| Trailing-edge(TE)noise is the main source of airfoil self-noise.At low-tomoderate Reynolds numbers,the intense tones will be commonly produced by the rigid TE,destroying the quieteness of aerial or underwater vehicles.A new solution of reducing the flow noise and improving the economical efficiency for vehicles can be provided by the deformable airfoils,therefore they have broadband application prospects.Elastic TE is one kind of fundamental deformable airfoil,whose geometry is relatively simple.However,the studies on the generating mechanisms and acoustic characteristics of elastic TE are rare.The effects of TE style,and the geometrical parameters of TE on the radiated noise are also not fully understood.In view of the above backbround,the acoustic radiating characterisictis of elastic TE are investigated for NACA 0012 airfoil.Direct noise simulation is adopted to resolve the evolution of acoustic propagating field of the elastic TE,in conjunction with a hybrid method to reveal the generating mechanisms of elastic-TE noise.The noise generation and propagation of TE noise at different angles of attack(?)are analysed in the simulations and experiments,meanwhile,the impacts of the elasticTE geometrical and material parameters on the noise reduction are also studied.In order to avoid the reflection of coustic waves from the computational boundaries induced by the high-amplitude perturbations in the weak,a hybrid nonreflective boundary condition(HNRBC)for the aoustic direct simulation is proposed.This HNRBC stretches the mesh along the wave front to achieve the maximum dissipation.An acoustic damping layer(ADL)is constructed to surround the acoustic simulation zone,in which the stretching direction of the mesh is matched with the propagating direction of sound waves,thereby achiveing the maximum disippation of waves.Then the ADL is combined with the NRBC applied on the outlet to enhance the capability of sound absorption.For the hybrid prediction of sound,a convective integral formualtion of generalized loading noise is proposed on the basis of convective FW-H formulations 1A,acclerating the acoustic prediction by about 37%.For the convective integreation formulation 1A,the effects of the virtual noise sources generated on the permeable data surface when vortex acrossing the surface are discussed,and it is proposed that using the open data surface and surface averaging method can suppress the virtual noise effectively.The acoustic simulation methods are vaildated.For the nosie generation in a typical fluid-structure system of forcedoscillating cylinder in uniform flow,three modes are identified under different cylider oscillating frequencies,and the range of noise self-suppression is provided.The acoustic field of rigid TE is simulated by direct method,by which intense tones are captured at the attack angles of 0° and 2°,featured by a main tone of highamplitude and several secondary tones equally spaced at the two sides of the main tone.With ? increasing,the main tone shifts to be secondary tones.The acoustic fields of rigid TE attached with an aft flat-plate(FP)extension are simulated,along with the investigations about the effects of FP length and thickness on the TE noise.The results show that the boundary layer separation is delayed by the FP extension,the size and strength of the separation bubbles located just upstream of TE is decreased,and the tones are suppressed completely,thereby the overall sound level.The noise reduction decreases as the increasing of ?.Within a critical length of FP,the radiated noie decreases with the increment of FP length.The TE bluntness noise is decreased by the thinner FP,nevertheless,the overall sound level changes little.For the purpose of improving the compatiblity between the TE large deformation and the requirement of mainting the high quality of small-scale strucural meshes near the TE,a combination of a diffusion-based dynamic mesh method and a ramping treatment of exchanged data among the CFD and CSD is ultilized for the fluidstructure interaction algorithm.Acoustic direct simulation and hybrid prediction is preformed for the partially elastic TE.The effects of the coupling between the fluid fluctuations and structural flexural deflections on sound generation are analyzed,obtaining the noise reduction of the elastic TE at various values of ?,structural damping and rigidity.The results show that the boundary layer separation is delayed by the elastic TE,the surface pressure fluctuation is decreased,and the LBL tones are eliminated.However,the low frequency nosie is increased.Especially when the deflection of the elastic TE is large,the frequency-lock of the fluid fluctuation and the sound by the structural bending waves is observed at the TE low-order natural frequencies,at which the tones appear.Increasing the structural damping can reduce the amplitude of tones.The noise reduction decreases as ? increases,and first increases and then decreases with the increase of elasticity,respectively.The radiatd sound fields from the elastic TE FP-extension are simulated by direct simulation and the hybrid predictions.The coupling between different noise sources and acoustic characteristics are analyzed for the elastic TE FP-extension at various angles of attack,together with the discussions of the effects the FP rigidity and thickness.In comparison with the rigid TE FP,the elastic ones can further reduce the noise radiation.At the locked frequencies,the loading noise generated by the surface pressure fluctations will induce secondary pressure waves on the airfoil surface,enhancing the LE noise.For the given values of rigidity,the noise reduction achieved by the elastic TE FP decreases as the rigidity decreases.The noise from elastic TE extension is measured in experiments,the noise sources of the main character frequencies are located using beamforming technique.The measured tonal source is located on the airfoil surface very near to the trailing edge.The tone can be effectively suppressed by attaching the elastic flat-plate TE extension,which also results in the noise source moving towards downstream to the flat-plate surface just upstream of the trailing edge.Agreements of the location of tone source,the suppression of tones by the elastic flat-plate TE extension and its effects on corresponding noise source are observed between the simulations and experiments.The measured effects of the variation of flat-plate thickness on the bluntness noise also verifies the simulations... |
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