Miyata M., Hayashi Y.

A two-dimensional rectangular bar with cross-section of 15 mm x 50 mm, placed perpendicular to, and with the longer side parallel to the flow direction, may generate aerodynamic sound whose frequency is proportional to the free stream velocity. If we make one of four edges round, for example, make it with r=3.0 mm, and place the bar with the rounded edge pointing the uniform flow direction, then a very high frequency aerodynamic sound can be emitted for a relatively narrow region of angle of attack. The frequency is one order higher than that generated by the usual Karman vortex just described, hence it corresponds to the frequency range of a few kHz. The dependence of the frequency on the free stream velocity is basically proportional to it, but the proportional coefficient changes stepwise into the higher mode with increasing the free stream velocity. The generation of the sound seems to be closely related to the separation and reattachment of laminar boundary layer developing along the rounded edge. If we put a tripping wire with 0.3 mm in diameter at a proper position parallel to the rounded edge, the emission of the sound stops completely, because the separated boundary layer cannot reattach on the bar surface anymore. The characteristics of the aerodynamic sound emitted from the rounded edge(s) and the behavior of the pressure fluctuation on the rectangular bar surface downstream of the rounded edge are investigated experimentally, under the free stream velocity from 4 m/s to 21 m/s. Flow behaviors of the reattaching or non-reattaching separated boundary layer around the rounded edge will be also investigated by means of LDV method.