The Sensitivity of Marginally Stratified Shelf Sea Fronts to Turbulent Mixing Processes

Turbulent mixing plays an important role in controlling the vertical structure of temperature, salinity and density in shelf seas. It is crucial in controlling the seasonal stratification in temperate shelf seas, though our incomplete knowledge of the processes involved prevents an accurate representation in numerical shelf sea modelling. This study made use of both observational field work and numerical turbulence modelling to identify first- and second-order mixing mechanisms in weakly stratified waters in the Celtic Sea, U.K. Field work was conducted both in spring (May, 2012) and summer (August, 2012). During the 11-day field work in spring, an overall warming of the well-mixed water column (0.7°C) was observed at a rate consistent with seasonal solar heat input at the sea surface. This was well-represented in numerical simulations for the same period, conducted using the two-equation k-epsilon statistical turbulence closure model implemented in the General Ocean Turbulence Model (GOTM). Comparative observations conducted over a 12- day period in summer (August, 2012) identified significant advective control with warming (1.7°C) substantially outpacing the calculated solar heat input (0.5°C). Field work in summer presented vertical thermal gradients as a result of the seasonal stratification (Ttop-Tbot = 2.5°C). The largest variability in stratification occurred over the neap-spring cycle; a breakdown in thermal stratification occurred during the transition to spring tides although the timing was controlled by strong surface forcing conditions which increased surface mixing and also advected well-mixed waters over the study site. The passage of non-linear internal waves were observed along the seasonal thermocline during slack water, increasing shear and reducing the dynamic stability of the water column. These were likely to be generated by hydraulic control and released during the transition from sub-critical to super-critical flow in the form of lee waves. Two packets of non-linear internal waves with a vertical displacement of the leading wave of 11 m and a period between successive troughs of approximately 23–36 minutes were recorded. Microstructure profiles of derived turbulent dissipation sampled over 12.4 hour tidal cycles were conducted at both neap and spring tides in spring and summer. Results show bottom boundary layer mixing to be the primary control on the water column structure with a dominant M2 tidal periodicity. The phase lag and tidal asymmetry observed was well reproduced by the 2-equation turbulence model. Phase lags were observed to increase with height above the bed with neap tides generating a larger phase lag at the top of the bottom boundary layer than at spring tides. The impact of a stratified water column was observed in the maximum height attained by the bottom boundary layer in summer although there was no observable increase in the phase lag in contrast to that reported in the literature. A lack of an internally stratified water column in the GOTM model meant that it did not reproduce the stratification effects on the growth of the bottom boundary layer. Turbulent dissipation levels in the numerical simulations also diverged from that observed in the interior supporting the notion of missing mixing mechanisms providing an additional source of turbulence to the shelf sea interior. The lack of interior mixing let to an over estimation in the strength of the thermocline in GOTM in comparison to the in-situ observations. The findings of this study concludes that in weakly stratified shelf seas typical of the conditions presented at this study site, the primary mechanism controlling the vertical structure of the water column is the strength of the tidal mixing that varies significantly over the spring-neap cycle. Increased surface forcing from strong wind events potentially can tip the balance between a stratified and well-mixed water column through increased vertical mixing in the surface layer near the thermocline and by generating enhanced horizontal advection as well as baroclinic instabilities. This study reaffirms the necessity for shelf sea numerical models to correctly parametrise interior mixing under stratified conditions since the lack of mixing led to an over estimation in the strength of the thermocline. One candidate mechanism identified in this study with the potential to enhance interior mixing were non-linear internal lee waves generated by the topography in the vicinity of the study site.NER

Numerical Study on the Propagation of Turbulent Fronts in Dilute Polymer Solutions

This work explores the mechanics of turbulent propagation in dilute polymer solutions with the objective of increasing the current understanding on turbulent propagation not only of this particular kind of flow, but also of Newtonian turbulent flows in general. By means of Direct Numerical Simulations of planar turbulent/non-turbulent interfaces, the phenomenon of turbulence propagation has been studied in its full range of turbulent scales

A hydrodynamical perspective on the turbulent transport of bacteria in rivers

The transport of bacteria in turbulent river-like environments is addressed, where bacterial populations are frequently encountered attached to solids. This transport mode is investigated by studying the transient settling of heavy particles in turbulent channel flows featuring sediment beds. A numerical method is used to fully resolve turbulence and finite-size particles, which enables the assessment of the complex interplay between flow structures, suspended solids and river sediment

