  Airfoils pitching in the stalled regime have been of keen interest in recent
years due to their desirable aerodynamic force characteristics. In this study,
we are numerically investigating the unsteady flow past a NACA 0012 airfoil
under sinusoidally pitching motion, using a finite volume based sharp interface
immersed boundary solver. The flow is investigated in the low Reynolds number
regime (Re=3000) for reduced frequencies of 0.1 and 0.5, at three different
pivot locations (c/3, c/2 and 2c/3 from the leading edge). The airfoil is
subjected to sinusoidal oscillations with its incidence angle varying from 15o
to 45o. Leading edge vortices (LEVs) that are formed during the pitching motion
dictate the transient aerodynamic characteristics. The flow field data is used
to identify individual LEVs in the flow field. The spatio-temporal evolution of
LEVs in terms of their strengths are traced throughout the pitching cycle to
obtain a quantitative estimate of its evolution. The effect of pivot location
and pitching frequency on the vortex evolution and aerodynamic forces is
investigated. Change in pitching frequency is found to have a drastic effect on
the vortex dynamics. LEV formation is delayed to higher fractional times and
LEVs are found to interact and merge into each other at higher frequencies. The
maximum lift and drag are also found to increase with pitching frequency. Pivot
location is found to have a higher influence on the magnitude and phase of
aerodynamic coefficients at higher frequencies. However, at both frequencies,
moving the pivot axis aftward is found to delay LEV evolution and cause a phase
lag on the aerodynamic forces.
