Acoustic and hydrodynamic analysis of the flow around an aerofoil with trailing-edge serrations

Abstract

Direct numerical simulations of the flow around a NACA-0012 aerofoil are conducted, employing an immersed boundary method to represent flat-plate trailing-edge extensions both with and without serrations. Properties of the turbulent boundary layer convecting over the trailing edge are similar for both cases. For cases with serrations, the trailing-edge noise produced by the flow over the aerofoil is observed to decrease in amplitude, and the frequency interval over which the noise reduction occurs differs depending on the serration length. The directivity and spanwise coherence of the trailing-edge noise appears largely unaffected by the serrations. The hydrodynamic behaviour in the vicinity of the trailing-edge extensions is investigated. The streamwise discontinuity imparted upon the turbulent flow by the straight trailing edge can clearly be observed in statistical quantities, whereas for the serrated case no spanwise homogeneous discontinuities are observed. The trailing-edge serrations appear to break up the larger turbulent structures convecting into the wake, and to promote the development of horseshoe vortices originating at the serrations themselves.

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

“…In presence of trailing-edge serration streamwise-coherent structures evolve toward more spanwise-oriented structures as demonstrated by the computations of Jones and Sandberg 21 thus affecting noise generation. For this reason, the magnitude-square coherence (γ 2 ) of the spanwise velocity fluctuations (w ′ ) is evaluated.…”

“…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. 12 However, understanding of the physical mechanisms behind trailing-edge serrations noise reduction has not been obtained yet.…”

“…So far only DNS (Direct Numerical Simulation) computations 1,21,22 have been able to reproduce the complex flow evolution and to provide data of the flow near the serration surface under realistic operating conditions. Jones and Sandberg 21 described the flow around a NACA 0012 airfoil at free-stream Mach number equal to 0.4 and at an angle of attack of 5…”

“…Jones and Sandberg 21 described the flow around a NACA 0012 airfoil at free-stream Mach number equal to 0.4 and at an angle of attack of 5…”

“…However, in the presence of trailing-edge serrations at an angle of attack, the strong three dimensionality of the flow 21 and the adverse pressure gradient 35 may strongly modify the statistical properties of the turbulent boundary layer approaching the trailing edge. A description of the flow features is then essential to uniquely identify the noise emission and eventually reduction obtained by the device.…”