Experimental study of transition to turbulence of a round thermal plume byultrasound scattering

In this study, we carried out the characterization of the transition to tur bulence of a thermal pure plume by using ultrasound scattering. For this, t he position, amplitude and broadening of the scattering peak are analyzed. The technique is based upon the scattering of an ultrasound wave coupling w ith an unstable how. The coupling between the acoustic mode with both vorti city and entropy modes is derived from non-linear terms of Navier-Stokes an d energy equations. When the scattering mechanism occurs, the characteristi c length scale of the flow structure under observation is comparable with t he wavelength of incoming sound. Thus, the flow can be probed at different length scales by only changing the frequency of incoming sound. The thermal plume rises from a heated disk immersed into a quiescent medium and can re ach transition and fully turbulent regimes. Criteria allowing the identific ation of both the beginning and the end of transition are derived from the results. The characteristics of the scattering process show evidence that a llows us to discern the beginning of transition. The analysis of the amplit ude of the scattering peak revealed a homogeneous behavior and led us to th ink of a possible principle of similarity. The evolution of both thermal an d velocity fluctuations has made it possible to establish the limits of bot h the beginning and the end of transition, in terms of local Grashof number Gr(z) and position of the measurement zone z/D. The limits for transition reported in this work are comparable in its magnitude order with those of t he literature. It was verified that thermal and velocity transition are phe nomena that begin and finish almost simultaneously. (C) 2000 Elsevier Scien ce Inc. All rights reserved.