shift, making the airfoil in a more stable state. When the turbulence intensity is 6.35%, the aerodynamic damping coefficient increased by 33.33% (light stall working condition) and 58.33% (deep stall working condition) after equipping the Gurney flap compared with the reference airfoil working condition. When the turbulence intensity is 10.18%, the corresponding increases are 32.20% (light stall working condition) and 34.13% (deep stall working condition).

[0065] (6) Comprehensively evaluating an aerodynamic performance control effect of the Gurney flap wind turbine airfoil. Considering the aerodynamic change characteristics of the wind turbine airfoil in the whole range of pitch motion variation, the form of weighted average is adopted. As in the embodiment of the present disclosure, the parameters of evaluation specifically include: static lift-drag ratio, maximum lift coefficient in a pitching period, lift change in the pitching period, stability performance after stall in the pitching period, and safety and stability performance in the pitching period. Through the comparison of the above parameters, the effect of Gurney on the dynamic stall can be described more comprehensively, and the comprehensive evaluation can be carried out by assigning weight and weighted average. According to the actual work, the above five parameters can be set to 30%, 30%, 15%, 15% and 15%, and then quantitative evaluation is performed.

[0066] (7) performing a prediction of dynamic stall characteristics of the reference airfoil by modified empirical constant of a parametric semi-empirical model obtained from the wind tunnel test. In the embodiment, taking the L-B dynamic stall semi- empirical model currently used in engineering practice as an example.

[0067] The pitch-up stall is corrected, because it takes a limited time for the disturbance in the airfoil boundary layer to make the vortex strong enough, so that the pressure coefficient value of the leading edge exceeds the critical value, leading to the occurrence of dynamic stall. So that, in step (7), in the prediction of the dynamic stall characteristics of the reference airfoil, for a correction of pitch-up stall, due to increase of vortex strength, the airfoil chord generates convection, causing additional overshoot of the normal force, and an overshoot value AC" y, of the normal force in a pitch-up stage is given by formula (3) and formula (4):

[0068] AC =B, (f"-f)V, 3) sin (5) 0<z<T,

Vo ty

[0069] V oe | z(2-T;) | ST, (4)

Ty

[0070] in the formulas, Bj is a relevant parameter of the overshoot value of the normal force, fi and fare a trailing edge separation point and a delay corrected final trailing edge separation point in a semi-empirical prediction model, and V is a shape function of a moving vortex influence on the normal force, T is a dimensionless time delay constant of the moving vortex, 7 is a decay time constant of the vortex, 71 is dimensionless time of the vortex propagation along a chord length, and subscript n is the n-th sampling time. At this time, the value of Bi is 1.5, the value of 7 is 6.0, and the value of 7 is 2.0.

[0071] At the same time, in the pitch-down stage of the airfoil, a secondary vortex is generated and convected during a reattachment process of airflow, and aerodynamic hysteresis leads to additional drop of the normal force, resulting in a low-key value. So that, in step (7), a prediction of dynamic stall characteristics of the reference airfoil is performed. for a correction of a reattachment of the airfoil during a pitch-down stage, a secondary vortex is generated and convected during a reattachment process of airflow, and aerodynamic hysteresis leads to additional drop of the normal force, resulting in a low-key value AC" a3, as shown in formula (5) and formula (6);

[0072] AC =B UAT, (5) sine() O<r <T, 27, 00731 Fe . = 2) or 27,

[0074] in the formulas, B: is a relevant parameter of the low-key value of the normal force in a pitch-down stage, 7ris a dimensionless time delay constant of the reattachment process, His a shape function of the influence on the normal force in an air reattachment stage, 7-1s the dimensionless time of a vortex motion on a airfoil surface during the reattachment stage of the airfoil. At this time, the value of Bz is 3.0, the value of 7: is 10.0.

[0075] Moreover, in the prediction of the dynamic stall characteristics of the reference airfoil, a predicted value of a critical normal force coefficient is consistent with the normal force corresponding to the abrupt angle of attack of a static pitching moment of the airfoil. The specific test results are shown in Fig. 7, and Fig.7 shows a comparison result of test value of the reference airfoil and predicted value of the original L-B model and the corrected model when T.1.=10.18% (oscillation amplitude is 10°, average angle of attack is 10°, reduced frequency is 0.0167). By comparison, it is found that the predicted value of the corrected L-B model in the working conditions involved in the figure 1s closer to the wind tunnel test value than the original L-B model, especially in the area before the flow reattachment in the pitch-up stall and pitch-down stage.

