Wards occupy significant proportions of hospital floor areas and due to their constant use, represent a worthwhile focus of study. Single-bed wards are specifically of interest owing to the isolation aspect they bring to infection control, including airborne pathogens, but threats posed by airborne pandemics and family-involvement in hospital care means cross-infection is still a potential problem. In its natural mode, ventilation driven by combined wind and buoyancy forces can lead to energy savings and achieve thermal comfort and high air change rates through secure openings. These are advantageous for controlling indoor airborne pathogens and external air and noise pollution. However, there is lack of detailed evidence and guidance is needed to gain optimum performance from available natural ventilation systems. This research is a proof of concept investigation into the feasibility and impact of natural ventilation systems targeting airflow rates, thermal comfort, heating energy and control of pathogenic bio-aerosols in hospital wards. In particular, it provides insights into the optimal areas of vent openings which could satisfy the complex three-pronged criteria of contaminant dilution, low heating energy and acceptable thermal comfort for occupants in a naturally ventilated single bed ward. The main aim of this thesis is the structured study of four systems categorised into three groups: Simple Natural Ventilation (SNV) in which single and dual-openings are used on the same external wall; Advanced Natural Ventilation (ANV) which is an emerging concept; and finally Natural Personalised Ventilation (NPV) which is an entirely new concept borne out of the limitations of previous systems and gaps in literature. The focus of this research is in the exploratory study of the weaknesses and potentials of the four systems, based on multi-criteria performances metrics within three architecturally distinct single-bed ward designs. In contributing to the body of existing knowledge, this thesis provides a better understanding of the performances of three existing systems while presenting the new NPV system. The analysis is based on dynamic thermal modelling and computational fluid dynamics and in the case of the NPV system, salt-bath experiments for validation and visualisation of transient flows. In all cases, wards were assumed to be free of mechanical ventilation systems that might influence the natural flow of air. The thesis meets three major objectives which have resulted in the following contributions to current knowledge: An understanding of the limitations and potentials of same-side openings, especially why and how dual-openings can be useful when retrofitted into existing wards. Detailed analysis of bulk airflow, thermal comfort, heating energy and room air distribution achievable from existing SNV and ANV systems, including insights to acceptable trickle ventilation rates, which will be particular useful in meeting minimum dilution and energy requirements in winter. This also includes qualitative predictions of the airflow pattern and direction obtainable from both systems. The innovation and study of a new natural ventilation system called Natural Personalised Ventilation (NPV) which provides fresh air directly over a patient s bed, creating a mixing regime in the space and evaluation of its comfort and energy performances. A low-energy solution for airborne infection control in clinical spaces is demonstrated by achieving buoyancy-driven mixing ventilation via the NPV system, and a derivative called ceiling-based natural ventilation (CBNV) is shown. A comparative analysis of four unique natural ventilation strategies including their performance rankings for airflow rates, thermal comfort, energy consumption and contaminant dilution or removal using an existing single-bed ward design as case study. Development of design and operational recommendations for future guidelines on utilising natural ventilation in single-bed wards either for refurbishment or for proposed designs. These contributions can be extended to other clinical and non-clinical spaces which are suitable to be naturally ventilated including treatment rooms, office spaces and waiting areas. The findings signify that natural ventilation is not only feasible for ward spaces but that there is opportunity for innovation in its application through further research. Future work could focus on related aspects like: impacts of fan-assisted ventilation for a hybrid flow regime; pre-heating of supply air; integration with passive heat recovery systems as well the use of full-scale experiments to fine-tune and validate findings.

