RANS/LES Simulation of Low-Frequency Flow Oscillations on a NACA0012 Airfoil Near Stall

The aerodynamic characteristics of wings and airfoils near the stall point are difficult to predict, but have an important impact on the performance and flight safety of aircraft. The present work investigates the natural low-frequency flow oscillation phenomenon of the airfoils that has the leading-edge stall type. Hybrid RANS/LES Simulations of flows around NACA0012 airfoil were conducted to obtain better understanding of this phenomenon at the angle of attack range of 13o-20o at the condition of Reynolds number 6×106 and Mach number 0.2. Results indicated that self-sustained natural low-frequency flow oscillations were taking place. Micro structures of flowfield were captured to reveal flow mechanism. A large vortex structure, formed by many small vortices, played a significant role in the phenomenon of low-frequency oscillation. A separation bubble was formed at the upper leading edge, and then the separation region expanded leading to a wide separation of the entire leeward side. The fluctuating lift coefficient showed very low-frequency unsteadiness and a large fluctuation, and the Strouhal number was about 0.01. Time-dependent flowfield showed irregular alternation between large separated flow and almost attached flow. The low-frequency oscillations were found to exist over a range of angles of attack near stall.