What is it about?

Spray systems and droplet streams are abound in a wide variety of technical, medical, and industrial applications. In many of these cases, the key to achieving the desired outcome, increase system efficiency or reduce air pollution emissions lies in the precise control of the spray or droplet stream and in a deep understanding of the motion and interaction of the droplets with the ambient medium and nearby droplets. In this work, the coming together of droplets - called grouping process - in monodisperse droplet streams is examined in detail by direct numerical simulation (DNS) using the multiphase code Free Surface 3D.

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Why is it important?

In technical applications of sprays or streams, the motion of droplets is governed by the flow field around them, which, in turn, is affected by the collection of droplets, leading to coupled flow dynamics. This, in combination with the initial conditions (droplet size, velocity, initial separation of the droplets in the stream/spray, ambient conditions, and flow properties), can lead to local formation of droplet clusters/groups under certain conditions. The tendency of initially distant droplets to approach each other and form larger droplets, denoted here as the phenomenon of droplet grouping, can potentially lead to an alteration of droplet size and velocity distribution, drag forces, final settling points, or evaporation rates. Although this phenomenon can significantly alter spray characteristics, either be beneficially or detrimentally depending on the application under consideration, the mechanics and evolution of the flow and droplet dynamics are not yet fully understood. This study contributes to a better understanding of grouping behavior within monodisperse droplet streams by extracting accurate information about the development of inter-droplet distance and velocity, drag forces on each single droplet, and detailed flow characteristics, at moderate Reynolds numbers in the range of Re=100-500.

Perspectives

The motion and interaction of droplets is an interesting topic, which really affects nearly every technical application where multi-phase flows are important or even the influence of ocean sprays on the formation of tropical cyclones (hurricanes). This article shows in detail the complex behavior and various outcome that can arise when droplets are interacting, a fact that is often neglected in literature.

Matthias Ibach
Institute of Aerospace Thermodynamics

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This page is a summary of: Investigation of droplet grouping in monodisperse streams by direct numerical simulations, Physics of Fluids, August 2022, American Institute of Physics, DOI: 10.1063/5.0097551.
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