Experiments on free-surface turbulence

Surprisingly little is known about the statistical nature of the shape of a free surface above turbulence and about how this shape depends on the properties of the turbulence. The main focus of this thesis is on experiments in which the statistical properties of both the surface and the turbulence are measured with a number of different techniques. The experiments are done in a free-surface water-channel, in which turbulence is generated with an active grid. This active grid consists of an array of horizontal and vertical rods through the channel, with small wings attached to them. The rods are individually driven by electric motors, according to a certain forcing protocol, thereby adding energy to the turbulence. A major advantage of an active grid is that, by changing this protocol, the properties, such as the intensity and the isotropy, of the generated turbulence can be changed. These properties were measured by means of Laser-Doppler Velocimetry. The turbulence behind the active grid is much more intense than turbulence generated by a more common static grid. The maximum Taylorbased Reynolds number reached with the active grid (at 40 times the mesh size behind the grid) was Re¿= 256, compared to Re¿ = 70 with a similarly dimensioned static grid. Consequently, the active-grid-generated turbulence shows clear Kolmogorov scaling behaviour over a relatively wide range of scales. The stronger turbulence also leads to stronger surface deformations. In order to characterise the shape of the surface, it is essential to measure the surface shape with a high resolution both in space and time. In order to achieve this, a novel technique has been developed, based on refraction of a laser beam that shines through the surface. The de ection of the beam due to the local surface slope is measured by means of an optical position sensing device. The beam is swept along a line by means of a rapidly oscillating mirror (with a frequency of close to 2 kHz). This allows measurements of the surface slope at multiple points along the line as a function of time. This surface scanning technique can be combined with Particle Image Velocimetry (PIV), which provides snapshots of the velocity field and the vertical component of vorticity in horizontal planes just below the surface. This combination allows us to simultaneously measure the velocity field and the surface deformations above it. PIV is based on the cross-correlation of the intensity distributions in images of particles suspended in the ow, that are illuminated by a thin laser light sheet. When applying PIV to turbulence, it is important to realise that the velocity field that can be obtained with PIV is a spatially averaged representation of the actual velocity field. The effect this averaging has on measured turbulence properties is investigated by means of kinematic simulations, in which realistic turbulent velocity fields, with a prescribed energy spectrum, are generated. Synthetic particle images derived from these ??elds are evaluated by means of a PIV algorithm and the velocity spectrum is calculated. Comparing this to the prescribed spectrum clearly shows the averaging, and allows us to predict its in uence on other measured turbulence properties. The turbulence generated by our grid is not strong enough to lead to very large deformations of the surface. The measured changes in elevation are less than 1 mm. In that case, somewhat naively, one would expect the surface deformations to be primarily associated with sub-surface vortices. In the core of a vortex the magnitude of the vorticity is high, while the pressure is low. This low pressure causes a dimple in the surface above the vortex. This effect can, for instance, be seen when stirring a cup of tea or in the wake behind bridge pillars in a river. Consequently, in simultaneous measurements of the surface shape and the sub-surface velocity field one would expect to find a relatively large correlation between the vertical component of vorticity and the surface elevation. Indeed, our measurements show that relatively strong vortices in the turbulence do deform the surface. However, the measured correlation coefficients are low (<0:1). Spectra of the surface slope in space and time show that, instead of being connected directly to sub-surface structures, much of the surface actually consists of gravity-capillary waves, i.e. regular surface waves. For surface waves, there is a clear relation between their wavelength and their frequency. This relation can be identified in our spectra. The presence of these waves is somewhat surprising, since resonant wave growth can only be expected to occur if the uctuation velocities in the turbulence are larger than the minimum phase velocity of the waves (˜ 0:23 m/s), while the measured fluctuation velocities in our turbulence are an order of magnitude smaller. A remarkable feature of the waves above the turbulence is that they travel in all directions across the surface. In fact, provided that the turbulence far below the surface is isotropic, the surface shape itself is isotropic as well. In other words, statistically, the waves on the surface are the same in every direction. We can change this by changing the forcing protocol of the active grid such that the turbulence becomes anisotropic. In that case the surface shape becomes anisotropic as well. This is a clear indication that the surface waves are excited locally by the turbulence. We have found evidence to suggest that the waves are excited by the largest structures in the turbulence. As a consequence of this, the surface shape does not re ect the wide range of scales in the sub-surface turbulence, but instead exhibits waves primarily with wavelenghts close to the integral scale of the turbulence

Turbulence of a free surface

We study the free surface of a turbulent channel flow, in particular, the relation between the statistical properties of the wrinkled surface and those of the velocity field beneath it. For an irregular flow shed off a vertical cylinder, surface indentations are strongly correlated with vortices in the subsurface flow. For fully developed turbulence this correlation is dramatically reduced. This is because the large eddies excite random capillary-gravity waves that travel in all directions across the surface. Both their predominant wavelength and their anisotropy are determined by the subsurface turbulence

