May 6, 2021
The master thesis on the subject of the DNS analysis of warm cloud microphysics and related mixing dynamics at the cloud-clear air interface has been finalized by Davide Codoni.
A simplified cloud-clear air interface is studied through a direct numerical simulation on a grid of 512 × 512 × 1024 points. The interface is simulated through a time-decaying turbulent shearless mixing layer between two homogeneous and isotropic turbulent fields with different turbulent kinetic energy. The Navier-Stokes equations in the Boussinesq approximation are solved for an incompressible fluid together with the advection-diffusion equation for the water vapor density, seen as a passive scalar. Water droplets dynamics are taken into account through the solution of the droplet's momentum equations together with the water droplets growth equation. The main water particle growth mechanisms are the water vapor diffusional growth and the collision coalescence growth which are both considered in the code. The feedbacks of the water droplets dynamics on the velocity, temperature, and vapor density fields are taken into account. Two simulations have been carried out. The first simulation describes a situation in which the cloud region (the high energy region) is supersaturated and the interface is initially saturated and the Taylor microscale Reynolds number is Reλ = 43. The second simulation analyses the case in which the cloud region is saturated and the interface is subsaturate, while the Taylor microscale Reynolds number is slightly higher than in the first simulation, Reλ = 53. In this work not only the main features of the particular turbulent shearless mixing simulated are described but also the temperature and the water vapor density transport across the mixing layer are analyzed together with the water droplets dynamics. In particular, the role of turbulence in advecting the inertial particles is investigated through the visualization of the clustering phenomenon. The time evolution of the droplet size distribution spectrum has been analyzed for both simulations. The aim of this work is the study of the water droplet dynamic in a saturated and supersaturated turbulent environment and the effect of the entrainment on water droplets at the cloud-clear air interface. In the saturated case, a very strong reduction in the liquid water content due to the intense evaporation is observed, while in the supersaturated case an increase in the liquid water content can be seen. The droplet size distribution analysis showed the iii same trends, a strong decrease of the mean droplet radius is observed for the saturated case and a slight increase of the mean radius is seen for the supersaturated case. Finally from the visualization of the water droplets' spatial distribution, a significant clustering is observed. Furthermore, it is shown that the water droplets concentrate in the low vorticity intensity regions. Only two eddy turnover time was simulated and a significant droplet growth cannot be appreciated, but the results obtained agree with the results in the literature.
Link: https://webthesis.biblio.polito.it/6872/1/tesi.pdf