J. zyxwvutsrq Fluid Mech. zyxwvutsrq (1993), vol. 254, pp. 363-374 Copyright zyxwvutsr 0 1993 Cambridge University Press 363 Observations on transition in plane bubble plumes By M. ALAM AND V. H. ARAKERI Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India (Received 31 January zyxwvu 1992 and zyxwv in revised form 24 March 1993) Flow visualization studies of plane laminar bubble plumes have been conducted to yield quantitative data on transition height, wavelength and wave velocity of the most unstable disturbance leading to transition. These are believed to be the first results of this kind. Most earlier studies are restricted to turbulent bubble plumes. In the present study, the bubble plumes were generated by electrolysis of water and hence very fine control over bubble size distribution and gas flow rate was possible to enable studies with laminar bubble plumes. Present observations show that (a) the dominant mode of instability in plane bubble plumes is the sinuous mode, zyxw (b) transition height and wavelength are related linearly with the proportionality constant being about 4, zy (c) wave velocity is about 40 zyxwvu % of the mean plume velocity, and (d) normalized transition height data correlate very well with a source Grashof number. Some agreement and some differences in transition characteristics of bubble plumes have been observed compared to those for similar single-phase flows. 1. Introduction When gas bubbles are released in the interior of an otherwise stagnant liquid, they rise owing to buoyancy entraining the surrounding liquid to form what can be termed as a ‘bubble plume’. Large-scale bubble plumes have found some applications like bubble breakwaters as originally suggested by Taylor (1955); they have also been used to prevent parts of the surface of a river or lake from freezing over (Baines 1961) and in containment of oil slicks (Jones 1972). In addition, a desire to understand the hydrodynamics of undersea blowouts has stimulated some recent studies on bubble plumes. A comprehensive review of the state of the art in understanding and predicting the mean flow characteristics of turbulent large-scale round bubble plumes has been provided by Milgram (1983). Typically, large-scale bubble plumes of the type studied by Milgram involve heights which are in the range of few metres and gas flow rates which are fractions of a cubic metre per second. At the other end of the spectrum are small-scale bubble plumes where heights are in the range of a few centimeters and gas flow rates are a few centimeters cubed per second or smaller. These find application in chemical and metallurgical processing industries (Leitch & Baines 1989). If the bubble sizes involved are a few millimeters, even the weak bubble plumes of the type investigated by Leitch & Baines turn out to be turbulent. In fact, there are hardly any studies involving laminar and transitional bubble plumes. It is felt that studies in this direction could provide a data base for comparison with modelling of two-phase flow equations where there are still some unsettled questions regarding the handling of the dispersed phase (van Wijngaarden 1972; Ishii 1975). These questions can perhaps be investigated in a more meaningful manner by studying laminar or transitional bubble plumes than their turbulent counterparts where there are additional complexities. These considerations have