In-line Vibration Research Articles

Forced Vibration Tests for In-Line Vortex-Induced Vibration to Assess Partially Strake-Covered Pipeline Spans

AbstractHelical strakes can suppress vortex-induced vibrations (VIVs) in pipelines spans and risers. Pure in-line (IL) VIV is more of a concern for pipelines than for risers. To make it possible to assess the effectiveness of partial strake coverage for this case, an important gap in the hydrodynamic data for strakes is filled by the reported IL forced-vibration tests. Therein, a strake-covered rigid cylinder undergoes harmonic purely IL motion while subject to a uniform “flow” created by towing the test rig along SINTEF Ocean's towing tank. These tests cover a range of frequencies, and amplitudes of the harmonic motion to generate added-mass and excitation functions are derived from the in-phase and 90 deg out-of-phase components of the hydrodynamic force on the pipe, respectively. Using these excitation- and added-mass functions in VIVANA together with those from experiments on bare pipe by Aronsen (2007 “An Experimental Investigation of In-Line and Combined In-Line and Cross-Flow Vortex Induced Vibrations,” Ph.D. thesis, Norwegian University of Science and Technology, Trondheim, Norway.), the IL VIV response of partially strake-covered pipeline spans is calculated. It is found that as little as 10% strake coverage at the optimal location effectively suppresses pure IL VIV.

Characteristics of In-line Flow-Induced Vibration in Case of Two Circular Cylinders

Characteristics of In-line Flow-Induced Vibration in Case of Two Circular Cylinders

Modelling of coupled cross-flow and in-line vortex-induced vibrations of flexible cylindrical structures. Part II: on the importance of in-line coupling

To illustrate the influence of the in-line coupling on the prediction of vortex-induced vibration (VIV), the simulation results of the coupled cross-flow and in-line VIVs of flexible cylinders- obtained with three different wake oscillator models with and without the in-line coupling- are compared and studied in this paper. Both the cases of uniform and linearly sheared flow are analysed and the simulation results of the three models are compared with each other from the viewpoints of response pattern, fluid force, energy transfer and fatigue damage. The differences between the simulation results from the three models highlight the importance of the in-line coupling on the prediction of coupled cross-flow and in-line VIVs of flexible cylindrical structures.

Out-of-plane vibration of an electrostatically actuated microbeam immersed in flowing fluid

The present work investigates the out-of-plane nonlinear vibration of an electrostatically actuated cylindrical microbeam immersed in flowing fluid. The lift and drag forces as the two basic flow-induced factors affecting the dynamics of the microbeam are modeled using Van der Pol equation. The Euler–Bernoulli beam theory is used to simulate the new nonlinear model of beam cross fluid and inline motion with considering geometrical nonlinearities effects. The coupled nonlinear equations governing the microbeam out-of-plane motion and the wake oscillation are solved using the Galerkin and the step-by-step linearization methods to evaluate the response of the coupled structure to a combined applied voltage and fluid flow. Response of the microbeam to different input voltages in the presence of fluid flow is investigated in two directions of normal and parallel to fluid flow. It is shown that applying voltage not only can be used to control the lock-in regime, but also can increase the maximum dynamic amplitude up to 100 percent for a given flowing fluid. Moreover, ignoring inline vibration and geometrical nonlinearities effects may decrease the accuracy of the obtained maximum amplitude up to 18 percent in the lock-in regime.

Dynamic characteristics of a slender flexible cylinder excited by concomitant vortex-induced vibration and time-varying axial tension

