CN114858405B - Method for determining simulation range of rough zone of airfoil test - Google Patents

Abstract

The present invention provides a method for determining the simulation range of the rough band of an airfoil test, which is used to accurately simulate the position of the rough band on the surface of the airfoil in a wind tunnel test, and can accurately show the difference in the leading edge pollution range caused by the difference in the spanwise position of the blade and the airfoil geometry. The method steps include: (1) In the wind wheel system, the motion trajectory of the pollutants is calculated using the Lagrangian particle tracking method; (2) The pollutant collection efficiency per unit area of the model surface per unit time is determined by the mass flux that hits the model surface within a unit span; (3) The pollutant collection efficiency curve on the airfoil surface is drawn; (4) A pollution collection efficiency cutoff threshold is given to determine the pollution roughness simulation range, and the range above this value is the roughness simulation range. The present invention can freely determine the leading edge pollution roughness simulation range according to the tester's acceptance of pollution.

Description

A method for determining the simulation range of rough zone in airfoil test

Technical Field

The invention particularly relates to a method for determining a rough zone simulation range of an airfoil test.

Background Art

Wind turbines rely on rotor blades to capture wind energy, and the aerodynamic performance of the blade airfoil is the fundamental factor that determines the power characteristics and load characteristics of wind turbines. When wind turbine blades work under natural conditions in the atmospheric troposphere, the blade surface is easily affected by dust pollution, insect pollution, and wind erosion, which damage the surface finish of the blade and make the blade surface rough; at the same time, blade manufacturing errors can also cause deformation of the airfoil profile and increase the surface roughness of the airfoil.

The susceptibility of a blade to damage due to leading edge contamination depends on two factors: the stability of the boundary layer in the presence of asperities and the tendency of the wing or blade to collect contaminants.

When the leading edge of the airfoil is rough, the transition position of the airfoil boundary layer will move forward, the thickness of the boundary layer after the transition will increase, and the equivalent curvature of the airfoil will be reduced, thereby reducing the maximum lift coefficient; at the same time, the surface roughness can cause the laminar boundary layer to transition to the turbulent boundary layer, increase the friction resistance, and reduce the lift-to-drag ratio of the airfoil in a large range; in addition, the roughness of the leading edge of the airfoil will also cause the blade to have double stall characteristics, which will seriously affect the aerodynamic characteristics of the airfoil and lead to a reduction in the output power of the wind turbine. Studies have shown that pollution from factors such as insects and dust can reduce the output power of the wind turbine by about 25% to 30%. Studies have also shown that the leading edge of the airfoil is the most sensitive area of the rough surface, so it is of great significance to study the influence of the roughness of the leading edge of the airfoil.

Wind tunnel testing is the most reliable means of obtaining the effect of leading edge roughness on airfoil performance. Currently, the main means of simulating leading edge roughness are: (1) Standard roughness strips. Use one or more standard roughness strips to simulate leading edge roughness, such as using Z-Z roughness strips or zigzag strips. This simulation method is the most widely used, but it is not accurate in simulating leading edge roughness. (2) Distributed roughness elements. For example, spraying cylindrical particle units or rough simulation stickers on the surface of the model. This type of particle unit usually has a low aspect ratio (k/d, k is the particle height, d is the particle diameter), and cannot accurately simulate leading edge contamination. (3) Use mosquito or sand and gravel sprayers to spray pollutants onto the surface of the test model. This method simulates the most accurately, but the test cost is too high and it is almost unrealistic for large-scale tests.

The test simulation range of the roughness band is generally determined by experience. The standard roughness band is in the form of a strip and is pasted at a fixed chord position: if the relative thickness of the airfoil is less than 30%, the roughness band is only pasted at the 5%c position on the upper surface; if the relative thickness of the airfoil is greater than 30%, the upper surface is pasted at 5%c and the lower surface is pasted at 10%c. For distributed roughness units, the roughness units are generally distributed within the 5%c range of the upper surface.