This thesis deals with the description and development of two classical turbulent shear flows, namely free jet and flat plate turbulent boundary layer flows. In both cases new experimental data has been obtained and in the latter case comparisons are also made with data obtained from data bases, both of experimental and numerical origin. The jet flow studies comprise three parts, made in three different experimental facilities, each dealing with a specific aspect of jet flows. The first part is devoted to the effect of swirl on the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of combined X-wire and cold-wire anemometry. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air. The second jet flow part was set out to test the hypothesis put forward by Talamelli & Gavarini (Flow, Turbul. & Combust. 76), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. The experiments confirm the hypothesis and show that the instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers as well as its effect towards increased turbulence activity has been shown. The third part of the jet flow studies deals with the hypothesis that so called oblique transition may play a role in the breakdown to turbulence for an axisymmetric jet.For wall bounded flows oblique transition gives rise to steady streamwise streaks that break down to turbulence, as for instance documented by Elofsson & Alfredsson (J. Fluid Mech. 358). The scenario of oblique transition has so far not been considered for jet flows and the aim was to study the effect of two oblique modes on the transition scenario as well as on the flow dynamics. For certain frequencies the turbulence intensity was surprisingly found to be reduced, however it was not possible to detect the presence of streamwise streaks. This aspect must be furher investigated in the future in order to understand the connection between the turbulence reduction and the azimuthal forcing. The boundary layer part of the thesis is also threefold, and uses both new data as well as data from various data bases to investigate the effect of certain limitations of hot-wire measurements near the wall on the mean velocity but also on the fluctuating streamwise velocity component. In the first part a new set of experimental data from a zero pressure-gradient turbulent boundary layer, supplemented by direct and independent skin friction measurements, are presented. The Reynolds number range of the data is between 2300 and 18700 when based on the free stream velocity and the momentum loss thickness. Data both for the mean and fluctuating streamwise velocity component are presented. The data are validated against the composite profile by Chauhan et al. (Fluid Dyn. Res. 41) and are found to fulfil recently established equilibrium criteria. The problem of accurately locating the wall position of a hot-wire probe and the errors this can result in is thoroughly discussed in part 2 of the boundary layer study. It is shown that the expanded law of the wall to forth and fifth order with calibration constants determined from recent high Reynolds number DNS can be used to fix the wall position to an accuracy of 0.1 and 0.25 l_ * (l_* is the viscous length scale) when accurately determined measurements reaching y+=5 and 10, respectively, are available. In the absence of data below the above given limits, commonly employed analytical functions and their log law constants, have been found to affect the the determination of wall position to a high degree. It has been shown, that near-wall measurements below y+=10 or preferable 5 are essential in order to ensure a correctly measured or deduced absolute wall position. A  number of peculiarities in concurrent wall-bounded turbulent flow studies, was found to be associated with a erroneously deduced wall position. The effect of poor spatial resolution using hot-wire anemometry on the measurements of the streamwise velocity is dealt with in the last part. The viscous scaled hot-wire length, L+, has been found to exert a strong impact on the probability density distribution (pdf) of the streamwise velocity, and hence its higher order moments, over the entire buffer region and also the lower region of the log region. For varying Reynolds numbers spatial resolution effects act against the trend imposed by the Reynolds number. A systematic reduction of the mean velocity with increasing L+ over the entire classical buffer region and beyond has been found. A reduction of around 0.3 uƬ, where uƬ is the friction velocity, has been deduced for L+=60 compared to L+=15. Neglecting this effect can lead to a seemingly Reynolds number dependent  buffer or log region. This should be taken into consideration, for instance, in the debate, regarding the prevailing influence of viscosity above the buffer region at high Reynolds numbers. We also conclude that the debate concerning the universality of the pdf within the overlap region has been artificially complicated due to the ignorance of spatial resolution effects beyond the classical buffer region on the velocity fluctuations. / QC 20100820

The description of the effects of the wind meandering in the scalar dispersion is a challenging task, since this type of flow represents a physical state characterized by multiple scales. In this study, a Lagrangian stochastic diffusion model is derived to describe the scalar transport during the horizontal wind meandering phenomenon, occurring in a PBL. The model is derived from the linearization of the Langevin equation and employs a heuristic functional form, which represents the autocorrelation functions of the meandering. The new solutions, which describe the longitudinal and lateral wind components, were used to simulate two experiments of contaminants dispersion in low-wind conditions, INEL (USA) and GRAZ (Austria). The results of the comparison indicate that the new model reproduces fairly well the observed concentrations of contaminants and, therefore, satisfactorily describes the enhanced dispersion due to the presence of meandering. / Descrever os efeitos provocados pelo meandro do vento na dispersão de escalares é uma tarefa desafiadora, uma vez que este tipo de escoamento representa um estado físico caracterizado por múltiplas escalas. Neste trabalho, deriva-se um modelo estocátisco Lagrangiano para descrever a dispersão de escalares, na camada limite planetária, durante o fenômeno de meandro do vento horizontal. O modelo é derivado a partir da linearização da equação de Langevin e emprega uma forma funcional heurística, que representa as funções de autocorrelação do meandro. As novas soluções, que descrevem as componentes longitudinais e laterais do vento, foram empregadas para simular dois experimentos de dispersão de contaminantes em condições de vento fraco, INEL (USA) e GRAZ (Áustria). Os resultados das comparações indicam que o novo modelo pode ser usado para reproduzir as concentrações observadas de contaminantes e, portanto descreve de forma satisfatória a difusão reforçada provocada pelo meandro do vento.

