Beat of sound generated from NACA0012 airfoil on application of suction-blowing excitation.

The present numerical study explores the effect of periodic suction-blowing excitation (SBE) on sound generation in the flow over a NACA0012 airfoil, with a focus on inducing beats in the resulting sound signals. Simulations are conducted at a Reynolds number (Re) of 5000 and an angle of attack of 5° using direct numerical simulation (DNS) approach where compressible Navier-Stokes equations are solved to compute both flow and sound fields directly. At low Re, vortex-shedding generates tonal noise at the Strouhal frequency (St). The introduction of periodic SBE adds sound sources at the excitation frequency (ff), with the resultant sound field arising from the interaction between vortex-shedding and excitation-induced sound sources. This study investigates the effects of excitation forcing frequency (ff) and amplitude (Ae), identifying conditions that lead to beat formation. Beats are observed when ff is close to St and the sound intensities of both sources are comparable. Distinct wave propagation patterns emerge during beat formation, significantly differing from non-beat scenarios. A key contribution of this work is to offer a guideline on the relationship between excitation amplitude and the formation of sound beats, either in the axial direction of flow or normal to it.