A method for designing a differential design of upper and lower blades to suppress inter-blade interference noise of a contra-rotating propeller

Abstract

This invention discloses a differentiated design method for upper and lower blades to suppress inter-blade interference noise in counter-rotating propellers, belonging to the field of aeroacoustic design technology for aero-power systems. The method includes: controlling the ratio of the axial spacing to the diameter of the counter-rotating propeller within the range of 0.25 to 0.35, and determining the optimal spacing with the goal of maximizing force efficiency; locating the interference noise source using a hybrid aeroacoustic method combining large eddy simulation and Ffowcs-Williams-Hawkings acoustic analogy; arranging rib structures in the trailing edge region of the suction surface of the upper blade, with a height designed based on the boundary layer thickness of 0.8δ to 1.5δ, a width 1.5 to 3 times the height, and a spanwise spacing of 2 to 5 mm; arranging a wavy leading edge structure in the leading edge region of the lower blade, with an amplitude of 3% to 5% of the average chord length and a wavelength of 7.5% to 12.5% ​​of the average chord length; this invention uses physical scale to drive the selection of noise reduction structural parameters, achieving effective suppression of inter-blade interference noise in counter-rotating propellers while keeping hovering efficiency loss within an acceptable range.

Description

Technical Field

[0001] This invention belongs to the field of aeroacoustic design technology of aviation power systems, specifically involving a differentiated design method for upper and lower rotor blades to suppress inter-blade interference noise of counter-rotating propellers. It is applicable to the low-noise design of counter-rotating propellers in UAVs, electric vertical take-off and landing aircraft and small turboprop power systems under hovering or near-hovering conditions. Background Technology

[0002] Counter-rotating propellers, with their advantages of recovering the kinetic energy of the upstream wake and achieving higher propulsion efficiency in a compact structure, are widely used in equipment such as UAVs and eVTOL. However, their excellent aerodynamic performance is accompanied by significant noise problems. During actual rotation, the counter-rotation of the front and rear rows of blades causes strong unsteady aerodynamic interference. The rear blades periodically cut the tip vortex and wake field of the front blades, generating severe pressure pulsations. This results in significant discrete single tones at the blade passing frequency (BPF) and its harmonics, accompanied by broadband noise components.

[0003] Existing interference noise control methods mainly include increasing axial spacing, optimizing blade number matching, blade sweep profile modification, and introducing acoustic liners. However, these methods suffer from limitations in noise reduction potential, limited adaptability to wide operating conditions, or the need to incur additional weight costs. In recent years, biomimetic flow control technologies (such as wavy leading edges, serrated trailing edges, and rib / groove structures) have provided new ideas for aerodynamic noise control. However, existing research mainly focuses on the verification of single structures on basic airfoils, lacking a systematic design process for real rotating components like counter-rotating propellers. More importantly, existing methods often rely on experience to determine the size and location of noise reduction structures, lacking a design basis driven by physical dimensions, leading to unstable noise reduction effects or excessive aerodynamic losses.

[0004] Therefore, a design method is needed that can unify "noise source location", "structural size determination" and "aerodynamic loss control" under the same physical framework in order to achieve efficient suppression of inter-blade interference noise of counter-rotating propellers. Summary of the Invention

[0005] The purpose of this invention is to provide a differentiated design method for upper and lower propellers to suppress interference noise between the blades of a counter-rotating propeller, so as to solve the problems of lack of physical basis and poor aerodynamic compatibility in the noise reduction structure design of the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for differentiating the upper and lower blades to suppress inter-blade interference noise in a counter-rotating propeller includes the following steps:

[0008] Step 1: Determine the distance between the upper and lower propellers to maximize the force effect.

[0009] The axial spacing d of the counter-rotating propellers z With diameter The ratio d z The force efficiency (T / P) of the counter-rotating propeller system was selected within the range of 0.25 to 0.35. This range is based on a trade-off between aerodynamic efficiency and interference intensity: too small a spacing will exacerbate unsteady interference between the upper and lower blades, leading to thrust loss and noise degradation; too large a spacing will make the structure too loose, and the marginal aerodynamic benefits will decrease rapidly. The force efficiency (T / P) of the counter-rotating propeller system under different axial spacings was measured by aerodynamic performance tests or CFD simulations, and the spacing that maximizes the force efficiency was selected as the optimal spacing.

