## 3 Experimental Results and Discussion
Considering the drastic change in \(C_{L}\) in the range of the AoA shown in Figure 6, the separation point of the boundary layer appears to move discontinuously to the leading edge at a 15\({}^{\circ}\) AoA for _Re_ = 1.9 \(\times\) 10\({}^{5}\) and 18\({}^{\circ}\) for _Re_ = 4.2 \(\times\) 10\({}^{5}\)-5.7 \(\times\) 10\({}^{5}\). These descriptions in such AoA and _Re_ conditions are consistent with the condition for determining a stall, in which the flow separates from the wing surface and the wing loses a significant amount of \(C_{L}\). Strong hysteresis is also observed between the pitch-up and pitch-down motions around angles of attack of 13-18\({}^{\circ}\) for _Re_ = 1.9 \(\times\) 10\({}^{5}\)-5.7 \(\times\) 10\({}^{5}\) (Figure 6). The hysteresis observed in the \(C_{L}\) curve is attributed to the bursting and reformation of a small laminar separation bubble near the leading edge of the wing, which depends on changes in the AoA. The position of the laminar separation bubble suggested by the shape of the \(C_{p}\) distribution is indicated by the red arrow in Figure 7 for _Re_ = 1.9 \(\times\) 10\({}^{5}\)-5.7 \(\times\) 10\({}^{5}\). The boundary layer separation occurs in the laminar state after the bursting of the laminar separation bubble in the aforementioned range of _Re_. In contrast, there is no hysteresis in the \(C_{L}\) curve for _Re_ = 9.4 \(\times\) 10\({}^{5}\). These results suggest that the laminar separation bubble is very small or is not formed, and a turbulent transition of the boundary layer occurs further upstream around the leading edge than that under the other _Re_ conditions. The separation points of the boundary layer determined from the particle image velocimetry (PIV) results are 0.07c, 0.065c, 0.065c, and 0.2c for _Re_ = 1.9 \(\times\) 10\({}^{5}\), 4.2 \(\times\) 10\({}^{5}\), 5.7 \(\times\) 10\({}^{5}\), and 9.4 \(\times\) 10\({}^{5}\), respectively. This result also supports the consideration that the turbulent transition occurred closer to the leading edge at _Re_ = 9.4 \(\times\) 10\({}^{5}\) than that at the other _Re_ values. According to the above description, the flow can be characterized into three different patterns: (1) the boundary layer completely attached on the suction side; (2) the boundary layer partially separated along the chord length; and (3) the boundary layer completely separated at the leading edge. In the first flow pattern, a negative \(C_{p}\) peak formed at the leading edge, and \(C_{p}\) recovered toward the trailing edge. The \(C_{p}\) distribution showing this flow pattern is indicated by the black lines in Figure 7a-d. In the second flow pattern, a negative \(C_{p}\) peak exists at the leading edge, \(C_{p}\) recovery occurs up to the middle chord of the wing model, and a flat distribution of \(C_{p}\) appears near the trailing edge. The \(C_{p}\) distributions corresponding to the second flow pattern are indicated by blue lines. In the third flow pattern, no negative \(C_{p}\) peak is observed, and the \(C_{p}\) distribution is flat over the suction side of the wing model. \(C_{p}\) distributions corresponding to the last characteristic are indicated by red lines.