摘要：

We report some part of the work done by the teams "Wakes and dynamics of vortex" at Marseille. The wake of a sphere rigidly held has been investigated. The results of experiments and of numerical simulations have been successfully compared. When increasing the Reynolds number, different bifurcations occur : first the symmetry of the wake around the streamwise direction is broken but the flow is steady (210 < Re < 270), then the flow becomes unsteady and vortex loops are shed [1]. The orientation of the wake is linked to a small upstream velocity gradient which allows to select it. The transition from the configuration of a cylinder to a sphere is possible through different ways of control such the transverse interaction of two spheres, the spanwise sinuous variation of a cylinder or the variation of the aspect-ratio of a circular cylinder from value close or larger than one.The dynamics of a tethered sphere in a flow parallel to the fixation with natural frequency has been studied in a vertical water channel. The two parameters of this problem are the reduced mass of the sphere (ratio of the density of the solid to that of the fluid) and the reduced velocity built on the upstream velocity, the diameter of the sphere and the natural frequency of this pendulum. The sphere exhibits oscillation at the natural frequency in the two perpendicular directions. The trajectory is elliptic or circular contrary to the case when the gravity is perpendicular to the flow and oscillations are in line [2].The triangular and elliptic instabilities in rotating cylinders have been studied by Eloy et al [3]. The elliptical instability of a spheroid is now the subject of experimental and analytical studies done by Lacaze et al. [4] and also reported in a presentation of the 5 Euromech of Toulouse.

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