[0010] Step 2: CFD to locate the interference noise source

[0011] A three-dimensional computational fluid dynamics model of a counter-rotating propeller was established. A hybrid aeroacoustic method combining Large Eddy Simulation (LES) and the FW-Hawkings (FW-H) acoustic analogy equations was employed to calculate the flow field and far-field acoustic field. The root mean square (RMS) values ​​of the surface pulsating pressures in the trailing edge region of the upper blade suction surface and the leading edge region of the lower blade were analyzed. p_rms Alternatively, the sound pressure level spectrum of the far-field monitoring point can be extracted to identify the spanwise and chordwise locations where the interference noise source intensity is greatest. The output of this step is: the arrangement zone of the upper blade rib structure (spanwise range, chordwise range) and the arrangement zone of the lower blade wavy leading edge structure (spanwise range).

[0012] Step 3: Design the upper blade rib structure based on boundary layer thickness

[0013] Based on the local Reynolds number Re and local chord length c of the upper blade tip region l The boundary layer thickness δ is estimated using the Blasius solution for a flat plate boundary layer.

[0014]

[0015] in Given the air kinematic viscosity and the blade tip velocity, The flow distance along the chord length is defined as h = (0.8~1.5)δ, which effectively constrains the near-wall high-shear layer without significantly increasing the equivalent roughness; the width is designed as w = (1.5~3)h to maintain spanwise flow continuity; the spanwise spacing is designed as d = 2~5mm. The ribs are arranged within the spanwise range (0.65D~0.95D) and the chordwise range (0.5c) determined in step 2. l ~0.95c l )Inside.

[0016] Step 4: Design the blade wavy leading edge structure based on the average chord length

[0017] Based on the average chord length c of the lower blade mean The amplitude A of the designed wavy leading edge structure is 3%~5% c mean The wavelength λ is 7.5%~12.5% ​​c mean The leading edge profile is defined by a cosine function:

[0018]

[0019] Arrange the wavy leading edge within the spanwise range (0.4D~D) determined in step 2.

[0020] Step 5: Performance Verification

[0021] The performance of a counter-rotating propeller combining a ribbed structure and a wavy leading edge structure was verified using the same hybrid aeroacoustic method as in step 2. The following conditions must be met simultaneously:

[0022] • Hovering efficiency loss does not exceed 1% (using force efficiency T / P as an indicator, i.e., the relative change rate of the thrust-power ratio between the prototype and the combined model).

[0023] • The total sound pressure level (10~10000 Hz) is reduced to the expected level compared to the prototype.

[0024] If the expected noise reduction target is not achieved, fine-tune the structural parameters in steps 3 and 4 (such as adjusting the rib spacing and the amplitude / wavelength of the wavy leading edge within a given range), and repeat steps 2 to 5 until the requirements are met.

[0025] Beneficial effects

[0026] 1. This method drives the selection of structural parameters based on physical scales (boundary layer thickness, average chord length), avoiding trial and error based on experience and improving design efficiency and reliability.

[0027] 2. By first determining the axial spacing and then locating the noise source, the noise reduction structure is ensured to act on the area with the strongest interference, thus achieving "precise deployment".

[0028] 3. The rib height is linked to the boundary layer thickness, and the amplitude of the wavy leading edge is proportional to the average chord length, so that the structural dimensions match the flow characteristic scale, effectively reducing noise while keeping aerodynamic losses within an acceptable range.

[0029] 4. The closed-loop verification in step 5 ensures the engineering practicality of the method and can be directly used for the design iteration of actual products. Attached Figure Description

[0030] Figure 1 : Flowchart of the design method of this invention.

[0031] Figure 2 : Schematic diagram of the differentiated design of the upper and lower propellers.

[0032] Figure 3 : Schematic diagram of the trailing edge region of the upper blade suction surface and the arrangement of the rib structure.

