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An experimental investigation on the vortex/wake-induced force of double unequal-diameter cylinders in tandem was performed. In the experiment, the upstream-to-downstream cylinder diameter ratio was 1.569, and the gap spacing ratio between the cylinder models ranged from 3 to 8. Under uniform flow with Reynolds number up to 11 328, the strains caused by the vortex/wake-induced vibrations were measured for the double flexible cylinders. Based on the measured strains, the displacements were reconstructed with the mode superposition method. Then, the vortex/wake-induced forces were obtained inversely. Subsequently, the time-varying excitation and added mass coefficients were identified by the forgetting factor least squares method. The results show that the wake-induced force coefficient and excitation coefficient of the downstream cylinder are smaller than those of the single cylinder, while the added mass coefficient becomes larger. In addition, the downstream cylinder under frequency capture can absorb a higher proportion of energy from the wake to excite larger vibration amplitudes, especially in the in-line direction. [ABSTRACT FROM AUTHOR]

The three-dimensional characteristics of the flow past four square cylinders in an in-line square configuration, with five spacing ratios ranging from 1.4 to 5, were studied in depth in this study. Direct numerical simulation of the spectral/hp element method was employed at Re = 150 and 200. The onset and evolution of various unstable modes were expounded in detail by means of three-dimensional vortices, energy curves, wake patterns, and force coefficients. At each spacing, the three-dimensional instability and the corresponding flow pattern were comprehensively analyzed to illustrate transitional features. Except for the existence of unstable mode A and mode B when spacing was considerably small and large, for most of the intermediate spacing ratios, the vortex structures were dominated by mode C instability, whose flow patterns all appeared as anti-phase synchronization. Through the evolution of flow patterns over time, the three-dimensional effects were already observed at a low Reynolds number of 150 because of the influence of the gap flow and the mutual interference of the wake. Under the transitional spacing for Re = 200, multiple modes were interfering fiercely with each other and appeared as chaotic states. Compared with other bluff body forms, the four square cylinders generated numerous discrepancies and new modal transitions in three-dimensional cases. [ABSTRACT FROM AUTHOR]

Many unequal-diameter risers have been widely applied to satisfy different operational requirements in ocean oil and gas drilling engineering, and adjacent tandem risers can undergo complex vibration responses and even collisions under ocean flow conditions. To investigate the wake interference responses for multiple risers in tandem configuration, an experiment of double and triple unequal-diameter flexible cylinders was conducted under uniform flow with Reynolds numbers Re ranging from 2832 to 11,328 for the larger cylinder and from 1805 to 7220 for the smaller cylinder. The two cylinders were 8 m in length with aspect ratios equal to 282.49 and 443.21, respectively, and the corresponding diameter ratio was 0.64. Two different tandem configurations were arranged with a large cylinder upstream and downstream, and the wall spacing between adjacent cylinders was fixed at 4 D , where D is the diameter of small cylinders. Fiber Bragg grating strain sensors were used to measure both in-line (IL) and cross-flow (CF) strain responses. The modal superposition method was applied to reconstruct displacement responses based on measured strains, and drag forces were further inversely identified from the mean IL displacements. Equivalent wake velocities were calculated from drag force reductions. The results show that the downstream large cylinder in a triple-cylinder system endures a stronger wake interference effect than that in a double-cylinder system. Wake interference can decrease the dominant mode of downstream cylinders and increase the vibration amplitudes due to the reduction in the local wake velocity. In addition, a downstream cylinder undergoing wake-induced vibrations has the same dominant frequency in the CF and IL directions, and its CF frequency is identical to that of the upstream cylinder. • An experiment of double and triple unequal-diameter flexible cylinders in tandem was conducted, with entire models immersed vertically in water. In addition, the large and small cylinders were 8 m in length with aspect ratios equal to 282.49 and 443.21, respectively, and the corresponding diameter ratio was 0.64. • Compared to a double-cylinder system, the downstream large cylinder in a triple-cylinder system endures a stronger wake interference effect from the two upstream small cylinders. • Wake interference can decrease the downstream cylinders' dominant mode orders and increase their vibration amplitudes compared to a single cylinder, which results from the reduction of local wake velocity. • Upstream cylinders undergo classic VIV responses with a frequency ratio f IL /f CF of approximately 2.0, while the downstream cylinders may experience WIV responses with f IL /f CF equal to 1.0. In addition, the strong vortex shedding from an upstream cylinder can cause downstream SCs to vibrate at the same CF frequency as the upstream cylinder. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the hydrodynamic characteristics of a floating cylinder are investigated via forced oscillation experiments in towing tank. The effects of Keulegan–Carpenter number, Reynolds number, reduced velocity and overtopping on hydrodynamics of the floating cylinder in oscillatory and steady flow are studied. The results show a considerably difference of the hydrodynamic characters between the floating and the fully immerged cylinders due to the influences of free surface. The growth of the reduced velocity, a proven notable effect on hydrodynamics, will lead to the increase of added mass coefficient and the decrease of drag coefficient. Meanwhile the overtopping, a particular phenomenon for the floating cylinder, render the added mass coefficients reach up to 3.6 while for the drag coefficient small influences were made. [Copyright &y& Elsevier]