Since the above rough simulation range is determined semi-empirically, it does not take into account the differences in the shapes of the various airfoils themselves and the differences in the spanwise positions of the airfoils on the wind rotor blades. Therefore, it is unable to reflect the performance impact of various airfoils under real leading edge contamination.

Summary of the invention

Based on the above shortcomings, the present invention provides a method for determining the simulation range of the rough band of an airfoil test, which breaks through the limitation of using a semi-empirical method to determine the rough simulation range in traditional wind tunnel tests, and is used to accurately simulate the position of the rough band on the surface of the airfoil in a wind tunnel test. The leading edge contamination roughness simulation range can be freely determined according to the tester's acceptance of contamination, and the difference in the leading edge contamination range caused by the difference in the spanwise position of the blade and the airfoil geometry can be more intuitively reflected.

The technical solution adopted by the present invention is as follows: A method for determining the simulation range of the rough zone of an airfoil test, the steps are as follows:

(1) Calculate the trajectory of pollution particles

Assuming that the pollution particles are initially stationary relative to the free flow speed, the initial velocity of the pollutants in the calculation model system is the superposition of the wind speed and the blade rotation speed; the initial velocity is assigned to the pollution particles at a position 3 to 5 times the chord length upstream of the leading edge of the airfoil. When the pollution particles approach the blades, the change in local velocity induces resistance, thereby changing the motion trajectory of the pollution particles. The influence of the resistance is determined by the motion equation of the pollution particles:

in:

K＝2m/(ρS _ref c) (3)

Wherein, _CD is the pollution particle resistance coefficient, and are parallel and perpendicular to the airfoil chord, respectively. α is the airfoil angle of attack. The angle between the pollution particle velocity and the relative wind speed is defined as γ. The mass parameter K is given in dimensionless form and is determined by the particle mass m, air density ρ, particle reference area S _ref and airfoil chord length c. The K value reflects the degree of fit between the pollution particle trajectory and the streamline.

The pollutant particles are set as spheres, and their drag coefficients are determined by the drag coefficients of the spheres under different Reynolds numbers. The Reynolds number of the pollutant particles is calculated using the particle diameter and wind speed. When the Reynolds number is 4000-200000, the drag coefficient of the sphere is approximately a constant. When the Reynolds number is <4000, the drag coefficient of the sphere is not a constant and is corrected using Stokes' law. The formula for the segmented drag coefficient is as follows:

Wherein, _CD is the drag coefficient of the polluted particles, Re is the Reynolds number of the particle motion, _Reref and _CD,ref represent the critical Reynolds number and the critical drag coefficient respectively;

The above drag coefficient formula is combined with the flow field data and the discrete forms of formulas (1) and (2) to calculate the trajectory of the pollution particles based on the initial conditions;

(2) Calculation of pollution particle collection efficiency

The pollution particle collection efficiency is described as the mutual motion trajectory of a pollution particle. The two trajectories are considered to form a particle channel between them. As the distance between the trajectories widens, the particle channel widens, and the particle propagation distance increases. The pollution particle collection efficiency β = dy ₀ /ds, which describes the widening of the particle channel through differential form, where dy ₀ is the distance between the two particles far upstream perpendicular to the equivalent incoming flow, and ds is their length on the airfoil surface;

(3) Draw the collection efficiency curve

The horizontal axis s is the length of the curve on the airfoil surface, and the vertical axis is the pollution particle collection efficiency. On the airfoil surface, the leading edge position is defined as 0, the upper surface is positive, and the lower surface is negative. There is also pollution in the middle and rear part of the airfoil, but as it develops downstream, the pollution particle collection efficiency decreases rapidly.

(4) Determine the scope of pollution simulation

All positions of the airfoil will be affected by pollution, but as it develops towards the trailing edge, the pollution collection efficiency decreases rapidly and the impact of pollution decreases rapidly. The cutoff threshold of the pollution particle collection efficiency is set according to the tester's acceptance of pollutants. When the pollution particle collection efficiency is lower than this threshold, it is considered that the impact of pollution can be ignored. The range above this value is the rough simulation range.

Furthermore, preferably, the cutoff threshold is β=0.2-0.4.