[0033] Figure 4 Schematic diagram of the arrangement of the leading edge region and wavy leading edge structure of the lower blade.

[0034] Figure 5 Example of a curve showing the change of boundary layer thickness δ with tip velocity and chordal position.

[0035] in: Figure 2 (1) is the upper blade, (2) is the blade clamp, (3) is the blade pad, (4) is the upper blade drive motor, (5) is the lower blade drive motor, (6) is the upper blade coaxial connecting frame, (7) is the lower blade coaxial connecting frame, (8) is the lower blade, (9) is the upper blade rib structure, and (10) is the lower blade wavy leading edge structure. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of this invention.

[0037] In this article, "Thrust / Power" (T / P) refers to the ratio of the thrust generated by the propeller to the input power, and is used to characterize hovering efficiency.

[0038] Example 1

[0039] This embodiment uses a certain type of diameter D = 330 mm and average chord length c. mean The design is based on a counter-rotating propeller with a diameter of 20.5mm and a design speed of n = 4000 rpm.

[0040] Step 1: Determine the axial spacing d z = 99mm, i.e., d z / D = 0.3. This value has been verified through previous experiments; within the range of 0.25 to 0.35, d z When / D = 0.3, the system force efficiency has reached saturation, and further increasing the spacing yields minimal benefits.

[0041] Step 2: Establish the LES / FW-H numerical model with a total of approximately 11.8 million grid cells and a time step Δt = 2.083 × 10⁻ 5 s. Calculation results show that: the trailing edge of the upper blade suction surface is at 0.75D~0.95D in the spanwise direction and 0.5c in the chordwise direction. l ~0.95c lThe root mean square value of the regional pulsating pressure is the highest; the leading edge of the lower blade in the spanwise region of 0.5D~D is the main contributor to BPF noise. Based on this, the rib arrangement zone and the wavy leading edge arrangement zone are determined.

[0042] Step 3: Tip velocity U t = πDn / 60 ≈ 69.2 m / s, taking the chord position x = 15 mm (approximately 0.75). air kinematic viscosity ν = 1.5 × 10⁻ 5 m² / s, calculated δ≈ 0.284 mm. Taking h = 0.25 mm (approximately 0.88δ), w = 0.5 mm (2h), and spanwise spacing d = 2.5 mm. There are 17 ribs in total, arranged at spanwise 0.75D~0.95D and chordwise 0.5c. l ~0.95c l area.

[0043] Step 4: Set the amplitude A = 0.8 mm (approximately 3.9% c). mean ), wavelength λ = 2 mm (approximately 9.8% c mean The wavy leading edge is arranged in the spanwise region of 0.5D~D.

[0044] Step 5: Perform LES / FW-H validation on the combined model (S1_X5), and the results are as follows:

[0045] • Hovering efficiency loss: approximately 0.35% (force efficiency decreases from the prototype value to the combined model value, with a relative change rate of 0.35%);

[0046] • Total sound pressure level (10 ~ 20000Hz) decreased by 4.05dB;

[0047] • 1st-order BPF noise reduction: 4.47dB; 2nd-order BPF noise reduction: 1.65dB; 3rd-order BPF noise reduction: 4.29dB.

[0048] All indicators meet the design requirements.

[0049] Example 2

[0050] To verify the universality of this method, the same propeller as in Example 1 was used, but the rotational speed was adjusted to 3000 rpm. In step 1, d... z / D is still set to 0.3; Step 2 recalculates the noise source location, and the result is basically consistent with the 4000rpm operating condition; In Step 3, U tWith a speed of approximately 51.9 m / s and δ ≈ 0.327 mm, h = 0.3 mm is set, and other parameters remain unchanged. Step 4 parameters remain unchanged. Step 5 verifies that the hovering efficiency loss is approximately 0.6%, and the total sound pressure level decreases by approximately 2.1 dB. After fine-tuning the wavy leading edge parameters (adjusting A to 0.6 mm and λ to 1.5 mm), the total sound pressure level decreases by 2.8 dB, approaching the target. This embodiment demonstrates that the method remains applicable at different speeds, and iterative optimization can be used to further approximate the target if necessary.

