Computational Aeroacoustic Prediction of Tonal Noise for Low Reynolds Number Airfoils

Abstract

Airfoils operating in low Reynolds number, Re, conditions can generate tonal aeroacoustic noise due to the laminar or transitional boundary layer (BL) at the airfoil trailing edge (TE). At modest Re of less than 1×10⁵, an elongated laminar separation bubble (LSB) can occur near the TE which adds complexity to the BL transition and also generates tonal noise. Computational fluid dynamic and computational aeroacoustic simulations of the SD 7037 airfoil at Re=4.1×10⁴ are completed to study this phenomenon. The numerical methods used are incompressible wall-resolved large eddy simulation (LES) and the Ffowcs-Williams and Hawkings acoustic analogy, with the results of both methods validated against experimental data. The LES simulation of the airfoil BL development is critical to the tonal noise prediction and the accuracy of the predicted tones were assessed for a series of mesh refinements in the near-wall and separated BL regions. The mesh refinements in the regions of BL separation resulted in the correct simulation of the LSB and the associated aeroacoustic tonal noise for 1° angle of attack (AOA). Simulations at higher AOAs showed the sensitivity of the transient BL behaviour to the mesh refinements in the BL, while the time-averaged BL behaviour remained stable. Spectral analysis of the velocity in the BL determined that the source of the tone originates from the Tollmien-Schlichting wave frequency in the attached laminar BL, which is then amplified by the Kelvin-Helmholtz instability that forms in the LSB. The accurate tonal noise prediction occurred in the absence of the acoustic feedback mechanism.