Flexible cylinder structures, such as top-tensioned risers (TTRs) and tethers of tension leg platforms (TLPs), display complicated dynamic characteristics under the combined excitation of vortex-induced vibration (VIV) and time-varying axial tension, which causes serious fatigue damage and is a major concern in ocean engineering fields. In this paper, model tests were conducted on a flexible cylinder with an aspect ratio of 350 and a mass ratio of 1.90 to study the effect of time-varying tension on the VIV response. Three tension amplitude ratios (Tv/Tc = 0.1, 0.2 and 0.3, where Tv is the amplitude of the varying tension and Tc is the constant tension) and six tension frequency ratios (fv/f1 = 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0, where fv is the frequency of the varying tension and f1 is the 1st-order natural structural frequency) were considered. The Reynolds number ranged from approximately 800 to 16,000. The displacements were reconstructed with the measured strains using a model analysis method. The effects of the tension amplitude ratio and tension frequency ratio on the dynamic characteristics were discussed from the maximum root mean square (RMS) of the displacements, power spectral density (PSD) plots, time-space-varying displacements, and motion trajectories. The tension excitation places the vibration into a higher-order mode earlier. For a large tension amplitude ratio (Tv/Tc = 0.3), the max RMS of the cross-flow (CF) displacement remarkably increases because the frequency of the varying tension is equal to the parametric resonance frequency (fv/f1 = 2.0). The effect of the tension frequency ratio is enhanced with the increase in the amplitude ratio. For the case of fv/f1 = 2.0, the sum and difference frequencies disappear, and the bandwidth of the dominant frequency widens. Figures in the shape of the number 8 are not found in the motion trajectory plots for Tv/Tc = 0.3, indicating that energy transfer is influenced by tension excitation. The in-line (IL) vibrations show an alternate conversion between the 5th-order mode vibrations and the analogous 3rd-order mode vibrations.

Two-degree-of-freedom vortex-induced vibrations of two square cylinders in tandem arrangement at low Reynolds numbers

Although flow past cylinders has a vast spectrum of applications in various fields of engineering, the existence of flow-induced vibrations (FIVs) add a need for a more precise and thorough approach when studying such flows. In this context, this work investigates the FIVs’ mechanism due to two square cylinders placed in tandem arrangement through a characteristics-based-split (CBS) finite element method. Both cylinders, with a low mass ratio (mr= 2), are free to oscillate in in-line and transverse directions. The center-to-center distance of 5D (D being the cylinder side width) is kept fixed between the cylinders. The Reynolds number (Re) is ranged from 40 to 200 with an increment of 40, and computations are carried out for the reduced velocity (Ur) ranging from 3 to 13 under each Re value. Results show that the effects of Re and Ur on the dynamic responses are qualitatively similar for both cylinders. However, effects on the downstream cylinder are stronger due to the upstream wake. For Re equal to or beyond 120, the in-line low-frequency characteristic becomes obvious at some higher values of Ur for the upstream cylinder, whereas it exists over a wide range of Ur for the downstream cylinder. For lower Re (up to 120), cylinders show steady behavior at Ur= 5. However, for higher Re, they vibrate with the maximum amplitudes (especially the in-line amplitude of the downstream cylinder), and show complex wake flow at the same Ur. Compared to higher Re, the maximum vibration amplitudes in the transverse direction of lower Re are relatively higher, while the opposite trend is observed for in-line vibration amplitudes. Several unstable trajectories are also identified other than the typical figure-eight trajectories. In context of vortex shedding, different permutation of steady state, 2S, P+S, and complex modes are observed for different values of Re. It is also noted that the vortex shedding pattern keeps changing in the far wake for higher Re.

Prediction of Vortex-Induced Vibration Response of Deep Sea Top-Tensioned Riser in Sheared Flow Considering Parametric Excitations

Abstract It is widely accepted that vortex-induced vibration (VIV) is a major concern in the design of deep sea top-tensioned risers, especially when the riser is subjected to axial parametric excitations. An improved time domain prediction model was proposed in this paper. The prediction model was based on classical van der Pol wake oscillator models, and the impacts of the riser in-line vibration and vessel heave motion were considered. The finite element, Newmark-β and Newton‒Raphson methods were adopted to solve the coupled nonlinear partial differential equations. The entire numerical solution process was realised by a self-developed program based on MATLAB. Comparisons between the numerical calculation and the published experimental test were conducted in this paper. The in-line and cross-flow VIV responses of a real size top-tensioned riser in linear sheared flow were analysed. The effects of the vessel heave amplitude and frequency on the riser VIV were also studied. The results show that the vibration displacements of the riser are larger than the case without vessel heave motion. The vibration modes and frequencies of the riser are also changed due to the vessel heave motion

Experimental investigation on coupled cross-flow and in-line vortex-induced vibration responses of two staggered circular cylinders