[0051] Supplementary Explanation

[0052] The LES / FW-H method used in step 2 of this method is not the only option. Decoupled eddy simulation (DES) or the Lattice Boltzmann method (LBM) combined with acoustic analogy can also be used for noise source localization. The boundary layer thickness estimation in step 3 can also use more accurate empirical formulas for turbulent boundary layers, but the Blasius solution is sufficient for engineering design accuracy under hovering conditions. Performance verification in step 5 can be performed using either numerical simulation or experimental measurements, with experimental data serving as the final criterion.

[0053] The rib and wavy leading edge geometric parameters obtained in this embodiment fall within the scope of the claims of the product structure patent filed at the same time. However, this method does not require the final product to fall within this range. Instead, it drives the geometric selection based on the scale relationship and criteria of Reynolds number, boundary layer thickness, and average chord length, and completes the aerodynamic / acoustic dual threshold verification in step 5. Therefore, this method and the product structure patent belong to different protected objects, and there is no issue of duplicate authorization.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for differentiated design of upper and lower propellers to suppress inter-blade interference noise in counter-rotating propellers, characterized in that, Includes the following steps: • Step 1: Determine the axial spacing of the counter-rotating propellers This makes its ratio to the propeller diameter D... It is within the range of 0.25 to 0.35; • Step 2: Establish a three-dimensional computational fluid dynamics model of the counter-rotating propeller. Use a hybrid aeroacoustic method that combines large eddy simulation with the Williams-Hawkings acoustic analogy equation to calculate the flow field and far-field sound field. Identify the intensity distribution of interference noise sources in the trailing edge region of the upper blade and the leading edge region of the lower blade, and determine the spanwise and chordwise locations of the maximum noise intensity. • Step 3: Based on the local Reynolds number Re and local chord length of the upper blade tip region The boundary layer thickness δ was calculated, and the height h of the rib structure was designed to be 0.8δ ~ 1.5δ, the width w to be 1.5 ~ 3 times h, and the spanwise spacing d to be 2 ~ 5 mm. The rib structure was then arranged in the tip and trailing edge region of the upper blade suction surface, with a spanwise distribution range of 0.65D ~ 0.95D and a chordwise distribution range of 0.5... ~ 0.95 ; • Step 4: Based on the average chord length of the lower blade The amplitude A of the designed wavy leading edge structure is 3%. ~5% The wavelength λ is 7.5%. ~ 12.5% The wavy leading edge structure is arranged in the leading edge region of the lower blade, with a spanwise distribution range of 0.4D ~ D; • Step 5: The performance of the counter-rotating propeller with the combination of rib structure and wavy leading edge structure is verified by the hybrid aeroacoustic method described above. The hovering efficiency and far-field total sound pressure level are obtained and compared with the prototype counter-rotating propeller.

2. The method for differentiated design of upper and lower propellers to suppress inter-blade interference noise of contra-rotating propellers according to claim 1, characterized in that, The axial spacing mentioned in step 1 The method for determining the optimal force efficiency is as follows: through aerodynamic performance tests or CFD simulations, the force efficiency T / P of the counter-rotating propeller system under different axial spacings is measured, and the spacing that maximizes the force efficiency is selected as the optimal spacing.

3. The method for differentiated design of upper and lower propellers to suppress inter-blade interference noise of contra-rotating propellers according to claim 1, characterized in that, The location with the highest noise intensity in step 2 is determined by the following method: extracting the root mean square value of the surface pulsating pressure in the trailing edge region of the upper blade suction surface and the leading edge region of the lower blade, or extracting the sound pressure level spectrum of the far-field monitoring point, and determining the region with the highest pulsating pressure or sound pressure level as the location of the strongest noise source.

4. The method for differentiated design of upper and lower propellers to suppress inter-blade interference noise of contra-rotating propellers according to claim 1, characterized in that, The boundary layer thickness δ mentioned in step 3 is estimated using the Blasius solution for a flat plate boundary layer. , where ν is the air kinematic viscosity, U is the tip velocity, and x is the flow distance along the chord length.

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