Noise generated from two flat plates in tandem configuration in turbulent mean flow

Abstract

This work is a fundamental investigation of the noise radiated by two flat plates in a tandem configuration in a turbulent flow. It is shown that this configuration is capable of significantly reducing turbulence-aerofoil interaction noise at low frequencies. The current study presents far-field noise experimental data and detailed high-resolution numerical simulations based on a spanwise vortex impinging on a flat-plate aerofoil at a zero angle of attack. The noise reduction spectra of tandem plates exhibit peaks of strong noise reduction at non-dimensional frequencies of $fl_d/U_c=n$, where $l_d$ is the distance between the leading edges of the two flat plates, $U_c$ is the convection velocity and $n$ is an integer. It is proposed that these are caused by interference between the two leading edges radiating with a phase shift of 180$^\circ$. In addition to the edge-to-edge interference, it is found that noise reductions are also caused by an effective shortening of the chord which leads to generally weaker radiation. These two mechanisms were recently proposed by the authors for aerofoils with downstream porosity and are investigated numerically in detail here for the case of flat plates in tandem configuration.