Research On The Control And Reduction Mechanism Of Airfoil Trailing Edge Broadband Noise

Inspired by the silent flight capability of owls,the serrated trailing edge has been considered as an effective method in controlling trailing edge noise.In this paper,the large-scale,high-fidelity implicit large eddy simulation was carried out for the airfoil with serrated trailing edge,focusing on the effect of sawtooth length and profile on the broadband noise and near-field flow.The intrinsic noise reduction mechanism was revealed in detail,and a new composite design was made accordingly,which achieved more noise reduction.The main works were summarized as follows.(1)A high-fidelity large eddy simulation(LES)was performed for the serrated trailing edge NACA-0012 airfoil at Mach number 0.1631 and Reynolds number 96,000.Since the trailing edge noise was more sensitive to the length of the serrations,the noise radiation of serrations with different lengths was investigated and it was found that the medium length serration achieved the best noise reduction with a maximum noise reduction of 1.94 dB in the main propagation direction of the acoustic wave.The dynamic mode decomposition showed that the sawtooth significantly changed the spatial distribution of the pressure mode structure near the upstream airfoil and induced the energy transfer of the pressure modes between different frequencies.In order to reveal the intrinsic physical mechanism of noise reduction,the coherence characteristics between acoustic waves radiated from sources on the airfoil surface were quantitatively analyzed based on the classical Ffowcs Williams&Hawkings acoustic analogy theory,and the results revealed that destructive interference was the main mechanism for noise reduction.However,the results of pure dipole source analysis showed that the longer serrations had a better noise reduction effect,which was different from the LES results,suggesting that the effect of flow structure was not negligible.Therefore,we established the first quantitative correlation between the flow structure near the sides of the sawtooth and the physical sound sources based on the multi-process acoustic theory,and revealed the dual effect of the sawtooth trailing edge on the flow sources.The results confirmed that the flow sources play an important role in the noise reduction of serrated trailing edges and should be fully considered in the design of serrations.(2)In order to optimize the noise reduction performance of the serration,the relatively systematic curved-edge serration design was achieved by using different curvature combinations in the front and back halves of the serration based on the existing serrated sawtooth.Firstly,the large eddy simulation was performed and the noise reduction characteristics of different curved-edge serrations were compared.It was found that all the curved-edge serrations could achieve a certain degree of noise reduction.The noise reduction effect of the feather-shaped serration was better than that of the traditional triangular serration,and the noise reduction in the main propagation direction was about 2.51 dB.Secondly,by extracting the vortex structure near the serrations,it was found that the feather-shaped serrations induced a similar vortex structure to that of the traditional triangular serrations at their side positions.The feather-shaped serrations delayed the flow interaction between the upper and lower side of the airfoil.Finally,the coherence analysis showed that destructive interference was still the dominant noise reduction mechanism for the noise reduction of curved-edge serrations,and the noise reduction was more obvious in the low-frequency range.Furthermore,the physical sourcebased diagnosis found that the side vortex pairs near the curved edges were also beneficial for trailing edge noise reduction.(3)Based on the understanding of the noise reduction mechanism of the sawtooth,we further designed two types of composite trailing edges to improve the noise reduction performance,i.e.,cutting-type poro-serrated(CPS)trailing edge and inserttype poro-serrated(IPS)trailing edge.The results showed that the IPS composite trailing edge could achieve more noise reduction compared to the traditional sawtooth serrations,and the maximum broadband noise reduction reached 6.09 dB.The noise reduction was mainly found in the low and mid-frequency range,while the noise increased in the downstream direction in the high-frequency domain.However,it should be noted that the IPS composite trailing edge had a negative impact on the aerodynamic performance,with an increase of the drag coefficient by 8.9%in the high porosity cases.Further flow analysis showed that the IPS composite trailing edge induced side vortex structures similar to that for the sawtooth trailing edge and the spanwise vortex structures specific to the trailing edge location.The energy proportion of the most energetic mode decreased,while the energy proportion of the high-frequency modes increased slightly.In order to reveal the intrinsic noise reduction mechanism of the composite control,we systematically analyzed the effect of the composite control on the near-field pressure fluctuation and found that the combined effects of the significant decrease in convective velocity at the IPS trailing edge position and the strengthened destructive coherence between the radiated acoustic waves from different positions on the airfoil surface determine the final noise reduction performance.