Hydrodynamic Instability in the Generation of Slat Noise

Slat noise constitutes today a major contribution to the total noise emitted by regional jets, and the mechanisms responsible for their generation are not completely understood. In this paper, we are concerned with the low-frequency peaks that are commonly found in the far- and near-field spectra. Lattice-Boltzmann simulations of a standard three-element high-lift wing configuration are carried out, and attention is focused on the turbulent flow in the slat region. Near- and far-field data from the simulations are compared with wind-tunnel experiments and results in the literature, showing good agreement. Proper Orthogonal Decomposition is used to identify large-scale structures related to the two dominant frequency peaks in the noise spectrum. Different correlations are used in the POD: the usual one based on kinetic energy, and others based on the pressure fluctuations, inside and outside the slat region. Considering the latter increased the relative importance of the leading POD mode, reaching almost 100% of the correlation for the given frequency, and its physical structure resembles the fluctuations typically associated with mixing-layer instability. Additionally, the analyses indicate that the structures with highest turbulent kinetic energy are not necessarily the ones most correlated with the noise radiated to the far field.