Random lock - in intervals for tubular structural elements subject to simulated natural wind ∗

The paper reports on wind tunnel experiments with an elastically suspended circular cylinder vibrating under the excitation of natural wind of high turbulence degree. The natural wind turbulence was simulated by superposing the low frequency part of the natural wind turbulence on the background high frequency turbulence of the wind tunnel flow. This was done by controlling the propeller rotation speed according to an artificially generated low frequency speed sample function drawn from a suitable random process model. The experiment provided statistical data on the intermittend random occurrence and size of strong lock-in vibrations in resonanse with the vortex shedding excitation. The purpose of the experiment was to obtain data support for the formulation of a sufficiently detailed stochastic model to allow computer simulation of reasonably realistic fatigue damage accumulation at ”hot spots” of tubular structural elements subject to the natural wind. The engineering relevance of the investigation is supported by comparing with the unrealistic highly conservative rules of wind induced fatique commonly given in codes of practice. The stochastic lock-in model as well as the related fatigue calculation procedure is reported in the paper.