The investigation of an unstable convective flow using optical methods

The convective flow field in a vessel is investigated by laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV, namely Particle Tracking Velocimetry-PTV). The vessel is heated from below along a linear element at a temperature higher than that of the fluid. Hot fluid raises up and generates two counterrotating vortices. For a given aspect ratio, the two vortices become unstable and start to oscillate on a vertical plane (orthogonal to the heating element). This regime is investigated for increasing Rayleigh numbers to analyze the transition from regular to irregular conditions. The main transition mechanism is observed to be mostly connected to type II intermittency, a mechanism not frequently observed in experiments. However, at some Rayleigh numbers the present data does not definitely rule out type III intermittency. The phenomenon is analyzed by looking at the main frequencies in the spectrum of the horizontal velocity component and their changes with the Rayleigh number at a point above the heating element. Modifications in the local energy spectrum are analyzed by using the Wavelet Transform (WT) tool. Data obtained by PTV measurements make it possible to point out the spatial configuration of the flow and to determine the two velocity components on the measurement plane. These data are used to clarify the fundamental mechanisms of the transition. Instabilities are observed as sudden changes between two regimes of oscillations of the two counterrotating vortices: the first is characterized by oscillations centered on the vertical axis and the second by nonsymmetrical oscillations. © 1998 American Institute of Physics.