Separation bubble and aerodynamic characteristics of S1223 airfoil at low Reynolds numbers

The surface pressure distribution of the S1223 airfoil at low Reynolds numbers (Re = 6.0×10⁴, 1.0×10⁵ and 2.0×10⁵) was measured using the surface pressure measurement technique. Both time-averaging and transient processing methods were used to obtain the steady and transient pressure/lift coefficients on the airfoil. Combined with the flow structure analysis, the influence mechanism of the Reynolds number and the angle of attack on the lift of the airfoil was studied. According to the variation of time averaged lift coefficient with the angle of attack, S1223 airfoil exibits a "static hysteresis" effect at low Reynolds numbers. The results show that under negative angles of attack, the lift coefficient curve shows a nonlinear phenomenon where its slope gradually increases. When the angle of attack is increased from a negative value, the lower wing surface changes from the state of complete separation to that of laminar separation bubbles, then the separation bubbles gradually shrink until disappeared, resulting in the slope of the lift coefficient curve gradually increasing with the angle of attack. When the angle of attack exceeds the stall value, the flow structures on the S1223 airfoil under different Reynolds numbers are essentially different. At Re = 6.0×10⁴ and 1.0×10⁵, the flow field around the airfoil rapidly undergoes large-scale flow separation, and the lift coefficient decreases sharply; at Re = 2.0×10⁵, short bubbles on the upper wing surface are generated periodically, causing low-frequency oscillations, and the lift coefficient exhibits a quasi-periodic variation. In the meantime, the averaged flow field on the upper wing surface exhibits a long bubble structure with a chord length of 40%.