The interference phenomenon among multiple flexible cylinders is an important factor in designing offshore structures, which has been extensively studied by researchers in the past decades. In this paper, Vortex-induced Vibration (VIV) responses of two flexible cylinders experiencing the stepped flow with tandem arrangement are mainly studied using the viv-Thick-SJTU solver, which is developed based on the open source toolbox OpenFOAM and the Bernoulli-Euler bending beam model combining with the thick strip model. Hydrodynamic forces are calculated in each three-dimensional (3D) fluid strip, while vibrations of the cylinder are computed using the Finite Element Method (FEM) through the Newmark -β algrithm. The dynamic mesh technique is adopted to update the computational mesh through the cubic spline interpolation algorithm at every time step. Effects of the gap ratio between two tandem cylinders on vibration characteristic are mainly studied, while gap ratios are set as 4 and 10. Firstly, the validation of the code is conducted. Then, the modal decomposition method and Fast Fourier Transform (FFT) method are used to get the dominant vibration mode and vibration features of both cylinders.

Experimental investigations on the hydrodynamic forces on an intermittently spanning pipeline exposed to steady currents were carried out. The effect of intermittent local spanning sections on the global hydrodynamic behavior was studied by changing the ratio between the non-spanning length (B) and the total length (L), namely the blocking ratio B / L. A range of gap height (G) to diameter (D) ratios, i.e. gap ratio G / D, and 4 different Reynolds numbers (Re) in the subcritical region were tested in the experiments. The results show: i) for a certain gap ratio, the mean drag increases gently with the decreasing blocking ratio at Re = 5.5 × 10 4 , whereas the mean lift decreases significantly with the decreasing blocking ratio at all values of Re tested; and ii) for a certain blocking ratio, increasing the gap ratio leads to an increase in mean drag and decrease in mean lift. Further, simple approaches are proposed based on the present dataset for estimating the global effects on hydrodynamic drag and lift forces due to local spanning geometry.

Flows past an inclined cylinder in the vicinity of a plane boundary are numerically investigated using direct numerical simulations. Parametric studies are carried out at the normal Reynolds number of 500, a fixed gap ratio of 0.8 and five inclination angles (α) ranging from 0° to 60° with an increment of 15°. Two distinct vortex-shedding modes are observed: parallel (α ≤ 15°) and oblique (α ≥ 30°) vortex shedding modes. The occurrence of the oblique vortex shedding is accompanied by the base pressure gradient along the cylinder span and the resultant axial flows near the cylinder’s base. The drag and lift coefficients decrease from the parallel mode to the oblique mode, owing to the intensified three-dimensionality of the wake flows and the phase difference in the vortex-shedding along the span. The Independent Principle (IP) is valid in predicting the hydrodynamic forces and the wake patterns when α ≤ 15°, and IP might produce unacceptable errors when α ≥ 30°. Compared to the mean drag force, the fluctuating lift force is more sensitive to the inclination angle. The IP validity range is substantially smaller than that for flows past a wall-free cylinder.