Vorticity and scalar transport in turbulent round jets with and without heating

Abstract

The round turbulent jets are ubiquitous in both natural and engineered systems such as clouds, engine exhausts, chimney plumes, and respiratory flows. We aimed to understand the vorticity and scalar transport in turbulent jets under various conditions. The thesis work is divided into four studies shown ahead. The first study attempts to gain insights into the vorticity structure of a canonical jet that is consistent with the mean vorticity fluxes. The mean vorticity fluxes are used as a bridge to connect mean Reynolds stresses and the instantaneous vorticity structure of the turbulent flow. Using an in-depth statistical analysis, “dominant” vorticity flux events are determined which have sparse occurrence but very large contribution to Reynolds stresses. By composing vortex elements of these “dominant” events together, we propose a closed “hairclip”- type structure as the most likely candidate for the coherent structures in a turbulent jet. The study briefly delves into the transition of the vorticity ring to understand the generation of vorticity fluxes in a unstable laminar jet. We next study temperature and scalar transport in an off-source heated jet (OSHJ), which represents the simplest of flows to model heat and water vapour dynamics of deep cumulus clouds, where heating away from the jet source mimics the latent-heat release in clouds during condensation of water vapor into liquid droplets. We find that the scalar-to-velocity width ratio approaches a roughly constant value near the end of the heating zone in literature including our work. The axial distance where this is realized is used as a characteristic scale to divide similar properties of studies (with large variations in heating details) in zones, which provides a unified framework for these studies. Moreover, the effect of heating on the disruption of flow structure is illustrated. In the third study, the focus shifts to aerosol transport during human speech flows (act like series of transient jets; puffs), particularly relevant in the context of virus transmission, such as SARS-CoV-2. Direct numerical simulations explore the turbulent transport of potentially infectious aerosols during short conversations, providing estimates of exposure for various speech configurations; like monologues and dialogues. The study reveals the significant impact of conversation dynamics on aerosol exposure, emphasizing the importance of lateral and axial separations to minimize transmission risk. The results have implications for epidemiological models and respiratory disease management. The final study simulates droplet dynamics in a transient jet to model cough flows. A computational approach is proposed, coarse-graining respiratory droplets into an Eulerian liquid field. The simulation captures key features reported in the literature, including initial supersaturation in the jet core and enhanced droplet lifetimes for high humidity. The approach offers a computationally less expensive alternative for studying longrange droplet transport in cough flows. In summary, the thesis contributes significantly to the understanding of vorticity and scalar transport in round turbulent jets, addressing both fundamental and practical aspects with implications for fields ranging from fluid dynamics to respiratory disease transmission.