Lionel Schouveiler

STABILITY OF A TRAVELING ROLL SYSTEM IN A ROTATING DISK FLOW

The stability of a traveling roll system, which results from the development of a primary instability of the flow between a stationary and a rotating disk, is experimentally studied. The characteristics of this traveling pattern and of the bifurcation from which it results are obtained. We show in particular that the band of the stable roll modes is limited by the Eckhaus secondary instability.

Hysteretic mode exchange in the wake of two circular cylinders in tandem

Our experimental study is devoted to the analysis of the flow past two tandem circular cylinders near the vortex shedding threshold. A recent bidimensional numerical analysis of this flow [Mizushima and Suehiro, Phys. Fluids 17, 104107 (2005)] has predicted that the bifurcation diagram should become complex in the vicinity of the instability threshold. Subcritical and saddle node bifurcations that lead to hysteretic exchanges between two different modes of vortex shedding were detected for particular distances of separation of the cylinders. We present here visualizations and velocity measurements of this flow in a water channel that prove the robustness of the complexity of the bifurcation diagram in real flows.

Flow-induced vibrations of high mass ratio flexible filaments freely hanging in a flow

The behavior of high mass ratio flexible filaments freely hanging in steady horizontal uniform flows is experimentally and theoretically investigated. When the flow velocity is small, static equilibrium states, where the filaments are inclined to the flow, are observed. Then, above a critical value of the wind velocity, the filaments exhibit periodic oscillations in the vertical plane. The problem is theoretically addressed considering the beam theory equations for the filament dynamics where the action of the flowing fluid is modeled using semiempirical expressions. These equations are first solved for the stationary equilibrium states. Then, the stability of these steady solutions relative to small perturbations is analyzed. A good agreement between experimental and theoretical results is found.

Experiments on the transition to turbulence of the flow between a stationary and a rotating disk

The goal of this experimental study is to describe the transition to turbulence of the flow between a rotating and a stationary disk. Our experimental apparatus consists of a rotating disk set in a cylindrical housing full of water. Visualizations that use a small amount of anisotropic reflective particles permit the description of the different instabilities which occur in the flow. Circular and spiral waves are observed when the layer of water is larger than the boundary layers of the disks (Batchelor velocity profiles). At higher rotation speed, turbulence arises by a complex mixing of these waves. On the contrary, when the boundary layers are merged, the basic flow is close to a torsionnal Couette flow. In this case, isolated spots or solitary waves are visualized. When the Reynolds number is further increased, turbulence occurs in this case by the progressive invasion of the whole flow by these structures. Finally, these observations are compiled in a transition diagram which gives for the first time a global and detailed view of the transition to turbulence of the flow between a rotating and a stationary disk.