Cross-wind excitation of a 2D square prism with vibrating leading edge flaps

This paper presents and discusses the results of wind tunnel measurements of the pressure distributions on the lateral sides of a 2D square prism with vibrating leading edge flaps. The flaps were driven with sinusoidal motion at different frequencies and amplitudes. Differential pressures across the lateral sides of the model were sampled simultaneously to give sets of signals at five different distances from the front surface. RMS lift coefficients and lift spectra were calculated by integrating pressures. It was found that increasing the flapping frequency and amplitude decreased the RMS lift coefficient up to 70%. An optimum mean flap angle existed with specific frequency and amplitude. Frequency domain analysis revealed that increased flapping amplitude disrupted vortex shedding, while increased flapping frequency suppressed pressure fluctuations in a wide frequency band. Co-spectra results showed that the flapping action affected the pressure on the side of the model as if the onset flow was made more turbulent. The results suggest that the vibrating flaps introduce a force fluctuation at the flap operating frequency. It appears as if the fluctuating velocity components introduced by the flapping motion work in a manner similar to small-scale turbulence in turbulent onset flow and change the characteristics of the shear layer. Preliminary tests using one flap gave results which confirmed the presence of completely different flows on the opposing faces.