A surface modification method applicable to the suppression of supercritical airfoil buffet

Abstract

The present invention provides a surface modification method suitable for suppressing buffet of supercritical airfoils. First, a basic airfoil is selected, and length parameters L, L1, and L2 are given to determine the lengths of the Bump ramp section, peak section, and trailing edge section applied to the basic airfoil. Then, the position parameter Xc of the Bump is determined, and the position parameter Xc is the chord length position of the starting point of the Bump ramp section. After that, a profile corresponding to the L2 length of the Bump peak section is intercepted from the supercritical airfoil surface and offset by an initial height H in the Z direction of the airfoil to obtain two end points C and D of the Bump peak section. Points A and C are connected to obtain the ramp section, and points D and B are connected to obtain the trailing edge section. By changing the offset height H, the optimal height H for the best buffet suppression is obtained. opt ; Through the parameters L, L1, L2, Xc, and H opt , the optimal Bump profile ABCD applied to the supercritical airfoil is obtained, and the surface modification is completed. The present invention reasonably arranges Bumps on the upper surface of the supercritical airfoil to suppress the transonic shock buffet phenomenon of the supercritical airfoil.

Description

Technical Field

[0001] The present invention relates to the fields of aerodynamic design and flow control, and specifically presents a surface modification method suitable for suppressing buffet of supercritical airfoils. Background Art

[0002] Supercritical airfoils can increase the drag divergence Mach number of an aircraft, thereby broadening the flight speed range of civil airliners. Therefore, supercritical airfoils are commonly used for the wings of large civil airliners. However, when the flight Mach number increases or the flight angle of attack increases, a shock-induced separation region will be generated on the upper surface of the airfoil. The interaction between the shock wave and the separation region leads to large-scale shock self-sustained oscillation. This shock buffet phenomenon will bring unsteady aerodynamic loads, resulting in aircraft structural fatigue and difficult control, and restricting the flight envelope. Therefore, it is necessary to perform flow control on supercritical airfoils to suppress buffet.

[0003] Some work on buffet flow control has been carried out at home and abroad, such as using flow control technologies such as blowing / suction technology, vortex generators, and trailing edge deflectors. These flow control technologies have achieved certain buffet suppression effects, but these flow control means require additional control devices or energy injection to work effectively. For this reason, some simple surface micro-modification technologies have emerged, such as adding bumps on the surface. The Bump technology has been successfully applied to some aerodynamic designs, such as adding a Bump bulge at the entrance of a supersonic inlet to isolate the boundary layer. In terms of buffet suppression, the feasibility of this surface micro-modification technology has been verified, but for the buffet suppression of supercritical airfoils, no corresponding surface micro-modification technology has been proposed yet. Summary of the Invention

[0004] To achieve the suppression of buffet of supercritical airfoils, the present invention proposes a surface modification method for supercritical airfoils, which reasonably arranges bumps on the upper surface of the supercritical airfoil to suppress the transonic shock buffet phenomenon of the supercritical airfoil. After adopting this method, it is possible to suppress buffet without additional control devices or energy injection.

[0005] The technical solution of the present invention is as follows:

[0006] A surface modification method suitable for suppressing buffet of supercritical airfoils, comprising the following steps:

[0007] Step 1: Select a basic airfoil and specify length parameters L, L1, and L2 to determine the lengths of the ramp section, peak section, and trailing edge section of the bump applied to the basic airfoil; where the length parameter L is the overall length of the bump along the chord length direction of the basic airfoil, the length parameter L1 is the length of the ramp section of the bump along the chord length direction of the basic airfoil, and the length parameter L2 is the length of the peak section of the bump along the chord length direction of the basic airfoil;

[0008] Step 2: Determine the position parameter Xc of the Bump. The position parameter Xc is the chord length position of the starting point of the Bump ramp section. The specific steps are as follows:

[0009] Step 2.1: Generate the computational grid of the basic airfoil, and conduct steady CFD numerical simulations on the reference airfoil at a fixed Mach number and different angles of attack. Based on the obtained steady CFD calculation results, use the method of changing the slope of the linear lift segment by 0.1 to estimate the buffet onset angle of attack.