Le biofilm épilithique (anciennement périphyton), agrégat phototrophe d’organismes se développant sur le fond des cours d’eau, joue un rôle essentiel dans le fonctionnement des hydroécosystèmes fluviaux comme la Garonne. Pour améliorer la modélisation à l’échelle du tronçon de rivière de ces systèmes, il est nécessaire de prendre en compte les caractéristiques locales de l’écoulement notamment pour la prédiction de l’évolution de la biomasse. Les méthodes expérimentales actuelles ne permettent pas d’accéder aux grandeurs hydrodynamiques locales proches du fond rugueux, dans la zone dite de ’canopée’. Le travail présenté ici vise donc à compléter ces résultats par des simulations numériques directes, avec une méthode de frontières immergées, d’écoulements turbulents de couche limite hydrauliquement rugueuse formée d’hémisphères. L’objectif est double : il s’agit d’une part de mettre en relation les motifs de colonisation et la croissance du biofilm observés expérimentalement aux caractéristiques locales de l’écoulement, et d’autre part, de quantifier les flux d’espèces chimiques entre la canopée et la pleine eau et ce, en fonction du type d’arrangement des obstacles formant le fond, du confinement et du nombre de Reynolds rugueux. Après avoir validé la méthode numérique sur des cas proches de ceux rencontrés dans les expériences, et vérifié que les grandeurs hydrodynamiques moyennes et turbulentes dans la pleine eau sont en bon accord avec les résultats expérimentaux, plusieurs campagnes de simulations ont été réalisées pour deux types d’arrangements du fond (aligné et décalé), plusieurs confinements et une gamme de nombre de Reynolds rugueux représentatifs des écoulements en rivière. Ce travail a pu mettre en évidence que le biofilm colonise préférentiellement les zones de faible cisaillement local et un critère de colonisation a été déterminé. Des simulations avec transfert d’espèce chimique ont été réalisées pour différentes valeurs du nombre de Schmidt et ont permis de quantifier les flux d’échange en fonction de ce paramètre. / The epilithic biofilm, aggregate set of phototrophic organisms growing on the bed of rivers, plays an essential role in the functioning of hydro-ecosystems such as the Garonne river. To improve the modeling of these systems it is necessary to take into account the local conditions in the region close to the rough bottom, named ’canopy’. Direct numerical simulations with a immersed boundary method are used to simulate turbulent boundary layer flows with a rough wall composed of hemipheres. The aim of these simulations is twofold : (1) investigate the influence of local flow conditions on the biofilm colonization and growth processes and (2) quantify the transfers of chemical species between the canopy region and the main water column, for various hemispheres arrangement, confinement and turbulent Reynolds number encountered in natural rivers. This work allowed to show that biofilm colonization first occurs in low-to-moderate local shear stress at the hemisphere surface. Simulations with the transport of a passive scalar were performed for various Schmidt number, and gave an estimation of the mass fluxes as a function of this parameter.

