The effect of mass ratio on the structural response of a freely vibrating square cylinder oriented at different angles of attack

This study reports on an experimental investigation of the effect of mass ratio on the transverse flow-induced vibration (FIV) response of an elastically mounted square cylinder placed at three different incidence angles: two symmetric with respect to the centreplane, α=0° and 45°; and one asymmetric at 20°. These three angles display different dominant FIV phenomena: transverse galloping and combined vortex-induced vibration (VIV) and galloping for α=0°, VIV for α=45°, and higher branch subharmonic (period-doubled) VIV for α=20° (Nemes et al., 2012). The mass ratio is defined as the ratio of the total oscillating mass (m) to the displaced fluid mass (m _d ), m ^∗ =m∕m _d . The present results show that the mass ratio (m ^∗ =2.64 – 15.00) has a significant influence on the structural vibration response for all FIV phenomena, and can dictate whether two of these modes exist at all. Three primary observations are presented: for the α=0° case, the combined VIV–galloping response is diminished as the mass ratio is increased, and it ceases to exist for m ^∗ ⩾11.31; for the α=45° case, the peak values of the normalised oscillation amplitude during VIV are only reduced marginally with increasing m ^∗ , however the body oscillation amplitude in the desynchronised regions is significantly attenuated; for the α=20° case, there exists a critical mass ratio (m _crit ^∗ ≃3.50) above which the higher branch subharmonic VIV does not persist.