Fine Scale Features of Turbulent Shear Flows

This thesis presents an investigation into kinematic features of fine scale turbulence in free shear flows. In particular it seeks to examine the interaction between the different length scales present in shear flow turbulence as well as the interaction between the strain-rate tensor and the rotation tensor, which are the symmetric and skew-symmetric components of the velocity gradient tensor respectively. A new multi-scale particle image velocimetry (PIV) technique is developed that is capable of resolving the flow at two different dynamic ranges, centred on inertial range scales and on dissipative range scales, simultaneously. This data is used to examine the interaction between large-scale fluctuations, of the order of the integral scale, and inertial and dissipative range fluctuations. The large-scale fluctuations are observed to have an amplitude and frequency modulation effect on the small scales, and the small scales are shown to have a slight effect on the large scales, illustrating the two way nature of the energy cascade. A mechanism whereby integral scale rollers leave behind a wake of intense small-scale fluctuations is proposed. The interaction between strain and rotation is examined with regards to the rate of enstrophy amplification (ωiSijωj). It is found that the mechanism that is responsible for the nature of enstrophy amplification is the alignment tendency between the extensive strain-rate eigenvector and the vorticity vector. This mechanism is also observed to be scale dependent for ωiSijωj > 0, but independent for ωiSijωj < 0. This is subsequently confirmed with new dual-plane stereoscopic PIV experiments performed as part of this study. Finally, computational data is used to examine the effect of experimental noise and variation of spatial resolution on the observation and understanding of this strain - rotation interaction

Spatial evolution of the turbulent/turbulent interface geometry in a cylinder wake

This study aims to examine the spatial evolution of the geometrical features of the turbulent/turbulent interface (TTI) in a cylinder wake. The wake is exposed to various turbulent backgrounds in which the turbulence intensity and the integral length scale are independently varied and comparisons to a turbulent/non-turbulent interface (TNTI) are drawn. The turbulent wake was marked with a high-Schmidt-number ($Sc$) scalar and a planar laser induced fluorescence (PLIF) experiment was carried out to capture the interface between the wake and the ambient flow from $x/d$ = 5 to 40 where $x$ is the streamwise coordinate from the centre of the cylinder and $d$ is the cylinder's diameter. It is found that the TTI generally spreads faster toward the ambient flow than the TNTI. A transition region of the interfaces' spreading is found at $x/d \approx 15$, after which the interfaces propagate at a slower rate than previously (upstream) and the mean interface positions of both TNTI and TTI scale with the local wake half-width. The location of both the TNTI and TTI have non-Gaussian probability density functions (PDFs) in the near wake because of the influence of the large-scale coherent motions present within the flow. Further downstream, after the large-scale coherent motions have dissipated, the TNTI position PDF does become Gaussian. For the first time we explore the spatial variation of the ``roughness'' of the TTI, quantified via the fractal dimension, from near field to far field. The length scale in the background flow has a profound effect on the TTI fractal dimension in the near wake, whilst the turbulence intensity only becomes important for the fractal dimension farther downstream

Energy exchanges between coherent modes in the near wake of a rotor model at different tip speed ratios

In this work we investigate the spatio-temporal nature of various coherent modes present in a rotor wake using a combination of new PIV experiments and data from Biswas and Buxton (2024). A multi-scale triple decomposition of the acquired velocity field is sought to extract the coherent modes and thereafter, the energy exchanges to and from them are studied using the multi-scale triple-decomposed coherent kinetic energy budgets developed by Baj and Buxton (2017). Different frequencies forming the tip vortex system (such as the blade passing frequency, turbine's rotational frequency and their harmonics) are found to be energised by different sources such as production from the mean flow or non-linear triadic interaction or both, similar to the primary, secondary or the mixed modes discussed in Biswas et al. (2022). In fact, the tip vortex system forms a complex network of nonlinear triadic energy transfers, the nature and the magnitudes of which depend on \lambda. On the other hand, the modes associated with the sheddings from the nacelle or tower and wake meandering are found to be primarily energised by the mean flow. We show that the tip vortex system exchanges energy with the mean flow primarily through the turbine's rotational frequency. In fact, the system transfers energy back to the mean flow through the turbine's rotational frequency at some distance downstream marking the onset location of wake recovery (x_{wr}). x_{wr} is shown to reduce with \lambda due to stronger interaction and earlier merging of the tip vortices at a higher \lambda

Hygiene along the continuum of care in the early post-natal period: an observational study in Nigeria.

