Vortex-induced Vibration Excitation Research Articles

Correlation and modal analysis techniques for the study of the VIV response of a twin-box deck based on 3D LES simulations

AbstractTwin-box decks are prone to vortex-induced vibration (VIV) at moderate wind speeds, and previous studies have shown a complex interplay between the flow and the two parallel box girders. OpenFOAM is used to model the aerodynamic and aeroelastic responses of the twin-box deck of the Stonecutters Bridge by means of 3D LES simulations. Two cases are thoroughly analysed under smooth incoming flow: (i) static deck and ii) heave peak amplitude response at VIV excitation. The computational data provided by the CFD simulations have been exploited aiming at better understanding the complex aerodynamic and aeroelastic phenomena taking place. First, spanwise correlation analyses are applied to understand the level of organisation of the forcing flow actions, and identify the regions of the deck affected by different flow structures. Second, three different mode decomposition techniques (POD, SPOD and DMD) are applied to the time-dependent pressure distributions, revealing the pivotal role played by the leeward box in the twin-box deck response under wind action. The modal analyses are of utmost interest in the development of Reduced Order Models (ROM) for the VIV response of twin-box long-span bridges.

Simultaneous energy utilization and vibration suppression study of a rolling-structured triboelectric nanogenerator for the vortex-induced vibration of a cylinder

Vortex-induced vibration (VIV) is a prevalent phenomenon observed in marine submersible buoys subjected to ocean currents, significantly compromising the structural fatigue life and sensor stability. Meanwhile, the provision of in-situ energy supply and long-term sustainability pose significant challenges for marine submersible buoys. To tackle these issues, this paper presents a novel approach that entails the simultaneous utilization of VIV energy and suppression of VIV through the implementation of a rolling-structured, freestanding triboelectric-layer-based nano-generator (RF-TENG). An experimental investigation has been conducted to explore the power generation mechanism of RF-TENG when subjected to VIV excitation, as well as the vibration suppression effect of RF-TENG on VIV. The following conclusions have been drawn: (1) The maximum power generation is observed within the filling rate range of 50%–60%, where the conversion of kinetic energy to electrical energy resulting from particle friction prevails. Correspondingly, the maximum vibration suppression rate is achieved within the filling rate range of 60%–90%, primarily attributed to energy dissipation due to particle collisions. Notably, at approximately 60%–70% filling rate, RF-TENG exhibits a relatively efficient and balanced performance in terms of both energy utilization and vibration suppression. (2) Particle motion irregularity is a prominent factor that greatly influences the power conversion efficiency, which is manifested as the presence of the third harmonic component in the output voltage and current. The power conversion efficiency of RF-TENG attains its peak value at a filling rate of 50%. (3) Within a certain range, an increase in particle diameter contributes to enhancing the robustness and VIV suppression efficiency of RF-TENG. However, it comes at the cost of reduced power generation capacity. In this study, under the excitation of VIV experienced by a cylinder with an effective amplitude of approximately 1.56 cm and a frequency of approximately 1.62 Hz, RF-TENG achieves a power generation density of 80 mW/m3 and a VIV amplitude suppression rate of 24%. These results suggest that RF-TENG is an effective method for synchronously collecting energy and suppressing VIV in marine structures.

Experimental investigation on vortex-induced force of a flexible pipe under oscillatory flow

In this paper, vortex-induced force (VIF) characteristics of a flexible pipe under oscillatory flow are experimentally investigated with different KC numbers ranging from 10 to 178, and reduced velocities from 4 to 7.9. The strain information caused by vortex-induced vibration (VIV) at different cross-sections on the flexible cylinder is measured in the model test. Combined with the modal superposition method and inverse analysis method for hydrodynamic force, the hydrodynamic forces on the flexible cylinder are inverted, including the excitation force in phase with velocity and added mass force in phase with acceleration. Then, through a least square method with forgetting factors, the excitation coefficients and added mass coefficients at each time step are identified. The results show that the VIV excitation coefficient of a flexible pipe under oscillatory flow has a significant “acceleration and deceleration” effect, that is, the excitation coefficients are mostly positive at the acceleration section of the oscillatory flow and negative at the deceleration section. This “acceleration and deceleration” effect also explains the response amplitude modulation feature of VIV under oscillatory flow. And this effect can be represented by the dimensionless acceleration parameter γ of the flow field, and the range of γ corresponding to the positive excitation coefficient is affected by the KC number and the maximum reduced velocity. In addition, the added mass coefficients present different time-varying features under oscillatory flow with different maximum reduced velocity. When the maximum reduced velocity is larger than 6.5, the time-varying trend of the added mass coefficient is opposite to that of the flow velocity. While when the maximum reduced velocity is smaller than 6.5, the added mass coefficient is maintained around 2.0 in VIV excitation region; and it is around 1.0 in the non-excited region which is close to the added mass coefficient of a cylinder in still water.

