Modulation of turbulent flow field in an oscillating grid system owing to single bubble rise

Abstract

Modulation of turbulent flow in an oscillating grid system due to controlled release of a bubble train was experimentally investigated using particle image velocimetry (PIV) technique wherein flow field modulation was reported in single bubble resolved manner. The bubble diameter was varied in the range ∼2.70 to 3.52 mm (∼26 to 34 times the single-phase Kolmogorov scale, and ∼1.05 to 1.35 times the single-phase integral length scale). The two-dimensional (2D) instantaneous velocity fields were obtained for both single–phase and bubble train cases at grid Reynolds numbers (Reg) ranging from 1080 to 10,800. The modulation of single-phase turbulence due to bubble was analysed based on the turbulence intensity, isotropy ratio (IR), length scales, specific energy dissipation rates and energy spectra. The single-phase turbulence fluctuating velocity increased ∼5–76% in the inertial subrange region in the presence of bubbles. Presence of bubbles also led to enhancement in the flow field isotropy ratio due to upward acting buoyancy force. It was noted that at high Reg (6480–10800), the IR value of the flow was found to be more dependent on the grid Reynolds number compared with bubble diameter. The integral length scale of the single-phase flow decreased following a power law dependency over the Reg. It was found that the specific energy dissipation rate of single–phase flow increased with an increase in bubble diameter. The energy spectra exhibited a slope less steep than −5/3 which indicates the additional turbulence production by the bubbles in the inertial subrange region. Energy transfer augmentation from large scale to small scale due to bubble was explianed by the dissipative spectrum which showed reduction of the energy in the small scale and an enhancement of the energy in both large scale and inertial subrange.