Upstream Effect of Trailing Edge Tonal Emissions on a Laminar Separation Bubble

Abstract

An experimental study was carried out to investigate feedback effects in a laminar separation bubble formed on the suction side of an airfoil in low Reynolds number flows. Wind tunnel experiments were performed on a NACA 0012 airfoil for a range of chord based Reynolds numbers 65,000 ≤Rec≤ 260,000 at an angle of attack of two degrees. The experimental conditions were selected so as to produce laminar boundary layer separation on both sides of the airfoil. Time–resolved, two-component particle image velocimetry measurements were performed simultaneously with surface and far field microphone measurements to characterize flow development and acoustic emissions. The results show that amplification of disturbances in separated shear layers on both the suction and pressure sides of the airfoil lead to shear layer roll-up and shedding of vortices from separation bubbles. When the vortices do not break up upstream of the trailing edge, the passage of these structures over the trailing edge produces periodic pressure fluctuations. Through a feedback mechanism between the produced fluctuations and disturbance amplification in the shear layers, the frequency band of amplified disturbances narrows significantly, effectively locking onto a particular frequency. The effect is similar to that due to external periodic forcing and results in notable changes in the overall separation bubble characteristics. The roll-up vortices forming on the pressure side, where the bubble is located closer to the trailing edge, are shown to define the characteristic frequency of pressure fluctuations, thereby affecting the disturbance spectrum on the suction side. However, when the bubble on the pressure side is suppressed via boundary layer tripping, a weaker feedback effect is also observed on the suction side. The results give a detailed quantitative description of the observed phenomenon and provide a new outlook on the role of coherent structures in separation bubble dynamics.