In order to better understand the vortex-induced vibration mechanism of multiple cylinders, this article takes a relatively simple case of two staggered circular cylinders as the embarkation point and investigates their vortex-induced vibration characteristics by model test. The experimental Reynolds number ranges from 22,000 to 88,000. The in-line gap L is set as 3.0 D, 3.6 D, 4.2 D and 5.5 D in turn, and the cross-flow gap T is set as 0.7 D, 1.1 D, 1.5 D, 1.9 D, 2.3 D and 2.7 D, respectively. By measuring the vibrating response in model test, the response differences between the two staggered cylinders and the isolated cylinder and the effects of the gaps are discussed. The results indicate that the variation trend of response of the upstream cylinder with reduced velocity is basically similar to that of the isolated cylinder. However, the downstream cylinder shows some great differences. When the in-line gap ratio L/ D is 3.6, the cross-flow amplitude curve of downstream cylinder changes from “single peak” to “double peaks” with the increase in cross-flow gap ratio T/ D, and in-line amplitude curve even shows four different kinds of forms. When L/ D is increasing, maximum amplitudes of the downstream cylinder in two directions also show an increasing trend, and the wake galloping phenomenon even appears in some conditions. Generally, the case of staggered cylinders is a generalized combination of two circular cylinders in tandem and side-by-side arrangements, and this article has extended the research scope of the double-cylinder vortex-induced vibration to arbitrary flow direction.

Study on Vortex-Induced Vibration of Deep-Water Marine Drilling Risers in Linearly Sheared Flows in consideration of Changing Added Mass

In order to more accurately predict the coupled in-line and cross-flow vortex-induced vibration (VIV) response of deep-water marine drilling risers in linearly sheared flows, an improved three-dimensional time-domain coupled model based on van der Pol wake oscillator models was established in this paper. The impact of the in-line and cross-flow changing added mass coefficients was taken into account in the model. The finite element, Newmark-β, and Newton–Raphson methods were adopted to solve the coupled nonlinear partial differential equations. The entire numerical solution process was realized by a self-developed program based on MATLAB. Comparisons between the numerical calculations and the published experimental tests showed that the improved model can more accurately predict some main features of the coupled in-line and cross-flow VIV of long slender flexible risers in linearly sheared flows to some extent. The coupled in-line and cross-flow VIV of a real-size marine drilling riser, usually used in the deep-water oil/gas industry in the South China Sea, was analyzed. The influence of top tension force and seawater flow speed, as well as platform heave amplitude and frequency, on the riser in-line and cross-flow VIV was also discussed. The results show that the platform heave motion increases the VIV displacements and changes the magnitudes of peak frequencies as well as the components of frequencies. The platform heave motion also has a significant influence on the vibration modes of the middle and upper sections of the riser. The impact level of each factor on the in-line and cross-flow VIV response of the riser is different. The improved model and the results of this paper can be used as a reference for the engineering design of deep-water marine drilling risers.

Suppression of in-line flow-induced vibration of a square prism by control plate

Suppression of in-line flow-induced vibration of a square prism by control plate

Measurement of CO2 and Ultrasonic Vibration Based on Tunable Fiber Ring Laser

In this paper, we propose a method to simultaneously measure carbon dioxide (CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) concentrations and ultrasonic vibration using fiber sensors with a tunable fiber ring laser based on a loop design. The in-line interferometric vibration sensor consists of a single-mode-multimode-single-mode fiber structure and detects continuous dynamic vibrations through intensity demodulation. The evanescent wave absorption based CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensor is composed of a no-core fiber coated with silica derived from tetraethylorthosilicate. The fiber ring laser output is tuned to the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> absorption wavelength of 1574.6 nm, which also best matches the quadrature bias point of the vibration sensor. The vibration frequency signals from 10 Hz to 50 kHz, and fast detection of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentrations from 10% to 100% are experimentally demonstrated. The proposed hybrid sensor system is a promising method for monitoring the gas pipelines operating state, gas turbines, nuclear power plants, and automotive engines, where both vibration and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration measurements are essential.

A single van der pol wake oscillator model for coupled cross-flow and in-line vortex-induced vibrations

In this study a new wake oscillator model is proposed to describe the coupled cross-flow and in-line vortex-induced vibrations of an elastically supported rigid cylinder. Different from many other studies where two wake oscillators have been applied, the current model uses only one wake oscillator coupled to both cross-flow and in-line motions. The new model is based on the van der Pol oscillator with the classic acceleration coupling between the wake and cross-flow motion, while the in-line motion is coupled with the wake variable in a nonlinear manner. The predictions of this new model are compared with the existing experimental data and shown to be in good agreement. In addition to the conventional lock-in range that corresponds to reduced velocities between 5 and 8, another lock-in is predicted around reduced velocity of 2.5 due to the in-line vibration. Most importantly, the new model is proved to be able to predict the appearance of the ‘super-upper’ branch at small mass ratios without changing the tuning parameters. The limitations of the model associated with unrealistic predictions of free vibrations with very small mass ratios and those of forced in-line vibrations at high frequencies are also discussed along with a possible remedy.

