Abstract
This paper employs serrated leading edges to inject streamwise vorticity to the downstream boundary layer and wake to manipulate the flow field and noise sources near the blunt trailing edge of an asymmetric aerofoil. The use of a large serration amplitude is found to be effective to suppress the first noise source-bluntness-induced vortex shedding tonal noise-through the destruction of the coherent eigenmodes in the wake. The second noise source is the instability noise, which is produced by the interaction between the boundary layer instability and separation bubble near the blunt edge. The main criterion needed to suppress this noise source is related to a small serration wavelength because, through the generation of more streamwise vortices, it would facilitate a greater level of destructive interaction with the separation bubble. If the leading edge has both a large serration amplitude and wavelength, the interaction between the counter-rotating vortices themselves would trigger a turbulent shear layer through an inviscid mechanism. The turbulent shear layer will produce strong hydrodynamic pressure fluctuations to the trailing edge, which then scatter into broadband noise and transform into a trailing edge noise mechanism. This would become the third noise source that can be identified in several serrated leading edge configurations. Overall, a leading edge with a large serration amplitude and small serration wavelength appears to be the optimum choice to suppress the first and second noise sources and, at the same time, avoid the generation of the third noise source.
Highlights
This paper employs serrated leading edges to inject streamwise vorticity to the downstream boundary layer and wake to manipulate the flow field and noise sources near the blunt trailing edge of an asymmetric aerofoil
The first is related to the bluntness-induced coherent and spanwise structures in the wake region where the velocity fluctuations caused by the shedding cycle would resonate to the far field in the form of narrowband radiation
The second noise source is related to the boundary layer instability scattering at the blunt edge
Summary
This paper employs serrated leading edges to inject streamwise vorticity to the downstream boundary layer and wake to manipulate the flow field and noise sources near the blunt trailing edge of an asymmetric aerofoil. The so-called leading edge serration is well known for its capability to improve the aerodynamic performance of aerofoil at the post-stall regime by generating streamwise vortices to transfer high momentum flow from the free stream to the boundary layer (Miklosovic et al, 2004; Johari et al, 2007; Hansen et al, 2011; Rostamzadeh et al, 2012) This process results in a shrinking or an outright suppression of the separation zone near the leading edge, minimising the tendency for stall (Van Nierop et al, 2008; Fish et al, 2011; Skillen et al, 2015)
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