Aeroacoustics of NACA 0018 Airfoil with a Cavity

This paper reports a detailed numerical study of the aeroacoustics of a NACA 0018 airfoil with a cavity at zero angle of attack, with chord-based Reynolds and freestream Mach numbers at 2 x 10(4) and 0.2, in two dimensions. For an airfoil without a cavity, vortex shedding from the trailing edge and low-frequency wake flapping are found to be the major aeroacoustic processes. The presence of a cavity introduces additional acoustic generation due to cavity oscillation modes but weakens the noise source at the airfoil trailing edge via a jittering of the flow. Three aeroacoustic mechanisms are identified with extensive fast Fourier transform, wavelet, and coherence analyses. The airfoil with cavity effectively increases the lift-to-drag ratio from almost zero (ordinary airfoil) to approximately 5.3. However, it radiates less acoustic power by 1.2 and by 2.6 dB from the airfoil upper and lower surfaces, respectively, even if the acoustic contribution of wake flapping of the ordinary airfoil is excluded. The outcomes of the study suggest that an airfoil with a cavity is a feasible configuration that gives high lift-to-drag ratio yet low noise generation, making it a promising quiet airfoil design at low Reynolds number.