On the noise and wake flow of an airfoil with broken and serrated trailing edges

This paper represents the results of an experimental program aiming at exploring the effectiveness of several broken, saw-tooth shaped trailing edges to suppress the instability tone noise at acceptable aerodynamic loss at low Reynolds number flow. This paper also studies the noise sources through acoustic and wake velocity measurements. The test cases consist of several angles of attack from 0 o to 5 o at a fixed Reynolds number of 150,000. For an airfoil with a sharp trailing edge, instability tone noises are detected. The noise mechanism is identified as the aeroacoustic feedback loop which involves the diffraction of the Tollmien-Schlichting waves at the trailing edge. When the trailing edge is replaced with a broken and serrated type, most of these instability tone noises are suppressed, resulting in large noise reduction. The effectiveness of the tonal noise reduction also increases with the serration angle of the saw-tooth. The wake velocity spectra produced by a sharp trailing edge could be very different depending on the angle of attack. Laminar wake at the zero angle of attack ensures that no wake tones could be produced. At 3 o attack angle, the wake becomes very unstable with the presence of a spectra hump and multi-discrete components. It is proposed that these multi-discrete components could eventually produce wake tones in addition to the instability tones produced by the aeroacoustic feedback loop at the pressure side. Because of this, laminar airfoil at a moderate angle of attack always possesses serious noise issue. The wake velocity spectra for a broken, serrated trailing edge are characterized by vortex shedding at 0 o attack angle with no apparent bluntness-induced tone. At 3 o attack angle, the pressure imbalance between the suction and pressure side at each saw-tooth creates counter-rotating vortices. These vortices inhibit both of the spanwise coherence of the bluntness-induced vortex shedding and the multi-discrete components in the wake that would have been produced by a sharp trailing edge. The results in this paper demonstrate that a versatile and low noise laminar airfoil could be achieved when a broken, serrated type trailing edge is used.