Noise generation of fluttering flag in a free stream

• Published in: Journal of Fluid Science and Technology Vol. 16; no. 1; p. JFST0005
• Main Authors: NISHIKAWA, Reon, TERASHIMA, Osamu, KONISHI, Yasufumi, OKUNO, Miyu
• Format: Journal Article
• Language: English
• Published: Tokyo The Japan Society of Mechanical Engineers 2021; Japan Science and Technology Agency
• Subjects: Constants; Directivity; Displacement; Displacement measurement; Downstream effects; Flag; Flags; Flutter; Inlets (waterways); Lasers; Microphones; Noise; Noise generation; Particle image velocimetry; Sound pressure; Vibration; Visualization; Wind tunnel; Wind tunnels
• ISSN: 1880-5558; 1880-5558
• DOI: 10.1299/jfst.2021jfst0005

An experimental study on the noise from a fluttering flag was performed in a low-noise wind tunnel. In the experiment, simultaneous measurements of noise from the flag and its motion were performed using microphones and a camera, respectively, to obtain the noise characteristics and their relation. Additionally, simultaneous measurements of noise and its displacement were performed to quantitively discuss their relation using seven laser displacement sensors. The experimental results indicated that a highly periodic noise with significant directivity in the vertical direction is generated from the flag, and the dominant frequency of the noise is linearly proportional to the inlet velocity. Additionally, the constants of proportionality are inversely proportional to the length of the flag and the square root of its thickness. The results also indicated that the downstream edge of the flag rolls up and down when significant periodic sound pressure is generated by the flow near the downstream edge of the flag. Furthermore, near the center of the downstream edge, the flag flutters at the dominant frequency of the emitted noise with high two-dimensionality. Therefore, the fluttering region was observed as the source of significant periodic noise from the flag. It is also found that the vibration of the downstream edge which brought the noise generation was caused by the strong upward or downward flow which occurs periodically.