A wind tunnel investigation of the effects of end and laminar/turbulent inflow conditions on cylinder vortex-induced vibrations

Abstract

The influence of end and inflow conditions on the vortex-induced vibrations of a cylinder is studied on the basis of wind tunnel experiments, combining measurements of the body displacement and flow velocity in its wake. The cylinder of length-to-diameter aspect ratio 5.5 has one end exposed to the current. It is elastically mounted in the cross-flow direction, with low damping. The structure to displaced fluid mass ratio is close to 1000. The behavior of the system is explored over a range of values of the reduced velocity, U⋆, defined as the inverse of the oscillator natural frequency, non-dimensionalized by the inflow velocity (U∞) and the cylinder diameter (D), typically between 4 and 8, at a Reynolds number of the order of 104, based on U∞ and D. Three end conditions are examined: two free-end conditions with either a flat or a hemispherical shape, and a condition where a fixed plate is placed close to the flat end. Three inflow states are considered: a laminar condition and two grid-generated turbulent conditions of different turbulence intensity levels (up to 10%) and comparable integral length scales, around 30% of D. Vibrations characterized by bell-shaped evolutions of their magnitude with U⋆ develop, under flow-body synchronization (lock-in), in all conditions, with peak amplitudes ranging from 4% to 12% of D. The free end causes a shift of the response bell-shaped curve towards higher U⋆ values. The shift may be such that there is no overlap between the vibration domains in the end-plate and free-end conditions. The passage from laminar to turbulent inflow conditions is found to attenuate this shift. A clear reduction of vibration amplitude is observed when the end plate is removed, and the amplitude is further reduced when the flat end is replaced by the hemispherical one. On the other hand, turbulent inflows induce a global enhancement of the vibration amplitudes, which may reach +45% relative to the laminar stream condition. The end/inflow conditions are also shown to impact the regularity of the structural responses.