[0010] Step 2.2: In the range of angles of attack including the estimated buffet onset angle of attack α buffet-onset , take points at the set angle of attack interval step size and conduct unsteady CFD calculations. According to the unsteady calculation results, obtain the buffet onset angle of attack α onset and the fully developed buffet angle of attack α established of the reference airfoil;

[0011] Step 2.3: Conduct unsteady flow field calculations at the oncoming flow angle of attack α established , and determine the position X of the most downstream interference point of the shock wave boundary layer from the leading edge of the reference airfoil under the condition of fully developed buffet;

[0012] Step 2.4: Arrange the Bump at the position Xc from the leading edge, where Xc < X < Xc + L1, so that the position X is in the ramp section of the Bump, and then obtain the positions of the leading edge point A and the trailing edge point B of the Bump;

[0013] Step 3: Cut off the airfoil surface corresponding to the peak section of the Bump with a length of L2 from the supercritical airfoil surface, and offset the initial height H along the Z direction of the airfoil to obtain the two end points C and D of the peak section of the Bump;

[0014] Step 4: Connect points A and C to obtain the ramp section, where the ramp section is tangent to the airfoil and the peak section at points A and C. Connect points D and B to obtain the trailing edge section, where the trailing edge section is tangent to the airfoil and the peak section at points B and D;

[0015] Step 5: By changing the offset height H, repeat Steps 3 to 4 to obtain the optimal height H opt for the best buffet suppression;

[0016] Step 6: Through the parameters L, L1, L2, Xc, H opt , obtain the optimal Bump airfoil surface ABCD applied to the supercritical airfoil to complete the surface modification.

[0017] Furthermore, in Step 1, the overall length L does not exceed 25% of the chord length of the basic airfoil. The corresponding length L1 of the ramp section is 10% - 12.5% of the airfoil chord length, and the corresponding length L2 of the peak section is 3% of the airfoil chord length.

[0018] Further, in step 2.1, the process of estimating the buffet onset angle of attack is as follows:

[0019] Obtain the lift-to-drag ratio curve and the lift curve slope curve according to the steady-state CFD calculation results. Take the maximum point of the lift-to-drag ratio curve, and its abscissa is α Kmax ; within the range of angle of attack α ≤ α Kmax , take the lift curve slopes at multiple points and calculate the average value as the slope of the linear lift segment (ΔC L / Δα) liner ; starting from α Kmax on the lift curve slope curve, gradually increase the value of α, and find the point where the lift curve slope is (ΔC L / Δα) liner - 0.1. The α coordinate α buffet-onset of this point is the estimated value of the buffet onset angle of attack.

[0020] Further, in step 2.2, within the range of ±1° of the estimated buffet onset angle of attack α buffet-onset , take points every 0.1°.

[0021] Further, in step 3, the initial height H is 0.5% of the airfoil chord length.

[0022] Further, in step 4, connect points A and C with a B-spline curve, and connect points D and B with a B-spline curve.

[0023] Further, in step 5, the optimal buffet suppression means that the oscillation amplitude of the lift coefficient is the smallest under the condition of fully established buffet.

[0024] Advantageous Effects

[0025] The surface modification strategy for buffet suppression applicable to supercritical airfoils provided by the present invention has the following advantages:

[0026] (1) By incurring a relatively low cost and performing a relatively simple surface modification, the same buffet suppression effect as methods such as active flow control can be achieved.

[0027] (2) This modification strategy is only a local modification and does not significantly change the original airfoil surface. Therefore, the buffet onset performance of the baseline airfoil is basically not lost.