Mathematical Methods, Modelling and Applications

This volume deals with novel high-quality research results of a wide class of mathematical models with applications in engineering, nature, and social sciences. Analytical and numeric, deterministic and uncertain dimensions are treated. Complex and multidisciplinary models are treated, including novel techniques of obtaining observation data and pattern recognition. Among the examples of treated problems, we encounter problems in engineering, social sciences, physics, biology, and health sciences. The novelty arises with respect to the mathematical treatment of the problem. Mathematical models are built, some of them under a deterministic approach, and other ones taking into account the uncertainty of the data, deriving random models. Several resulting mathematical representations of the models are shown as equations and systems of equations of different types: difference equations, ordinary differential equations, partial differential equations, integral equations, and algebraic equations. Across the chapters of the book, a wide class of approaches can be found to solve the displayed mathematical models, from analytical to numeric techniques, such as finite difference schemes, finite volume methods, iteration schemes, and numerical integration methods

A hydrodynamical perspective on the turbulent transport of bacteria in rivers

The transport of bacteria in river systems is a phenomenon which occurs on a multitude of length scales ranging from the size of individual microbes up to the size of an entire estuary. At the same time the understanding of the spreading of microbial populations after a localised contamination event such as a combined sewer overflow is crucial for the prediction of the water quality downstream of the source, which is in turn essential to managing public health. It is well-established that microbial populations in fluvial systems may preferably be found on the surface of small particles rather than solely freely suspended in the water body. The attachment to particles provides an environment beneficial to the survival of bacteria due to the improved access to nutrients and the shielding from environmental stressors, but also alters their dispersion characteristics as the transport of bacteria is then coupled to the trajectories of heavy particles. The importance in the distinction between the particle-attached and the freely-suspended mode of transport has been recognised in the mechanistic modelling of bacteria fate and transport. However, due to the multiscale nature of the problem, the mechanisms which govern the transport of particles in river-like flows are never resolved explicitly, and hence, the models profoundly rely upon the availability of accurate descriptions thereof. The associated problem of particles settling in a turbulent carrier flow is an active topic of research by itself, and is rich in emerging phenomena such as the emergence of spatial inhomogeneities or non-trivial modifications of the settling characteristics compared to quiescent environments. In particular, the transient settling of particles in horizontal open channels, which serves as an abstraction of particle-attached bacteria transport in rivers, has hitherto received only little attention in the literature. As a consequence, the knowledge on the impact of its defining features such as boundedness, anisotropy and vertical inhomogeneity on the settling characteristics is limited and needs to be addressed to enable the formulation of reliable models thereof. The aim of this thesis is to fill the knowledge gap on the transport characteristics of heavy particles in turbulent horizontal open channel flows, and to identify phenomena which may be of importance in the context of bacteria transport modelling. For this purpose, the incompressible Navier--Stokes equations and the momentum balance equations for dispersed particles are solved using direct numerical simulations and the immersed boundary method. This approach resolves all relevant scales of turbulence and the microscopic flow around each particle explicitly, and thus, describes the particle-fluid interaction from fundamental principles of physics without the need of additional modelling. Apart from the contaminated particles, which are introduced near the free surface of the flow, the simulation domain includes approximately 100,000 fully resolved particles at the bottom of the domain, which form a realistic sediment bed, and enable the examination of the interaction between contaminated particles and mobile sediments. Concerning the parameter space, the value of the friction Reynolds number is varied within the range $Re_{\tau} \in [241,838]$, while the contaminant parameter space is chosen such that the resulting relative turbulence intensities---defined as the ratio between the friction velocity and the undisturbed terminal velocity---lie within the range $I_{\tau} \in [0.47,2.88]$. Moreover, two types of sediment bedforms are investigated in order to assess their effect on contaminant transport, namely a macroscopically flat bed and a bed featuring ripples. The analysis of the simulation data shows that the settling velocity of the contaminant particles is enhanced in the ensemble-averaged sense, yet, the time from beginning of the settling until the initial deposition is prolonged when compared to the ratio between the channel height and the terminal velocity. The enhancement is demonstrated to be a result of the preferential sampling of turbulent sweep events, which also implies that the streamwise component of the particle velocity is increased compared to the mean fluid velocity at the same position. A closer examination of the spatial organisation of contaminated particles reveals that they tend to accumulate in large-scale high-speed velocity streaks in the outer region of turbulence. Due to this focusing mechanism, the mean-squared lateral displacement of the settling particles stagnates in the lower half of the channel such that contaminants are not further dispersed in cross-stream direction until shortly before deposition. The same behaviour could be reproduced using a time-invariant exact coherent flow state resembling a hairpin vortex as a proxy for turbulence, and an extended parameter sweep in this setup suggests that this transport barrier effect persists even at high relative turbulence intensities. It is speculated that this phenomenon might confine contaminated particles to a region close to the river bank over a considerable downstream distance in the aftermath of a combined sewer overflow event, which might seriously impact decisions regarding public health measures. Near the sediment bed, the barrier effect of the large-scale motions is inactive and contaminants are found to disperse laterally at a rate which presumably depends on the Shields parameter. The interaction between the sediment and the contaminants is distinct for the two bed topologies under investigation. In the case of macroscopically flat beds, the contaminated particles are transported towards sediment ridges which are in turn known to be a result of the action of large-scale fluid motions, and the mixing of contaminants and sediment particles is restricted to the thin layer of sediment near the interface. In contrast, the presence of ripples leads to a capturing effect where contaminated particles are preferentially deposited in the trough of the ripple, and subsequently buried by a thick layer of sediment due to the propagation of the bed feature. This mechanism temporarily immobilises a large share of all contaminated particles until the displacement of the ripple has sufficiently progressed for them to be eroded on the windward side. During the immobilisation, the associated bacteria are shielded from solar radiation to a substantial degree, which likely has a significant impact on their inactivation, especially in shallow waters. Moreover, the cyclic nature of this phenomenon may provide one of many explanations for bacteria storages which are known to exist in river sediments and may cause bursts in fecal bacteria indicator levels even in absence of immediate contamination events. It is concluded that direct numerical simulation can be a valuable tool for the analysis of bacteria transport, and recommendations are made on how the conjectures compiled in this thesis can be targeted in laboratory experiments to examine their relevance