The acceptability of intermittent preventive treatment of malaria in infants (IPTi) delivered through the expanded programme of immunization in southern Tanzania

BACKGROUND\ud \ud Intermittent preventive treatment of malaria in infants (IPTi) reduces the incidence of clinical malaria. However, before making decisions about implementation, it is essential to ensure that IPTi is acceptable, that it does not adversely affect attitudes to immunization or existing health seeking behaviour. This paper reports on the reception of IPTi during the first implementation study of IPTi in southern Tanzania.\ud \ud METHODS\ud \ud Data were collected through in-depth interviews, focus group discussions and participant observation carried out by a central team of social scientists and a network of key informants/interviewers who resided permanently in the study sites.\ud \ud RESULTS\ud \ud IPTi was generally acceptable. This was related to routinization of immunization and resonance with traditional practices. Promoting "health" was considered more important than preventing specific diseases. Many women thought that immunization was obligatory and that health staff might be unwilling to assist in the future if they were non-adherent. Weighing and socialising were important reasons for clinic attendance. Non-adherence was due largely to practical, social and structural factors, many of which could be overcome. Reasons for non-adherence were sometimes interlinked. Health staff and "road to child health" cards were the main source of information on the intervention, rather than the specially designed posters. Women did not generally discuss child health matters outside the clinic, and information about the intervention percolated slowly through the community. Although there were some rumours about sulphadoxine pyrimethamine (SP), it was generally acceptable as a drug for IPTi, although mothers did not like the way tablets were administered. There is no evidence that IPTi had a negative effect on attitudes or adherence to the expanded programme on immunisation (EPI) or treatment seeking or existing malaria prevention.\ud \ud CONCLUSION\ud \ud In order to improve adherence to both EPI and IPTi local priorities should be taken into account. For example, local women are often more interested in weighing than in immunization, and they view vaccination and IPTi as vaguely "healthy" rather preventing specific diseases. There should be more emphasis on these factors and more critical consideration by policy makers of how much local knowledge and understanding is minimally necessary in order to make interventions successful

Ant colony system for a VRP with multiple time windows and multiple visits

The vehicle routing problem with time windows is frequently found in literature, while multiple time windows are not often considered. In this paper a mathematical formulation of the vehicle routing problem with multiple time windows is presented, taking into account periodic constraints. A meta-heuristic based on Ant Colony System is proposed and implemented. Computational results related to a purpose-built benchmark are finally reported

Hiding or hospitalising? On dilemmas of pregnancy management in East Cameroon

Current international debates and policies on safe motherhood mainly propose biomedical interventions to reduce the risks during pregnancy and delivery. Yet, the conceptualisations of risk that underlie this framework may not correspond with local perceptions of reproductive dangers; consequently, hospital services may remain underutilised. Inspired by a growing body of anthropological literature exploring local fertility-related fears, and drawing on 15 months of fieldwork, this paper describes ideas about risky reproduction and practices of pregnancy protection in a Cameroonian village. It shows that social and supernatural threats to fertility are deemed more significant than the physical threats of fertility stressed at the (inter)national level. To protect their pregnancies from those social and supernatural influences, however, women take very physical measures. It is in this respect that biomedical interventions, physical in their very nature, do connect to local methods of pregnancy management. Furthermore, some pregnant women purposefully deploy hospital care in an attempt to reduce relational uncertainties. Explicit attention to the intersections of the social and the physical, and of the supernatural and the biomedical, furthers anthropological knowledge on fertility management and offers a starting point for more culturally sensitive safe motherhood interventions

Fear and rumours regarding placental biopsies in a malaria-in-pregnancy trial in Benin.

BACKGROUND: A multi-country, community-based trial on scheduled screening and treatment for malaria in pregnancy was conducted in Benin, The Gambia and Burkina Faso. Despite standardized procedures and outcomes, the study became subject to rumours and accusations of placenta being sold for mystical and financial gain by trial staff, leading to drop-out rates of 30% and the consequent halting of placental biopsy sampling in Benin. This paper explores the role of socio-cultural beliefs related to placenta and identified additional factors contributing these rumours. METHODS: A qualitative comparative emergent-theory design was used to assess social factors related to trial implementation and uptake in the three countries. Data from participant observation, informal conversations, group discussions and interviews were triangulated and analysed with NVivo Qualitative Analysis software. RESULTS: Despite similar sociocultural beliefs about the sacred nature of the placenta in all three study countries, these beliefs did not affect participation rates in Burkina Faso and The Gambia and placenta-related rumours only emerged in Benin. Therefore, the presence of beliefs is not a sufficient condition to have generated placenta-selling fears. The rumours in Benin reflected the confluence of placenta-related beliefs and factors related to the implementation of the trial (including a catalysing adverse event and miscommunication during the informed consent procedure). Furthermore, distinct socio-political factors contributed to the emergence of rumours, including the historical distrust in governmental organizations and the tense relationship between some of the actors involved in the trial. CONCLUSION: Transdisciplinary social science research designs should accompany the implementation of the trial. The integration of multiple stakeholders' knowledge and involvement is required to define and solve upcoming barriers

