Passive control of the flow around a fixed circular cylinder is achieved using a porous layer between the obstacle and the fluid. The various media are easily handled by means of the penalization method. The computational domain is reduced to a close neighbourhood of the body thanks to efficient nonreflective boundary conditions. The porous layer changes the vortex shedding and induces a strong reduction of the vorticity magnitude and of the root mean-square lift coefficient.

INTRODUCTION

In the vicinity of bluff bodies, the shedding of vortices can induce unsteady forces of small amplitude with excitation close to a structural resonant frequency that provoke structural failures (Williamson and Govardhan, 2004). Therefore, the study and the control of vortex shedding have a crucial importance in engineering applications such as the offshore oil industry. In this case, the vortex-induced vibrations (VIV) can affect the risers. As the environmental conditions are a given and cannot be changed, the only way to reduce the VIV is to use an efficient control technique adapted to the riser framework. Several control methods are already proposed to reduce the drag and lift forces, or to regularize the vortex shedding around the 2-dimensional and 3-dimensional circular cylinders (Wong, 1979; Williamson and Govardhan, 2004). Most of them use active control strategies (Gatulli and Ghanem, 2000; Gillies, 1998; Zhijinn, 2003) that are Very difficult to implement in riser geometry. In fact, such a geometry needs passive devices which don't need additional energy supply in the system. Some fruitful research has been already performed in such a case. For example, some authors have added dimples (Bearman and Harvey, 1993) or splitter plates (Kwon and Choi, 1996) to control and regularize the flow around a circular cylinder. In some other cases, the control technique is performed using a secondary small cylinder (Mittal and Raghuvanshi, 2001), or an appropriately distributed electromagnetic field (Posdziech and Grundmann, 2001).

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