Numerical Study of Owls’ Leading-Edge Serrations

The silent flight ability of owls is often attributed to their unique wing morphology and its interaction with their wingbeat kinematics. Among these distinctive morphological features, leading-edge serrations stand out – these are rigid, miniature, hook-like patterns located at the leading edge of the primary feathers of their wings. It had been hypothesized that these leading-edge serrations serve as a passive flow control mechanism, influencing the aerodynamic performance and potentially affecting the boundary layer development over the wing, subsequently influencing wake flow dynamics. Despite being the subject of research spanning multiple decades, a consensus regarding the aerodynamic mechanisms underpinning owls’ leading-edge serrations remains elusive. While the literature extensively explores the aerodynamic and aeroacoustic properties of serrated wing geometries, the predominant focus had been on owl-like serrations, including sawtooth patterns, wavy configurations, cylindrical shapes, and slitted variations. This emphasis has often overshadowed the authentic geometry of owl wing serrations, which are notably shorter than the wing\u27s chord and oriented at an angle relative to the freestream airflow. In order to shed light on the flow dynamics associated with owls\u27 leading-edge serrations, this study delves into numerically simulating the flow field surrounding an owl wing, meticulously replicating the serrated leading-edge geometry, at an intermediate chord-based Reynolds number (40000). A direct numerical simulation (DNS) approach is employed to simulate the fluid flow problem, where the Navier-Stokes equations for incompressible flow are solved on a Cartesian grid with sufficient resolution to resolve all the relevant flow scales, while the wing is represented using an immersed boundary method. Two wing planforms are considered for numerical analysis: one featuring leading-edge serrations and another without them. The findings suggest that the serrations improve suction surface flow by promoting sustained flow reattachment via streamwise vorticity generation at the shear layer, prompting weaker reverse flow, and thus augmenting stall resistance. However, aerodynamic performance is negatively impacted due to the shear layer passing through the serration array which results in altered surface pressure distribution over the upper surface. It is also found that serration increases turbulence level in the downstream flow. Turbulent momentum transfer near the trailing edge is significantly increased due to the presence of serrations upstream the flow which also influences the mechanisms associated with separation vortex formation and its subsequent development over the upper surface of the wing. Turbulent budget analysis at the leading-edge shear layer demonstrates that serration reduces turbulence production in the immediate vicinity; however, the reduction effect does not persist further downstream when the shear layer rolls up, and eventually merges with a large separation vortex. In the wake of the serrated wing, integral scale was found to be larger than the smooth wing which implies that serrations at the leading-edge does not promote scale reduction at the wake

