OSCILLATIONS AND BREAKUP OF A BUBBLE IMMERSED IN A TURBULENT FIELD

This work is an experimental study of the deformation and breakup of a bubble in a turbulent flow. A special facility was designed to obtain intense turbulence without significant mean flow. The experiments wer e performed under microgravity conditions to ensure that turbulence wa s the only cause of bubble deformation. A scalar parameter, characteri stic of this deformation, was obtained by video processing of high-spe ed movies. The time evolution and spectral representation of this scal ar parameter showed the dynamical characteristics of bubble deformatio n. The signatures of the eigenmodes of oscillation predicted by the li near theory were clearly observed and the predominance of the second m ode was proved. In addition, numerical simulations were performed by c omputing the response of a damped oscillator to the measured turbulenc e forcing. Simulations and experiments were found to be in good agreem ent both qualitatively, from visual inspections of the signals, and qu antitatively, from a statistical analysis. The role of bubble dynamics in the deformation process has been clarified. On the one hand, the t ime response of the bubble controls the maximum amount of energy which can be extracted from each turbulent eddy. On the other hand, the vis cous damping limits the energy that the bubble can accumulate during i ts fluctuating deformation. Moreover, two breakup mechanisms have been identified. One mechanism results from the balance between two opposi ng dominant forces, and the other from a resonance oscillation. A new parameter, the mean efficiency coefficient, has been introduced to tak e into account the various aspects of bubble dynamics. Used together w ith the Weber number, this parameter allows the prediction of the occu rrence of these two mechanisms. Finally, the influence of the residenc e time of the bubble on the statistics of the deformation has been ana lysed and quantified.