Coherent Structures and Pressure Fluctuations over an Airfoil Using Time-Resolved Measurements

Abstract

This study focused on the unsteady behaviors of large-scale vortical structures and wall pressure fluctuations and their coupling mechanism on a low-Reynolds-number airfoil. Three incidence angles were chosen for comparison: [Formula: see text], and [Formula: see text] (representing two flow regimes, namely, the separation bubble formation and separation without subsequent reattachment). Simultaneous measurements of velocity fields and wall-pressure fluctuations were performed using a high-resolution time-resolved particle image velocimetry operating at 2 kHz and an array of flush-mounted microphones sampled at 10 kHz. The results provided a detailed description of the shear layer transition and the coherent structures development on the airfoil. The roll-up process, pairing process, spatial-temporal growth of the vortical structure, and vortex break-up were clarified through a long-period analysis. Spectral analysis demonstrated that vortical structures at the fundamental frequency were spatially and temporally localized and that their subharmonics were significantly enhanced. Frequency–wavenumber spectra revealed that the energy was dominantly concentrated along the convective ridge. With increasing incidence angle, the energy level of the inclined ridge increased, with a lower extension in the frequency domain and a higher dispersion in the wavenumber domain. The conditional averaging and quadrant analysis results demonstrated that strong events in the near-wall region were dominated by the sweep motion and ejection motion, which enhanced the interaction between the inner and outer layers. This also indicated the existence of a strong relationship between the wall-pressure peaks and turbulence production mechanisms.