Malcolm J. Cook

CFD Modelling of Natural Ventilation: Combined Wind and Buoyancy Forces

Abstract Results of a CFD simulation of the wind-assisted stack ventilation of a single-storey enclosure with high and low-level ventilation openings are presented and compared with both the laboratory measurements and the analytical model of the flow and thermal stratification developed by Hunt and Linden (2001). Comparisons show that close quantitative agreement is obtained between the thermal stratification predicted by the CFD and the analytical model and experimental measurements. A key consideration in the CFD modelling work is how to specify appropriate boundary conditions at the inlet and outlet locations of the enclosure. This paper investigates the use of constant pressure boundary conditions imposed over an opening whose physical area has been reduced to account for the effects of discharge and expansion. The close agreement with the analytical predictions and experimental results gives confidence in this approach and offers guidance on how to model wind-assisted stack ventilation flows using CFD.

Buoyancy-driven displacement ventilation flows: Evaluation of two eddy viscosity turbulence models for prediction:

A computational fluid dynamics code has been used to model buoyancy-driven displacement ventilation flows. The results of using two eddy viscosity turbulence models are compared with those of mathematical theory and salt bath experiments carried out at the University of Cambridge. The work highlights some of the difficulties involved in modelling buoyancy-driven flows and identifies a preferable turbulence model for predicting such flows.

Numerical studies of displacement natural ventilation in multi-storey buildings connected to an atrium

This paper describes computational fluid dynamics (CFD) simulations used to investigate displacement natural ventilation in simple multi-storey spaces connected to an atrium. The purpose of the work is to demonstrate the potential of CFD for modelling these airflows using solutions from simple mathematical models and salt bath experiments to provide an indication of the accuracy that can be attained. The storeys are connected to an atrium and air flows into them via top-down-chimneys. The driving force is provided by localised point heat sources on each floor which generate buoyant plumes that entrain the surrounding air and transport warm air upwards forming a warm, stratified layer in each storey. The mathematical models are used to describe the main flow features, such as stratification height, temperature gradient and ventilation flow rate. Results showed that, using the RNG k — ε turbulence model, the predicted airflow patterns, temperature profiles and ventilation flow rates agreed favourably with the mathematical models, demonstrating the potential of using CFD for modelling buoyancy-driven displacement ventilation in multi-storey spaces connected to an atrium. Practical applications: Computer simulation programs have become valuable tools in the building design process, particularly of innovative buildings. This paper looks at the ability of CFD techniques to model buoyancy-driven natural ventilation in simple multi-storey spaces. The methods used in this paper provide a basis for others to use CFD for predicting natural ventilation in more complex, realistic building structures and are useful for both building designers and CFD practitioners.

Application of passive downdraught evaporative cooling (PDEC) to non-domestic buildings

The applicability of Passive Downdraught Evaporative Cooling (PDEC) for reducing energy consumption in hot dry climates is reviewed. A new EC Joule project explaining the application of PDEC in non-domestic buildings is described. The building performance assessment methodology which employs dynamic thermal simulation programs for thermal analysis, and computational fluid dynamics (CFD) codes for airflow modelling is discussed. The role which wind tunnel tests and field measurements have in producing improved models is noted. Preliminary results from CFD benchmark trials are presented.

Coupling a model of human thermoregulation with computational fluid dynamics for predicting human–environment interaction

This article describes the methods developed to couple a commercial computational fluid dynamics (CFD) program with a multi-segmented model of human thermal comfort and physiology. A CFD model is able to predict detailed temperatures and velocities of airflow around a human body, whilst a thermal comfort model is able to predict the response of a human to the environment surrounding it. By coupling the two models and exchanging information about the heat transfer at the body surface, the coupled system can potentially predict the response of a human body to detailed local environmental conditions. This article presents a method of exchanging data, using shared files, to provide a means of dynamically exchanging simulation data with the IESD-Fiala model during the CFD solution process. Additional code is used to set boundary conditions for the CFD simulation at the body surface as determined by the IESD-Fiala model and to return information about local environmental conditions adjacent to the body surface as determined by the CFD simulation. The coupled system is used to model a human subject in a naturally ventilated environment. The resulting ventilation flow pattern agrees well with other numerical and experimental work.

Natural Ventilation and Low Energy Cooling of Large, Non-Domestic Buildings – Four Case Studies

Abstract The air conditioning of large non-domestic buildings is becoming an increasing trend, even in moderately mild climatic zones. This is often needed to avoid overheating that results from high internal heat gains and solar radiation. This paper describes work, undertaken in the United Kingdom, aimed at minimizing the need for conventional air conditioning in such buildings. Four case studies are presented that demonstrate how dynamic thermal and computational fluid dynamics analysis have been used to assist in the design of a diverse range of naturally ventilated and passively cooled buildings. Cooling solutions included natural ventilation with night cooling (case studies 1 and 2), pre-cooling of the supply air using an underground labyrinth (case study 3), and passive cooling combined with ‘top up’ chilled water cooling of the supply air (case study 4). The first two buildings are now occupied and demonstrate good occupant satisfaction. This work demonstrates that numerical modelling techniques played a successful role in the design of innovative, energy efficient buildings.

