Session D18: Vortex Dynamics: Flow Induced Vibrations and Interactions

Flow-induced oscillations of tandem tethered cylinders in a channel flow

In single degree-of-freedom (DOF) flow-induced oscillation studies of tandem rigid cylinders, the system most often consists of a front fixed cylinder and a trailing cylinder that is constrained to move perpendicular to the flow. We have conducted experiments in a water channel to investigate the behavior of a single DOF system of cylinders in which the trailing cylinder is constrained to move in a circular arc about the leading cylinder. We will discuss the dynamic response of the trailing cylinder for Reynolds numbers ranging from 10,000 to 20,000 and for inter-cylinder spacings from 3D to 5D, where D is the diameter of the cylinders. The experiments show a multi-frequency response that cannot be classified as a simple harmonic oscillator, as is assumed in typical tandem cylinder models. We compare our results with existing work on transversely constrained cylinders to determine the effect of tethering the cylinders.   

Effect of mass ratio on fluid induced motions of a circular cylinder with strips

The objective of the current experimental work is to investigate the effects of mass ratio on Fluid Induced Motions, such as vortex induced vibration (VIV) and galloping, of elastically mounted circular cylinders attached with strips to their outer surface. Although the effect of mass ratio on VIV of a smooth circular cylinder is well documented in literature, however, their effects on circular cylinders with strips, capable of inciting galloping oscillations haven't been investigated and could have potential applications in the domain of vibration based energy harvesters. In the current work, three different mass ratios were tested, out of which, one falls below the critical mass in vortex induced vibration of a circular cylinder. The strips used for the experiments included sandpaper strips of prescribed roughness and smooth strips with no roughness, both of which served as surface protrusion based mechanisms of altering the flow around the cylinder. Interesting variations were observed in the amplitude, frequency response and the power spectrum, depending on the mass ratio of the oscillating system tested.   

Wake dynamics of streamwise oscillating cylinders with one and two degrees of freedom

The effect of a second degree of freedom on the structural response and wake modes of a cylinder undergoing streamwise Vortex-Induced Vibrations (VIV) was studied using DPIV. The 2-DOF oscillating cylinder was found to exhibit similar amplitude response to a cylinder allowed to oscillate only in the streamwise direction, i.e. containing two response branches separated by a low amplitude region, as reported in the literature. The first branch was characterised by negligible transverse motion and the appearance of both alternate (A-II) and symmetric (S-I) vortex-shedding which competed in an unsteady manner. However, this mode competition did not appear to have a significant effect on either the streamwise or transverse motion. The additional DOF was found to simplify the overall dynamics of the system in the second response branch by reducing the number of states that the wake can exhibit: while the 1 -DOF oscillating cylinder was found to exhibit 3 different wake states and hysteresis in the second branch, the 2-DOF one was found to exhibit only one wake mode in the second branch (the SA mode) and the cylinder response was no longer hysteretic. Figure-of-eight motion trajectories were observed throughout the lock in range and the phase angle between the streamwise and transverse motion was found to decrease in a linear manner with reduced velocity, with a sudden jump when the wake changed from the A-II to the SA mode.   

The effects of the external cross-flow excitation forces on the vortex-induced-vibrations of an oscillating cylinder

Vortex induced vibrations can significantly affect the effectiveness of structures in aerospace as well as offshore marine industries. The oscillatory nature of the forces resulting from the vortex shedding around bluff bodies can result in undesirable effects such as increased loading, stresses, deflections, vibrations and noise in the structures, and also reduced fatigue life of the structures. To date, most studies concentrate on either the free oscillations or the prescribed motion of the bluff bodies. However, the structures in operation are usually subject to the external oscillatory forces (e.g. due to the platform motions in offshore industries). In this work, we present the effects of the external cross-flow forces on the vortex-induced vibrations of an oscillating cylinder. The effects of the amplitude, as well as the frequency of the external force on the fluid-forces on the oscillating cylinder are carefully studied and presented. Moreover, we present the transition of the response to be dominated by the vortex-induced-vibrations to the range where it is mostly dictated by the external oscillatory forces. Furthermore, we will discuss how the external forces can affect the flow structures around a cylinder.   

Effect of forced vibration on vortex evolution behind side-by-side cylinders

Vortex patterns in the wake of two side-by-side circular cylinders in stationary state and under forced cross-flow vibration were compared through experimental study. The hydrogen bubble visualization technique was employed for flow visualization. The Reynolds number was fixed at Re $=$ 250 for all the experiments. Two center-to-center pitch ratios were examined: P/D $=$ 3 and 6. For the two cylinders under forced vibration, the vibration frequency was chosen to match with the vortex shedding frequency in stationary state, and the vibration amplitude (A) was fixed at A/D $=$ 0.25. Under forced in-phase vibration, very strong in-phase vortex shedding behind the two cylinders was observed for both P/D $=$ 3 and 6. But the vortices evolve differently in the wake at different P/D. Under forced anti-phase vibration, both in-phase and anti-phase vortex shedding were observed for the two values of P/D. The effect of in-phase and anti-phase vibration on vortex evolution was characterized through comparison with the stationary case.   