BACKGROUND: Newborns delivered in healthcare facilities in low- and middle-income countries are at an increased risk of healthcare associated infections. Facility-based studies have focused primarily on healthcare worker behaviour during labour & delivery with limited attention to hygiene practices in post-natal care areas and largely ignore the wide variety of actors involved in maternal and neonatal care. METHODS: This exploratory mixed-methods study took place in six healthcare facilities in Nigeria where 31 structured observations were completed during post-natal care, discharge, and the first 6 hours after return to the home. Frequency of hand hygiene opportunities and hand hygiene actions were assessed for types of patient care (maternal and newborn care) and the role individuals played in caregiving (healthcare workers, cleaners, non-maternal caregivers). Qualitative interviews with mothers were completed approximately 1 week after facility discharge. RESULTS: Maternal and newborn care were performed by a range of actors including healthcare workers, mothers, cleaners and non-maternal caregivers. Of 291 hand hygiene opportunities observed at health facilities, and 459 observed in home environments, adequate hand hygiene actions were observed during only 1% of all hand hygiene opportunities. Adequate hand hygiene prior to cord contact was observed in only 6% (1/17) of cord contact related hand hygiene opportunities at healthcare facilities and 7% (2/29) in households. Discharge advice was infrequent and not standardised and could not be remembered by the mother after a week. Mothers reported discomfort around telling non-maternal caregivers to practice adequate hand hygiene for their newborn. CONCLUSIONS: In this setting, hand hygiene practices during post-natal care and the first 6 hours in the home environment were consistently inadequate. Effective strategies are needed to promote safe hand hygiene practices within the post-natal care ward and home in low resource, high-burden settings. Such strategies need to target not just mothers and healthcare workers but also other caregivers

A fast-running physics-based wake model for a semi-infinite wind farm

This paper presents a new generation of fast-running physics-based models to predict the wake of a semi-infinite wind farm, extending infinitely in the lateral direction but with finite size in the streamwise direction. The assumption of a semi-infinite wind farm enables concurrent solving of the laterally-averaged momentum equations in both streamwise and spanwise directions. The developed model captures important physical phenomena such as vertical top-down transport of energy into the farm, variable wake recovery rate due to the farm-generated turbulence, and also wake deflection due to turbine yaw misalignment and Coriolis force. Of special note is the model's capability to predict and shed light on the counteracting effect of Coriolis force causing wake deflections in both positive and negative directions. Moreover, the impact of wind-farm layout configuration on the flow distribution is modelled through a parameter called the local deficit coefficient. Model predictions were validated against large-eddy simulations extending up to 45 kilometres downstream of wind farms. Detailed analyses were performed to study the impacts of various factors such as incoming turbulence, wind-farm size, inter-turbine spacing, and wind-farm layout on the farm wake

A fast-running physics-based wake model for a semi-infinite wind farm

This paper presents a new generation of fast-running physics-based models to predict the wake of a semi-infinite wind farm, extending infinitely in the lateral direction but with finite size in the streamwise direction. The assumption of a semi-infinite wind farm enables concurrent solving of the laterally averaged momentum equations in both streamwise and spanwise directions. The developed model captures important physical phenomena such as vertical top-down transport of energy into the farm, variable wake recovery rate due to the farm-generated turbulence and also wake deflection due to turbine yaw misalignment and Coriolis force. Of special note is the model's capability to predict and shed light on the counteracting effect of Coriolis force causing wake deflections in both positive and negative directions. Moreover, the impact of wind farm layout configuration on the flow distribution is modelled through a parameter called the local deficit coefficient. Model predictions were validated against large-eddy simulations extending up to 45 km downstream of wind farms. Detailed analyses were performed to study the impacts of various factors such as incoming turbulence, wind farm size, inter-turbine spacing and wind farm layout on the farm wake

Simulating and investigating compressible flows interaction with fractal structures:71st Annual Meeting of the APS Division of Fluid Dynamics

