This paper concerns the application of a linear microphone array in the quantitative evaluation of blade trailing-edge (TE) noise reduction. The noise radiation from the blades with straight and serrated TEs is measured in an indoor open-jet wind tunnel. The array data are processed using the inverse method based on the Clean algorithm based on spatial source coherence (Clean-SC). In order to obtain correct application and achieve the best effect for the microphone array test, the computing software for array data reduction is firstly developed and assessed by Sarradj’s benchmark case. The assessment results show that the present array data processing method has a good accuracy with an error less than 0.5 dB in a wide frequency range. Then, a linear array with 32 microphones is designed to identify the noise source of a NACA65(12)-10 blade. The performance of the Clean-SC algorithm is compared with the Clean algorithm based on point spread functions (Clean-PSF) method for experimentally identifying the noise sources of the blade. The results show that there is about a 2 dB error when using the Clean-PSF algorithm due to the interference of different aerodynamic noise sources. Experimental studies are conducted to study the blade TE noise reduction using serrated TEs. The TE noise for the blade with and without sawtooth configurations is measured with the flow speeds from 20 m/s to 70 m/s, and the corresponding Reynolds numbers based on the chord are from 200,000 to 700,000. Parametric studies of the sawtooth amplitude and wavelength are conducted to understand the noise reduction law. It is observed that the TE noise reduction is sensitive to both the amplitude and wavelength. The flow speed also affects the noise reduction in the serrated TEs. To obtain the best noise suppression effect, the sawtooth configuration should be carefully designed according to the actual working conditions and airflow parameters.

View Video Presentation: https://doi.org/10.2514/6.2021-2258.vid A hybrid URANS/FW-H method is applied to simulate the tone noise reduction of a single-stage axial-flow fan with- and without- Wavy Leading-Edge (WLE) vanes configurations. The fan is with rotation speed of 7000rpm, and the inflow averaged Mach at the leading-edge of vanes is 0.49 and the corresponding vanes chord based Reynolds number is about 1040000. The fans with base vane configuration, three whole wavy leading-edge vanes configurations and three partial wavy leading-edge vanes configurations were simulated in the study. The numerical simulation results show that the main noise sources of the rotor-vanes interaction tone are concentrated in the leading edge of downstream vanes. Results also show that the WLE vanes can substantially reduce the rotor-vanes interaction tone noise with a little effects on the fan aerodynamic performance. The study indicates a reasonable wave distribution region is benefit to both aerodynamic performance and noise reduction effect. The aerodynamic and aero-acoustic mechanism of the rotor-vanes interaction tone noise reduction using WLE were also studied in this paper. It is found that the wave leading-edge configuration could reduce the intensity of the noise source significantly, which is related to noise reduction. Meanwhile, the wave leading-edge vanes configuration could increase the difference of phase along vanes span, which reduce the correlation of noise source region and reduce rotor-vanes interaction tone noise.

In a numerical study, a shorter span extent than experiment is often used to save the computational resource. The predicted sound pressure level should be corrected before compared with the experimental results. This study concerns the extended aeroacoustic spanwise correction method for the noise prediction radiated from large-span objects. Four new types of spanwise correction models are derived based on the assumption of the spanwise coherence function taking the form of a rectangular function, a trigonometric function, a Laplacian function and a Gaussian function, respectively. The large eddy simulation (LES) combined with the acoustic analogy theory is used for the aerodynamic noise prediction. The predicted far-field sound levels are then corrected by the proposed spanwise correction methods for the large-span objects. Far-field acoustic measurements and near-field hot-wire measurements are also performed in an anechoic wind tunnel for validation purpose. The predicted aerodynamic and aeroacoustic results are found in good agreement with the experiments with the proposed spanwise correction method. The present models based on the Laplacian function and Gaussian function are unified models taking the advantage of that there is no need to compare the relative extent of the numerical length, experimental length, and coherence length. The results also indicate that although there is no significant difference between the various functions, corrections based on the Gaussian profile seem to perform better compared with other functions.

Future UHBR (Ultra-High Bypass-Ratio) engines might cause serious ‘turbine noise storms’ but, at present, turbine noise prediction capability is lacking. The large turning angle of the turbine blade is the first major factor deserving special attention. The RANS (Reynold Averaged Navier–Stokes equation)-informed (here called RI) method and LINSUB (the bound vorticity 2D model LINearized SUBsonic flow in cascade), developed to predict fan broadband noise, coupled with a two-flat-plates (here called TP) assumption for the turbine blade, is applied here, and one autonomous rapid RI-TP model for predicting turbine wake interaction broadband noise has been developed. Firstly, taking the single axial turbine test rig NPU-Turb as the object, both the experimental data and the DDES/AA (delayed Detached Eddy Simulation/Acoustic Analogy) hybrid model have been used to validate the RI-TP model. High consistency in the medium and high frequencies among the three designed and off-designed rotation speeds indicates that the RI-TP model has the ability to predict turbine broadband noise rapidly. And compared with the original RANS-informed method, with one thin-flat-plate assumption on the blade, the RI-TP model can enhance the PWL (sound power level) in almost the whole spectral range below 10 KHz, which, in turn, is closer to the experimental data and the DDES/AA prediction results. The PWL trend with a ‘dividing point’ position is also studied. The spectrum would move up or down if the location is away from true value. In addition, the extraction location for turbulence as an input for the RI-TP model is negligible. In the future, multi-stage characteristics and the blade thickness effect should be further considered when predicting turbine noise.