The switching process of trailing edge dominant tones on the different ladders

Abstract

Airfoil trailing edge noise in low-to-moderate Reynolds number has multiple tonal noise. The frequency of the dominant tone has a ladder structure for the dependency on the free stream velocity, while the presence of the secondary tones is explained as an amplitude modulation by the primary single tone. However, during the jumping process, when the main tone is selected from two nearly equal strength tonal noise on the different ladders, the choice of the main tone and amplitude modulation theory are poorly investigated and lacks specific experiments at present. Hence, the switching process of trailing edge dominant tones of a NACA0012 airfoil at a Reynold number of around 130,000 is focused on by a simultaneous measurement of microphone and hot-wires at Beihang D7 aeroacoustics wind tunnel. The far field noise measurement confirms presence of two tones on time-average with a velocity range from 19.8m/s to 20.2m/s. For a freestream velocity of 20m/s, after selecting the time span by short-term Fourier transform, the wavelet contour maps of microphone present a random selection of the strongest tone, which is a switching tones pattern contradictory to the periodic amplitude modulation of only one tone. The simultaneous measurement results by two hotwires also show similar patterns with differences on the spanwise locations. This non-uniform spanwise alternation between frequencies rather than a consistent modification along the spanwise direction might contribute to the switching tones pattern in the microphone signals. A statistical analysis of the occurrence of two tones indicates that the switching tones pattern in sound pressure signals is greatly influenced by the spatial distribution of velocity on the spanwise direction as sound sources during the jumping process.