Vortex-induced vibrations of two cylinders with different diameters close to a horizontal plane boundary at low Reynolds number

A numerical study is performed on the vortex-induced vibrations of two rigidly coupled cylinders with different diameters placed in a proximity of a horizontal plane boundary. The two cylinders are elastically supported and free to vibrate in two degrees-of-freedom. The Reynolds number is kept constant at Re=200. The influence of the small cylinder placement relative to the large cylinder is systematically studied. The effects on the vibration amplitudes and hydrodynamic forces are analyzed. The flow structures around the cylinders are investigated to elucidate the variations in observed structural responses. At investigated gap ratio (e/D=0.9), the bottom boundary effects are found to affect the behavior of the structure significantly. The suppression of the vortex shedding from the bottom surface of the large cylinder is observed, leading to in-line vibration lock-in. When the small cylinder is located in the α=135° configuration, the lock-in range is significantly wider than that of a single cylinder configuration. When the small cylinder is located in the α=90° configuration the effect of the small cylinder is manifested by increased mean drag and change in the direction of the mean lift force to negative (directed towards the bottom plane boundary). For the α=45° configuration, reduced vibration response and narrower lock-in range are identified compared with those of the single cylinder, and the other investigated configurations.

Stationary and flow-induced vibration of two elliptic cylinders in tandem by immersed boundary-MRT lattice Boltzmann flux solver

Numerical method, combining immersed boundary method and multi-relaxation-time lattice Boltzmann flux solver, is used to carry out flow-structure interaction problems in this paper. Without dynamic mesh technique and body fitted grid re-generation, the flow-structure interaction problems can be solved by Cartesian mesh in order to simplify calculation process and save computation time. The accuracy and rationality of this method are verified by comparison of force coefficients, vibration amplitudes and flow characteristics with previous numerical results. Numerical simulations of two stationary and flow-induced vibration elliptical cylinders in tandem are carried out in laminar flow. For two stationary elliptical cylinders, the existing of downstream elliptical cylinder effectively prevents the unsteady characteristic of upstream wake at small gap spacing. With the increase of aspect ratio, the critical gap augments. After reaching the critical gap, the upstream force coefficients decrease and the downstream ones increase as aspect ratio increasing. Flow characteristic presents three regimes, including steady flow, shear layer attachment and 2S shedding vortex and 2S or C(2S) shedding vortex. For two flow-induced vibration elliptical cylinders, in comparison with stationary simulations, upstream transverse vibration responses are strengthened and downstream ones are strengthened at larger gap spacing and aspect ratio. The in-line vibration responses are both weaken, especially for downstream bluff body. Flow characteristic show different flow-regimes: (a) shear layer attachment for upstream and 2S shedding vortex for downstream; (b) 2S shedding vortex for upstream and C(2S) shedding vortex for downstream; (c) 2S shedding vortex for upstream and vortex pair presenting in the downstream wake. Greater L/D and AR, the effects of upstream wakes on downstream elliptical cylinder are more intense.

Hydrodynamic features of three equally spaced, long flexible cylinders undergoing flow-induced vibration

