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Title: Turbulent convection from an area source in a confined space
Author: Ciriello, Francesco
ISNI:       0000 0004 7968 423X
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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The study of turbulent convection is fundamental to our understanding of many flows that occur in the natural and built environment. This dissertation focusses on the density stratification produced by a turbulent lazy plume that is formed by the release of buoyant fluid from a circular source in a cylindrical container. This configuration allows us to explore a host of convection problems that uncover important features of the dynamics of turbulent plumes, Rayleigh-Taylor convection, gravity currents and stratification processes. Initially, the dynamics of lazy plumes are investigated, i.e. plumes formed by sources for which the Richardson number exceeds Γ₀ > 1. Particular attention is dedicated to quantifying the near-source entrainment of very lazy plumes. For Γ₀ ≳ 10³, newly-acquired experimental data presented herein reveals that an excess of dilution occurs in the proximity of the source relative to that predicted by classic integral models. Observationally, the increased dilution coincides with the appearance of features resembling the growth and breakdown of Rayleigh-Taylor fingers in the near-source region. Simple estimates to quantify the enhancement in entrainment are deduced based on classic quadratic laws for the growth of Rayleigh-Taylor convective layers. These estimates find good agreement with measurements of volume flux in the plume based on a separate suite of flow visualisation and particle image velocimetry experiments. A discussion on the shape of the mechanism is presented to discuss how the mechanism is expected to vary with increasing degree of plume 'laziness' (viz. via an increase Γ₀). The study is subsequently extended to examine the flow that a lazy plume produces within the confines of a container, referred to herein as the 'filling-box' problem. En route to describing the filling behaviour of a lazy plume, the dynamics of pure (Γ₀ = 1) plumes whose source radius is much smaller than the height and width of the container are first examined. An experimental campaign was launched to examine the filling-box flow patterns that occur for a plume in cylindrical containers of radius-to-height aspect ratio 0.25 ≲ φ : = R/H ≲ 2.0. New physical insights into the internal structure of these flows were acquired by simultaneously interrogating them with light-induced fluorescence and particle image velocimetry. A classification of filling-box regimes is reported in which five distinct flow patterns are identified: 'breakdown', 'rolling', 'slumping', 'blocking' and 'displacement filling'. Finally, this classification is extended to the stratification produced by a lazy plume. A theoretical model is developed to describe filling-box flows for a wide range of possible combinations of plume source conditions, characterised by the source Richardson number of the plume, '1 ≤ Γ₀ ≲ 10⁶, the container aspect ratio, 0.7 ≲ φ ≲ 1.3 and relative size of the source compared to the height of the environment, 0.01 ≲ β₀ : = b₀/H ≲ 0.4, b₀0 being the source radius. An experimental campaign of flow visualisations and particle image velocimetry is conducted to address key assumptions contained within the theoretical model.
Supervisor: Hunt, Gary Richard Sponsor: EPSRC ; Dyson Technology Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: natural convection ; forced convection ; lazy plume ; filling box ; turbulent convection ; plume ; buoyant plume ; buoyant jet ; gravity current ; radial gravity current ; plume outflow ; distributed plume ; area source plume ; environmental flows ; environmental fluids ; forced plume ; smoke fire ; room heating ; building physics ; thermal convection