The near-wall modelling of turbulent flows has been an active field of research due to the computational cost associated with the direct numerical simulations of such flow, which are characterized by a wide range of length and time scales. With the recent advancements in technological capabilities, the availability of high-fidelity data has enabled the construction of data-driven approaches to model turbulence. In this thesis, deep-learning models are used to model the dynamically important near-wall region in a turbulent boundary layer. As a first step, a direct numerical simulation (DNS) of an incompressible zero-pressure-gradient (ZPG) turbulent boundary layer (TBL) over a flat plate is performed using a pseudo-spectral code, SIMSON (Chevalier et al., 2007). The Reynolds number based on free-stream velocity and inlet displacement thickness is 450 and the passive scalars are simulated at Prandtl numbers of 1, 2, 4 and 6. Turbulence statistics for the flow and thermal fields are computed and compared against the numerical simulations at a similar Reynolds number. To generate the training, validation and test datasets for the neural network, the turbulent velocity fluctuation fields are sampled at various wall-normal locations, y+ = 15, 30, 50, 100 at a constant sampling time of ∆t+ = 0.99, in addition to the streamwise and spanwise wall-shear-stress fields, pressure field and heat flux fields at the wall. A fully convolutional network (FCN) based model is proposed for the prediction of two-dimensional velocity-fluctuation fields farther from the wall using the sampled fields at the wall. The quality of predictions from the network is assessed based on (i) the mean-squared error (MSE) between the predictions and the DNS fields, (ii) the relative percentage error in prediction of root-mean-squared (RMS) of fluctuations or fluctuation intensity and (iii) the correlation coefficient between the predicted and the DNS fields. Different types of predictions are performed, where the three components of the velocity-fluctuation fields are predicted simultaneously by the FCN, and these predictions are classified based on the input fields to the FCN. Three different types of predictions are presented in this study, and an auxiliary-loss-function approach is also introduced to improve the performance of the FCN. The results from the proposed data-driven model for ZPG TBL shows a good capability in the prediction of both the instantaneous fluctuation fields and the turbulent statistics like fluctuation intensity. In particular, the prediction of velocity-fluctuation fields at y+ = 30 using only the heat-flux field at Pr = 6 exhibits less than 12% error in the prediction of streamwise fluctuation intensity. The results obtained in this study indicate the potential of FCN in serving as a computationally effective tool to predict turbulent-velocity-fluctuation fields close to the wall using the inputs from the wall and finds useful application in flow-control problems. / Nära väggmodelleringen av turbulenta flöden har varit ett aktivt forskningsfält på grund av beräkningskostnaderna i samband med de direkta numeriska simuleringarna av sådant flöde, som kännetecknas av ett brett spektrum av längd- och tidsskalor. Med de senaste tekniska framstegen har tillgången på data i hög kvalitet möjliggjort konstruktion av datadrivna metoder för modellturbulens. I denna avhandling används djupinlärningsmodeller för att modellera det dynamiskt viktiga området nära väggen i ett turbulent gränsskikt. Som ett första steg utförs en direkt numerisk simulering (DNS) av ett inkomprimerbart nolltryck-gradient (ZPG) turbulent gränsskikt (TBL) över en platt platta med hjälp av en pseudo-spektral kod, SIMSON (Chevalier et al., 2007). Reynolds-talet baserat på friströmshastighet och inloppsförskjutningstjocklek är 450 och de passiva skalarna simuleras vid Prandtlnumbers på 1, 2, 4 och 6. Turbulensstatistik för flödet och termiska fält beräknas och jämförs med de numeriska simuleringarna vid ett liknande Reynolds -nummer. För att generera utbildnings-, validerings- och testdatauppsättningar för det neuralanätverket samplas turbulenta hastighetsfluktuationsfält på olika väggnormala platser, y+ = 15, 30, 50, 100 vid en konstant provtagningstid på ∆t+ ≈ 0, 99, dessutom till strömmande och spanvisa väggskjuvspänningsfält, tryckfält och värmeflödesfält vid väggen. En helt konvolutionsnät (FCN) baserad modell föreslås för förutsägelse av tvådimensionella hastighetsfluktuationsfält längre från väggen med hjälp av de samplade fälten vid väggen. Kvaliteten påförutsägelser från nätverket bedöms baserat på (i) medelkvadratfelet (MSE) mellan förutsägelserna och DNS-fälten, (ii) det relativa procentuella felet vid förutsägelse av rot-medelkvadrat (RMS) för fluktuationer eller fluktuationsintensitet och (iii) korrelationskoefficienten mellan de förutsagda och DNS fälten. Olika typer av förutsägelser utförs, där de tre komponenterna i hastighetsfluktuationsfälten förutspås samtidigt av FCN, och dessa förutsägelser klassificeras baserat på inmatningsfälten till FCN. Tre olika typer av förutsägelser presenteras i denna studie, och en metod för hjälp-förlustfunktion introduceras också för att förbättra prestanda för FCN. Resultaten från den föreslagna datadrivna modellen för ZPG TBL visar en god förmåga i förutsägelsen av både momentana fluktuationsfält och den turbulenta statistiken som fluktuationsintensitet. I synnerhet uppvisar förutsägelsen av hastighetsfluktuationsfält at y+ = 30 med endast värmeflödesfältet vid Pr = 6 mindre än 12% fel i förutsägelsen av strömningsvis fluktuationsintensitet. Resultaten som erhållits i denna studie indikerar FCN: s potential att fungera som ett beräkningsmässigt effektivt verktyg för att förutsäga turbulenta hastighetsfluktuationsfält nära väggen med hjälp av ingångarna från väggen och finner användbar tillämpning i flödeskontroll -problem.