Vortex-induced vibrations of a cylinder with tripping wires

Abstract

Thin wires are attached on the outer surface and parallel to the axis of a smooth circular cylinder in a steady cross-stream, modelling the effect of protrusions and attachments. The impact of the wires on wake properties, and vortex-induced loads and vibration are studied at Reynolds numbers up to 4.6 X 10^4, with 3.0 X 10^4 as a focus point. For a stationary cylinder, wires cause significant reductions in drag and lift coefficients as well as an increase in the Strouhal number to a value around 0.25-0.27. For a cylinder forced to oscillate harmonically, the main observed wire effects are: (a) an earlier onset of frequency lock-in, when compared with the smooth cylinder case; (b) at moderate amplitude/cylinder diameter (A=D) ratios (0.2 and 0.5), changes in the phase of wake velocity and of lift with respect to motion are translated to higher forcing frequencies, and (c) at A=D = 1:0, no excitation region exists; the lift force is always dissipative. The flow-induced response of a flexibly mounted cylinder with attached wires is significantly altered as well, even far away from lock-in. Parameterizing the response using nominal reduced velocity Vrn = U/fnD, we found that frequency lock-in occurs and lift phase angles change through 180deg at Vrn=4.9; anemometry in the wake confirms that a mode transition accompanies this premature lock-in. A plateau of constant response is established in the range Vrn = 5.1-6.0, reducing the peak amplitude moderately, and then vibrations are drastically reduced or eliminated above Vrn = 6.0. The vortex-induced vibration response of the cylinder with wires is extremely sensitive to angular bias near the critical value of Vrn = 6.0, and moderately so in the regime of suppressed vibration.