The beneficial effects and advantages of the present invention are as follows: the present invention can accurately determine the roughness simulation range at different spanwise positions and on different airfoils of the wind wheel blades; at the same time, the difference in pollution range can be finely considered according to the difference in pollutant particles; and the difference in pollution range can be finely determined according to the difference in pollution tolerance of the designer; the present invention improves the test efficiency and the accuracy of the test.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagram showing the movement trajectory of pollution particles when K = 0.01;

Figure 2 is a diagram showing the movement trajectory of pollution particles when K = 1;

FIG3 is a schematic diagram of the distance between two particles far upstream perpendicular to the equivalent incoming flow and the distance on the airfoil surface;

Figure 4 is a schematic diagram of the flow distance between the upper and lower surfaces of the airfoil

FIG5 is a graph showing the collection efficiency of three airfoils with different thicknesses.

FIG6 is a schematic diagram of the roughness range that needs to be simulated on the leading edge of the airfoil corresponding to β=0.2;

Figure 7 shows the airfoil profiles for NACA63 ₃ -418 and NREL S814 wind forces;

Figure 8 is a cloud diagram of the impact velocity distribution at different positions along the span of the NACA63 ₃ -418 blade;

FIG9 is a cloud diagram of the impact velocity distribution at different positions of the NREL S814 blade span;

Figure 10 is a cloud diagram showing the pollution collection efficiency distribution of NACA63 ₃ -418 components at different positions along the blade span;

Figure 11 is a cloud diagram showing the pollution collection efficiency distribution of NREL S814 at different positions along the blade span;

FIG12 is a graph showing the collection efficiency of the blade section at a spanwise position of 30 m from the hub;

FIG13 is a schematic diagram of the roughness range of the airfoil leading edge that needs to be simulated when β is set to 0.3;

DETAILED DESCRIPTION

The present invention is further described below with reference to the accompanying drawings:

Example 1

A method for determining the rough zone simulation range of an airfoil test, the steps are as follows:

(1) Calculate the trajectory of pollution particles

In the wind turbine blade system, the Lagrangian tracking method is used to calculate the trajectory of the pollution particles. Assuming that the pollution particles are initially stationary relative to the free stream velocity, the initial velocity of the pollutants in the calculation model system is the superposition of the wind speed and the blade rotation speed. The initial velocity is assigned to the pollution particles at a position 3 to 5 times the chord length upstream of the leading edge of the airfoil. When the pollution particles approach the blades, the change in local velocity induces drag, thereby changing the trajectory of the pollution particles. The influence of drag is determined by the motion equation of the pollution particles:

in:

K＝2m/(ρS _ref c) (3)

In the equation, _CD is the drag coefficient of the pollutant particles, and Parallel and perpendicular to the airfoil chord, respectively. α is the airfoil angle of attack, and the angle between the pollution particle velocity and the relative wind speed is defined as γ. The mass parameter K is given in dimensionless form and is determined by the particle mass m, air density ρ, particle reference area S _ref (reference area used to calculate the drag coefficient) and airfoil chord length c. The K value reflects the degree of fit between the pollution particle trajectory and the streamline; the smaller the K value, the closer the particle trajectory is to the streamline; K→0 means that the pollution particle and the streamline are completely consistent. A high K value means that the particle is less affected by changes in the surrounding flow field and tends to fly along a straight path. Figures 1 and 2 show the trajectory of the pollution particles when K=0.01 and K=1.

In the calculation, the pollutant particles are simplified as particles, and the initial direction cannot be determined. It is also assumed that the pollutant particles will not consciously respond to changes in the relative wind speed near the blades. These assumptions cause the influence of lift to be ignored in equations (1) and (2). In the calculation, the pollutant particles are assumed to be spheres, and their drag coefficients are determined by the drag coefficients of the spheres at different Reynolds numbers. The Reynolds number calculation of the pollutant particles uses the particle diameter and wind speed. At larger Reynolds numbers (4000-200000), the drag coefficient of the sphere is approximately a constant. At smaller Reynolds numbers (<4000), the drag coefficient of the sphere is not a constant and is corrected using Stokes' law. The formula for the segmented drag coefficient is as follows:

Wherein, _CD is the drag coefficient of the polluted particles, Re is the Reynolds number of the particle motion, _Reref and _CD,ref represent the critical Reynolds number and the critical drag coefficient respectively;

The above drag coefficient formula, combined with the flow field data and the discrete forms of formulas (1) and (2), calculates the trajectory of the pollution particles based on the initial conditions.

