On the lift increments with the occurrence of airfoil tones at low Reynodls numbers

The aeroacoustic effects on the aerodynamics of an NACA 0006 airfoil are investigated experimentally at relatively low Reynolds numbers, $Re=30,000$ - $70,000$. By employing two wind-testing airfoil models at different chord lengths, $L = 40$ and $100$ [mm], the aerodynamic dependence on Mach number is examined at a given Reynolds number. In a particular range of Reynolds number, tonal peaks of trailing-edge noise are obtained from a shorter-chord airfoil, while no apparent tones are observed with longer chord length at a lower Mach number. Surprisingly, the occurrence of a tonal noise leads to a greater lift slope in the present wind-tunnel experiment, evaluated via a PIV approach. The lift curves obtained experimentally at higher Mach numbers agree well with two-dimensional numerical simulations, performed at $M = 0.2$. At the Mach number, the numerical results clearly indicate the occurrence of an acoustic feedback loop with discrete tones, within a range of angle of attack. A few three dimensional numerical results are also presented. In the simulation at $Re = 50,000$, the suppression of tonal noise corresponds to the development of a turbulent wedge in the suction-side boundary layer at the angle of attack $4.0$ [deg.], which agrees with the experiment.