Study of acoustic disturbances effect on laminar separation bubble by IR thermography

Abstract

This work concerns the study of sound effect on laminar separation bubble phenomenon which happens on the upper surface of aerodynamic bodies having small thickness and operating at low Reynolds number. The laminar bubble increases the aerodynamic drag of the wing: so the boundary layer control becomes important, in order to avoid the separation problem. In addition to control methods such as turbulators or boundary layer blowing/suction systems it would be possible in accordance with linear stability theory to induce the formation and amplification of the Tollmien–Schlichting waves introducing a sound wave having a particular frequency as a disturbance in the freestream. In this study a thin airfoil is tested at a Reynolds number of 60,000. The airfoil is installed inside a low speed wind tunnel test chamber; at the inlet of the wind tunnel is positioned a sub woofer sound source connected to an amplifier and a signal generator. In this way it was possible to generate a pure tone in the freestream. The experiments were conducted using a quantitative IR thermography method in order to get the positions of the laminar bubble characteristic points (separation, transition and reattachment point) with and without the acoustic disturbance. The frequency range between 200 and 800 Hz with step of 100 Hz was inspected. The airfoil angle of attack was varied between 2° and 8°; inside this interval the laminar bubble was visible by thermography.