Study on Wind-Induced Vibration of a Super-High Tower

In this paper, wind-induced vibration of a super-high tower is numerically studied. The natural frequencies of the tower are calculated. Karman’s Vortex Street is simulated and the alternate lateral forces across the wind are obtained. It is found that with the wind speed range of 0–52.3m/s acting on the tower, the maximum vortex shedding frequency is lower than the second natural frequency of the tower. Resonance of the tower could occur at the first natural frequency with the horizontal force amplitude 241.5N/m. For high towers, it is suggested that the wind actions in across the wind and fatigue strength checks should also be considered in the design approach.

Experiments on an aerofoil at high angle of incidence in longitudinal oscillations

Details of flow visualization, aerodynamic forces and pitching moment, static pressure and skin friction measurements have been carried out on a symmetrical aerofoil at fixed angle of incidence in longitudinal oscillations parallel to the uniform airstream of a wind-tunnel.This investigation shows weak unsteady effects at incidences below that of static stall. For higher incidences, strong unsteady effects appear and depend on the frequency and amplitude of the oscillations. The measurements indicate an overshoot of the instantaneous lift and drag which is explained by a strong vortex shedding process occurring during the dynamic stall encountered by the aerofoil in decelerated motion, as observed for profiles oscillating in pitch through stall. When the aerofoil is going forward in accelerated motion dynamic reattachment may be observed at very high incidence over a short part of the period of oscillation.Dynamic stall and dynamic reattachment contribute to a favourable effect of unsteadiness on the mean lift coefficient, which increases as compared to the steady state one, and which is expressed through an empirical formula involving incidence, frequency and amplitude of oscillations. At given incidence, optimization of this feature is achieved by matching the frequency and the amplitude of oscillation, respectively with the frequency linked with the highest peak of energy in the wake, and with the distance between two consecutive vortices in the mean wake when modelled as a von Kármán's vortex street.