On the spatial-temporal development of synthetic turbulent boundary layer on a serrated trailing edge

Abstract

This paper presents results from an experimental study to investigate the flow mechanism that enables the add-on saw-tooth serrations to achieve the reduction of broadband trailing edge noise produced by a NACA 0008 aerofoil. The current work traces the temporal-spatial development of an artificially generated synthetic turbulent boundary layer (SBL) on a straight and serrated aerofoil trailing edge. The experiment was performed at Re = 0.96 × 10 5 based on the aerofoil chord. Temporal variations in the acoustic pressure radiated as a result of the flow interaction between the aerofoil trailing edge and the SBL were studied by conducting a wavelet analysis of the far-field noise measurement data. The results successfully capture a short duration variation in the radiated acoustic pressure associated with the passing of the SBL over the trailing edges. Comparison of the results obtained for the straight and serrated trailing edges identifies noise reduction over a short time domain that is exactly linked to a reduced level of wallnormal turbulence intensity close to the surface of trailing edge serrations. A distinct high momentum vortical structure is formed in the near wake of serrations at a later stage of the SBL. This secondary flow is responsible for the reduction of the fluctuations in the wall-normal velocity component at the serration, as well as the increase of the anisotropic level of the turbulence at the serration. As a result, turbulence scattering at the oblique edges is weakened, leading to the reduction of the radiated noise. The results presented in this paper provide a new outlook on the turbulence interaction with trailing edge serrations and the associated trailing edge broadband noise generation.

Reconstruction of the deterministic turbulent boundary layer for the study of aerofoil self-noise mechanisms

“…This is indeed confirmed in Fig. 12a-c of Juknevicius and Chong (2019). An important aerofoil design consideration is emphasised here.…”