The passive control of the flow around a 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 non-reflective 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.
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. Therefore, the study and the control of vortex shedding has a crucial importance in engineering applications like offshore oil industry. In this case, the vortex-induced vibrations (VIV) can affect the risers. As the environmental conditions are given and can not 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 2D and 3D circular cylinders (Wong 1979). Most of them use the active control strategies (Gatulli and Ghanem 2000, Gillies 1998, Zhijinn 2003) that are very difficult to implement in the riser geometry. In fact, such a geometry needs passive devices which don't need additional energy supply in the system. A few 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).
