Abstract
The present work investigates the natural low-frequency flow oscillation phenomenon using large-eddy simulation. Four simulations were carried out for flow around a NACA 0012 airfoil with Reynolds number of 50,000 and Mach number M-infinity = 0.4 at angles of attack of 9.25, 9.29, 9.4, and 9.6 deg. The results clearly indicate that natural low-frequency flow oscillation is taking place. The oscillations are self-sustained and caused by periodic bubble forming and bubble bursting on the suction side. The low-frequency oscillation phenomenon was found to exist over a range of angles of attack near stall. Computed maximum reverse flow inside the bubble indicates an absolute instability mechanism. The time- and span-averaged flowfleld as well as the instantaneous turbulent flowfield illustrated the dynamics of low-frequency oscillation near stall. The location of transition is shown to move downstream during bubble bursting. Spectral analysis of the surrounding acoustic field reveals that the observed phenomenon affects the circulation around the entire airfoil. Analysis of the flowfield led to the proposition of a mechanism for low-frequency flow oscillations near stall in which the separated shear layers from the leading edge and the trailing edge play a major role in driving the separated flow to reattachment.