[0028] The additional aspects and advantages of the present invention will be partially given in the following description, partially become apparent from the following description, or be understood through the practice of the present invention. Description of the Drawings

[0029] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, wherein:

[0030] Figure 1 Schematic diagram of the structure of the Bump

[0031] Figure 2 Specific schematic diagram of applying the Bump to a supercritical airfoil

[0032] Figure 3 Flow chart

[0033] Figure 4 Airfoil computational grid, wherein: a) Overall schematic diagram of the grid; b) Local enlarged view of the near-wall surface

[0034] Figure 5 Estimated buffet boundary diagram

[0035] Figure 6 Schematic diagram for determining the buffet onset angle of attack and the fully developed buffet angle of attack

[0036] Figure 7 Position of the most downstream shock-boundary layer interaction

[0037] Figure 8 Comparison between the final modified airfoil and the initial airfoil

[0038] Figure 9 Lift coefficient of the airfoil after changing the height H

[0039] Figure 10 Buffet amplitude of the airfoil after changing the height H

[0040] Figure 11 Buffet onset lift coefficient of the airfoil after changing the height H Detailed implementation manners

[0041] The surface modification method for suppressing buffet of a supercritical airfoil proposed by the present invention performs surface modification based on the Bump basic configuration. The basic structure of the Bump is as shown Figure 1 , and it mainly consists of three parts: ramp section, crest section, and tail section. The surface modification method provided by the present invention is to optimally apply the Bump configuration to the supercritical airfoil, as shown in Figure 2 and Figure 3 , and includes the following steps:

[0042] Step 1: Select a basic airfoil and specify length parameters L, L1, and L2 to determine the lengths of the ramp, crest, and tail sections of the Bump applied to the airfoil. Among them, the length parameter L is the overall length of the Bump along the chord direction of the basic airfoil, the length parameter L1 is the length of the ramp section of the Bump along the chord direction of the basic airfoil, and the length parameter L2 is the length of the crest section of the Bump along the chord direction of the basic airfoil; the overall length L does not exceed 25% of the chord length of the basic airfoil, the corresponding length L1 of the ramp section is 10% - 12.5% of the chord length of the airfoil, and the corresponding length L2 of the crest section is 3% of the chord length of the airfoil.

[0043] Step 2: Determine the position parameter Xc of the Bump. The position parameter Xc is the chord position of the starting point of the ramp section of the Bump; it specifically includes the following steps:

[0044] Step 2.1: Generate a computational grid for the basic airfoil and perform steady-state CFD numerical simulations on the reference airfoil at a fixed Mach number and different angles of attack.

[0045] Based on the obtained steady-state CFD calculation results, the method of changing the slope of the linear lift segment by 0.1 is used to estimate the buffet onset angle of attack. The specific operation process is as follows: Obtain the lift-to-drag ratio curve (the curve of the lift-to-drag ratio K versus the angle of attack α) and the lift curve slope curve (the curve of the lift curve slope ΔC L / Δα versus the angle of attack α) from the steady-state CFD calculation results. Take the maximum point of the lift-to-drag ratio curve, and its abscissa is α Kmax ; within the interval of α ≤ α Kmax , take the average value of the lift curve slopes at multiple points as the slope of the linear lift segment (ΔC L / Δα) liner ; starting from α Kmax on the lift curve slope curve, gradually increase the value of α and find the point where the lift curve slope is (ΔC L / Δα) liner - 0.1. The α coordinate α buffet-onset of this point is the estimated value of the buffet onset angle of attack, as shown in Figure 5 .

[0046] Step 2.2: Take points at intervals of 0.1° within the range of ±1° of the estimated buffet onset angle of attack α buffet-onset and perform unsteady CFD calculations. According to the unsteady calculation results, obtain the buffet onset angle of attack α onset and the fully developed buffet angle of attack α established of the reference airfoil;

[0047] Step 2.3: Conduct calculations with the incoming flow angle of attack being α establishedThe unsteady flow field calculation under the condition of complete establishment of buffeting is performed to determine the most downstream shock wave position, that is, the position X from the most downstream interference point of the shock wave boundary layer to the leading edge of the reference airfoil;

[0048] Step 2.4: Arrange the Bump at a distance of Xc from the leading edge, ensuring that the most downstream interference point X in the shock boundary layer in step 2.3 is in the ramp section of the Bump, that is, Xc is satisfied. <X<Xc+L1,从而得到Bump的前缘点(点A)和后缘点(点B);

[0049] Step 3: A profile of length L2 corresponding to the crest of the bump is intercepted from the surface of the supercritical airfoil, and offset by a certain height H along the Z direction of the airfoil. The initial offset height is preset to be 0.5% of the chord length of the airfoil to obtain the crest of the bump, that is, point C and point D.

