Dynamics of the large-scale structures and associated noise emission in airfoil slats

Abstract

We investigate the slat narrowband peak noise generating mechanism. Unsteady flow data were generated by a lattice-Boltzmann-based commercial code for four configurations, accounting for variations of the airfoil angle of attack and slat overlap. Comparison with experimental results indicates that the aspects of the flow field relevant for the generation of the narrowband peaks were accurately captured. Frequency-domain proper orthogonal decomposition (POD) is applied to identify dominant large-scale structures in the frequency range dominated by the peaks. The combined use of the two POD metrics, namely, the turbulent kinetic energy in the turbulent flow region and the acoustic pressure in the far field, demonstrated that the structures most correlated with the noise resemble spanwise coherent Kelvin–Helmholtz vortices which dominate the slat cove only at the frequency of the narrowband peaks. Time evolution of the structures educed using the acoustic pressure correlation provides detailed evidence of the hydrodynamic and acoustic steps of a Rossiter-like feedback mechanism between the slat cusp and trailing edge. The combined analysis of results for the different slat configurations provides an explanation for the effect of the slat configuration on the amplitude of the narrowband peaks observed in previous studies, particularly the influence of the main-element suction peak.

SPOD analysis of noise-generating Rossiter modes in a slat with and without a bulb seal

“…2019; Souza et al. 2019). PowerFLOW is based on the lattice Boltzmann method in which macroscopic gas properties such as pressure, velocity and temperature are expressed as functions of the statistical behaviour of the molecules contained in a control volume for a given distribution function .…”

“…It is important to note that, despite three-dimensional simulations being necessary to fully reproduce the slat flow dynamics, the emitted noise is correlated to spanwise-aligned (two-dimensional) structures immersed in the flow, as shown by Souza et al. (2019).…”

“…Souza et al. (2019) conducted lattice Boltzmann numerical simulations for different configurations of slat gap and overlap relative to the wing main element. Results were analysed via proper orthogonal decomposition (POD) in the frequency domain (today commonly referred to as spectral POD or SPOD; e.g.…”

“…Finally, Souza et al. (2019) concluded that, since the aerodynamic design of the slat aims at reducing the main-element suction peak, there is a conflict between good aerodynamics and low noise emission.…”

“…The same techniques used by Souza et al. (2019) are employed here, namely lattice Boltzmann simulations and SPOD. We show that the higher recirculation of turbulence in the two-bubble scenario accelerates the evolution of the coherent structures and enhances the lower-order Rossiter modes.…”