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Of Rainfall and Marine Snow: How Small-Scale Physical Interactions have Large-Scale Consequences
Environmental flows contain many constituents other than the flowing fluid phase itself. The atmosphere sustains clouds, consisting of tiny drops of liquid water and/or miniscule ice crystals. Additionally, the winds transport aerosols, dust and pollen. In oceanic environments currents transport huge quantities of sediment and plankton. A common theme is that the dispersed phase is tiny in comparison with the smallest flow length scales, but interactions of the dispersed phase can have large-scale consequences: examples include the formation of rainfall or the clustering of organic material to form marine snow.
Given that these topics all deal with the turbulent transport and mutual interactions of tiny particulate matter, it is natural to bring together a multidisciplinary audience active in these fields. The problems described here are investigated on vastly different scales (from micrometers to thousands of kilometers) and using different approaches (experimental, theoretical, numerical). We wanted to bring these communities in contact to learn from each other and to foster interdisciplinary cooperations. This includes the formulation of common research goals.
During the workshop there were ten keynote lectures, which aimed at providing insight into capabilities, diagnostics and insights of that particular field as well as provoking discussions. The PhD students involved in the FOM research program “Droplets in Turbulent Flow” were prominently featured as well, to present the contributions to the field in the Netherlands. There were several discussion sessions, in which new research goals were defined. Additionally, the time schedule allowed for plenty of individual discussions amongst the participants.
During the discussion sessions some perspectives were set forth. Experimental flow control and diagnostics have improved enough to perform measurements in realistic parameter ranges. Numerical methods and computer power have reached a level to allow for a next step away from the idealized point-particle models to finite-sized representations. There are new possibilities to include additional physical effects in both experiments and simulations, like charged particles and evaporation/condensation. In all fields the tendency is to move beyond the paradigm of homogeneous isotropic turbulence towards more realistic inhomogeneous systems. While the individual fields and scales are not yet overlapping, they are surely approaching. The most pressing need appears to be for an experiment to validate collision rates of droplets or particles in turbulence.
We wish to thank the Lorentz Center for the outstanding organizational support, both before and during the workshop. We acknowledge the financial support for this workshop from Stichting FOM and the J.M. Burgerscentrum.