Experimental Research on Vortex-Induced Force Characteristics of Flexible Riser with Buoyancy Module and Strakes

In this paper, the vortex-induced force characteristics of a flexible riser with buoyancy modules and strakes were experimentally investigated. The strain information caused by vortex-induced vibration (VIV) at different cross-sections on the flexible cylinder was measured in the model test. Combined with the modal superposition method and inverse analysis method for vortex-induced force, the vortex-induced forces on the flexible cylinder were inverted, including the excitation force in phase with velocity and added mass force in phase with acceleration. Then, the excitation coefficients and added mass coefficients at each time step were identified via a least-squares method. The results show that the added mass coefficient and excitation coefficient of the flexible riser covering buoyancy modules present a “sawtooth” distribution along the length of the pipe, while the total mass (sum of structural mass and added mass) is consistent along the pipe. The strake has little effect on the added mass coefficient of the buoyancy module and the bare pipe section, but when it covers the bare pipe section, the excitation coefficient of the bare pipe section decreases to a negative value, showing a damping effect. In addition, when the bare pipe section of the flexible riser is completely covered with helical strakes, only the vortex-shedding frequency component of the buoyancy modules remains in the VIV response of the pipe. The reason for this phenomenon is that helical strakes completely suppress the vortex shedding of the bare pipe section, resulting in the reduction in the VIV excitation energy of the original bare pipe section to 0.

Revealing the effects of damping on the flow-induced vibration of flexible cylinders

This study reveals how damping shapes the global vortex-induced vibration (VIV) response of flexible cylinders. Global behavior may vary from full-length standing waves to traveling waves on infinite cylinders. Structural damping rules the standing wave case whereas radiation damping regulates VIV response on very long cylinders. A single scalar equation expresses the balance of power flowing through the structure. In that equation, Arms, which is the root-mean-square response in the VIV excitation region, is shown to be an excellent indicator of global response because of its relation to power flow. Under steady-state conditions, the net power flow must be zero, which directly leads to three independent dimensionless damping parameters, namely α,βR,andc*.βR indicates when radiation damping is important, α reveals the relative importance of structural versus radiation damping, and c∗ locates the global VIV behavior on the spectrum of lightly to strongly damped systems. Structural, hydrodynamic, and wave radiation damping are all taken into account. Plots of Arms∗ versus c∗ show the global effects of damping on response. Uncontrolled factors often reveal themselves as graphical anomalies, leading to new insights on VIV. Data from experiments and numerical simulations are presented to support the conclusions.

Modeling the interference of vortex-induced vibration and galloping for a slender rectangular prism

Several bluff bodies in an airflow, such as rectangular cylinders with moderate side ratio, in particular conditions of mass and damping can experience the interference of vortex-induced vibration (VIV) and galloping. This promotes a combined instability, which one may call “unsteady galloping”, with peculiar features and possibly large vibration amplitudes in flow speed ranges where no excitation is predicted by classical theories. The mathematical model proposed between the 70's and the 80's by Prof. Y. Tamura to simulate this phenomenon was considered here for the case study of a two-dimensional rectangular cylinder with a side ratio of 1.5, having the shorter section side perpendicular to the smooth airflow. This wake-oscillator model relies on the linear superposition of the unsteady wake force producing VIV excitation and the quasi-steady force that is responsible for galloping. The model formulation was slightly modified, and the way to determine a crucial parameter was changed, revealing a previously unexplored behavior of the equations. In the present form, the model is able to predict the dynamic response of the rectangular cylinder with a satisfactory qualitative and, to a certain extent, quantitative agreement with the experimental data, although the limitations of the present approach are clearly highlighted in the paper. The mathematical modeling of unsteady galloping and the analysis of the results offer a deep insight into this complicated phenomenon and its nonlinear features. The model also represents a useful engineering tool to estimate the vibration of a structure or structural element for which the interference of VIV and galloping is envisaged.

Vortex induced vibration excitation competition between bare and buoyant segments of flexible cylinders

This paper addresses a practical problem: “Under what fractional coverage of buoyancy modules, would the vortex induced vibration (VIV) excitation on buoyant segments dominate the response?” The source of data is a recent model test on a 38m long flexible cylinder, densely instrumented with fiber optic strain gauges and accelerometers. A pipe model with five staggered buoyancy configurations was tested. The paper emphasis is on exploring the winner of excitation competition between bare and buoyant segments and offering useful insights on the design of a pipe with staggered buoyancy modules in uniform flow. Four particular topics are covered: (i) the identification of VIV excitation regions, (ii) the experimental determination of the winner of excitation competition between bare and buoyant segments, (iii) the prediction of the winner of excitation competition, and (iv) the factors affecting fatigue damage rate for pipes with staggered buoyancy modules. Potential areas for promising additional research and a general guidance in designing staggered buoyancy pipes are proposed.

On shear flow single mode lock-in with both cross-flow and in-line lock-in mechanisms

A long flexible cylinder exposed to ocean currents is known to undergo vortex-induced vibration (VIV). In a spatially sheared flow the response of a riser to VIV can vary from single mode lock-in to multimodal. A new experimental facility was designed and built to investigate the above-mentioned areas. The facility consisted of a long flexible cylinder in either a uniform or a simplified vertically sheared flow. The instrumentation consisted of direct local fluid force measurement at two locations on the cylinder as well as accelerometers spaced along the cylinder axis. The simplified shear flow was a 2-slab flow, with each slab having uniform velocity. Test conditions included forcing the cylinder simultaneously at resonance in both regions to investigate modal competition issues and multimodal response patterns. Resonant VIV excitation of two different modes simultaneously, was conducted which revealed single mode lock-in of the higher frequency through an unexpected mechanism. The higher frequency mode's damping region underwent in-line excitation at four times the predicted shedding frequency that provided a power-in effect to support the dominant mode's cross-flow response.