Computational analysis and flow structure of a transitional separated-reattached flow over a surface mounted obstacle and a forward-facing step

Large-eddy simulation (LES) of transitional separating-reattaching flow on a two-dimensional square surface mounted obstacle and a forward facing step has been performed using a dynamic sub-grid scale model. The Reynolds number based on the uniform inlet velocity and the obstacle/step height is 4.5 × 103. The mean LES results for both the obstacle and step flow compare reasonably well with the available experimental and DNS data. The flow structures upstream of the surface-mounted obstacle (referred to hereafter as obstacle) and the forward-facing step (referred to hereafter as FFS) consist of unstable two-dimensional structures and coherent rib-shaped structures. These structures with the aid of 3D streamline visualisation strongly indicate that the upstream separation bubble is a closed one rather than an open one in the sense that there is little evidence to suggest that there is fluid injection from the upstream separation region into the downstream separated region for the two geometries. The spectra and time history for the velocities and pressure fields at locations immediately upstream of the obstacle and FFS (including the recirculation region) were analysed using both the Fourier and wavelet transforms and revealed the unsteady nature of the recirculation region upstream of the obstacle and FFS. The transition process has been elucidated using both 2D and 3D flow visualisation of the flow. In both geometries (obstacle and FFS), the separated boundary layer downstream of the leading edge shows 2D nature and roll-up shortly downstream of the separation line leading to 2D K-H rolls to be shed from the leading edge. Coherent structures such as the λ-shaped and rib-like vortices commonly associated with a flat plate boundary layer and also found in the separated-reattached flow of a blunt leading edge plate aligned horizontally to a flow are not common in the separated-reattached flow over the obstacle and FFS.

The role of diffusion on the interface thickness in a ventilated filling box.

We examine the role of diffusivity, whether molecular or turbulent, on the steady-state stratification in a ventilated filling box. The buoyancy-driven displacement ventilation model of Linden et al. (J. Fluid Mech., vol. 212, 1990, p. 309) predicts the formation of a two-layer stratification when a single plume is introduced into an enclosure with vents at the top and bottom. The model assumes that diffusion plays no role in the development of the ambient buoyancy stratification: diffusion is a slow process and the entrainment of ambient fluid into the plume from the diffuse interface will act to thin the interface resulting in a near discontinuity of density between the upper and lower layers. This prediction has been corroborated by small-scale salt bath experiments; however, full-scale measurements in ventilated rooms and complementary numerical simulations suggest an interface that is not sharp but rather smeared out over a finite thickness. For a given plume buoyancy flux, as the cross-sectional area of the enclosure increases the volume of fluid that must be entrained by the plume to maintain a sharp interface also increases. Therefore the balance between the diffusive thickening of the interface and plume-driven thinning favours a thicker interface. Conversely, the interface thickness decreases with increasing source buoyancy flux, although the dependence is relatively weak. Our analysis presents two models for predicting the interface thickness as a function of the enclosure height, base area, composite vent area, plume buoyancy flux and buoyancy diffusivity. Model results are compared with interface thickness measurements based on previously reported data. Positive qualitative and quantitative agreement is observed.

Passive Downdraught Evaporative Cooling I. Concept and Precedents

This is the first of a series of four papers that describe a 3-year EU-funded research project into the application of passive downdraught evaporative cooling to non- domestic buildings. In this paper various evaporative cooling techniques are reviewed. By spraying fine drop lets of water at the top of atria, a downdraught of air cooled by evaporation can be produced. Such direct eva porative cooling using an evaporation tower appears to be a suitable approach for partly displacing the need for air-conditioning in hot, dry climates. It can satisfy fresh air requirements and reduce or eliminate demand for mechanical cooling. Examples of this cooling technique in Southern Europe and the Middle East have already demonstrated its operation and potential energy sav ings. However, limitations, primarily due to control of the system, have been identified. This introductory paper presents the theoretical basis of evaporative cooling, reviews some historical precedents, and discusses their relative strengths and weaknesses. Three further papers in this series will disseminate the main findings of the project.

Low energy refurbishment strategies for health buildings

Public health buildings contribute significantly to UK carbon emissions. New build initiatives have received more attention than the considerable opportunities to reduce carbon emissions within the retained health estate. The research reported here has considered the environmental performance of a typical medium rise, medium depth, concrete-framed, late 1960s acute hospital following low energy environmental design interventions. The interventions are made to optimize daylighting and natural ventilation/cooling whilst reducing overheating caused by summer time solar gains. Three options are investigated: advanced natural ventilation using plena and exhaust stacks; fan-assisted natural ventilation in which fans are used in the exhaust stacks; and mechanical ventilation/cooling with heat recovery. Computer simulations have been carried out to predict the influence on thermal performance (overheating risk) and energy consumption of each of these options on the original design. For each case, current weather data, and future weather data for the years 2020, 2050 and 2080, have been used.