Abstract Three equally spaced flexible cylinders are frequently applied in many engineering fields. The flow-induced vibration (FIV) hydrodynamic features of three such cylinders are not the same as those of an isolated single one and remain unknown. In this paper, the hydrodynamic coefficients for three flexible cylinders subjected to FIV in an equilateral-triangular arrangement with a centre-to-centre spacing of 6 diameters were identified using an inverse analysis method according to the displacement response data obtained from model tests. The lift coefficient, varying drag coefficient and added mass coefficient at the dominant frequency were calculated by decomposing the cross-flow (CF) and in-line (IL) fluctuating forces. Three typical cases of an equilateral-triangular configuration (A, B, and C), which correspond to flow incidence angles of 0 ∘ , 30 ∘ , and 60 ∘ , were studied and discussed (the incidence angle is defined as the angle between the flow orientation and the line linking the centre points of one cylinder and the equilateral-triangular configuration). The hydrodynamic coefficients of the upstream cylinders are insignificantly influenced by the downstream cylinders. In contrast, the wake of the upstream cylinders impose a notable effect on the IL hydrodynamic coefficients of the downstream cylinders. Two robust frequency components ( f v , I L and f v , I L _ 1 ∕ 2 ) were observed in the IL vibrations of the downstream cylinders. The IL hydrodynamic forces at f v , I L have similar features to the classical vortex shedding forces of an isolated flexible cylinder. However, the IL hydrodynamic forces at f v , I L _ 1 ∕ 2 exhibit distinct behaviours that are closely related to the unique response characteristics in the IL direction. In addition, the IL fluctuating force coefficients and varying drag coefficients at f v , I L _ 1 ∕ 2 are relatively small and change slowly with the reduced velocity.

Wake structures and vortex-induced forces of a controlled in-line vibrating circular cylinder

A two-dimensional numerical study was performed to investigate wake structures and fluid forces on a circular cylinder with prescribed vibrations in the in-line direction. Reynolds number was fixed at 500, and the SST k-w model was used to simulate turbulence in the flow. A wake map for in-line vibrations, which includes six wake patterns named AS, SS, S(AS), S(2Po), S(P + S) and S(2P), is drawn in the plane of non-dimensional frequency from 0.2 to 0.9 and an amplitude ratio from 0.01 to 0.275. The S(2Po) mode is a newly discovered wake pattern that consists of two pairs of symmetric vortex shedding per cycle. The S(P + S) mode and S(2P) mode follow similar wake patterns as the P + S mode and the 2P mode for cross flow vibrations, and it takes two in-line cycles to complete one S(P + S) or S(2P) vortex shedding cycle. Extensive numerical investigations show that similar wake structures can be produced if the maximum and minimum values of relative velocity between the flow and the vibrating cylinder are kept the same, regardless of the motion directions. One positive excitation coefficient region is observed for the non-dimensional frequency ranging from 0.38 to 0.43, and the peak amplitude ratio of the zero contour line for the excitation coefficient reaches 0.056. The changing sign of energy transfer between flow and structure is not necessarily caused by the change of wake modes, but it results from the changing time of vortex shedding. Typical 'figure eight' profiles are captured for simultaneous values of lift and drag forces within the positive excitation region.

Influence of Frequency Ratio on the Hydroelastic Response of a Cylinder with Degrees of Freedom under Vortex Induced Vibration

Vortex induced vibration of cylindrical structures is an extensively researched topic. Most of the studies have concentrated on the response of the cylinder in the cross flow (CF) direction. In a realistic ocean environment, structures such as drilling and marine risers are more or less free to vibrate both in CF and in line (IL) directions. It has also been observed that the IL vibrations have significant influence on the CF response. Interaction between the responses in inline and cross flow directions has still been not fully understood. This paper addresses the same through a simplified numerical method for understanding the interaction between these two responses using two dimensional computational fluid dynamics (CFD) simulations. Here analyzes two cases have been considered; where in the cylinder is modeled with two different values of ratio of natural frequency of the cylinder in the IL direction to that in the CF direction. The trends of variation of hydrodynamic and structural parameters have been analyzed to comprehend the effect of directional natural frequency ratio on the cylinder response and hydrodynamic force coefficients. The shedding pattern has also been studied in this paper. An increase by 18% in the value of the lift coefficient and 38 % of that in the drag coefficient has been observed when the frequency ratio is increased from 1 to 2. The results show that the cylinder with frequency ratio 2 is more prone to lock in vibration. This phenomenon may be related to the shifting of shedding pattern from 2S to P + S mode when the frequency ratio is 2.

