Instantaneous Surface Pressure Measurements of Quasi-Periodic Behavior of Laminar Separation Bubble in the Angles of Attack Near Airfoil Stall

Abstract

To investigate quasi-periodic behavior of laminar separation bubble near airfoil stall, instantaneous surface pressure distributions of NACA0012 airfoil were measured. Reynolds number based on the airfoil chord length was 1.3 × 10⁵ and the measured angles of attack were 11.3^º, 11.6^º and 12.0^º. At α = 11.6^º, the flow around the airfoil indicated quasi-periodical switching behavior between an attached flow with laminar separation bubble (short bubble) and a large separated flow at a frequency of about 2Hz. At α = 11.3^º, the flow attaches to the airfoil for a longer period and switches between these two flow patterns irregularly. At α = 12.0^º, two different flow phases were observed. One is the phase when the flow switches between the large separated flow and the attached flow with short bubble. At the other phase which was observed longer than the first phase, the flow oscillates without forming the short bubble. Wavelet analysis was performed on the instantaneous surface pressure distributions. Results indicated that pressure fluctuations at frequency of 20--40Hz were observed when the flow was switching from an attached flow with short bubble to large separated one. These fluctuations were observed for all the three tested angles of attack.