Manipulation of Aerodynamic Forces on an Aerofoil using Synthetic Jet Circulation Control

Abstract

This thesis reports on the experimental and numerical studies of a NACA0015 circulation control aerofoil using synthetic jets. In the current study, the rounded trailing edge (Coanda surface), which is the distinctive feature of a circulation control aerofoil, has a diameter-to-chord ratio d/c of 0.047. The wind tunnel study of the aerofoil is conducted at a free-stream velocity of 10 m/s, corresponding to a chord Reynolds number of 1.10×105, with the free-stream turbulence intensity of approximately 2% and at the angle of attack of 0° to 15° (stall angle). The synthetic jet actuators on the aerofoil are characterised and calibrated for the frequency range of 100 to 1000 Hz to yield the desired range of momentum coefficient. An experimental investigation is initially carried out on the unsteady flow over the aerofoil to reveal the dominant frequencies which provide insights into effective flow control. The pressure and velocity measurements reveal that there are two flow instabilities, namely shear-layer instability and vortex shedding, which exhibit their dominant frequencies near the laminar separation bubble and in the wake region, respectively. The vortex shedding frequencies at α = 0° and 15° scale to Sr ≈ 0.17 and 0.15, respectively. Based on these dominant frequencies, the effects of synthetic jet actuation on the aerodynamic characteristics of the aerofoil are studied. According to the wind tunnel results, the excitation frequency of 175 Hz (F+ = 0.14) is found to yield the highest lift coefficient which amounts to approximately 25% increase over the aerofoil at α = 3°. The numerical results show good agreement with the experimental results. Thus, the effects of higher momentum coefficient on the lift and skin friction coefficients are investigated. The relationship between the lift and momentum coefficients is relatively linear at the low momentum coefficient range (Cμ < 0.0044) and begins to lose its linearity at the moderate and high momentum coefficient ranges (Cμ > 0.0044). Upon actuating the synthetic jet, the length of the laminar separation bubble could be reduced while the jet detachment angle on the Coanda surface could be further delayed.