Experimental investigation of the suppression of vortex-induced vibration in four cylinders arranged in a square under different spacing ratios

A vortex-induced vibration (VIV) is a typical flow-structure interference phenomenon that causes an unsteady flow pattern owing to vortex shedding at or near the structure’s natural frequency, leading to resonant vibrations. VIVs can cause the failure of marine risers owing to premature fatigue, because of which a growing amount of research has attended to them. To investigate the effects of suppressing the VIV of splitter plates on a square array of cylinders under different spacing ratios, cylinders undergoing VIV in a flow with a Reynolds number ranging from 3300 to 6750 were experimentally studied in this paper. The results show that the splitter plate can suppress cross-flow (CF) vibrations as well as in-line flow (IF) vibrations in the cylinders. For cross-flow vibration, the suppression effects of the splitter plate on the upstream cylinders were at their maximum at $$S/D = 5$$ while the effects on downstream cylinders reached this at $$S/D = 6$$, where $$S/D$$ refers to the ratio of the distance between the centers of cylinders to its diameter. For in-line flow vibrations, the effects on upstream and downstream cylinders all reached the maximum at $$S/D = 4$$. The vibrational frequency of the cylinder with splitter plate was also lower than that of the bare cylinder in the flow.

Dynamic response of three long flexible cylinders subjected to flow-induced vibration (FIV) in an equilateral-triangular configuration

A three-cylinder system with an equilateral-triangular configuration represents one of the most common, basic units of multi-cylinder systems in many engineering fields. In this paper, an experimental investigation of the flow-induced vibration (FIV) of a three-cylinder system in an equilateral-triangular arrangement was conducted in a towing tank to study their response characteristics. Three identical flexible cylinder models with a centre-to-centre distance of 6.0 times the cylinder diameter were towed along the tank to generate a uniform flow. The Reynolds number ranged from approximately 1600 to 16000 depending on the towing velocity, which was in the range of 0.10–1.00 m/s. The time-varying bending strains of the flexible cylinders in the cross-flow (CF) and in-line (IL) directions were measured using strain gauges at different measurement positions. The displacement responses were reconstructed by a modal analysis method based on the acquired strain signals. The experimental results (e.g., displacement amplitudes, response frequencies, dominant modes and motion trajectories of the flexible cylinders) were presented to reveal the FIV response features. Three typical cases of the equilateral-triangular configuration (A, B and C) corresponding to flow incidence angles of 0°, 30°, and 60° were selected and discussed. (The incidence angle is the angle between the flow direction and the line joining the centre of one cylinder to the centre of the equilateral-triangular configuration.) In Cases A and B, the upstream cylinder is insignificantly affected by the downstream cylinders and behaves like an isolated flexible cylinder; meanwhile, the upstream cylinders in Case C vibrate similarly to two side-by-side flexible cylinders with weak mutual interference. Striking departures exist in the IL oscillations of the downstream cylinders due to the notable effects of the upstream cylinders. The phenomenon of mixed modes appears in the IL vibrations of the downstream cylinders. Furthermore, the IL response spectra always contain a robust lower frequency component equal to the dominant frequency in the CF direction. Moreover, eight-, half-moon- and C-shaped vibration figures are comparatively uncommon for the downstream cylinders.

Vortex-induced vibration of a piggyback pipeline half buried in the seabed

Experiments were conducted in a tow tank on the steady current-induced vibration of the smaller flexible cylinder (aspect ratio L∕d = 61 and mass ratio m0 = 1.38) in a piggyback configuration with the main pipe half-buried in the seabed. The effect of the gap-to-diameter ratio (G∕d) based on the smaller cylinder diameter is studied for five different configurations from 1.0 to 4.0 and ∞ (open flow) at different reduced velocities (Ur) from 2 to 10 based on the 1st modal natural frequency. An underwater optical measurement system was used in the current experiment to acquire both cross-flow (CF) and in-line (IL) vibration with a both spatially and temporally dense format. The results show that the existence of the half-buried larger cylinder will increase the mean drag coefficient for the smaller flexible cylinder and induce a positive mean lift coefficient pushing it away from the larger cylinder. For the smallest gap-to-diameter ratio of G∕d = 1.0 when the interaction is the strongest, experiments show that the root mean square (RMS) of the CF vibration will increase due to the appearance of the stronger higher harmonic motion compared to the open flow case. The results of the inversely calculated vortex forces reveal that the 3rd harmonic fluid force in the CF direction is even stronger than the 1st harmonic fluid force for G∕d = 1.0. In addition, it is found that there is a strong correlation between the positive lift coefficient in phase with velocity (Clv) and the counter-clockwise trajectory between the CF and the IL vibrations of the smaller flexible cylinder, even with the existence of the large half-buried main pipeline.