Observations on the Early Transition Process in a Closed Cylindrical Container with Rotating Bottom

Abstract

In recent years the flow generated by a rotating end wall inside a closed cylindrical container has become a new prototype flow to study the fundamental transition mechanisms in a confined dynamical flow system. Similar to the Taylor-Couette flow a large number of different steady and unsteady flow patterns can be observed. Experiments of Escudier (1984) and numerical simulations of Lugt & Abboud (1987), Lopez (1990) among others, illustrated the existence of multicellular flows, including flows with steady central recirculation bubbles very similar to vortex breakdown. More recently Lopez & Perry (1992) and Sørensen & Christensen (1995) examined in more detail how the bubbles become unsteady. According to both numerical studies periodic axial oscillations of the central recirculation bubbles mark the onset of the unsteady flow regime. However, the nature of this transition is not yet clear. Tsitverblit (1993) and Christensen et al. (1993) found unstable stationary solutions which coexist with stable unsteady solutions, indicating the existence of a supercritical Hopf bifurcation. In contrast to this Lopez & Perry (1992) found no such unstable steady solutions and, therefore, excluded the transition mechanism via a supercritical Hopf bifurcation. Experimental observations on the transition to unsteady flow remain scarce and are mainly based on the description of the bubble behavior with varying flow parameters. In this paper we present some new experimental observations which illustrate the transition process for different flow configurations with and without bubbles.