A numerical investigation of the vortex-induced vibration (VIV) in a side-by-side circular cylinder arrangement has been performed in a two-dimensional laminar flow environment. One of the cylinders is elastically mounted and only vibrates in the transverse direction, while its counterpart remains stationary in a uniform flow stream. When the gap ratio is sufficiently small, the flip-flopping phenomenon of the gap flow can be an additional time-dependent interference to the flow field. This phenomenon was reported in the experimental work of Bearman and Wadcock ["The interaction between a pair of circular cylinders normal to a stream," J. Fluid Mech. 61(3), 499-511 (1973)] in a side-by-side circular cylinder arrangement, in which the gap flow deflects toward one of the cylinders and switched its sides intermittently. Albeit one of the two cylinders is free to vibrate, the flip-flop of a gap flow during VIV dynamics can still be observed outside the lock-in region. The exact moments of the flip-flop phenomenon due to spontaneous symmetry breaking are observed in this numerical study. The significant characteristic vortex modes in the near-wake region are extracted via dynamic modal analysis and the interference between the gap flow and VIV is found to be mutual. In a vibrating side-by-side arrangement, the lock-in region with respect to reduced velocity becomes narrower due to the interference from its stationary counterpart. The frequency lock-in occurs and ends earlier than that of an isolated vibrating circular cylinder subjected to an identical flow environment. Similar to a tandem cylinder arrangement, in the post-lock-in region, the maximum vibration amplitudes are escalated compared with those of an isolated circular cylinder configuration. On the other hand, subjected to the influence from VIV, the biased gap flow deflects toward the vibrating cylinder quasi-stably during the frequency lock-in process. This behavior is different from the reported bi-stable regime in a stationary side-by-side arrangement. The analyses show that the flip-flop is associated with a characteristic low flip-flopping frequency, which is dependent upon the values of gap ratio, Reynolds number and the symmetry of the gap flow strength in a time-averaged sense. The disappearance of the flip-flop during the frequency lock-in of vibrating side-by-side arrangements is further investigated through a critical-point concept and a critical vortex merging distance.

In this work, the coupled dynamics of the gap flow and the vortex-induced vibration (VIV) on a side-by-side (SBS) arrangement of two circular cylinders is numerically investigated at moderate Reynolds number 100Re 800. The influence of VIV is incorporated by allowing one of the cylinders to freely vibrate in the transverse direction, which is termed as a vibrating side-by-side (VSBS) arrangement. A comparative analysis is performed between the stationary side-by-side (SSBS) and the VSBS arrangements to investigate the characteristics of the complex coupling between the VIV and the gap flow in a three-dimensional flow. The results are also contrasted against the isolated stationary and the vibrating configurations without any proximity and gap flow interference. Of particular interest is to establish a relationship between the VIV, the gap flow and the near-wake instability behind bluff bodies. We find that the kinematics of the VIV regulates the streamwise vorticity concentration, which accompanies with a recovery of two-dimensional hydrodynamic responses at the peak lock-in stage. On the other hand, the near-wake instability may develop around an in-determinant two-dimensional streamline saddle point along the interfaces of a pair of imbalanced counter-signed vorticity clusters. The vorticity concentration difference of adjacent vorticity clusters and the fluid momentum are closely interlinked with the prominence of streamwise vortical structures. In both SSBS and VSBS arrangements, the flip-flopping frequency is significantly low for the three-dimensional flow, except at the VIV lock-in stage for the VSBS arrangement. A quasi-stable deflected gap flow regime with negligible spanwise hydrodynamic (i.e., two-dimensional) response is found at the peak lock-in stage of VSBS arrangements. Owing to the gap-flow proximity interference, a high streamwise vorticity concentration is observed in its narrow near-wake region. The increase of the gap-flow proximity interference tends to stabilize the VIV lock-in, which eventually amplifies the spanwise correlation length and weakens the streamwise vortical structures. We employ the dynamic mode decomposition procedure to characterize the space-time evolution of the primary vortex wake.

