| Trailing-edge tonal noise of airfoils is strong under low to moderate Reynolds number condition, at which a lot of machines operate in engineering applications. The bionic serrated trailing edge has been found to have the ability to reduce the airfoil aerodynamic noise when the Reynolds number is high, also a few experiments have indicated that serrations on the trailing edge have the potential to control the tonal noise under low to moderate Reynolds number. The effects introduced by the serrations on the flow and the noise source around airfoils are still to be further investigated, so as to understand the mechanism of noise reduction associated with the serrated trailing edge and provide instructions to the design of low-noise airfoils.This thesis presents a numerical study of the flow around NACA-0018 airfoils with serrated trailing edges under 51.6 1 0cR e ? ? and 6 ? angle of attack. Two models with serrations which have different lengths and the same wavelength were designed to investigate the effects introduced by the serrations on the flow related to the tonal noise. The zonal RANS-LES method was applied to compute the flow around airfoils, and the WALE model was chosen as the LES subgrid scale model, meanwhile the SST k ?? turbulent model was applied within the RANS zones. The mesh near the wall was refined and could perform a medium-resolution LES. Furthermore, the aerodynamic noise was predicted by the FW-H formulas.Under the computational condition, the laminar boundary layers on the suction sides of the airfoils separate at about x/ C ? ?0.1 near the leading edge, and reattach as turbulent boundary layers around the mid chord, resulting separation bubbles with a length of0.2C. The turbulence on the suction sides near the trailing edges are strong. Comparisons show that the trailing edge serrations have no distinct effect on the development of the boundary layer upstream on the suction side. On the pressure side, the boundary layer of airfoil A1 with short serrations is similar to that of the original airfoil A0, i.e. the laminar boundary layers separate at about x/ C ?0.49. While the boundary layer on airfoil A2 with long serrations does not separate until the flow arrives about x/ C ?0.55. The pressure sides near the trailing edge are within separation zones under transitional condition.Results from the unsteady simulation indicate that, the lift and drag of the original airfoil A0 fluctuate periodically with a frequency of 2270 Hz. While the forces on serrated airfoils A1 and A2 have no apparent periodicity. Analysis on the instantaneous flow show that near the trailing edge of A0, the vortices and positive vorticity shed off from the pressure side periodically, leading to the periodic fluctuation of the lift and drag. And the shed vorticity structures of A1 and A2 break down more rapidly.It could be concluded that, the long serrations on the trailing edge of A2 provide the channel for the longitudinal flow, within which vortices that rotate clockwise exist. So the strong turbulence from the suction side could meet and mix with the flow from the pressure side further upstream, making the turbulence stronger within the backflow area on the pressure side and shear layer in the wake. The stronger turbulent flow near the trailing edge delays the separation of the boundary layer on the pressure side, resulting a narrower backflow area. Thus the periodic shedding of the positive vorticity near the trailing edge is changed and the periodicity of the pressure fluctuation is broken, so that the fluctuating pressure on the whole airfoil is weaken.Under the computational condition, the flow structure on the pressure side near the trailing edge is influenced by the long serrations, which show the potential to control the trailing edge tonal noise. Prediction based on the FW-H formulas indicates that the long serrations suppress the tone of the trailing edge noise and reduce the overall sound pressure level. |
PDF Full Text Request