HAM Solutions for Vortex-Induced Vibration of a Circular Cylinder

The vortex-induced vibration (VIV) phenomenon is result of fluid-structure interaction which occurs in many engineering fields. The study of VIV of a circular cylinder is of practical importance (such as in marine cables and flexible risers in petroleum production). In this paper, one classical phenomenological VIV model — the motion of the cylinder is modeled by a simple linear equation, and the fluctuating nature of the vortex wake oscillation is modeled by a van der Pol oscillator, is analyzed. Firstly, the homotopy analysis method (HAM), a powerful technique for highly nonlinear problems, is developed to solve the coupled fluid-structure dynamical system with the convergence of the homotopy series solutions being demonstrated. Based on the HAM solutions, some properties of the fully nonlinear classical coupled VIV model are presented. All the results proved that the proposed HAM scheme has potential to be an effective analytic technique to study the VIV problems.

Flow Past a Sphere With Surface Blowing and Suction

The effect of uniform surface blowing and suction on the wake dynamics and the drag and lift forces on a sphere is studied using a high-resolution direct numerical simulation technique. The sphere Reynolds number Re, based on its diameter and the freestream velocity, is in the range 1–300. The onset of recirculation in the sphere wake occurs at higher Re, and the transition to nonaxisymmetry and unsteadiness occurs at lower Re in the presence of blowing. The size of the recirculation region increases with blowing, but it nearly disappears in the case of suction. Wake oscillation also increases in the presence of blowing. The drag coefficient in the presence of blowing is reduced compared to that in uniform flow, in the range 10<Re<250, whereas it is increased in the presence of suction. The reduction in the wake pressure minimum associated with the enhanced vortical structures is the primary cause for drag reduction in the case of blowing. In the case of suction, it is the increased surface vorticity associated with the reduction of the boundary layer that results into increased drag. The fluctuations in the instantaneous lift and drag coefficients are significant for blowing, and they result from the asymmetric movement of the wake pressure minimum associated with the shedding process.

Experimental Study on the Stability of the Flow Behind an Accelerating Circular Cylinder

Disturbance growth in the wake of a circular cylinder moving at a constant acceleration is examined experimentally. The cylinder is installed on a carriage moving in the still air. The results show that the critical Reynolds number for the onset of the global instability leading to a self-sustained wake oscillation increases with the magnitude of acceleration, while the Strouhal number of the growing disturbance at the critical Reynolds number is not strongly dependent on the magnitude of acceleration. It is also found that with increasing the acceleration, the Ka´rma´n vortex street remains two-dimensional even at the Reynolds numbers around 200 where the three-dimensional instability occurs to lead to the vortex dislocation in the case of cylinder moving at constant velocity or in the case of cylinder wake in the steady oncoming flow.