(2) Calculation of pollution particle collection efficiency

The content of pollutant particles in the atmosphere is defined as the mass of pollutant particles per unit volume in the free stream, which expresses the density of pollutant particles. The collection efficiency also describes the trajectories of a pollutant particle moving relative to each other. The two trajectories can be considered to form a particle conduit between them. As the distance between the trajectories widens, the particle conduit widens and the distance that the particles travel increases. The collection efficiency describes the widening of the particle conduit through the differential form of β = dy ₀ /ds, where dy ₀ is the distance between the two particles far upstream perpendicular to the equivalent incoming flow, and ds is their length on the airfoil surface. This length is described as shown in Figure 3.

(3) Draw the collection efficiency curve

The horizontal axis s is the length of the curve on the airfoil surface, and the vertical axis is the collection efficiency of polluted particles. On the airfoil surface, the leading edge position is defined as 0, the upper surface is positive, and the lower surface is negative. The definition is shown in Figure 4. In theory, there is also a pollution problem in the middle and rear part of the airfoil, but as it develops downstream, the collection efficiency decreases rapidly, that is: when the curved surface length s/c→the trailing edge of the airfoil, the collection efficiency β→0. Figure 5 shows the collection efficiency curves of three airfoils with different thicknesses.

(4) Determine the scope of pollution simulation

All positions of the airfoil will be affected by pollution, but as it develops towards the trailing edge, the efficiency of collecting polluted particles decreases rapidly, and the impact of pollution decreases rapidly. In actual experiments, a cutoff threshold for the collection efficiency of polluted particles can be specified. When it is lower than this threshold, it is considered that the impact of pollution can be ignored. The selection of the threshold is determined by the tester's acceptance of pollution, and it can generally be set to around β = 0.2 to 0.4. If β = 0.2 is set as the cutoff threshold for Figure 6, the horizontal coordinate position of the upper and lower surfaces corresponding to β = 0.2 is the roughness range that needs to be simulated on the leading edge of the airfoil.

Example 2

As shown in FIG. 7 , this embodiment selects NACA63 ₃ -418 and NREL S814 wind turbine airfoils as examples. The relative thicknesses of the two airfoils are 18% and 24% respectively. This embodiment assumes that the wind turbine blades use the same airfoil composition from the blade root to the blade tip, and the blade span is 60m.

A method for determining the rough zone simulation range of an airfoil test, the steps are as follows

(1) Calculate particle motion trajectory

Figures 8 and 9 are the impact velocity distribution cloud diagrams of NACA63 ₃ -418 and NREL S814 blades at different positions along the span. Since the blades on the outside rotate at a higher speed, the particle impact velocity is also higher.

(2) Calculation of pollution collection efficiency

Figures 10 and 11 are respectively the cloud diagrams of the collection efficiency of polluted particles at different positions of the blade span of NACA63 ₃ -418 and NREL S814. The thick black line in the figure is the position corresponding to β = 0.3.

(3) Draw the collection efficiency curve

Figure 12 shows the particle collection efficiency curve of the blade section at a spanwise position of 30m from the hub. It can be seen that on the upper surface, the collection efficiency curves of the two airfoils are relatively close, and the collection efficiency decreases rapidly when away from the leading edge. The collection efficiency of the lower surface shows a large difference: the collection efficiency of the S814 airfoil is higher at the leading edge of the lower surface, which is caused by the larger leading edge radius of the S814 airfoil; but away from the leading edge, the collection efficiency of the S814 airfoil is quickly effective, while the NACA63 _3-418 has a larger range of pollution collection, which reflects the difference in airfoil collection.