All concrete structures contain reinforcement spacers, and deep sections can be affected by bleeding and segregation without displaying visible indications during casting. However, their effects on mass transport and long-term durability are not well studied. In this paper, reinforced concrete columns were prepared with plastic and cementitious spacers to achieve 50 mm cover, and compacted at different vibration frequencies and durations. 28d cured samples were extracted along the height, conditioned to equilibrium (21 °C, 75% RH or 50 °C, 7% RH), and then subjected to water absorption, electrical conduction, epoxy impregnation and fluorescence imaging. Samples from the top of the column consistently gave higher accessible porosity and mass transport compared to samples from the bottom. Presence of spacers caused additional increases in mass transport because of preferential flow through the spacer-concrete interface which is more porous and microcracked compared to bulk concrete farther away. Image analysis on cross-sections showed that the columns experienced some aggregate segregation despite care taken to avoid over-compaction. The resistance of concrete to ingress of aggressive agents decreases with increasing height due to the combined negative effects of reinforcement spacers and segregation.

When a riser array system is subjected to a uniform flow, an unstable flow-induced vibration commonly occurs among cylinders, generally called fluid-elastic instability. It can cause long-term or short-term damage to the riser array system. A numerical investigation has been performed in the present study. Generally, flow-induced vibrations include vortex-induced vibration (VIV), wake-induced vibration (WIV), jet switching, turbulent buffeting and fluid-elastic instability. The dynamic interactions among the fluid-induced vibrations, wake interference and proximity interference pose difficulties in the design and operation of the riser array system. The dynamics of a riser array system is very different from that of basic canonical configurations such as side-by-side, tandem and staggered arrangements. In a riser array system, the interferences come from all possible nearby constituent risers. There is a synchronization phenomenon among the cylinders, which may lead to detrimental collisions and short-term failures. It is known that the vortex-induced vibration (VIV) of an isolated circular cylinder is self-limiting. An extensive vibration occurs in the lock-in region within which the frequency of the vortex shedding matches the structural frequency of the immersed structure. In a riser array system, there is a point at which the vibration of cylinder suddenly increases. The vibration of the constituent risers increases without bound with the increment of the free-stream velocity. This free-stream velocity is defined as the critical velocity. The interference not only comes from the inline and cross-flow directions, but also the wake interference from the diagonal upstream risers. In a riser array system, each riser vibrates independently. However, there is symmetry of frequency spectrum observed about the inline direction along the middle row of the risers. In this study, the dynamic response of the different risers in the array system is investigated with the help of the amplitude response results from the canonical arrangements (side-by-side and tandem) and wake flow structures. The long top-tensioned riser system can be idealized by two-dimensional elastically mounted cylinders to solve the complex fluid-structure interaction problem. The dynamic response of a typical riser array system has been analyzed at low and high Reynolds number. It is encouraging to see that the results reported in the present investigation can provide useful insight and suggestions in the design and optimization of riser systems to avoid collisions and various long-or short-term failures.

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System identification is crucial to predict the maneuverability of the ship. In this work, ε-support vector regression (ε-SVR) is implemented to identify hydrodynamic derivatives of Abkowitz maneuver model. A proposed technique, batch learning, is implemented with the addition of Gaussian white noise to reconstruct the samples and alleviate the parameter drift in the system identification of the ship maneuvering model. The predicted results are compared with results obtained from Planar Motion Mechanism (PMM) test. Standard maneuvers, 35° turning circle, 10°/10° and 20°/20° zigzags, are simulated and compared with the predicted model by ε-SVR. The presented results show that the proposed batch learning technique with Gaussian white noise is an effective technique, which improves the accuracy and robustness of ε-SVR in system identification. The results obtained from the predicted model match well with the those obtained from PMM results, which shows its excellent generalization performance. The developed model is applied to understand control requirements for vessels under different conditions.