The effects of a pair of vibrating leading edge flaps on the wind-induced structural vibration of tall building

Abstract

This thesis presents a thorough study of the aerodynamic excitation mechanism due to a pair of vibrating leading edge flaps installed on a 2D square prism. Excessive wind-induced structural vibrations can often cause discomfort to the occupants of modern skyscrapers. This study investigates the potential for a pair of vibrating leading edge flaps to stabilise tall buildings against wind- induced vibration. The thesis starts with an extensive survey on the wind excitation mechanisms on structures and the current solutions to structural vibrations. It reveals that motion of tall buildings of today is due mainly to wake excitation, and aerodynamic modifications to buildings offer a brighter future for tall buildings of ever-increasing height and size compared to the conventional mechanical methods of mitigating wind-induced motion. This leads to the present study of the vibrating leading edge flaps, which attempt to change the pressure fluctuation over a large area by using a relatively small aerodynamic surface, hence generating a large force to counter the motion of a building. During the experimental part of this study a High Speed Synchronous Digital Pressure Scanning System was developed. This special piece of equipment of advanced design allowed the correlation between each individual pressure signal with the measured flap motion to be determined. In order to isolate the effect of the flaps on the pressure fluctuations for detailed investigations, two analytical tools were developed to extract the flap-induced component of the pressure fluctuations. The effects of the flap motion on the pressure fluctuations were broken down into a magnitude and a phase difference and these were correlated with flap operating parameters, angle of attack, and the level of oncoming turbulence. The computation of the effects of the flap motion on the force fluctuations was achieved by integrating the effects of flap motion on the pressure fluctuations over the area of the building. A series of validations experiment were conducted using an aeroelastic model equipped with leading edge flaps. It was found that the vibrating flaps significantly reduced the wind-induced motion in the cross-wind direction. The empirical pressure model, however, only predicted the correct optimal parameters under certain conditions. The results of the experiments were applied to a hypothetical tall building and it was found that the vibrating flaps contribute to an additional equivalent damping of 8.1% of critical, which reduced the cross-wind response of the building by 67%.