(4) Determine the scope of pollution simulation

Determine β = 0.3 as the pollutant particle collection efficiency cutoff threshold, and determine the rough simulation range of the two blade sections in Figure 13. The simulation ranges of the upper surfaces of the two airfoils are similar, and the simulation ranges of the lower surfaces are quite different. Therefore, the rough simulation range of the S814 airfoil is from the upper surface s/c = 0.045 to the lower surface s/c = 0.192, and the rough simulation range of the NACA63 ₃ -418 airfoil is from the upper surface s/c = 0.045 to the lower surface s/c = 0.125.

Claims (2)

1. A method for determining the simulation range of a rough belt for an airfoil test, comprising the steps of:

(1) Calculating the motion trail of the polluted particles

Setting the pollutant particles to be static relative to the free flow velocity initially, wherein the pollutant is in a calculation model system, and the initial velocity is the superposition of the wind speed and the rotation speed of the blades; assigning an initial speed to the pollutant particles at a chord length position 3-5 times upstream of the front edge of the airfoil, and when the pollutant particles are close to the blade, inducing resistance by the change of the local speed, so that the movement track of the pollutant particles is changed, wherein the influence of the resistance is determined by the movement equation of the pollutant particles:

Wherein:

K＝2m/(ρS_refc) (3)

wherein C _D is the resistance coefficient of the pollution particles, AndThe method is characterized in that the method is respectively parallel to and perpendicular to airfoil chord lines, alpha is an airfoil attack angle, an angle between a polluted particle speed and a relative wind speed is defined as gamma, a quality parameter K is given in a dimensionless form and is determined by a particle mass m, an air density rho, a particle reference area S _ref and an airfoil chord length c, and the K value reflects the coincidence degree of a polluted particle track relative streamline;

The pollutant particles are set into spheres, the resistance coefficients of the pollutant particles are determined by the resistance coefficients of the spheres under different Reynolds numbers, the Reynolds numbers of the pollutant particles are calculated by using the particle diameter and the wind speed, the resistance coefficients of the spheres are approximately constant at the Reynolds numbers of 4000-200000, and the resistance coefficients of the spheres are not constant at the Reynolds numbers of <4000 and are corrected by using Stokes' law, and the segmentation resistance coefficients are calculated by the following formula:

Wherein, C _D is the resistance coefficient of the polluted particles, re is the Reynolds number of the particle movement, re _ref and C _D,ref respectively represent the critical Reynolds number and the critical resistance coefficient;

Calculating the motion trail of the pollution particles from initial conditions by combining the resistance coefficient formula with flow field data and discrete forms of formulas (1) and (2);

(2) Calculation of contaminating particle collection efficiency

Describing the efficiency of collection of contaminating particles as a trajectory of mutual movement of contaminating particles with respect to each other, the two trajectories being considered to form a particle conduit between them, the particle conduit widening as the distance between the trajectories widens, the distance of particle propagation increases, the particle conduit widening is described by a differential version in which dy ₀ is the distance between two particles far upstream perpendicular to the equivalent incoming flow, and ds is their length on the airfoil surface;

(3) Drawing a collection efficiency curve

The abscissa s is the curve length on the surface of the airfoil, the ordinate is the collection efficiency of the pollutant particles, the position of the front edge is defined to be 0 on the surface of the airfoil, the upper surface is a positive value backwards, the lower surface is a negative value backwards, the middle and rear parts of the airfoil are polluted, but the collection efficiency of the pollutant particles is rapidly reduced along with downstream development;

(4) Determining pollution simulation range

All positions of the wing section are affected by pollution, but as the wing section progresses to the trailing edge, the pollution collection efficiency is rapidly reduced, the pollution influence is rapidly reduced, a cutoff threshold value of the pollution particle collection efficiency is set according to the receiving degree of a tester on the pollutant, and when the pollution particle collection efficiency is lower than the threshold value, the influence of the pollution can be ignored, and a range higher than the threshold value is a rough simulation range.

2. A method of determining an airfoil test asperity band simulation envelope as claimed in claim 1 wherein: the cutoff threshold is beta=0.2 to 0.4.