Wake structures and vortex-induced vibrations of a long flexible cylinder—Part 2: Drag coefficients and vortex modes

Drag coefficients and vortex structures in the wake of a vertical long flexible cylinder vibrating at low mode numbers are presented in this paper. A model with an external diameter of 16 mm and a total length of 1.5 m giving an aspect ratio of about 94 was used to perform more than 100 runs in which Reynolds numbers ranged between 1200 and 12 000. Only the lower 40% of its length was exposed to the water current in the flume and applied top tensions varied from 15 to 110 N giving fundamental natural frequencies in the range from 3 to 7.1 Hz. Reduced velocities based on the fundamental natural frequency up to 16 were reached. The mass ratio was 1.8 and the combined mass-damping parameter about 0.05. The largest drag coefficients were found related to the largest x–y synchronised motions. Digital particle image velocimetry was used to investigate the vortex structures in the wake of the cylinder model. Two modes of vortex shedding were observed, depending on the response branch and the position along the length of the model at which the interrogations were performed.

[1]  Francisco Javier.,et al.  Multi-mode vortex-induced vibrations of a flexible circular cylinder , 2006 .

[2]  A. Laneville,et al.  On vortex-induced vibrations of cylinders describing X–Y trajectories , 2006 .

[3]  Jaap J. de Wilde.  Laboratory Investigation of Long Riser VIV Response , 2004 .

[4]  M. Gharib,et al.  ON CIRCULAR CYLINDERS UNDERGOING TWO-DEGREE-OF-FREEDOM FORCED MOTIONS , 2001 .

[5]  John Grue,et al.  Two-degree-of-freedom vortex-induced vibrations of a spring-mounted rigid cylinder with low mass ratio , 2008 .

[6]  A. Laneville,et al.  Vortex-induced vibrations of a long flexible circular cylinder , 1993, Journal of Fluid Mechanics.

[7]  C. Williamson,et al.  Fluid Forces and Dynamics of a Hydroelastic Structure with Very Low Mass and Damping , 1997 .

[8]  Peter W. Bearman,et al.  Wake structures and vortex-induced vibrations of a long flexible cylinder—Part 1: Dynamic response , 2009 .

[9]  M. J. Prosser,et al.  On vortex excitation of model piles in water , 1973 .

[10]  Franz S. Hover,et al.  Vortical patterns behind a tapered cylinder oscillating transversely to a uniform flow , 1998, Journal of Fluid Mechanics.

[11]  F. Huera-Huarte.  Drag Coefficients of a Long Flexible Circular Cylinder with Wake Interference , 2009 .

[12]  C. Williamson,et al.  MOTIONS, FORCES AND MODE TRANSITIONS IN VORTEX-INDUCED VIBRATIONS AT LOW MASS-DAMPING , 1999 .

[13]  C. Williamson,et al.  The effect of two degrees of freedom on vortex-induced vibration at low mass and damping , 2004, Journal of Fluid Mechanics.

[14]  Charles H. K. Williamson,et al.  Vortex-induced vibrations of a pivoted cylinder , 2005, Journal of Fluid Mechanics.

[15]  Eduard Naudascher,et al.  Flow-Induced Structural Vibrations , 1974 .

[16]  Peter W. Bearman,et al.  On the force distribution along the axis of a flexible circular cylinder undergoing multi-mode vortex-induced vibrations , 2006 .

[17]  A. Roshko,et al.  Vortex formation in the wake of an oscillating cylinder , 1988 .

[18]  Charles H. K. Williamson,et al.  Vortex-induced vibration of a cylinder with two degrees of freedom , 2003 .

[19]  Kostas F. Lambrakos,et al.  Blind predictions of laboratory measurements of vortex-induced vibrations of a tension riser , 2005 .

[20]  J. Kim Vandiver,et al.  Drag Coefficients of Long Flexible Cylinders , 1983 .