Time-Resolved Surface Pressure Measurements Within A Separated Flow Region on An Airfoil

Abstract

Laminar boundary layer separation and separated shear layer development on a NACA 0025 airfoil at low Reynolds numbers were studied experimentally. Flow visualization, hot-wire velocity measurements, and time-resolved surface pressure measurements were employed in this investigation. The results for two distinct flow regimes, namely, flow separation without subsequent shear layer reattachment and separation bubble formation, are discussed in detail. For both flow regimes, the transition occurs due to the amplification of natural flow disturbances in the separated shear layer. Initially, disturbances within a band of frequencies centred at some fundamental frequency are amplified. Further downstream, non-linear interactions set in, leading to a breakdown to turbulence. Based on the spectral and correlation analysis of velocity and surface pressure fluctuations, it is demonstrated that the amplification of disturbances and the attendant fluctuations in the flow velocity give rise to distinct surface pressure fluctuations at the fundamental frequency. Thus, time-resolved surface pressure measurements can be employed to estimate important unsteady characteristics of the separated flow region on an airfoil operating in low Reynolds flows.