[0076] (8) Completing a prediction of dynamic stall characteristics of the Gurney flap wind turbine airfoil by the modified empirical constant of the parametric semi-empirical model obtained from the wind tunnel test. Wherein in, in the step (8), a prediction of dynamic stall characteristics of the Gurney flap wind turbine airfoil is completed. The lift increase caused by equipping flap in a pitch oscillation period is regarded as a result of a combined effect of an airfoil circulation and the moving vortex, as shown in formula (7) and formula (8), “ " pn

[0077] Con = B; f Cx ato (7)

Coury ("1 [sin GH"

[0078] / (8)

[0079] in the formula, Cir» is defined as an additional normal force coefficient of the circulation due to the equipped flap, with a value of 1.0, B: is an additional normal force test parameter of the circulation, CN pro is defined as an additional normal force coefficient at a static zero angle of attack moment, Fris a shape function of the influence of the additional vortex motion on the normal force after equipping the Gurney flap, tr is a dimensionless time parameter of the vortex motion caused by the Gurney flap, Ba is a normal force test parameter for the increase of the vortex strength caused by the equipped flap, with a value of 3.0.

[0080] Similarly, in order to verify the accuracy of the prediction method of the present disclosure, the aerodynamic prediction value of the method of the present disclosure is compared with the test value, and the result is shown in Fig. 8. Fig. 8 shows the comparison result between the predicted value of additional lift of Gunny flap and the test value during the pitching period (oscillation amplitude is 10° , average angle of attackis 10° | reduced frequency is 0.0167, turbulence intensity is 0.5%). It is observed that the change trend of the predicted value of aerodynamic model is similar to that of the wind tunnel test value. The lift caused by the equipped flap during the attached flow is mainly due to the increase of circulation, so the lift change is small. With the increase of the angle of attack, the additional lift becomes dominated by the vortex motion at this time, and the additional lift coefficient decreases, until the air reattaches, the additional lift increases again.

[0081] Compared with Fig. 9, it can be found that the aerodynamic model can accurately predict the lift coefficient of the Gurney flap airfoil, but there is still a small deviation between the prediction model and the test value in the deep stall stage, this is due to the fact that L-B model is not accurately modeled and described in the deep stall stage.

[0082] The design of the present disclosure is applicable to the evaluation and prediction of the dynamic and static aerodynamic data of the airfoil under different working conditions, and the control effect of the dynamic aerodynamic performance of the Gurney flap is evaluated from multiple angles with high accuracy, which has important engineering significance for the research of the aerodynamic characteristics of the wind turbine airfoil. The present disclosure is not limited to the above embodiments. On the basis of the technical solution disclosed by the present disclosure,

those skilled in the art can make some replacement and deformation of some of the technical features without creative work according to the disclosed technical content, which are within the protection scope of the present disclosure.

Claims

Conclusions l. Dynamic stall evaluation and prediction method for a Gurney flap wind turbine aerofoil, which includes the following steps: (1) building a dynamic-aerodynamic stall test bench for a wind turbine aerofoil in a wind tunnel test section, vertically mounting a reference aerofoil section used for the test in the wind tunnel test section, placing a grid at the entrance of the wind tunnel test section, and modulating and simulating a grid turbulent field under real wind conditions; (2) adjusting parameters of a servomotor control, wherein the servomotor is controlled to determine an angle of attack of an aerofoil section, and setting a sampling frequency and sampling time of a pressure sensor; (3) starting a wind tunnel, adjusting a control box frequency to obtain a set wind speed, and collecting dynamic and static aerodynamic data of the aerofoil section under different turbulence intensities and angles of attack; (4) closing the wind tunnel, equipping a Gurney flap on the trailing edge of a pressure surface of a test aerofoil section, repeating step (3), and closing the wind tunnel after the test is completed; (5) processing data, calculating aerodynamic force of the aerofoil under each working condition, and comparing and analyzing aerodynamic characteristics of the equipped Gumey flap-rear aerofoil; (6) to comprehensively evaluate an aerodynamic performance control effect of the Gurney flap wind turbine aerofoil in the form of a weighted average; (7) performing a prediction of dynamic stall characteristics of the reference aerofoil using an adjusted empirical constant of a parametric semi-empirical model obtained from the wind tunnel test; (8) completing a prediction of dynamic stall characteristics of the Gurney flap wind turbine aerofoil by means of the adjusted empirical constant of the parametric semi-empirical model obtained from the wind tunnel test.

