On the aeroacoustic and flow structures developed on a flat plate with a serrated sawtooth trailing edge

Abstract

a b s t r a c tResults of an experimental study on turbulent flow over a flat plate with a serrated sawtooth trailing edge are presented in this paper. After tripping the boundary layer to become turbulent, the broadband noise sources at the sawtooth serrated trailing edge is studied by several experimental techniques. Broadband noise reduction by the serrated sawtooth trailing edge can be realistically achieved in the flat plate configuration. The variations of wall pressure power spectral density and the spanwise coherence (which relates to the spanwise correlation length) in a sawtooth trailing edge play a minor role in the mechanisms underpinning the reduction of self noise radiation. Conditional-averaging technique was applied in the boundary layer data where a pair of pressure-driven oblique vortical structures near the sawtooth side edges is identified. In the current flat plate configuration, the interaction between the vortical structures and the local turbulent boundary layer results in a redistribution of the momentum transport and turbulent shear stress near the sawtooth side edges as well as the sawtooth tip, thus affecting the efficiency of self noise radiation.

Three-dimensional flow field over a trailing-edge serration and implications on broadband noise

“…Up to x/c = 0.04, a smaller ϕ ′ corresponds to a stronger near-wall outward motion induced by the streamwise-oriented vortices. According to Chong and Vathylakis,7 this outward motion may locally result in a stronger contribution to the broadband scattered noise when compared to an unaffected boundary layer convecting along the serrated edge at angle ϕ. Towards the tip of the serrations, near-wall streamlines remain almost aligned with the x-coordinate, or they are subjected to an inward motion, thus potentially resulting in a local lower contribution to the scattered noise with respect to the root location (ϕ ′ approximately equal or even higher than ϕ).…”

“…For the aforementioned applications, the most relevant mechanisms are related to the interaction of the turbulent boundary layer with the airfoil trailing edge, especially at low Mach numbers. 3 In the past, several passive noise-mitigation solutions have been proposed with the goal to alleviate this contribution, e.g., trailing-edge brushes, 4 sinusoidal and sawtooth serrations, [5][6][7][8][9][10][11][12] slits, 13,14 and randomly shaped trailing edges. 15 In the past decade, many analytical, [16][17][18][19] experimental, 5,9,11,20 and numerical studies 1,[21][22][23] have appointed sawtooth serrations as one of the most promising solutions.…”

“…One of the most relevant attempts, though with substantial flow perturbation, was presented by Chong and Vathylakis. 7 In their study, they measured surface-pressure fluctuations on serrations installed at the trailing edge of a flat plate. Quiescent conditions were maintained on one side of the plate.…”

Near-wall pressure fluctuations over noise reduction add-ons

“…For this reason, research was focused on several passive noise-mitigation solutions, which could be easily installed with a relatively low effect on the overall turbine performance. 4 Amongst the most simple ones, sinusoidal and sawtooth geometries [4][5][6][7][8] are already installed at the trailing edge of many industrial wind turbines. More complicated geometries have been designed by employing slits, 9 brushes, 10 randomly 9 and iron-shaped trailing edges.…”

“…Additionally, it is practically impossible to install pressure sensors on thin serrations without perturbing the flow. 5,15 An alternative approach consists in estimating pressure from velocity in turbulent-boundary layers, 16,17 by solving the Navier-Stokes momentum equation in the Poisson form. This particular technique typically allows obtaining higher spatial resolution with respect to the one that can be achieved via surface microphone.…”