Experimental investigation of vortex- and wake-induced vibration of tandem cylinders

This paper presents the experimental investigation of wake-induced vibration (WIV) and vortex-induced vibration (VIV) acting on two circular cylinders. In particular, a riser installed in a offshore field may experience flow interferences between adjacent risers. Accordingly, scaled models were arranged in a tandem configuration in order to observe the flow interference between the cylinders. While an upstream cylinder is fixed or allowed to have one degree of freedom(1-DOF) motion, a downstream cylinder is allowed to have two degree of freedom(2-DOF) motion. An experimental test is carried out in a circulating water channel with cylinders which are connected by linear springs in two horizontal directions. A three-dimensional displacement meter and a load cell were used to measure cylinder's motion and hydrodynamic force of a cylinder, respectively. In order to implement single and dual resonance condition, natural frequency ratio of in-line to cross-flow directions, f*=fnx/fny, was set as 1.0 and 1.9. To validate an experimental result, it is compared with that of previous study in terms of cylinder's motion response amplitude and oscillation frequency. From the experiment, we have identified that there are relative effects between VIV and WIV with respect to selected parameters. Influence of WIV are also quantitatively observed with in-line motion power spectral density and cross-flow motion time history.1. Introduction 1.1 Background 1 1.2 Literature review 3 1.3 Objectives and Scopes 6 2. Experimental set-up 2.1 Circulating Water Channel 7 2.2 Experimental equipment 8 2.3 Rigid circular cylinder models 10 2.4 Fixed and two degree of freedom 12 2.5 One and two degree of freedom 14 3. Test matrix for experiment 3.1 Parameters set-up 15 3.2 Cases for model experiment 18 4. Amplitudes and frequencies of cylinder's motion 4.1 Single cylinder with 1-DOF and 2-DOF motions 20 4.2 Downstream cylinder of WIV 24 4.2.1 Fixed upstream cylinder & 2-DOF downstream cylinder 24 4.2.2 1-DOF upstream cylinder & 2-DOF downstream cylinder 32 5. An effect of WIV excitation mechanism 5.1 Time history and Power spectral density 37 5.2 Trajectories of motion 42 5.3 Flow interference between tandem cylinders 45 5.4 Force coefficients with Wake shielding effect 47 6. Conclusions References 50Maste