Abstract: In the study described in this paper, the drag and lift forces on a circular cylinder undergoing forced oscillation in the transverse direction near a wall were experimentally investigated in a towing tank at a Reynolds number of 2×105. The first four orders of the components of the oscillating drag and lift forces were calculated. The effects of the gap ratio, oscillation frequency, and oscillation amplitude were studied, with particular emphasis on the influence of the nearby wall on the high-order force components. A rigid cylinder was placed near a flat plate, which was used to simulate the seabed, and was towed forward in the water together with the flat plate. The cylinder was forced into sinusoidal oscillation in the cross-flow direction over a wide range of oscillation frequency and amplitude. The hydrodynamic drag and lift forces were measured by three-dimensional force transducers attached to the ends of the cylinder. [Copyright &y& Elsevier]

With the development of ocean engineering, freely hanging water intake riser (WIR) has aroused widespread interests. The WIR with one free end would be easier to deflect remarkably, along with more complex vortex-induced vibration (VIV) responses and hydrodynamic forces. To elucidate the VIV characteristics of WIR under uniform flow, a series of VIV experiments for a scaled WIR model are carried out. Then a hydrodynamic identification method for VIV along with large offset is developed. The response and hydrodynamic characteristics of the WIR in cross-flow (CF) and in-line (IL) directions are further analyzed. Results indicate that VIV displacements with higher exciting modes are unstable when uniform flow velocity gets large. The IL-to-CF dominant frequency ratio of VIV is found to be 1.0, not 2.0 in conventional VIV knowledge and the IL harmonic at nearly twice the dominant CF frequency is observed more complex for flow velocity greater than 0.3 m/s. The vortex-induced force coefficients are found to be time-varying. Moreover, it is found that within the spectrum of Reynolds number from 2 × 103 to 1.1 × 104, the drag coefficient distribution changes along the WIR model, and the average drag coefficient is in range of 1.0–1.5, which is slightly smaller than the results predicted by the other published empirical formulae. • A series of VIV experiments for a scaled water intake riser model were carried out. • A hydrodynamic identification method for VIV of water intake riser was developed. • The vortex-induced force coefficients of WIR under uniform flow were time-varying. • The drag coefficient is slightly smaller than the results of empirical formulae. [ABSTRACT FROM AUTHOR]

A reliable load-to-response prediction for flexible risers under drag forces can protect the multiple-riser system from collision and optimize its spatial arrangement. To investigate the drag coefficients for double risers in tandem, a series of experiments for two unequal-diameter flexible cylinders in different initial gap spacings was performed under uniform flows with Reynolds numbers up to 11,328. Based on the reconstructed mean drag displacement from measured strains, a distributed drag force amplified by vortex-induced vibrations can be identified reversely. Meanwhile, a modified wake model was developed to estimate the mean wake velocity for the downstream cylinder. With this mean wake velocity, the drag coefficients of the downstream cylinder were obtained. The features of drag coefficients and vortex/wake-induced vibrations were found to be closer to the corresponding features of an isolated cylinder. Furthermore, an empirical drag coefficient model for double unequal-diameter cylinders in tandem was proposed, which accounts for the drag amplification effect by vibrations, tandem spacing, upstream-to-downstream cylinder diameter ratio, mean wake velocity, etc. The results show that this empirical drag coefficient model can obtain the prediction results with relatively higher precision. • A series of tests were performed to study the drag coefficients of double unequal-diameter flexible cylinders in tandem. • A modified wake model was proposed to predict the wake velocity, considering the VIV of the upstream cylinder. • A new drag coefficient prediction model for double unequal-diameter flexible cylinders in tandem was proposed. [ABSTRACT FROM AUTHOR]