2. Dynamic stall evaluation and prediction method for a Gurney

flap wind turbine aerofoil according to claim 1, wherein the reference aerofoil section is manufactured by 3D printing, and three rows of pressure transducers are placed on the upper and lower surfaces of the aerofoil section; the pressure transducers are connected to vent holes at the bottom of the aerofoil through an internal channel of the aerofoil, and a base of the reference aerofoil section is arranged with grooves; the reference aerofoil section has a straight through hole at 1/4 of the leading edge; the Gurney flap takes a rectangular shape and undergoes 3D printing, and grooves are provided on the top and bottom edges of the Gurney flap to securely connect it to the trailing edge of the reference aerofoil section.

A dynamic stall evaluation and prediction method for a Gumey flap wind turbine aerofoil according to claim 1, wherein step (5) of calculating the aerodynamic force of the aerofoil under each operating condition specifically includes: static lift-drag ratio, maximum lift coefficient in a pitch period, lift change in the pitch period, stability performance after stalling in the pitch period, and safety and stability performance in the pitch period, where the stability performance M after stalling and the safety and stability performance of a pitching aerofoil be calculated with formula (1) and formula (2); M = max J Gmax) Cr {1} 0.1 'da aA 2) where in the formulas, a is an actual angle of attack of the aerofoil, Osta is a stall angle of the aerofoil, A is an amplitude of the oscillation of the aerofoil , Cl 1s is the lift coefficient in the pitch period, Cu is the moment coefficient of the pitch movement in the pitch period, max{ C's} is the maximum lift coefficient in the pitch period, and { is an aerodynamic damping coefficient used to determine safety and quantify stability performance of a pitching dynamic aerofoil.

A dynamic stall evaluation and prediction method for a Gurney flap wind turbine aerofoil according to claim 1, wherein the step (7) of the modified empirical constant specifically includes: for a pitch-up stall correction, due to an increase in the vortex strength, the aerofoil-chord generates convection, causing additional excess of the normal force; and an excess value AC", of the normal force in a pitch-up phase is given by formula (3) and formula (4); ACT =B (f"- fF, 3) sin") O<r<T, = or @ ) cost] ZR) >, Ty where in the formulas, B; is a relevant parameter of the normal force exceedance value, f; and f”' are a trailing edge separation point and a trailing edge corrected endpoint in a semi-empirical prediction model, respectively, and Jy is a shape function of a moving vortex effect on the normal force, T is a dimensionless time delay constant of the moving vortex, 73 is a decay constant of the vortex, 7 is the dimensionless time of vortex propagation along a chord length and subscript n is the nth sampling time.

The dynamic stall evaluation and prediction method for a Gumey flap wind turbine aerofoil according to claim 1, wherein the step (7) of the adjusted empirical constant specifically comprises: for a correction of a sticking of the aerofoil during a pitch-down stage, a secondary vortex is generated and convected during an airflow adhesion process, and aerodynamic hysteresis leads to additional drop in normal force, resulting in a low value AC" yy, as shown in formula (5) and formula (6); ACY =B, ("1H ¥, (5) cost Ls Tan where in the formula, B: is a relevant parameter of the low-value of the normal force in a pitch-down phase, 7: is a dimensionless time delay of the bonding process , Vx is a shape function of the influence on the normal force in an air adhesion phase, and 7 is the dimensionless time of a vortex movement on an aerofoil surface during the aerofoil adhesion phase.

The dynamic stall evaluation and prediction method for a Gurney flap wind turbine aerofoil according to claim 1, wherein for the step (7) of performing a prediction of the dynamic stall characteristics of the reference aerofoil, a predicted value of a critical normal force coefficient consistent with the normal force corresponding to the sudden angle of attack of a static pitch moment of the aerofoil.

The dynamic stall evaluation and prediction method for a Gurney flap wind turbine aerofoil according to claim 1, wherein the step (8) of completing a prediction of dynamic stall characteristics of the Gurney flap wind turbine aerofoil specifically includes: the lift increase caused by resting the flap in a period of pitch oscillation is considered as a result of a combined effect of an aerofoil circulation and the moving vortex, as shown in formula (7) and formula (8); Care =B, f ’ Cx aro (7) . ee AT Cos =B, (f= SO) sin] (8) 27, where in the formula, Cex, is defined as an additional normal force coefficient of the circulation due to the equipped flap, B: is an additional normal force test parameter of the circulation , Cy gro is defined as an additional normal force coefficient at a static moment of zero degrees angle of attack, I is a shape function of the influence of the additional vortex motion on the normal force after equipping the Gurney flap, z is a dimensionless time parameter of the vortex motion that causes by the Gurney flap, Bs is a normal force test parameter for the increase in vortex strength caused by the fitted flap.