| Wind power generation plays an important role in the process of carbon peaking and carbon neutralization.Green environment and wind turbine blade refined design put forward higher requirements for researchers.The wind turbine noise level is a key indicator of environmental friendliness,and it is also one of the important factors that characterize structural characteristics and running blade efficiency of wind turbines.Therefore,it is of great significance to analyze the noise mechanism of sound flowing about running for blade optimization design.Approaching wind turbine aerodynamic noise generation mechanism research for noise reduction,precisely locating sound sources is key.However,due to the hysteresis effect,the azimuth angle of rotating sound source is difficult to accurately be located,and the detailed characteristics of the flow-induced sound derivation are not yet clear.These restrict in-depth study of sound source generation mechanism.To solve this problem,based on the classical beamforming theory,this paper proposes an algorithm for tracking and identifying accurately the rotating sound source in real time,which successfully realizes the precise location of the rotating sound source.A hot-wire anemometer was used to capture the transient flow field in the dominant sound source area.By analyzing the flow field and sound field data,process parameters of vortex correlation were derived.The internal correlation evaluation system of vortex-sound was established to reveal the sound source noise process mechanism.It provides methods and data support for blade optimization design and noise reduction.The precise location sound sources around wind turbine blades is a basis for revealing sound mechanism,and the algorithm is a core for locating rotating sound source precisely.Based on classical beamforming theory,an algorithm for tracking and identifying rotating sound sources is proposed with instantaneous speed and delayed azimuth angle rotation correction.First,the accuracy and reliability of the algorithm were verified using a rotating simulation bench.The delayed azimuth angles at different speeds were counted.The formula between rotation speed and delay azimuth angle was derived,and the algorithm was corrected.Locate the sound source using the correction algorithm.There are still errors between the actual sound source and the located sound source.And the maximum radial error was 0.0214 m,the maximum azimuth angle error was 2.5°.By comparison test,the error can meet test accuracy requirements.Then the identification algorithm was used to locate the dominant sound source of the running blade.Analyze the experimental data and draw the following conclusions: The dominant streaming sound source radial positions of the rotating blade are concentrated at 0.57R?0.71 R,and the chord positions are concentrated at 0.75C?1.25 C.It is found that the sound pressure level increases first,then decreases and then increases with the azimuth angle.The sound source position fluctuation is affected by wind speed and tip speed ratio in an increasing relationship.Which caused by fluctuations in the rotating speed and characteristic vibrations of the wind turbine.The fluctuations were consistent with the changed rotating speeds of the rotor.In order to reveal the noise mechanism of flow around rotating blade,its flow characteristics analysis are particularly critical.Hence,after determining dominant streaming sound source area of the blade,its transient flow field was captured by the hot wire anemometer.Its flow structure and dynamic evolution of Velocity,Reynolds stress and Root mean square are studied.It is found that interval structures of high-speed and low-speed stripes are transported outwards in the radial direction of the blade.The high-speed and low-speed stripes appear in pairs.With wind speed and tip speed ratio increasing,the spacing between the high speed and low speed stripes becomes smaller.The spacing between the high speed and low speed stripes do not change with time(azimuth angle).Based on the theory of hairpin vortex dynamics,the alternating high-low stripe flow structure was partial procedure of hairpin vortex.The fluid ejection positions were found.And its positions were concentrated at 0.57R?0.71 R in the radial direction and0.7C?0.8C in the chord direction.Quantitative Analysis,the Reynolds stress peak points,the maximum velocity fluctuation positions and the fluid ejection positions are concentrated at the same position.With increasing wind speed and tip speed ratio,they all move towards the blade tip in the radial direction and towards the trailing edge in the chord direction.With increasing azimuth angle,the positions remained almost unchanged under different working conditions.Both Reynolds stress value and velocity fluctuation decrease first,then increase and then decrease in the chord direction.The maximum value of Reynolds stress and velocity fluctuation initially increased with increasing azimuth angle,followed by a decrease and,then,increased with continued rotation process.Based on the flow-induced sound theory,the intermediate process parameters for evaluating the correlation of vortex sound are found.The Reynolds stress is the force of hairpin vortex to make breakage.the root mean square can characterize the degree of energy exchanged between hairpin vortexes.Based on sound field and flow field data analysis,the intermediate process parameters of vortex sound correlation were determined.A correlation evaluation system with three elements of frequency,position and energy are established.The three elements of the sound source were found to be consistent with the hairpin vortex.The important hairpin vortex structure of the sound source generation mechanism was determined.Analyze the sound source characteristics in the process of hairpin vortex derivation.The sound source generation mechanism is revealed.The dominant noise of rotating blade comes from the ejection stage during the hairpin vortex derivation process of the blade wall,and the sound source position lags behind the ejection location.The sound source frequency spectrum and velocity fluctuation frequency spectrum have the same changing law and present wide frequency characteristics.The sound source positions are consistent with the maximum velocity fluctuation positions with wind speed and tip speed ratio changed.But the positions of sound source do not coincide with the maximum velocity fluctuation positions.The sound source positions were closer to the blade tip in the radial direction and closer to the trailing edge in the chord direction than the maximum velocity fluctuation position.Sound source sound pressure and velocity fluctuation are linearly positively correlated with wind speed and tip speed ratio changed.The experimental results show that the dominant sound source of running blade originate from the strong momentum exchange at the fluid ejection position,which drives the hairpin vortex break,leads to random pressure pulsations around the blades,and finally produces a wide band of trailing edge noise.Relevant research results provide methods and data support for blade optimization design and noise reduction. |
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