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Overheating in buildings can be mitigated during periods of excessively warm weather through the application of a night purge ventilation strategy. Building on the mathematical model of Waterson and Hunt (2024), we investigate the emerging field of hybrid ventilation – specifically the rates of airflow achieved by the simultaneous combination of 'stack-driven' natural displacement ventilation and a mechanical supply. Whilst the focus of Waterson and Hunt (2024) was specifically on predicting the time taken to complete a purge, herein we focus on airflow rates and in doing so elucidate the somewhat counter intuitive role that the mechanical supply has on the natural ventilation flow rates. Our results challenge the perception that a hybrid approach must be superior to both solely natural and mechanical strategies. Focusing on the variation in airflow rates during a hybrid purge, our model reveals: (i) a hybrid purge offers an improvement over a solely mechanical purge only during one of the two distinct flow regimes identified; and (ii) the manner in which the stack-driven and mechanical components combine is highly nuanced and cannot be predicted by the linear superposition of the constituent stack-driven and mechanical contributions. [ABSTRACT FROM AUTHOR]

Many low-energy buildings rely on a night purge strategy during periods of excessively warm weather in order to flush out excess heat and thereby help prevent overheating on the following day. A typical purge utilises either a mechanical or a natural displacement ventilation strategy. However, we investigate purging by the simultaneous application of natural and mechanical ventilation, i.e. using a hybrid strategy. Hybrid ventilation is widely promoted as a means of overcoming the inherent limitations of both natural and mechanical ventilation, although relatively little is understood about its operation on a fundamental level. Such was the lack of knowledge on hybrid ventilation that even the time taken to complete a purge was not known at the outset of this work. Through the development of a mathematical model, supported by a series of laboratory experiments, we show that the time taken to purge a room using hybrid ventilation can be established analytically, and is controlled by two ratios: (i) the flow rate of the mechanical supply (the 'forcing') relative to the initial flow rate through the vents in the absence of mechanical forcing, and (ii) the ratio of the floor-level and ceiling-level vent areas. The analytical predictions show close agreement with our measurements. Supplementing a natural displacement ventilation strategy with even a modest mechanical supply can reduce significantly the time taken to purge warm air from a room. Some of the implications of our findings for preventing overheating in buildings by increasing the efficacy of a night cooling strategy are discussed. Practical Application This research is directly applicable to practitioners involved in the first-order design of hybrid ventilation systems – particularly designs which incorporate a 'night purge' regime. Governing equations are derived and the key parameters controlling the rate of purging of warm air are revealed, allowing informed first-order design decisions to be made without the need for expensive and time-consuming computational simulations. [ABSTRACT FROM AUTHOR]

Emptying-filling boxes have been studied in a wide range of configurations for decades, but the flow created in the box by two plumes rising from sources of arbitrary strength and elevation was previously unsolved. Guided by experiments and simplified analytical modelling, we reveal a rich array of two- and three-layer stratifications across seven possible flow regimes. The governing equations for these regimes show how the prevailing regime and stratification properties vary with three key parameters: the relative strength of the plumes, the height difference between their sources and a parameter characterising the resistance of the box to emptying. We observe and explain new behaviours not described in previous studies that are crucial to understanding emptying-filling boxes with multiple plumes. In particular, we demonstrate that the oft-assumed premise that n plumes leads to a stratification with n + 1 layers is not necessarily true, even in the absence of mixing. Two emptying-filling box models are developed: an analytical model addressing all combinations of the governing parameters and an extended model for three-layer stratifications that incorporates two mixing processes observed in the experiments. The predictions of these two models are in generally excellent agreement with measurements from the experimental campaign covering 69 combinations of the governing parameters. This study improves our understanding of emptying-filling boxes and could facilitate improvements to natural ventilation building design, as demonstrated by an example scenario in which occupants feel cooler upon the addition of a second source of heat. [ABSTRACT FROM AUTHOR]

This article presents a theoretical modelling framework for the previously unconsidered case of turbulent wall plumes that detrain continually with height in stably stratified environments. Built upon the classic turbulent plume model, our approach incorporates turbulent detrainment with a variable coefficient of detrainment. Based on a linear constitutive relationship between the ratio of the detrainment to entrainment coefficients and the ambient buoyancy gradient, it is found that for linear ambient stratifications, a dynamic quasi-equilibrium region, characterised by a near invariant local plume Richardson number, is achieved, downstream of which this equilibrium rapidly breaks down. With increasing ambient buoyancy gradient, while the plume becomes increasingly slender with weaker vertical motions, the level at which the plume breaks down to form a horizontal intrusion first decreases and then increases. Moreover, distinct from classic purely entraining plumes, a detraining wall plume can swell within the pre-equilibrium adjustment stage provided the local Richardson number is sufficiently low (Ri x 6), behaviour which is in accordance with observations made in filling-box experiments. [ABSTRACT FROM AUTHOR]

However, for a sufficiently weak co-flow, Graph $\varGamma (\zeta)$ is an increasing function of Graph $\zeta$ and leads to Graph $\varGamma 0. Specifically, we are interested in modelling how the co-flow alters the well-established behaviours of forced (Graph $\varGamma), pure (Graph $\varGamma =1$) and lazy (Graph $\varGamma > 1$) releases. As co-flow strength increases, Graph $\varGamma n$ decreases, rapidly for small Graph $\varGamma 0$ and slowly by comparison as Graph $\varGamma 0$ increases. As previously established, in quiescent surroundings Graph $\varGamma 0 = 1$ corresponds to a pure plume release, Graph $\varGamma 0 to forced releases and Graph $\varGamma 0 > 1$ to lazy releases (e.g. van den Bremer & Hunt 2014). The case $\varGamma 0 = 1$ As the sole restriction placed on the derivation of (4.9) is Graph $\varGamma /\beta \neq 0$, the expression for Graph $\varOmega 0^{*}$ holds across all three classes of release with the exclusion of the jet (Graph $\varGamma 0=0$). [Extracted from the article]

The article presents a case study exploring philosophical aspects of applying biomimicry principles in structural design. Biomimicry is an emerging field that proposes alternative ways of creating sustainable engineering solutions. A functional naturally ventilated office building was built using the principles of biological systems and applying them to large-scale conceptual design that could offer sustainability throughout its design life.