Vortex-induced vibrations of a cylinder with tripping wires

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, an d 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 a s an increase in the Strouhal number to a value around 0.25-0.27. For a cyl inder forced to oscillate harmonically, the main observed wire effects are: (a) an earlier onset of frequency lock-in, when compared with the smooth c ylinder 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 t o 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 wire s is significantly altered as well, even far away from lock-in. Parameteriz ing the response using nominal reduced velocity V-rn = U/f(n)D, we found th at frequency lock-in occurs and lift phase angles change through 180 degree s at V-rn approximate to 4.9; anemometry in the wake confirms that a mode t ransition accompanies this premature lock-in. A plateau of constant respons e is established in the range V-rn = 5.1-6.0, reducing the peak amplitude m oderately, and then vibrations are drastically reduced or eliminated above V-rn = 6.0. The vortex-induced vibration response of the cylinder with wire s is extremely sensitive to angular bias near the critical value of V-rn = 6.0, and moderately so in the regime of suppressed vibration.