[0050] Step 4: Connect points A and C with a B-spline curve to obtain a ramp segment (ramp), which is tangent to the airfoil and the crest segment (crest) at points A and C. Connect points D and B with a B-spline curve to obtain a tail segment (tail), which is tangent to the airfoil and the crest segment (crest) at points B and D.

[0051] Step 5: The reduction value of the lift coefficient oscillation amplitude under the condition of complete establishment of the buffeting is used as the criterion for judging the degree of buffeting suppression, that is, the smaller the lift coefficient oscillation amplitude, the higher the degree of buffeting suppression. Change the offset height H of the Bump, repeat steps 3 to 4 to obtain the optimal height, where H is the value when the buffeting amplitude can achieve the best suppression or even be completely suppressed. opt It is the optimal height.

[0052] After the above five steps, five parameters are determined: L, L1, L2, Xc, H opt , the optimal bump finally applied to the supercritical airfoil is the profile ABCD, and the surface modification is completed.

[0053] The following is a description of the surface modification example based on the OAT15A supercritical airfoil:

[0054] Step 1: Select the basic airfoil as OAT15A airfoil and give the length parameters L, L1, L2:

[0055] The chord length of the OAT15A airfoil is C, and the three length parameters L, L1, and L2 are set to 25%C, 10.7%C, and 3%C respectively.

[0056] Step 2: Determine the Bump location parameter Xc:

[0057] 1) Select the baseline airfoil OAT15A supercritical airfoil and generate an airfoil computational grid as shown in Figure 4 . The height of the first layer of grid near the wall surface meets the requirement of y + < 1. Use the open-source solver NASA CFL3D to perform steady CFD numerical simulation on the baseline airfoil. The calculation conditions in this embodiment are: Ma = 0.73, Re c = 3e6, C = 0.23m, P0 = 10 5 Pa, T0 = 300k, α = -4°, -2°, 0°, 1°, 1.2°, 1.4°, 1.6°, 1.8°, 2°, 2.2°, 2.5°, 2.8°, 2.9°, 3°, 3.1°, 3.2°, 3.3°, 3.5°, 3.8°, 4°, 4.5°, 5°, 6°.

[0058] 2) Based on the obtained steady CFD calculation results, use the method of changing the slope of the linear segment of the lift line by 0.1 to estimate the buffet boundary.

[0059] The specific operation is as shown in Figure 5 . Among them, the K curve is the lift-to-drag ratio curve, and the ΔC L / Δα curve is the lift line slope curve. Take the maximum point of the K curve, and its abscissa is α Kmax . Within the interval of α ≤ α Kmax , take multiple points on the lift line slope curve, and find the average value of the lift line slope ΔC L / Δα, that is, the slope of the lift linear segment (ΔC L / Δα) liner , and its value is about 0.18. Translate the straight line (ΔC L / Δα) liner = 0.18 downward by 0.1 to get the straight line (ΔC L / Δα) liner = 0.08. The abscissa of the intersection point of the straight line (ΔC L / Δα) liner = 0.08 and the ΔC L / Δα curve is the buffet boundary prediction value α buffet-onset , and its value is about 2.85°.

