An analytical approach to the rise velocity of periodic bubble trains in non-Newtonian fluids

Turbulent Bubble-Laden Channel Flow of Power-Law Fluids: A Direct Numerical Simulation Study

The influence of non-Newtonian fluid behavior on the flow statistics of turbulent bubble-laden downflow in a vertical channel is investigated. A Direct Numerical Simulation (DNS) study is conducted for power-law fluids with power-law indexes of 0.7 (shear-thinning), 1 (Newtonian) and 1.3 (shear-thickening) in the liquid phase at a gas volume fraction of 6%. The flow is driven downward by a constant volumetric flow rate corresponding to a friction Reynolds number of Reτ≈127.3. The Eötvös number is varied between Eo=0.3125 and Eo=3.75 in order to investigate the influence of quasi-spherical as well as wobbling bubbles and thus the interplay of the bubble deformability with the power-law behavior of the liquid bulk. The resulting first- and second-order fluid statistics, i.e., the gas fraction, mean velocity and velocity fluctuation profiles across the channel, show clear trends in reply to varying power-law indexes. In addition, it was observed that the bubble oscillations increase with decreasing power-law index. In the channel core, the bubbles significantly increase the dissipation rate, which, in contrast to its behavior at the wall, shows similar orders of magnitude for all power-law indexes.

Venting dynamics of an immersed granular layer

Air is injected locally at the base of an immersed granular bed. The gas, which is forced to flow gently through the material, creates several paths between the grains. We observe that the latter gas venting results in the emission of bubbles in a localized region at the free surface. Additional experiments, performed in two dimensions, permit a direct visualization of the paths, and a theoretical approach shows that the typical size of the region at the free surface can be accounted for by a diffusionlike process. The diffusion coefficient is expressed as a function of the system parameters.

Acoustic waveform of continuous bubbling in a non-Newtonian fluid

We study experimentally the acoustic signal associated with a continuous bubble bursting at the free surface of a non-Newtonian fluid. Due to the fluid rheological properties, the bubble shape is elongated, and, when bursting at the free surface, acts as a resonator. For a given fluid concentration, at constant flow rate, repetitive bubble bursting occurs at the surface. We report a modulation pattern of the acoustic waveform through time. Moreover, we point out the existence of a precursor acoustic signal, recorded on the microphone array, previous to each bursting. The time delay between this precursor and the bursting signal is well correlated with the bursting signal frequency content. Their joint modulation through time is driven by the fluid rheology, which strongly depends on the presence of small satellite bubbles trapped in the fluid due to the yield stress.

Aggregation behavior of two spheres falling through an aging fluid

We study the behavior of two spheres that settle along their line of center in a yield stress fluid that "ages:" a Laponite suspension. In such a fluid, the fluid flow behind a falling single particle can either exhibit a negative wake, i.e., an upward motion, or not, according to the stress exerted by the particle on the fluid. We show that, if their initial separation distance is smaller than 15 radii, two identical particles cluster whatever the wake's structure. In addition, in the conditions within which a negative wake is observed, we evidence an unexpected lateral motion of the spheres.