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Gyung Min Choi
Pusan National University

Ye Li
Mazda Motor Corporation Shinchi, Fucyumachi, Aki, Hiroshima 730-8670, Japan

Mamoru Tanahashi
Department of Mechanical and Aerospace Engineering Tokyo Institute of Technology 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan

Toshio Miyauchi
Dept. Mechanical and Aerospace Eng., Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan; Organization for the Strategic Coordination of Research and Intellectual Properties Meiji University 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa, Japan


Direct numerical simulation (DNS) has been performed to clarify the mechanism of sound generation in three-dimensional turbulent mixing layer. The amplitude of pressure fluctuation increases dramatically in the period of vortex pairing, and the sound wave is radiated from the mixing layer. In contribution of acoustic source components in Lighthil's acoustic analogy, the magnitude of entropy component becomes comparable to that of Reynolds stress, while the Reynolds stress component is a major factor which dominates total acoustic source in the two-dimensional calculation. The roles of coherent fine scale structures on sound generation are investigated. The structure of Reynolds stress component is identical to that of coherent fine scale eddies even in the core region where large number of coherent fine scale structures appears. On the other hand, the entropy component shows sheet-like structure around coherent fine scale eddies, and is dominated by the energy dissipation rate. In addition, the far-field sound was predicted using acoustic analogies and compared with the DNS result. For the case of source size of 4Λ, where Λ is the most unstable wavelength, Lighthill's analogy can predict far-field sound excellently, however, the amplitude of pressure fluctuation predicted by Powell's analogy shows significantly large values compared with the DNS result. For the case of source size of 3Λ, while the pressure fluctuation predicted by Lighthill's analogy shows agreement with the DNS result before the period of vortex pairing, it shows slightly small value compared with the DNS result after the mixing transition.