False Starts, Wrong Turns and Dead Ends: Reflections on Recent Developments in Criminology

The nature and direction of criminology has changed significantly over the past two decades. The subject area has also grown exponentially and become more diverse. New fields of inquiry are opening up as new issues are added to the criminological agenda. However, at the same time there are some unwelcome developments in the discipline that impact on the orientation of the subject and which detract from its overall viability and standing. The aim of this paper is to identify these unwelcome trends in order to contribute to the development of a more critical and coherent criminology

Experiments on free-surface turbulence

Surprisingly little is known about the statistical nature of the shape of a free surface above turbulence and about how this shape depends on the properties of the turbulence. The main focus of this thesis is on experiments in which the statistical properties of both the surface and the turbulence are measured with a number of different techniques. The experiments are done in a free-surface water-channel, in which turbulence is generated with an active grid. This active grid consists of an array of horizontal and vertical rods through the channel, with small wings attached to them. The rods are individually driven by electric motors, according to a certain forcing protocol, thereby adding energy to the turbulence. A major advantage of an active grid is that, by changing this protocol, the properties, such as the intensity and the isotropy, of the generated turbulence can be changed. These properties were measured by means of Laser-Doppler Velocimetry. The turbulence behind the active grid is much more intense than turbulence generated by a more common static grid. The maximum Taylorbased Reynolds number reached with the active grid (at 40 times the mesh size behind the grid) was Re¿= 256, compared to Re¿ = 70 with a similarly dimensioned static grid. Consequently, the active-grid-generated turbulence shows clear Kolmogorov scaling behaviour over a relatively wide range of scales. The stronger turbulence also leads to stronger surface deformations. In order to characterise the shape of the surface, it is essential to measure the surface shape with a high resolution both in space and time. In order to achieve this, a novel technique has been developed, based on refraction of a laser beam that shines through the surface. The de ection of the beam due to the local surface slope is measured by means of an optical position sensing device. The beam is swept along a line by means of a rapidly oscillating mirror (with a frequency of close to 2 kHz). This allows measurements of the surface slope at multiple points along the line as a function of time. This surface scanning technique can be combined with Particle Image Velocimetry (PIV), which provides snapshots of the velocity field and the vertical component of vorticity in horizontal planes just below the surface. This combination allows us to simultaneously measure the velocity field and the surface deformations above it. PIV is based on the cross-correlation of the intensity distributions in images of particles suspended in the ow, that are illuminated by a thin laser light sheet. When applying PIV to turbulence, it is important to realise that the velocity field that can be obtained with PIV is a spatially averaged representation of the actual velocity field. The effect this averaging has on measured turbulence properties is investigated by means of kinematic simulations, in which realistic turbulent velocity fields, with a prescribed energy spectrum, are generated. Synthetic particle images derived from these ??elds are evaluated by means of a PIV algorithm and the velocity spectrum is calculated. Comparing this to the prescribed spectrum clearly shows the averaging, and allows us to predict its in uence on other measured turbulence properties. The turbulence generated by our grid is not strong enough to lead to very large deformations of the surface. The measured changes in elevation are less than 1 mm. In that case, somewhat naively, one would expect the surface deformations to be primarily associated with sub-surface vortices. In the core of a vortex the magnitude of the vorticity is high, while the pressure is low. This low pressure causes a dimple in the surface above the vortex. This effect can, for instance, be seen when stirring a cup of tea or in the wake behind bridge pillars in a river. Consequently, in simultaneous measurements of the surface shape and the sub-surface velocity field one would expect to find a relatively large correlation between the vertical component of vorticity and the surface elevation. Indeed, our measurements show that relatively strong vortices in the turbulence do deform the surface. However, the measured correlation coefficients are low (<0:1). Spectra of the surface slope in space and time show that, instead of being connected directly to sub-surface structures, much of the surface actually consists of gravity-capillary waves, i.e. regular surface waves. For surface waves, there is a clear relation between their wavelength and their frequency. This relation can be identified in our spectra. The presence of these waves is somewhat surprising, since resonant wave growth can only be expected to occur if the uctuation velocities in the turbulence are larger than the minimum phase velocity of the waves (˜ 0:23 m/s), while the measured fluctuation velocities in our turbulence are an order of magnitude smaller. A remarkable feature of the waves above the turbulence is that they travel in all directions across the surface. In fact, provided that the turbulence far below the surface is isotropic, the surface shape itself is isotropic as well. In other words, statistically, the waves on the surface are the same in every direction. We can change this by changing the forcing protocol of the active grid such that the turbulence becomes anisotropic. In that case the surface shape becomes anisotropic as well. This is a clear indication that the surface waves are excited locally by the turbulence. We have found evidence to suggest that the waves are excited by the largest structures in the turbulence. As a consequence of this, the surface shape does not re ect the wide range of scales in the sub-surface turbulence, but instead exhibits waves primarily with wavelenghts close to the integral scale of the turbulence