Detection of Laminar Flow Separation and Transition on a NACA 0012 Airfoil Using Surface Hot Films

Abstract

A method for mapping the separation and transition of flow over a slowly pitching airfoil with high angular resolution is presented. An array of surfacemounted hotfilm sensors is used to record simultaneous corresponding voltages. The method makes use of windowed correlation and spectral signatures of hotfilm sensor voltages in synchronization with a servomotor controlling airfoil pitch angle. Results are given for a NACA0012 airfoil at three airspeeds at pitch angles of less than 6 deg. The airspeeds correspond to a region of known aeroelastic instability; they are situated between chord Reynolds numbers of 50,000 and 130,000. Tests in static and quasistatic pitch motion schedules were conducted. The quasistatic airfoil was sinusoidally pitching at 0.025 Hz between −6 deg and +6 deg (corresponding to a halfchord based reduced frequency between 0.0011 and 0.0020) and the detected separation and transition agreed very well with the static case. These results constitute a verification of the method used and provide insight into the size and location of the laminar separation bubble at transitional Reynolds numbers.