[0060] 3) Within the range of ±1° of the predicted buffet onset angle of attack α buffet-onset = 2.85°, take points every 0.1°, and perform unsteady calculations for each point. The calculation conditions are: Ma = 0.73, Re C = 3×10 6 , C = 0.23m, P0 = 10 5 Pa, T0 = 300K. The power spectral density (PSD) curve of the pressure pulsation at the 45% chord length position on the upper surface monitored by the unsteady calculation is as shown inFigure 6 As shown. When α = 3°, there is basically no peak in the low-frequency PSD, and the pressure remains stable; when α = 3.1°, there is a small peak in the low-frequency PSD, there is pressure pulsation, but the buffet has not been fully established; when α > 3.5°, there is a relatively stable high peak in the low-frequency PSD, there is a large periodic pressure pulsation, and the buffet is fully established. Thus, the buffet onset angle of attack α of the baseline airfoil is judged onset = 3.1°, and the buffet fully established angle of attack α established = 3.5°.

[0061] 4) Unsteady flow field calculations are carried out at α established = 3.5° to obtain the position of the most downstream shock wave under the condition that the buffet is fully established, as Figure 7 shown, that is, the position of the most downstream interference point of the shock wave boundary layer from the leading edge is X = 51%C;

[0062] 5) The Bump is arranged at the position Xc = 49.7%C from the leading edge. At this time, Xc < X = 51%C < Xc + L1, which meets the requirement of "the position X of the most downstream interference point of the shock wave boundary layer is in the ramp section", so points A and B are obtained.

[0063] Step 3, at the position corresponding to the crest section on the upper surface of the OAT15A airfoil, intercept a corresponding airfoil section with a length of L2 = 3%C, and offset it in the Z direction by the initial height H = 0.55%C to obtain the crest section of the Bump, that is, points C and D are obtained;

[0064] Step 4, connect points A and C with a B-spline curve to obtain the ramp section, which is tangent to the airfoil and the crest section, and connect points D and B with a B-spline curve to obtain the tail section, which is tangent to the airfoil and the crest section;

[0065] Step 5, determine the final height H opt :

[0066] Taking the reduction value of the lift coefficient oscillation amplitude ΔCL under the condition that the buffet is fully established as the discrimination criterion for the buffet suppression degree, change the Bump height H, and increase the height from 0.55%C in a set step size. Repeat steps 3 to 4 until the buffet is completely suppressed, and obtain H opt = 0.75%C. The final parameters are determined as: L = 25%C, L1 = 10.7%C, L2 = 3%C, Xc = 50%C, H opt = 0.75%C. Figure 8 The comparison between the modified airfoil and the baseline airfoil is shown.

[0067] Figure 9The time curves of the lift coefficient at different Bump heights are shown. Figure 10 The oscillation amplitude of the lift coefficient at different Bump heights is shown. It can be seen that during the process of increasing from 0.55%C to 0.75%C, the oscillation of the lift coefficient is gradually eliminated and the buffeting amplitude gradually decreases. opt When H = 0.75%C, the buffeting is basically completely suppressed.

[0068] Figure 11 The buffeting onset lift coefficients at different heights are shown. Table 1 shows the comparison of the buffeting onset performance between the final modified airfoil and the baseline airfoil.

[0069] Table 1

[0070] Configuration CL L / D Basic airfoil 0.9061 28.23 Final modified airfoil 0.9673 28.04

[0071] After surface modification, the buffeting onset lift coefficient of the airfoil increases from 0.9061 to 0.9673, and the lift-to-drag ratio remains at about 28. It can be seen that before and after modification, the buffeting onset lift-to-drag ratio performance of the airfoil remains basically unchanged, and the buffeting onset lift coefficient has been significantly improved.

