VIV and galloping instability of a rectangular cylinder with a side ratio of 1.5

The interaction between galloping and vortex-induced vibration was experimentally investigated in the particular case of an infinitely long rectangular cylinder with a side ratio of 1.5 in smooth flow. This geometry was found to be very unstable and large oscillations were observed also at high Scruton number in a range where no excitation was expected according to the classical theory. A ratio of the quasi-steady galloping critical wind speed to Karman-vortex resonance velocity larger than a minimum value in the range 7 - 9 seems to be necessary in order to avoid such a combined instability. This result, which finds a support from the literature, is in strong disagreement with Eurocode 1, which unconservatively claims that interaction can be excluded for a ratio of critical velocities outside the range 0.7 - 1.5. The effect of Reynolds number was also partially investigated by testing two section models at different scales. Finally, the experiments highlighted several nonlinear features of the dynamic system, such as secondary and superharmonic resonances, hysteresis and frequency demultiplication.