Droplet-turbulence interaction in moist tube convection*

The role of turbulence in the growth of droplet size and spectrum in cumulus clouds leading to precipitation has been a topic of intense research. However, due to the difficulty in simulating the turbulence in that of clouds in laboratory scale, there have been very few experimental studies in this area. To study some aspects of droplet behaviour in clouds, we have set up an experiment with turbulent moist tube convection in a vertical square tube connecting two reservoirs maintained at an unstable density difference (due to difference in both temperature and vapour concentration). While heated water at the bottom reservoir provides hot moist air, slow-moving cold air at the tube top in a closed-loop cold chamber acts as the cold reservoir. Due to the mixing of different air parcels of varying temperature and moisture, the vapour condenses to form droplets on the available aerosols and interact with the turbulence created in the tube. Preliminary experiments reveal a near-linear mean temperature gradient exist along the tube length. Imaging of the droplet field is done using laser and high-speed camera to quantify droplet velocity and inter-particle distance. The evolution of the droplet diameter and velocity spectra are studied to understand the behaviour of droplets in turbulence.