[0072] In summary, the modification strategy proposed by the present invention can suppress buffeting without making major changes to the basic airfoil, ensure that the buffeting onset lift-to-drag ratio performance remains basically unchanged, and increase the buffeting onset lift coefficient. The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A surface modification method applicable to suppressing buffet of supercritical airfoils, characterized in that: It includes the following steps: Step 1: Select a basic airfoil and specify length parameters L, L1, and L2 to determine the lengths of the Bump ramp section, peak section, and trailing edge section applied to the basic airfoil; Among them, the length parameter L is the overall length of the Bump along the chord length direction of the basic airfoil, the length parameter L1 is the length of the Bump ramp section along the chord length direction of the basic airfoil, and the length parameter L2 is the length of the Bump peak section along the chord length direction of the basic airfoil; Step 2: Determine the position parameter Xc of the Bump. The position parameter Xc is the chord length position of the starting point of the Bump ramp section; specifically, it includes the following steps: Step 2.1: Generate a computational grid for the basic airfoil and perform steady-state CFD numerical simulations on the reference airfoil at a fixed Mach number and different angles of attack; based on the obtained steady-state CFD calculation results, use the method of changing the slope of the linear lift segment by 0.1 to estimate the buffet onset angle of attack; Step 2.2: Within the angle of attack range including the estimated buffeting onset angle of attack α buffet-onset , points are taken at a set angle of attack interval step size, and unsteady CFD calculations are performed. According to the unsteady calculation results, the buffeting onset angle of attack α onset and the fully established buffeting angle of attack α established of the baseline airfoil are obtained; Step 2.3: Conduct unsteady flow field calculation at an oncoming flow angle of attack of α established to determine the position X of the most downstream interference point between the shock wave and the boundary layer from the leading edge of the reference airfoil under the condition of fully established buffeting; Step 2.4: Arrange the Bump at the position Xc from the leading edge, where Xc < X < Xc + L1, so that the position X is in the ramp section of the Bump, and then obtain the positions of the leading edge point A and trailing edge point B of the Bump; Step 3: Cut a profile corresponding to the L2 length of the Bump peak section from the supercritical airfoil surface and offset it by the initial height H in the Z direction of the airfoil to obtain the two end points C and D of the Bump peak section; Step 4: Connect points A and C to obtain the ramp section. The ramp section is tangent to the airfoil and the peak section at points A and C. Connect points D and B to obtain the trailing edge section. The trailing edge section is tangent to the airfoil and the peak section at points B and D; Step 5: By changing the offset height H, repeat Steps 3 to 4 to obtain the optimal height H for the best buffeting suppression. opt ; Step 6: Obtain the optimal Bump profile ABCD applied to the supercritical airfoil through the parameters L, L1, L2, Xc, and H opt , and complete the surface modification.

2. The surface modification method applicable to suppressing buffet of supercritical airfoils according to claim 1, characterized in that: In Step 1, the overall length L does not exceed 25% of the chord length of the basic airfoil. The corresponding length L1 of the ramp section is 10% - 12.5% of the chord length of the airfoil, and the corresponding length L2 of the peak section is 3% of the chord length of the airfoil.

3. The surface modification method applicable to suppressing buffet of supercritical airfoils according to claim 1, characterized in that: In Step 2.1, the process of estimating the buffet onset angle of attack is: The lift-drag ratio curve and the lift curve slope curve are obtained according to the steady-state CFD calculation results. The maximum point of the lift-drag ratio curve is taken, and its abscissa is α Kmax ; within the range of angle of attack α ≤ α Kmax , the lift curve slopes at multiple points are taken and averaged to obtain the slope of the linear lift segment (ΔC L / Δα) liner ; starting from α Kmax on the lift curve slope curve, the value of α is gradually increased to find the point where the lift curve slope is (ΔC L / Δα) liner - 0.

1. The α coordinate α buffet-onset of this point is the estimated value of the buffet onset angle of attack.

4. The surface modification method applicable to suppressing buffet of supercritical airfoils according to claim 1, characterized in that: In step 2.2, points are taken every 0.1° within the range of ±1° of the predicted buffet onset angle of attack α buffet-onset .

5. The surface modification method applicable to suppressing buffet of supercritical airfoils according to claim 1, characterized in that: In Step 3, the initial height H is 0.5% of the chord length of the airfoil.

6. The surface modification method applicable to suppressing buffet of supercritical airfoils according to claim 1, characterized in that: In Step 4, connect points A and C with a B-spline curve, and connect points D and B with a B-spline curve.

7. The surface modification method applicable to suppressing buffet of supercritical airfoils according to claim 1, characterized in that: In Step 5, the so-called best buffet suppression means that the oscillation amplitude of the lift coefficient is the smallest under the condition of fully established buffet.

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