Study of discrete tonal noise of NACA0015 airfoil at low reynolds numbers

Abstract

The aim of this research is to investigate the discrete tonal noise generated at low Reynolds number with a variation of angle of attack and freestream velocity. In order to meet the research objectives, experimental testing on NACA0015 airfoil was done in the anechoic wind tunnel to measure the sound spectrum at low Reynolds number of $7.0×10^4≤Re≤9.5×10^4$ and at angle of attack of $0^\circ≤α≤5^\circ$. Amplification of TS-waves by laminar separation bubble on the airfoil pressure side is known to be a necessary criterion in the generation of high intensity tonal noise whereas acoustic feedback mechanism contributes in the generation of the discrete tonal noise. It is shown in this thesis that previously proposed empirical models have limitations in predicting primary frequency at low Reynolds number when angle of attack varies. It is also shown that the airfoil tonal noise is of the highest intensity at $α=0^°$ and gradually decreases before disappearing beyond $α=5^\circ$ due to insufficient amplification of TS-waves at higher angle of attack. Each secondary tonal frequency is associated with acoustic feedback mechanism having velocity dependency of $~U^{0.8}$ while the primary frequency is associated with the most amplified TS-wave and is prone to exhibit ladder structure behavior with velocity dependency of $~U^{1.3}$. The work continues to study the effect of external acoustic excitation on the airfoil discrete tonal noise. The effect of excitation amplitude of 70dB and 90dB and excitation frequencies of 3000Hz and 2000Hz on the airfoil discrete tonal noise were studied at angle of attack of $0°, 3°, and 5°$. External acoustic excitation at proper excitation amplitude and excitation frequency is found able to suppress airfoil tonal noise. Acoustic excitation of 3000Hz and 90dB was found best to suppress tonal noise at angles of attack $3° and 5°$. In general, acoustic excitation only affects the behavior of the primary frequency. The primary frequency dependency at $α=0°$ was found to change to $~U^{2.0}$ with acoustic excitation at 3000Hz and 90dB as well as 2000Hz and 70dB. However, each discrete frequency was found to have $~U^{0.8}$ dependency with freestream velocity, which is similar to the case without excitation. Additionally, linear stability analysis was carried out to understand the behavior of TS-wave, which is the primary cause of the tonal noise. The effect of acoustic forcing on the naturally amplified TS-waves is investigated. The study was done by introducing acoustic forcing term into the OSE and the speed of propagation of the most amplified TS-wave was solved using the shooting method.