Trailing-Edge Blowing on Tandem Airfoils: Aerodynamic and Aeroacoustic Implications

This paper deals with an experimental investigation of two low-speed airfoils in tandem arrangement for a noise reduction study. The upstream airfoil of that tandem is equipped with a blowing slot on its suction side through which momentum is injected into the airfoil wake. This situation corresponds to a simplified rotor-stator arrangement and investigates the possibilities of wake manipulation and associated broadband noise reduction. The primary goal is to fill the wake sufficiently with momentum and thereby reduce the mean wake deficit, which is the cause of tonal noise in a rotor-stator stage. At the same time it is desired to also reduce the wake turbulence so that the broadband interaction noise resulting from the impingement of the turbulent wake on the downstream stator vane or airfoil is reduced. Different blowing geometries and blowing mass-flow rates are investigated using particle image velocimetry, leading edge surface pressure measurements, wake hot-wire measurements and finally acoustic far-field measurements. The major aim of this study is to quantify the possible broadband noise reduction and to identify the difference between alternative blowing designs.