Unsteady boundary layer measurement on an oscillating (pitching) supercritical airfoil in compressible flow using multiple hot-film sensors

Abstract

Aerodynamic characteristics of an airfoil in a dynamic motion are highly affected by the behavior of the viscous boundary layer. Boundary layer condition, and the related phenomena, depends on such different parameters as the oscillation type, flight Mach number, angle of attack, etc. In compressible and transonic regimes, the shock-boundary layer interaction causes complex phenomena such as λ-shock and separation bubble on the airfoil. When oscillation and unsteady effects are also added, a complicated flow is resulted which could not be easily investigated using theoretical and computational methods. Therefore, in this paper a series of static and dynamic (pitch) tests at pre stall angle of attacks were performed in a high speed wind tunnel to study the steady and unsteady behavior of the compressible boundary layer. Measurements involved pressure coefficient distribution and qualitative analysis of output voltage of hot films. Some sinusoidal pitching motions were performed on the SC (2)-0410 airfoil at Mach numbers of 0.4, 0.5, and 0.6 with the maximum Reynolds number of 8.8 million per meter. The effects of compressibility, reduced frequency, mean angle and oscillation amplitude and free stream Mach number were investigated. Results show asymmetry between transition and relaminarization during pitching motion, and delay in transition with respect to steady conditions. Also, the location of the shock, shock induced separation bubble, and their motions due to the change of instantaneous angle of attack were detected and interpreted using the outputs of hot-film sensors.