Airfoil Tonal Noise Generation in Resonant Environments

Abstract

To clarify tonal noise generation, an experimental study on airfoil tonal noise was undertaken using a conventional wind tunnel, which allows acoustic reflection on test section walls. A two-dimensional wing model with the NACA0015 cross-section was used at 5 degrees angle of attack. Most previous experiments conducted in anechoic environments commonly show that the tonal noise frequency is selected in an overall trend of U 1:5 (U is uniform velocity) locally consisting of a step-like structure, and Tollmien-Schlichting disturbances are rapidly amplified in the backflow region near the trailing edge of the pressure surface. The present experiments in an acoustically resonant environment show that the tonal noise emanates in accordance with the aforementioned features. However, the ladder-like structure has a different local slope from that observed in anechoic flow. These characteristics suggest that acoustic resonance does not play a fundamental role in tonal noise generation. Observation by hot-wire and smoke visualization techniques shows that unsteady disturbances rather than Tollmien-Schlichting waves are rapidly magnified by the Kelvin-Helmholtz instability in the backflow region. The frequency selection mechanism at tonal noise generation still remains unsolved.

Suppression of Trailing-Edge Noise Emitted by Two-Dimensional Airfoils

“…8, the moderate peak consisting of broadband disturbances peaking at about 270 Hz, which is very close to the frequency of the tonal noise, is obviously not attributable to T-S instabilities. This is consistent with our previous observations, 1) which concluded that the broadband disturbances originate from hydrodynamic instability of the inflectional velocity profile and as a consequence, visualized flow motions show that unsteady disturbances seem to be sequentially magnified during a half period, after the turbulent boundary layer is shed from the suction side of airfoil. The top spectrum is the natural case and the bottom with À20 dB shift is the artificially disturbed case.…”

“…The reason why acoustic sound is discrete is ascribed to feedback loops between the T-E noise and T-S waves, although T-S instability is inherently unstable with respect to broadband disturbances. However, this explanation does not cover the case where T-S instabilities are subcritical at very low Reynolds numbers, as reported by Nakashima et al 6) In addition, the experiments by Atobe et al 1) and Kobayashi et al 7) showed no T-S waves in generation of tonal noise, even for natural moderate Reynolds numbers. It appears that the mechanism of tonal noise generation from the trailing edge might be complex.…”

“…This is different from the Strouhal number of 2D bluff bodies. Figure 3 also shows that there is a ladder-like structure with a constant slope on the general trend curve, which has been found by numerous researchers [1][2][3] in conventional closed test sections to be prone to sound reflections from hard walls, resulting in acoustic resonance. Contrarily, Nash et al 8) demonstrated that each step does fit the U 0:8 dependence in anechoic environments.…”

Experimental investigation of aerofoil tonal noise generation

“…A notable exception is the data set of Chong & Joseph (2012), which shows a dependency on u 0.8 scaling over very small ranges of Reynolds number only, indicating a finer 'ladder' structure. Also the data of Atobe et al (2009) (not shown here) shows a different, namely constant scaling. Therefore, it appears that different types of 'ladder' structures might be associated with different mechanisms, as noted before by Tam & Ju (2012).…”

“…The study of Atobe, Tuinstra & Takagi (2009), conducted in a resonant environment, showed a fundamentally different velocity dependence. In contrast to the experiments conducted in anechoic facilities, tonal frequencies were reported to remain constant with increasing velocity, which was attributed to resonance effects.…”