Numerical simulations of vortex-induced vibrations on vertical cylindrical structure with different aspect ratios

This paper presents a computational fluid dynamics (CFD) study of vortex-induced vibration (VIV) for different aspect ratio (L/D) cylinder. Of particular interest was to measure hydrodynamic forces and numerically investigate the wake behaviour of VIV while varying the aspect ratio. The simulation models represented the actual experimental conditions with idealised free-surface boundary condition to capture the responses from fluid-structure interaction phenomenon. The simulations were performed in the subcritical flow region (7.4×103 < Re < 2×105), corresponding to a range of reduced velocity (Ur) from 2 to 14. The results of the cases studied were discussed and compared with the experimental data to verify the accuracy and validity of the present simulation. The comparisons have shown a similar curved-shape drag coefficient plot, and however underestimated the value of the drag coefficients over the reduced velocity. Additionally, the simulations seemed to capture a higher lift force response compared with the experimental data for a low aspect ratio. The correlation length was observed to be longer for larger aspect ratio and proportionally decreases as the aspect ratio decreases

Hydrodynamic interaction of two neutrally-buoyant smooth spheres suspended in plane Poiseuille flow: the BEM simulations versus the MoR approximations

The hydrodynamic interaction between two particles suspended in shear flows is fundamental to the macroscopic characterization of suspension flows. Although such interaction in quiescent or linear shear flow is well understood, studies on that in a nonlinear shear field are rare. In this study, the hydrodynamic interaction between two neutrally-buoyant smooth spheres moving at negligible Reynolds numbers in an unbounded plane Poiseuille flow has been calculated by three-dimensional boundary element method (BEM) simulations. The BEM results have been compared with the analytical results obtained with the method-of-reflection (MoR) approximations. The BEM simulations have been found to provide satisfactory predictions if the number of elements on the spheres are more than 200, whereas the MoR approximations provide satisfactory predictions only when the minimum separation\ud between the spheres is relatively large although this\ud MoR method has the advantage to easily calculate the\ud hydrodynamic interaction between two spheres freely moving at negligible Reynolds numbers in unbounded quadratic flow by solving ordinary differential equations. Furthermore, it is found that there is a preferential cross-streamline migration of the center-of-gravity of the sphere-pair in the plane of shear in plane Poiseuille flow which does not arise in simple shear flow. This migration is always directed towards low shear regions when the sphere having larger translational velocity approaches the other sphere, and reverses towards high shear regions when the faster sphere leads the other sphere in the plane of shear. There is also a cross-streamline migration of the center-of-gravity of the sphere-pair in the plane of vorticity, but this migration does not have a preferential direction. These migrations are symmetric about the point where the spheres are at the minimum separation, and are only significant when the hydrodynamic interaction of the spheres is strong. These results show that the migration of the center-of-gravity of the sphere-pair can be attributed to the nonlinearity of the shear field, which agrees with the MoR approximations. The hydrodynamic interaction between the two spheres has been quantified under various conditions by the BEM simulations for both identical and disparate spheres

Boundary-element simulation of hydrodynamic interaction of two smooth spheres suspended in an unbounded Couette flow

The hydrodynamic interaction between two particles, suspended in shear flows, is fundamental to the macroscopic characterization of suspension flows. Although the understanding of the hydrodynamic interaction between two particles suspended in a quiescent or linear shear flow is mature, studies of the interaction in a non-linear shear field are rare. The current study calculates such\ud interactions between two neutrally-buoyant smooth spheres moving at negligible Reynolds numbers in an unbounded wide-gap Couette flow by three-dimensional boundary-element method (BEM) simulations. Both the identical sphere-pair and the disparate sphere-pair are considered. The numerical\ud results show that there is a preferential cross-streamline migration of the center-of-gravity of the sphere-pair in the plane of shear in the unbounded wide-gap Couette flow that does not arise in simple shear-flow. This migration is always directed towards low-shear regions when the sphere having the larger translational velocity approaches the other sphere, and reverses towards high-shear regions when the faster sphere leads the other sphere in the plane of shear. There is also a crossstreamline migration of the center-of-gravity of the sphere-pair in the plane of vorticity, but this migration does not have a preferential direction. These migrations are symmetric about the point\ud where the spheres are at the minimum separation, and are only significant when the hydrodynamic interaction of the spheres is sufficiently strong. These results show that the migration of the center-of-gravity of the sphere-pair can be attributed to the non-linearity of the shear field. The hydrodynamic interaction between the two spheres has been quantified under various conditions by the BEM simulations for both identical and disparate spheres

Boundary-element method simulation of the impact of bounding walls on the dynamics of a particle group freely moving in a wide-gap Couette flow

In this study, the impact of the bounding walls on the dynamics of a group of neutrally-buoyant identical rigid spheres freely moving at negligible Reynolds numbers in a wide-gap Couette flow, which is important for understanding the particle migrations presented in concentrated suspensions subjected to inhomogeneous shear flows, is simulated by a three-dimensional boundary-element method\ud (BEM) code. The results show that the particle interactions very close to the bounding walls cause the particle group to migrate away from the walls. As the distance of the bounding walls from the group increases, the migration changes direction and the group then move towards the walls. As this distance continues to increase, the migration of the group decreases and beyond a specific distance from the bounding walls the migration of the group is negligible. In addition, the BEM simulations show that\ud the extent and rate of the migration of the group increase as the inter-particle distance decreases

Boundary-element simulation of hydrodynamic interaction of two smooth spheres suspended in an unbounded Couette flow

The hydrodynamic interaction between two particles, suspended in shear flows, is fundamental to the macroscopic characterization of suspension flows. Although the understanding of the hydrodynamic interaction between two particles suspended in a quiescent or linear shear flow is mature, studies of the interaction in a non-linear shear field are rare. The current study calculates such interactions between two neutrally-buoyant smooth spheres moving at negligible Reynolds numbers in an unbounded wide-gap Couette flow by three-dimensional boundary-element method (BEM) simulations. Both the identical sphere-pair and the disparate sphere-pair are considered. The numerical results show that there is a preferential cross-streamline migration of the center-of-gravity of the sphere-pair in the plane of shear in the unbounded wide-gap Couette flow that does not arise in simple shear-flow. This migration is always directed towards low-shear regions when the sphere having the larger translational velocity approaches the other sphere, and reverses towards high-shear regions when the faster sphere leads the other sphere in the plane of shear. There is also a cross-streamline migration of the center-of-gravity of the sphere-pair in the plane of vorticity, but this migration does not have a preferential direction. These migrations are symmetric about the point where the spheres are at the minimum separation, and are only significant when the hydrodynamic interaction of the spheres is sufficiently strong. These results show that the migration of the center-of-gravity of the sphere-pair can be attributed to the non-linearity of the shear field. The hydrodynamic interaction between the two spheres has been quantified under various conditions by the BEM simulations for both identical and disparate spheres.Hydrodynamic interaction Boundary-element method Wide-gap Couette flow Non-linear shear field Particle migration