PhD Defence Duco van Buuren | Experiments in jet turbulence - Single-phase measurements and bubble-particle three-phase flow

Experiments in jet turbulence - Single-phase measurements and bubble-particle three-phase flow

Duco van Buuren is a PhD student in the Department of Physics of Fluids.Promotors are dr. D.J. Krug and prof.dr. D. Lohse from the Faculty of Science & Technology.

How do bubbles and particles interact in turbulent flows? This question is central to many natural and industrial processes, including water treatment and chemical engineering. This thesis addresses this problem through the development of a novel experimental platform and a series of targeted flow measurements.

A random jet array, named TWISTER, was implemented in the Twente Water Tunnel to generate controlled turbulence. By introducing a new jet operating protocol based on gradient noise, the spatial distribution of active jets can be tailored. Experiments show that this distribution strongly governs the resulting turbulence: active jets form clusters that determine the turbulence intensity and energy dissipation rate. These observations are captured by a model that describes the turbulence from the clustered jets as originating from an effective single source. In parallel, a theoretical extension was developed that broadens the applicability of Laser Doppler Velocimetry in high-intensity turbulence.

Building on this controlled environment, bubble–particle interactions were investigated using high-speed imaging. Turbulence was found to reduce bubble rise velocities while increasing relative approach velocities between bubbles and particles. Despite the strong turbulence, the interaction dynamics often resemble a settling-like process driven by gravity. This apparent contradiction is resolved by proposing a local relative settling mechanism, in which buoyancy dominates at small scales while turbulence enhances particle motion.

Together, these results highlight the importance of the multi-scale nature of turbulence and provide new experimental and theoretical tools for studying complex multiphase flows.