Fatigue damage from random vibration pulse process of tubular structural elements subject to wind

In a wide range of the Reynolds number an elastically suspended circular cylinder surrounded by a homogeneous wind velocity field will generate vortex shedding of a frequency that by and large is proportional to the far field wind velocity. However, if the cylinder is free to vibrate, resonance will occur when the vortex shedding frequency equals an eigenfrequency of the cylinder. This vibration causes a feedback to the velocity field such that the vortex shedding frequency becomes "locked in" at the considered eigenfrequency within a rather wide interval of the wind velocity (Sarpkaya 1979). Recent wind tunnel experiments show that the position and width of the interval depends on whether the wind velocity increases or decreases. Also the experiments show that the presence of even a low degree of turbulence in the wind causes the interval bounds to be rather uncertain. For the degree of turbulence observed in the natural wind the undisturbed local wind velocity directly upstream to the cylinder varies as a sample from a random process. Thus the local wind velocity will cross in and out of the "iock in"-intervals in a random fashion causing pulse like bursts of strong vibrations. The paper describes a random pulse process model of this vibration behavior supported on the experimental work of the first author. Moreover, it is shown how the mean accumulated material fatigue damage per time unit according to the Palmgren-Miner rule can be evaluated by simulation.