Coupled oscillations in the aeroacoustics of a Katana blade

Abstract

The paper investigates experimentally the tonal characteristics of the aerodynamic sound of a Katana blade handled at high speed. Katana is the name of the Japanese sword used in martial arts. It is designed with a sharp cutting edge, a thick black‐edge, and sometimes so‐called blood‐grooves. When handled through the air, the blade radiates sound at different frequencies. First a low‐frequency, dominant sound is heard due to the von Karman vortex shedding in the wake of the blade. Second high‐frequency tones are produced in the case of a Katana with grooves. The tones result from the self‐sustained oscillations of the detached shear layers over the grooves which behave like cavities under a grazing flow. In the case of a Katana with no groove, other high‐frequency tones are heard due to acoustic back‐reaction on the Tollmien‐Schlichting waves growing in the boundary layers. The variations of the acoustic signature with varying angle of attack and relative flow speed are analyzed in the paper, with emphasis on the coupling between the different mechanisms. The main trends are compared to simple self‐sustained oscillation formulae. Finally the possible use of the sound as a criterion of inaccuracies in the handling of the Katana is discussed.