Symmetry breaking in vortex-induced vibration of a rotating cylinder

Vortex-induced vibration (VIV) of a flexibly-mounted circular cylinder, free to oscillate in the crossflow direction with imposed rotation around its axis, is studied experimentally. In particular, the influence of asymmetry that is introduced into the system by the forced rotation of the cylinder is considered. The rotation rate, $\alpha $, defined as the ratio of the surface velocity and free stream velocity, was varied from 0 to 2.6 in small steps. The amplitudes and frequencies of oscillations as well as the flow forces were measured in a Reynolds number range of Re$=$350-1000. The maximum amplitude of oscillation was found to be limited to values less than a diameter of the cylinder at high rotation rates. Also the lock-in range was found to become narrower at higher rotation rates and finally the oscillation ceased beyond $\alpha =$2.4. Vortex shedding pattern was found to change from 2S and 2P shedding (two single and two pairs of vortices shed per cycle of oscillation) for a non-rotating cylinder to P shedding (one pair of vortices shed in a cycle of oscillations) for the rotating cylinder. Also, the phase difference between the flow forces and displacement of the cylinder in the crossflow direction was influenced as the rotation rate was increased. At high reduced velocities the phase difference decreased from 180 degree for a non-rotating cylinder to values close to 90 degree for a rotating cylinder.   

Vortex-induced vibrations under oblique shedding

A slender flexible body with bluff cross-section placed at normal incidence within a current may be subjected to vortex-induced vibrations (VIV). In practical applications, the structures (e.g. marine risers, towing cables) are often inclined with respect to the direction of the oncoming flow, sometimes at large angles. The vibrations that may appear in such configurations are investigated in the present work on the basis of direct numerical simulation results. We find that a flexible cylinder inclined at 80 degrees exhibits regular large-amplitude vibrations and that the structural responses are excited under the lock-in condition, i.e. synchronization between body oscillation and vortex formation, which is the central mechanism of VIV. We show that the lock-in condition may involve parallel vortex shedding, where the vortex rows are aligned with the body axis, but also oblique vortex shedding patterns. The excited structural wavenumber and the spanwise wavenumber of the obliquely shed vortices coincide; therefore, the flexible structure and the wake are locked both temporally and spatially. In addition, we find that the VIV occurring under oblique shedding may reach very high frequencies compared to the vibrations observed under parallel shedding.   

Motion Response of 2 DOF Circular Cylinder in Bundle Arrangment

This study focuses on the motion response of a freely vibrating in streamwise and crossflow circular cylinder in the wake of two leading stationary circular cylinders. Studies on the effects of spatial positioning of the trailing circular cylinder to its amplitude and frequency response are conducted. In order to explain the effects of mass ratio and phase angle on the motion response of the structure, analytical model based on tandem cylinder arrangement are presented. For almost all reduced velocities, the results indicated larger crossflow amplitude of oscillation as the trailing cylinder is aligned to the centerline of one of the leading circular cylinder. Two dominant response frequencies are found on the trailing circular cylinder. Switching between the two response frequencies as the dominant response frequency is found to be strongly related to the natural frequency of the system. Additionally, the mass ratio played a significant role in determining the intermittent domination of the Vortex-Induced Vibrating (VIV) frequency of the structure. For low mass ratio, larger mass ratio is found to increase its amplitude of oscillation.   

Flow behaviour around tripped circular cylinders

An experimental study is carried out to investigate if the effects of multiple spanwise tripwires fitted on a circular cylinder in subcritical flow can be explained and predicted using the knowledge accumulated on the influence of only one spanwise tripwire. For this purpose, this study compares the vortex shedding frequency behind circular cylinders fitted with one, two and four spanwise wires. In the two-wire and four-wire fitted cases, the separation between the wires on the cylinder surface is arbitrarily selected as 90$^{\circ}$. Frequency measurements were done for varying tripwire locations. The Reynolds number was kept at the subcritical value of 10,000 (based upon cylinder diameter). The tripwires were approximately 6{\%} the diameter of cylinder. Results showed that a correlation exists between the effects of single- and multi-wire tripping, and a set of rules can be devised to predict the vortex shedding frequencies around a circular cylinder with complex two-dimensional tripping configurations.   

Investigation of the Wake Interactions in Tandem Cylinder Arrangements

Vortex-induced vibrations of a single cylinder in a cross-flow are compared with the wake-induced oscillations of the downstream cylinder of a tandem cylinder arrangement in a cross-flow. It is known that the synchronization of frequency of vortex shedding with the natural frequency of the structure leads to large amplitude motions. For larger reduced velocities beyond the lock-in region, the cylinder displacement is abruptly reduced due to the inertia dominated region where the frequency of vortex shedding is larger than the natural frequency of the structure. However, in the case of tandem cylinders, the large amplitudes of the downstream cylinder is found at the reduced velocities greater than that of lock-in region. In this work, we show that the wake from the upstream cylinder interacts with the downstream cylinder which influences the response of the coupled system. Extensive numerical experiments have been performed on a single cylinder and tandem cylinder arrangement in cross-flow. Here, the wake interactions in connection to the forces generated are systematically studied. The upstream cylinder is fixed and the downstream cylinder is free to oscillate in transverse direction.