On contour crossings in contour-advective simulations - part 2 - analysis of crossing errors and methods for their prevention

This is the second of two papers devoted to the analysis of contour crossing errors that occur in contour-advective simulations of fluid motion, where either vorticity or potential vorticity is represented by contours. We begin with a detailed discussion on some of the potential mechanisms for contour crossing. Past work has suggested that the formation of contour crossings is due to inadequate spatial resolution of contours [1]. The implementation of two schemes for preventing contour crossings within the framework of the Contour-Advective Semi-Lagrangian (CASL) algorithm is detailed here. We then present an analysis of contour crossing errors in simulations of quasigeostrophic turbulence on the f-plane and the quasigeostrophic motion of an initially circular vortex patch on the b-plane using the algorithm detailed in Part 1. We find that in general individual crossings occur at scales smaller than the inversion grid scale on which velocity is calculated, but at scales larger than that of the surgical scale that defines the smallest resolved features (vorticity) of a flow. If the resolution of a quasigeostrophic turbulence simulation on the f-plane is increased by doubling the number of grid points in each coordinate direction used in the calculation of the velocity field, then the total area in error due to contour crossings remains unchanged; a smaller number of crossings introducing larger scale area errors is replaced by a greater number of smaller local errors. Uniformly increasing the density of nodes along all contours and placement of nodes at points of close approach on contours are both effective methods for limiting contour crossings

TOPEX/POSEIDON Science Investigations Plan

TOPEX/POSEIDON is a satellite mission that will use the technique of radar altimetry to make precise measurement of sea level with a primary goal of studying the global ocean circulation. The mission represents the culmination of the development of satellite altimetry over the past two decades. The major thrust of the mission is a commitment to measuring seal level with an unprecedented accuracy such that the small-amplitude, basinwide sea level changes that bear significant effects on global change can be detected. The mission will be conducted jointly by the United States National Aeronautics and Space Administration and the French space agency, Centre National d'Etudes Spatiales. The 3- to 5-year mission will study the long-term mean and variability of ocean circulation. This document provides brief descriptions of the planned investigations as well as a summary of the major elements of the mission

Climate Change and Restoration of Degraded Land

The United Nations Climate Change Conference, Durban 2011, delivered a breakthrough on the international community's response to climate change. In the second largest meeting of its kind, the negotiations advanced, in a balanced fashion, the implementation of the Convention and the Kyoto Protocol, the Bali Action Plan, and the Cancun Agreements. The outcomes included a decision by Parties to adopt a universal legal agreement on climate change as soon as possible, and no later than 2015. One of the decisions adopted by COP 17 and CMP 7 regard to the land use, land-use change and forestry, and invites the Intergovernmental Panel on Climate Change to review and, if necessary, update supplementary methodologies for estimating anthropogenic greenhouse gas emissions by sources and removals by sinks resulting from land use, land-use change and forestry activities under Article 3, paragraphs 3 and 4, of the Kyoto Protocol. Land degradation is a human-induced or natural process which negatively affects the productivity of land within an ecosystem. The direct causes of land degradation are geographically specific. Climate change, including changes in short-term variation, as well as long-term gradual changes in temperature and precipitation, is expected to be an additional stress on rates of land degradation. Book Topics: • Introduction to Climate Change and Land Degradation • Change Mitigation • Climate Change and Waste Land Restoration • Water Management and Planning • Erosion and Hydrological Restoration • Forest Fire Land Restoration • Polluted Soils Restoration • Combating Climate Change by Restoration of Degraded Land • Research Matters – Climate Change Governance • Advanced Statistics Climate Change and Restoration of Degraded Land is of interests to academics, engineers, consultans, designers and professionals involved in restoration of degraded lands projects