Applied Mathematics Colloquium – Tian Ma, Helmholtz-Zentrum Dresden–Rossendorf

Title: Bubble clusters and particle-pair dispersion in bubble-laden flows

Speaker: Tian Ma, Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden–Rossendorf (HZDR), Dresden, Germany

Abstract:

Bubble clusters

We use experiments to study the evolution of bubble clusters in a swarm of freely rising,
deformable bubbles. A new machine learning-aided algorithm allows us to identify and track
bubbles in clusters and measure the cluster lifetimes. The results indicate that contamination in
the carrier liquid can enhance the formation of bubble clusters and prolong the cluster lifetimes.
The mean bubble rise velocities conditioned on the bubble cluster size are also explored, and
we find a positive correlation between the cluster size and the rise speed of the bubbles in the
cluster, with clustered bubbles rising up to 20% faster than unclustered bubbles.

Particle-pair dispersion

Bubble-induced turbulence (BIT) plays an important role in mixing, transport and collision of
small particles in many natural and industrial applications. A crucial component of the
Lagrangian description of BIT is the relative pair dispersion, based on the relative separation
of pairs of fluid particles. In the present work, we report the first experimental study of tracer particle-pair dispersion
in BIT for varying initial separations. To this end, 3D Lagrangian Particle Tracking
measurements are carried out in an octagonal bubble column, where the flow is generated by a
homogeneously distributed bubble swarm rising in water. Our results show that the relative
dispersion follows the classical ballistic regime at short times and is almost independent of the
initial separation 𝑟0, when 𝑟0 = 𝑂(10𝑑𝑏), with 𝑑𝑏 the bubble diameter. In order to fully
characterize the particle-pair dispersion in BIT, we also measure the anisotropy of the
dispersion using the barycentric map and show that it increases with increasing initial separation.

Biosketch: Tian Ma is a research group leader at Helmholtz-Zentrum Dresden – Rossendorf (HZDR) in
Germany, working on bubble-turbulence interaction. He received his diploma degree in
engineering from Universität Duisburg-Essen in Germany in 2011, and his Ph.D. from
Technische Universität Dresden in Germany in 2017. His group’s research covers three areas:
turbulence modeling and multiscale properties of bubble-induced turbulence, bubble clustering
and dispersion in turbulence, and novel measurement techniques for both liquid and gas
phases. Ma is the recipient of a Junior Research Group Grant from the HZDR and a Research
Fellowship from German Research Foundation.