Session: 17-01-01 Research Posters

Paper Number: 77447

Start Time: Thursday, 02:25 PM

77447 - Experimental Study on the Relation Between Flow-Induced Vibration and Noise Generation of a Fluttering Flag 

A fluttering flag in a flow has been investigated for a long time from both a physical and an engineering point of view. Various research topics, such as, its drag coefficients, characteristics of its oscillation, unsteady fluid force acts on it, and its effect on the heat transfer have been studied. However, the relation between its flow-induced vibration and sound generation has not been explored in detail. Because of the difficulties to simulate the flow-induced noise from the fluttering flag with large amplitude in a flow, in this study, we performed experiments in a wind tunnel to investigate the characteristics and the generation mechanism of flow-induced noise from the flag.

              First, the flow-induced noise and the motion of the flag were measured simultaneously in a low-noise wind tunnel to discuss the characteristics of the emitted noise from the flag, and the relation between the noise and its motion. Then, the flow-induced noise and the displacement of the flag were measured simultaneously, also in the low-noise wind tunnel to discuss the relation quantitatively. Measurements of the displacement of the flag were performed by means of the laser displacement sensors.

              The experiments were performed in a suction type low-noise wind tunnel at the Institute of Fluid Science, Tohoku University. The test section corresponds to an open type configuration surrounded by an anechoic chamber. The motion of the nozzle exit is rectangular. The width and vertical length are 300 mm and 500 mm, respectively. The streamwise length of the test section (i.e., length between the nozzle exit and edge of the collector nozzle at the downstream side) is 1,000 mm. The background noise level in the test section is less than 65 dB [A] where [A] denotes the A-weighted sound pressure level when the streamwise velocity of the free stream is 45 m/s. The flag used in this study is made of polyester and its streamwise length, width, and thickness are 410 mm, 300 mm, and 0.4 mm, respectively. The support bar of the flag is made of aluminum and its diameter is 8 mm; the weight of the flag without the bar is 0.011 kg. The flow velocity at the nozzle exit was 20, 25, and 30 m/s.

              The experimental results indicated that a highly periodic flow-induced noise with significant directivity in the vertical direction was generated from the flag, and the dominant frequency of the noise was linearly proportional to the inlet velocity. Additionally, constants of the proportionality were linearly 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 rolled up and down when significant periodic sound pressure was generated. Furthermore, near the center of the downstream edge, the flag fluttered at the dominant frequency of the emitted flow-induced noise with high two-dimensionality which showed the high-spatial correlation of the displacement for the spanwise direction of the flag. Therefore, the fluttering region was observed as the source of the significant periodic flow-induced noise from the flag.

Presenting Author: Miyu Okuno Kanazawa University

Authors:

Miyu Okuno Kanazawa University
Reon Nishikawa Toyama Prefectural University
Koki Shige Toyama Prefectural University
Osamu Terashima Toyama Prefectural University
Yasufumi Konishi Tohoku University
Toshihiko Komatsuzaki Kanazawa University