Flow visualizations around a bubble detaching from a particle in turbulent flows

• PIV was used to measure flow field around a detaching bubble from a particle.
• Bubbles were dragged or sheared off the particle.
• Bubbles can detach due to entrainment into a rotating eddy.
• Detachment occurred under the combined effects of eddies of different scales.

The thrust of this paper concerns flow visualizations around a bubble–particle aggregate in oscillating grid turbulence where no net flux is presented. The influences of a turbulent liquid’s motion on the detachment of a bubble from a particle are explored. The methods of analysing the two dimensional velocity fields are compared, including: Reynolds’ decomposition, Galilean decomposition, Gaussian filtering and discrete wavelet transform (DWT). The decomposed velocities that are extracted using the different methods on a snapshot of a velocity field series are illustrated and compared. In terms of the flow structure visualizations, the DWT proved to be advantageous over the other methods for the reasons that no a priori cut-off frequency needed to be defined and the flow structures could be decomposed, scale to scale. The DWT was a sufficient method to decompose the velocity fields and to achieve flow structure visualizations, which is conducive to understanding the mechanisms of bubble–particle detachment in a turbulent field. This method was extended to analyse the instantaneous velocity fields around a bubble detaching from a stainless steel particle and the influences of eddies on the detachment. Under the same operating conditions, different detachment modes were identified from the instantaneous velocity fields around the detaching bubble in the oscillating-grid turbulence, including:
• the sliding of a bubble resulting from the shearing force in the turbulent flow field;
• the stretching of a bubble resulting from the pulling force in the turbulent flow field;
• the entrainment of a bubble into an eddy, so that the bubble followed the movement of the eddy.

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