Flow Evolution and Unsteady Spectra of Dynamic Stall at Transitional Reynolds Numbers

Abstract

Dynamic stall experiments were performed on a NACA 0012 airfoil subjected to a constant pitch rate maneuver at three transitional Reynolds numbers of , , and . The primary objectives of this investigation were to describe the evolution of the flow and to quantify the characteristic timescales of dominant fluctuations in the off-body flowfield. These objectives were achieved through a combination of time-resolved particle image velocimetry and spectral analysis using empirical mode decomposition. Using spectral analysis, the velocity fluctuations near the leading edge were observed to be in an amplified state during the initial ejection of vortical structures. During this amplified phase, the most dominant velocity fluctuation modes were associated with displacement-thickness-based Strouhal numbers between and . The variation in the frequency of the dominant fluctuation modes was also analyzed as a function of the angle of attack. A numerical implementation of the Orr–Sommerfeld equation was used to extract the spatially unstable modes associated with the velocity profiles at the airfoil leading edge. The amplified frequencies from linear stability analysis were found to be consistent with those determined from the spectral analysis of the acquired velocity field.