Previous experimental and numerical studies have investigated incompressible flow interactions with multi-scale fractal structures with the objective of generating turbulence at multiple scales. Depending on various flow conditions, it was found that these fractal structures are able to enhance mixing and scalar transport, and in some cases to contribute to the reduction of flow generated sound in certain frequency ranges. The interaction of compressible flows with multi-scale fractal structures did not receive much attention as the focus was entirely on the incompressible regime. The objective of this study is to conduct large eddy simulations of flow interactions with various fractal structures in the compressible regime and to extract and analyze different flow statistics in an attempt to determine the effect of compressibility. Immersed boundary methods will be employed to overcome the difficulty of modeling the fractal structures, with adequate mesh resolution around small features of the fractal shapes

Simulating and investigating compressible flows interaction with fractal structures

Previous experimental and numerical studies have investigated incompressible flow interactions with multi-scale fractal structures with the objective of generating turbulence at multiple scales. Depending on various flow conditions, it was found that these fractal structures are able to enhance mixing and scalar transport, and in some cases to contribute to the reduction of flow generated sound in certain frequency ranges. The interaction of compressible flows with multi-scale fractal structures did not receive much attention as the focus was entirely on the incompressible regime. The objective of this study is to conduct large eddy simulations of flow interactions with various fractal structures in the compressible regime and to extract and analyze different flow statistics in an attempt to determine the effect of compressibility. Immersed boundary methods will be employed to overcome the difficulty of modeling the fractal structures, with adequate mesh resolution around small features of the fractal shapes

Understanding factors associated with the translation of cardiovascular research: A multinational case study approach

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.This article has been made available through the Brunel Open Access Publishing Fund.Background: Funders of health research increasingly seek to understand how best to allocate resources in order to achieve maximum value from their funding. We built an international consortium and developed a multinational case study approach to assess benefits arising from health research. We used that to facilitate analysis of factors in the production of research that might be associated with translating research findings into wider impacts, and the complexities involved. Methods: We built on the Payback Framework and expanded its application through conducting co-ordinated case studies on the payback from cardiovascular and stroke research in Australia, Canada and the United Kingdom. We selected a stratified random sample of projects from leading medical research funders. We devised a series of innovative steps to: minimize the effect of researcher bias; rate the level of impacts identified in the case studies; and interrogate case study narratives to identify factors that correlated with achieving high or low levels of impact. Results: Twenty-nine detailed case studies produced many and diverse impacts. Over the 15 to 20 years examined, basic biomedical research has a greater impact than clinical research in terms of academic impacts such as knowledge production and research capacity building. Clinical research has greater levels of wider impact on health policies, practice, and generating health gains. There was no correlation between knowledge production and wider impacts. We identified various factors associated with high impact. Interaction between researchers and practitioners and the public is associated with achieving high academic impact and translation into wider impacts, as is basic research conducted with a clinical focus. Strategic thinking by clinical researchers, in terms of thinking through pathways by which research could potentially be translated into practice, is associated with high wider impact. Finally, we identified the complexity of factors behind research translation that can arise in a single case. Conclusions: We can systematically assess research impacts and use the findings to promote translation. Research funders can justify funding research of diverse types, but they should not assume academic impacts are proxies for wider impacts. They should encourage researchers to consider pathways towards impact and engage potential research users in research processes. © 2014 Wooding et al.; licensee BioMed Central Ltd.RAND Europe and HERG, with subsequent funding from the NHFA, the HSFC and the CIHR. This research was also partially supported by the Policy Research Programme in the English Department of Health

Detecting Abnormal Vehicular Dynamics at Intersections Based on an Unsupervised Learning Approach and a Stochastic Model

This investigation demonstrates an unsupervised approach for modeling traffic flow and detecting abnormal vehicle behaviors at intersections. In the first stage, the approach reveals and records the different states of the system. These states are the result of coding and grouping the historical motion of vehicles as long binary strings. In the second stage, using sequences of the recorded states, a stochastic graph model based on a Markovian approach is built. A behavior is labeled abnormal when current motion pattern cannot be recognized as any state of the system or a particular sequence of states cannot be parsed with the stochastic model. The approach is tested with several sequences of images acquired from a vehicular intersection where the traffic flow and duration used in connection with the traffic lights are continuously changed throughout the day. Finally, the low complexity and the flexibility of the approach make it reliable for use in real time systems