Switch of tonal noise generation mechanisms in airfoil transitional flows

TL;DR

This study uses large eddy simulations to analyze how tonal noise generation mechanisms in an airfoil change with Reynolds number, revealing a switch from laminar separation bubble dynamics to vortex shedding as the dominant noise source.

Contribution

It identifies the transition of noise generation mechanisms in airfoil flows from laminar separation bubble dynamics to vortex shedding with increasing Reynolds number.

Findings

Flow patterns vary with Reynolds number, affecting noise sources.

Lower Re: intermittent vortex dynamics and laminar bubbles dominate.

Higher Re: vortex shedding becomes the main noise mechanism.

Abstract

Large eddy simulations are performed to study tonal noise generation by a NACA0012 airfoil at an angle of attack ${\alpha = 3}$ deg. and freestream Mach number of ${M_{\infty} = 0.3}$. Different Reynolds numbers are analyzed spanning ${0.5 \times 10^5 \le Re \le 4 \times 10^5}$. Results show that the flow patterns responsible for noise generation appear from different laminar separation bubbles, including one observed over the airfoil suction side and another near the trailing edge, on the pressure side. For lower Reynolds numbers, intermittent vortex dynamics on the suction side results in either coherent structures or turbulent packets advected towards the trailing edge. Such flow dynamics also affects the separation bubbles on the pressure side, which become intermittent. Despite the irregular occurrence of laminar-turbulent transition, the noise spectrum depicts a main tone with multiple equidistant secondary tones. Increasing the Reynolds number leads to a permanent turbulent regime on the suction side that reduces the coherence level causing only small scale turbulent eddies to be observed. Furthermore, the laminar separation bubble on the suction side almost vanishes while that on the pressure side becomes more pronounced and permanent. As a consequence, the dominant noise generation mechanism becomes the vortex shedding along the wake.