Deformation of ellipsoidal droplets in homogeneous and isotropic turbulence

We study the statistics of deformation of neutrally buoyant droplets in homogeneous isotropic turbulence (HIT), wherein the characteristic droplet size R is smaller than the characteristic Kolmogorov scale eta of the turbulent flow. We systematically focus on the characterization of droplet deformation statistics obtained with various phenomenological (reduced-order) ellipsoidal models (EMs)-assuming that the droplet preserves the ellipsoidal shape at all times-with the droplet moving as a passive tracer in the turbulent flow. The predictions of the EMs are compared with ground-truth data obtained with three-dimensional fully resolved simulations without any ad hoc assumption on the droplet shape. Our work helps in elucidating the applicability of the EMs in describing droplet deformation in HIT at changing the capillary number Ca = tau sigma/tau eta, weighting the relative importance of the droplet characteristic time tau sigma with respect to the turbulent flow characteristic time tau eta.