ORALS

Drop size distribution of aerosols controls the efficiency of crucially important environmental processes, e.g. transfer fluxes greenhouse gases at air-sea interface, and industrial processes, e.g. the energetic and environmental efficiency of energy production from liquid fuels. In these processes, all the relevant momentum, heat and mass transfer fluxes occur across the tiny interfaces, separating the drops by the carrier fluid. Interfaces are an inherently hard to define non-place! The accurate determination of interface position, shape and interaction with the fluid turbulence, however, is crucial to predict the overall behavior of the involved macroscopic physical phenomena. Although considerable observational and theoretical attention is being focused on this topic, a clear and comprehensive picture on formation and properties of the dispersed phases is not currently available yet. Our effort is to provide a general theoretical framework to describe the evolution of dispersed multiphase systems in turbulent flows. To this aim, Direct Numerical Simulation (DNS) of turbulence and accurate tracking of the interface are required, but the range of scales involved for most of practical environmental and industrial applications is so wide that performing this task is a formidable challenge for present day computers. These challenges include the grid resolution for DNS of turbulence which is of the order of the Kolmogorov scale, but of course, physical interfaces have a much smaller scale (order of few molecules) making the direct resolution unfeasible. In this talk, we will briefly review the historical pioneering studies and current experimental findings and computational methodologies used to describe interfaces and then, we will focus on the phase-field approach in turbulent flows. In this Eulerian approach, the phase distribution is described by the order parameter ϕ. We will examine several flow instances and phenomena ranging from turbulent stratified flows to turbulent dispersion of drops and bubbles to reveal potentials and limitations of the phase-field method. Interface interactions with turbulence, coalescence and breakup phenomena for different important physical phenomena involving changes of fluids density and viscosity and presence of surfactants (Marangoni effects) will be discussed in connection with the characteristics of turbulence. Finally, potentials to upscale current results and comparison with current theoretical and experimental findings will be presented.

References:
Deane, G.B. & Stokes, M.D. 2002 Scale dependence of bubble creation mechanisms in breaking waves. Nature 418 (6900), 839.
Garrett, C., Li, M. & Farmer, D. 2000 The connection between bubble size spectra and energy dissipation rates in the upper ocean. J. Phys. Oceanogr. 30 (9), 2163-2171.
Soligo, G., Roccon, A. & Soldati, A. 2019a Coalescence of surfactant-laden drops by Phase Field Method. J. Comput. Phys. 376, 1292-1311.
Soligo, G., Roccon, A. & Soldati, A. 2019b Breakage, coalescence and size distribution of surfactant laden droplets in turbulent flow. J. Fluid Mech.