PURPOSE AND FOCUS:

The production of random fragments (or particles) by the continuous breakup of clusters is known as fragmentation - a new domain in the modern physics. In fluid mechanics, the fragmentation is often a key-process, as for example the liquid jet fragmentation in the internal combustion engines, where such a process (often referred to as atomization) controls the efficiency of combustion – if the injected liquid bulk is not well-atomized, the part of it is withdrawn from the combustion chamber. Another important example of the liquid bulk fragmentation concerns the airborne disease transmission. Sneezes or coughs produce the saliva droplets, and the size distribution of those droplets is an extremely important factor – the large droplets settle on the ground before significant evaporation, while the small droplets remain airborne on significant distances. Also as an example, we may address stratocumulus clouds, in which grazing collisions between droplets and the turbulent shear of drops may result in the production of typical rain droplets. As to fragmentation of bubbles, the examples include cavitation and boiling processes, as well as different flows with microscopic bubbles in medical and biological applications.

In all mentioned situations, the general objective is first, to understand the essential interfacial phenomena leading to fragmentation and second, to characterize the fragmentation itself. Among different interfacial mechanisms of fragmentation, the two-way interactions between the interface and the turbulence in the immiscible fluids is of our particular interest. How the turbulent structures may induce two-phase interactions, and what may be the signature of the interface properties on those flow structures – this is one of the questions to be addressed. Also, there are questions of practical relevance, such as how the turbulent structures may induce the bubbles generation in cavitation? how these structures may control the air-blast or the pressure-assisted atomizing spray? in boiling process, how to characterize the link between the heat exchange intensity, the bubbles production and the corrosion? Although the very representative conferences on liquid atomization and sprays are organized annually (ILASS), or each two years (ICLASS), the idea of organizing the relatively small workshops, with the target on only few specific problems, seems to be useful. This motivated the organization of the SIG FAS. There is another interest in this SIG. The numerical simulation of turbulent flows with free interface is a very difficult task – too many length-scales are involved, from micro-scales characterizing the interface and the breakup to large scales of turbulent fluid dynamics. The combination of direct numerical front-tracking methods (such as Volume of Fluid or Level-Set) with LES approaches and stochastic modeling of the interface on residual scales is a way to account for variety of scales. By bringing together physicists and CFD scientists, the SIG FAS may help the development of such hybrid approaches.

The following objectives are on the target:

• Organization of workshops and discussions on specific aspects of fragmentation and sprays, such as:

- LES and alternative numerical approaches for flow simulation with the interface

- Spray meshless models; kinetic and stochastic approaches

- Sprays emitted upon violent exhalations

- Turbulence, Cavitation, Boiling

• Organization of summer/winter schools “Turbulence & Interface” in the close collaboration with GDR “Transfert & Interface” and GDR “Navier-Stokes 2.00
• Creation of experimental data bases for atomizing sprays and comparison with simulations
• Collaboration with ILASS-Europe (Institute for Liquid Atomization and Spray Systems)

ACTIVITIES OF THE SIG FAS

ERCOFTAC Workshop “Turbulence and Interface”, 15th - 17th June 2022, Ecole Centrale in Lyon, France https://www.ercoftac.org/events/turbulence-and-interface/