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

Abstract

The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a positive angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relates the velocity field to pressure fluctuations, suggests that the contribution of the serrations to far-field broadband noise is a function of the streamwise location. This observation is congruent with the spectra of the wall-normal and spanwise velocity fluctuations, which typically show low intensity close to the tips of the individual serrations. It follows that analytical models must take into account the local contribution to the far-field noise induced by the streamwise variation of the hydrodynamic pressure on the serration surface.

Near-wall pressure fluctuations over noise reduction add-ons

“…A clear reduction of the Reynolds uu component can be seen in presence of the solid serrations. This is in line with the experimental investigation from the authors, 7 showing that even at zero angle of attack the mild acceleration experienced by the flow to adapt to the serration surface allows for a dampening of the stream-wise Reynolds fluctuations with respect to the straight case. For the test case comparing the both STE and Sr20R21 a reasonable agreement of the mean flow profiles and Reynolds stresses can be seen already from y + = 20 (corresponding to one window size for the PIV fields).…”

“…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.…”

“…It was recently showed that one of the limitation of the theoretical model is the assumption of frozen turbulence along the serration length. 7,8 As a matter of fact, the intensity of the pressure fluctuations and their spectra vary in the stream-wise direction.…”

Concave serrations on broadband trailing edge noise reduction

“…Previous studies [10,24,25] showed that the sawtooth trailing-edge serrations induce distortion of the near-wall streamlines. Following Chong & Vathylakis [21], the distortion of the streamlines may reduce the effective angle seen by the turbulent flow convecting over the serrations and thus may locally increase the scattered noise with respect to an ideal flow over a sawtooth geometry.…”

“…[7,10,21,22]) have shown that the flow past serrated airfoils is strongly three dimensional, with vortical structures developing along the edges of the serrations. Flow measurements [7,10,24] and computations [22,25] showed that, even at small angles of attack, the turbulent flow tends to seep into the empty space in between serrations. This flow distortion is found to reduce the effective angle seen by the turbulent flow convecting over the edge of the serrations thus reducing the effectiveness of the serrations in mitigating far-field noise [21].…”