• Significant mean downward lift forces in the CF direction caused by the free surface effects are first observed. • An empirical formula for predicting whether "overtopping" occurs is proposed based on the experimental results. • Two distinct features of the mean hydrodynamic coefficients in the non-overtopping and overtopping regions are observed. The hydrodynamic forces on the stationary partially submerged cylinder are investigated through towing test with Reynolds number ranging from 5 × 104 to 9 × 105. Three test groups of partially submerged cylinders with submerged depths of 0.25 D, 0.50 D, and 0.75 D and one validation group of fully submerged cylinders are conducted. During the experiments, the hydrodynamic forces on the cylinders are measured using force sensors. The test results show a considerable difference in the hydrodynamic coefficients for the partially submerged cylinders versus the fully submerged cylinders. A significant mean downward lift force is first observed for the partially submerged cylinders in a steady flow. The maximum of the mean lift coefficients can reach 1.5. Two distinct features are observed due to the effects of overtopping: random distributions in the mean drag coefficients and a clear quadratic relationship between the mean lift coefficients and the Froude number appear in the non-overtopping region. However, the novel phenomenon of a good linear relationship with the Froude number for the mean hydrodynamic coefficients is clearly shown in the overtopping region. In addition, fluctuating hydrodynamic coefficients are further proposed and investigated. These results are helpful to have a better understanding of the problem and to improve related structural designs. [ABSTRACT FROM AUTHOR]

The three-dimensional flow around the four-cylinder and five-cylinder arrays are numerically studied at Re = 3900 by large eddy simulation (LES). By selecting eight spacing ratios (L/D = 2.0–7.0) and three incidence angles (α = 0°–45°), the influence of the center cylinder on the flow characteristics and interference effects of the four-cylinder array is discussed. Different spacing ratios can be divided into three categories with respect to flow regions of the multi-cylinder: small gap flow regime (L/D = 2.0–3.5), critical flow regime (L/D = 3.5–5.0) and large gap flow regime (L/D = 5.0–7.0). For cases with small spacing ratios, the influence of the center cylinder is very notable on the flow characteristics of the four-cylinder array. As the ratio increases, the influence of the center cylinder gradually decreases on the surrounding cylinders in the multi-cylinder array. On the other hand, the direction of the incoming flow also affects the flow characteristics of both four-cylinder and five-cylinder arrays. The development of the shear layer is strongly influenced by the center cylinder for the upstream cylinders at α = 45°. Boundary vortex flux components, flow characteristics and fluid force coefficients change significantly with adding the center cylinder for the four cylinders. • Numerical simulations of the flow around a four-cylinder and a five-cylinder are carried out. • The influence of the center cylinder on the flow characteristics of the multi-cylinder array is analyzed. • The flow field, boundary vortex flux, fluid force coefficients, and PSD are mainly studied for the multi-cylinder array. • The center cylinder has great effect on the flow characteristics of the multi-cylinder array at small spacing ratio. [ABSTRACT FROM AUTHOR]

The cross-flow vortex-induced vibration features of a submerged floating tunnel element, which is composed of two rigidly connected cylinders in a tandem configuration, were investigated via a self-oscillation model test in a steady flow. The Reynolds number ranged from 2 × 104 to 9 × 104, and the ratio of the center-to-center distance between the two cylinders and cylinder diameter varied within a range of 2-4. The vortex induced vibration responses and lift forces on the up- and downstream cylinders were studied under different spacing ratios and compared with those on a single cylinder. The results show that the spacing ratio plays an important role in VIV until the ratio reaches 4. For a small spacing ratio, a significant difference between the lift forces on the up- and downstream cylinders appears and induces a large torsional moment. For the convenience of engineering application, a torsional coefficient was proposed. The maximum torsional coefficient can reach 2.9, 1.2 and 0.98 for spacing ratios of 2, 3 and 4 at the reduced velocity of 5, respectively. Considering the vortex induced vibration responses as well as the torsional moment, a spacing ratio of 3 was recommended for tandem floating tunnel design. • Self-oscillation test of a twin-tube submerged floating tunnel segment model. • Lift forces on stationary and self-oscillation twin-cylinder are compared. • A torsional moment is induced due to the difference of lift force on the cylinders. • Spacing ratio heavily affects the vortex induced response, lift and torsion. [ABSTRACT FROM AUTHOR]