COMPLETE ITN - ETN NETWORK

The recent paper has been published on Physics of Fluids with a following title:

"Diffusion of turbulence following both stable and unstable step stratification perturbations" 

   Physics of Fluids (2022); https://doi.org/10.1063/5.0090042

     Luca Gallana, Shahbozbek Abdunabiev, Mina Golshan,  Daniela Tordella

The evolution of a two-phase, air and unsaturated water vapor, time decaying, shearless, turbulent layer has been studied in the presence of both stable and unstable perturbations of the normal temperature lapse rate. The top interface between a warm vapor cloud and clear air in the absence of water droplets was considered as the reference dynamics. Direct, three dimensional, numerical simulations were performed within a 6m x 6m wide and 12m high domain, which was hypothesized to be located close to an interface between the warm cloud and clear air. The Taylor micro-scale Reynolds’ number was 250 inside the cloud portion. The squared Froude’s number varied over intervals of [0.4; 981.6] and [-4.0; -19.6]. A sufficiently intense stratification was observed to change the mixing dynamics. The formation of a sub layer inside the shearless layer was observed. The sub-layer, under a stable thermal stratification condition, behaved like a pit of kinetic energy. However, it was observed that kinetic energy transient growth took place under unstable conditions, which led to the formation of an energy peak just below the center of the shearless layer. The scaling law of the energy time variation inside the interface region was quantified: this is an algebraic law with an exponent that depends on the perturbation stratification intensity. The presence of an unstable stratification increased the differences in statistical behavior among the longitudinal velocity derivatives, compared with the unstratified case. Since the mixing process is suppressed in stable cases, small-scale anisotropy is also suppressed.

The paper on the microphysical time scales has been accepted and is already published on Physics of Fluids (AIP).

Recent results have shown that there is an acceleration in the spread of the size distribution of droplet populations in the region bordering the cloud and undersaturated ambient. We have analyzed the supersaturation balance in this region, which is typically a highly intermittent shearless turbulent mixing layer, under a condition where there is no mean updraft. We have investigated the evolution of the cloud - clear air interface and of the droplets therein via direct numerical simulations. We have compared horizontal averages of the phase relaxation, evaporation, reaction and condensation times within the cloud-clear air interface for the size distributions of the initial monodispersed and polydisperse droplets. For the monodisperse population, a clustering of the values of the reaction, phase and evaporation times, that is around 20-30 seconds, is observed in the central area of the mixing layer, just before the location where the maximum value of the supersaturation turbulent flux occurs. This clustering of values is similar for the polydisperse population but also includes the condensation time. The mismatch between the time derivative of the supersaturation and the condensation term in the interfacial mixing layer is correlated with the planar covariance of the horizontal longitudinal velocity derivatives of the carrier air flow and the supersaturation field, thus suggesting that a quasi-linear relationship may exist between these quantities.

The supersaturation local balance equation is considered at a cloud boundary, and the structure of its turbulent production term is inferred from numerical experiments. The transient decay of an unstable-stratified, shearless-mixing layer at a warm cloud top is modeled as an initial value problem, in which the turbulent velocity, the active scalars, and the droplet dynamics are solved concurrently. The evolution of the cloud-clear air interface and the droplets therein is investigated via direct numerical simulations. Both initial monodisperse and polydisperse droplet size distribution cases are treated. We attempt to model the turbulent production of supersaturation - defined as the difference between the time derivative of the supersaturation and the condensation terms - in the mixing layer. The planar averages of the supersaturation production term are clearly correlated with the planar covariance of the computed horizontal velocity derivative and supersaturation fields, thus suggesting that a quasi-linear relationship may exist between these quantities. The value of the non-dimensional proportionality constant is deduced by i) modeling the production term as the interaction of two sinusoidal perturbations with local and global small-scale frequencies, and ii) spatially averaging the production term and the covariance of the supersaturation and the longitudinal velocity derivatives. It has been verified that the value of such proportionality constant does not vary considerably throughout the transient decay and is not sensitive to the initial droplet distribution dispersity.

POLITO group has published a paper in the International Journal of Multiphase Flow (IJMF). The paper concentrates on intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments.