Serrations, which imitate the sawteeth-like structures in the leading edge of owl wing, enable to reduce noises and alter the aerodynamic performance. However, the mechanism is not yet completely understood because the flow fields have three-dimensionality. In this study, in order to investigate three-dimensional mean flow structures on a flat plate with leading-edge serrations, flow fields viewed from the spanwise and vertical directions were measured by two-dimensional time-resolved PIV and stereo PIV at the Reynolds number of 5.0×10~3. When the angle of attack was 6 degree, the interval of the flow pattern was equal to the width of one sawtooth of the serrations. Whereas the flow over the trough was separated, and the flow over the vertex passed along the surface of the flat plate. This is because pairs of longitudinal vortices induced by the serrations suppress the flow over the vertex on the uppersurface of the flat plate. When the angle of attack was 14 degree, several flows over the vertexes were gathered, and specific flow patterns were formed. The flow over the vertex passed along the surface of the flat plate, so that separation points viewed from the spanwise direction moved to the downstream side. Reverse-flow regions behind the troughs could be divided into two categories, "open" and "closed" ones. Pairs of longitudinal vortices were seen in the closed reverse-flow regions only, and growth of the open reverse-flow regions forces the vortices to move upwards.
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