Scale dependent anisotropy & intermittency in bubble laden turbulent flows

-- Anisotropy
DNS data of disperse bubbly flows in a vertical channel are used to study scale-dependent anisotropy for flows dominated by bubble-induced turbulence. We developed a new method, based on an extension of the barycentric map approach, that allows to quantify and visualize the anisotropy and componentiality of the flow at any scale. Using this we found that the bubbles significantly enhance anisotropy in the flow at all scales compared with the unladen case, and that for some bubble cases, very strong anisotropy persists down to the smallest scales of the flow. The strongest anisotropy observed was for the cases involving small bubbles.

-- intermittency
We experimentally explore the properties of bubble-laden turbulent flow at different scales, considering how the bubbles effect the extreme events of the flow. To this end, high-resolution Particle Shadow Velocimetry measurements are carried out in a bubble column in which the flow generated by a homogeneous distributed bubble swarm rising in water for two different bubble diameters (2.7 mm & 3.9 mm) and moderate gas volume fractions (0.26% - 1.31%). We considered the intermittency quantified by the normalized probability density functions of the longitudinal velocity increments. The results reveal that extreme values in the velocity increments become more probable with decreasing Reynolds number, the opposite of the behavior in single-phase turbulence. We visualize those extreme events and find that regions of intense small-scale velocity increments occur near the turbulent/non-turbulent interface at the boundary of the bubble wake.