Pressure wavelet analysis of pitching oscillating airfoils in tandem configuration at low Reynolds number

In this paper, the flow field around a tandem arrangement of two identical oscillating NACA (National Advisory Committee for Aeronautics) 0012 airfoils was investigated using the continuous wavelet transform. Wind tunnel experiments were conducted on a test stand that provided a wide range of sinusoidal pitching motion with frequencies up to 10 Hz. This study aims to explore the flow physics of the tandem airfoils that oscillate with independent reduced frequencies. For this sake, experiments were performed at a reduced frequency of 0.15 for the front airfoil and five different reduced frequencies for the rear airfoil, ranging from 0.05 to 0.3. The chord-based Reynolds number was 6 × 104, and the horizontal distance between airfoils was equal to one chord length. The unsteady surface pressure was measured, and the wavelet transform was employed to analyze the pressure fluctuations. Findings indicate that the presence of the rear airfoil in the wake of the front airfoil prevents the formation of the laminar separation bubble. Also, the ratio of upstream/downstream airfoil reduced frequencies appears as one of the dominant frequencies of pressure fluctuations on the rear airfoil. Furthermore, when the reduced frequency ratio of the airfoils is lower than one, the normal force on the rear airfoil is often less than that experienced by an isolated single airfoil. Specifically, at equal reduced frequencies of 0.15 for both upstream/downstream airfoils, the maximum value of the normal force coefficient on the rear airfoil decreases by 30% compared to the single airfoil.