Effects of end condition and aspect ratio on vortex-induced vibration of a 5:1 rectangular cylinder

Abstract

A series of wind tunnel tests were carried out to study the end condition and aspect ratio effects of a sectional model on the vortex-induced vibration (VIV) of a 5:1 rectangular cylinder. Five aspect ratios varying from 9 to 32 as well as eight endplates with different sizes were examined in the tests. Static and elastically mounted sectional models were employed to measure the surface pressure and VIV response of the cylinder, respectively. Sectional fluctuating lift forces, shedding frequency, as well as their spanwise distribution and correlation on the sectional model were obtained and discussed. Results indicated that the aspect ratio had a significant effect on VIV amplitudes, and small aspect ratios tended to remarkably underestimate the VIV amplitudes. There existed an end interference region with a length of 6–8 D, where the sectional lift forces could be, on average, half the value for the non-interference region. Besides, the aspect ratio was positively related to the spanwise correlation of sectional lift forces. Both end condition and VIV had little effect on the length of the end interference region. In the VIV ‘lock-in’ range, with the increasing reduced wind velocity the fluctuating lift force first increased and reached a maximum value earlier than the VIV amplitude, and after that further increase of reduced wind velocity led to decreased fluctuating lift force (toward a value even smaller than that of the stational model). This complicated variation of sectional lift forces implied that vortex shedding was highly nonlinear and self-limiting. Finally, an empirical method based on the test results was explored to account for the influence of aspect ratio on VIV amplitude.