On bubble clustering and energy spectra in pseudo-turbulence

TL;DR

This study investigates bubble clustering, velocities, and energy spectra in pseudo-turbulence driven solely by rising bubbles, revealing vertical clustering, non-Gaussian velocity PDFs, and an energy spectrum decay close to theoretical predictions.

Contribution

It provides detailed measurements of bubble clustering, velocity distributions, and energy spectra in pseudo-turbulence, highlighting differences from classical turbulence.

Findings

Bubbles cluster vertically within a few radii.

Velocity PDFs are non-Gaussian with intermittency.

Energy spectrum decays with a -3.2 power law.

Abstract

3D-Particle Tracking (3D-PTV) and Phase Sensitive Constant Temperature Anemometry in pseudo-turbulence--i.e., flow solely driven by rising bubbles-- were performed to investigate bubble clustering and to obtain the mean bubble rise velocity, distributions of bubble velocities, and energy spectra at dilute gas concentrations ($\alpha \leq2.2$%). To characterize the clustering the pair correlation function $G(r,\theta)$ was calculated. The deformable bubbles with equivalent bubble diameter $d_b=4-5$ mm were found to cluster within a radial distance of a few bubble radii with a preferred vertical orientation. This vertical alignment was present at both small and large scales. For small distances also some horizontal clustering was found. The large number of data-points and the non intrusiveness of PTV allowed to obtain well-converged Probability Density Functions (PDFs) of the bubble velocity. The PDFs had a non-Gaussian form for all velocity components and intermittency effects could be observed. The energy spectrum of the liquid velocity fluctuations decayed with a power law of -3.2, different from the $\approx -5/3$ found for homogeneous isotropic turbulence, but close to the prediction -3 by \cite{lance} for pseudo-turbulence.