A CONTINUUM MIXTURE THEORY APPROACH TO SEDIMENT TRANSPORT WITH APPLICATION TO TURBULENT OSCILLATORY BOUNDARY LAYERS

Two aspects of the modelling of suspended sediment transport are investigated. One is the development of a theoretical base for sediment transport models starting from the continuum theory of immiscible mixtures (also know as two-phase flow theories). The other is a comparison with experimental data of numerical predictions from a number of turbulence models for oscillatory, turbulent boundary layer flow containing suspended sediment. A review is given of previous work that has applied continuum mixture theories to the field of sediment transport. Turbulent averaged forms of the mixture equations are presented and, in the dilute particle concentration limit and neglecting the effects of particle inertia, the equations are shown to reduce to those encountered in traditional approaches to modelling suspended sediment concentrations. Likewise, the equations governing the motion of the fluid phase reduce to standard forms, with the effect of the sediment particles appearing as a buoyancy term in the fluid momentum equation. Particle inertia is taken into account by expanding in terms of a non-dimensional parameter, the ratio of the response time of the particle to a characteristic time of the flow. Terms arising from particle inertia are then reduced to correlations for which models are available in the literature. The assumption of dilute particle concentrations is made throughout the derivation. An extensive comparison between a number of turbulence models is made by comparing numerical predictions with experimental data, whilst making the conventional assumption of zero particle inertia. The k - c model was found to perform well, with simpler models also giving reasonable agreement with experiment. Also investigated is the sensitivity of the solution to a number of factors, including: boundary conditions, empirical turbulence constants, and the stratifying effect of the suspended sediment. The effect of including terms associated with particle inertia are investigated in turbulent oscillatory boundary layer flows. This is found to lead to an enhancement of the vertical particle volume flux. However, given the uncertainties of specifying the boundary condition for the concentration at the bed, the effect is probably not of significance for small particles (diameter - 0.1 mm). Larger particles (diameter 0.25 mm) show more significant effects due to their inertia. The difference in mean horizontal velocity between the fluid and particle phases which results from the inclusion of inertia in the particle momentum equations is calculated. This difference is found to be very small.Sir William Halcrow and Partner

Assessment of nutrients and macroalgae growth in Poole Harbour, UK

Water Qualit

Microscale elastic properties of interphases in polymer matrix composites: correlating spatial mapping with cure history

Polymer matrix composites with textile reinforcement are used in a wide range of aerospace and industrial applications. Continuum mechanical predictions of the composite behaviors have been inaccurate and resorted to empirical corrections, because of the lack of polymer materials property information. The length scales involved make experimental measurement of the elastic properties of the matrix within fiber tows and proximity to individual fibers difficult. However, micro-Brillouin and Raman light scattering provide sufficiently high spatial resolution to probe the mechanical properties and chemical composition of the matrix, without interfering with the thermo-mechanical equilibrium of the material. The elastic properties of epoxy resin have been measured between and within the fiber tows of a composite with this technique, and compared to a bulk epoxy resin. Using this approach, the elastic properties have also been monitored in situ, during epoxy cure under different thermal and chemical conditions. To interpret and enhance these results, experiments are complemented with molecular dynamics simulations of the interface extrapolating findings to nanometer length scales. We observe that matrix materials in close proximity to fibers have a diminished elastic modulus compared with both bulk epoxy and material between tows. To explain the underlying reason for this finding we identify the extent to which residual stresses, chemical inhomogeneities, or purely structural rearrangements near the interface contribute to this effect. Finally, we correlate the spatial distribution of mechanical properties with the cure history

Distribution of natural disturbance due to wave and tidal bed currents around the UK

The UK continental shelf experiences large tidal ranges and winter storm events, which can both generate strong near-bed currents. The regular tidal bottom currents from tides plus wind driven ‘benthic storms’ (dominated by wave-driven oscillatory currents in shallow water) are a major source of disturbance to benthic communities, particularly in shallow waters. We aim to identify and map the relative impact of the tides and storm events on the shallower parts of the North West European continental shelf. A ten-year simulation of waves, tides and surges on the continental shelf was performed. The shelf model was validated against current meter observations and the Centre for Environmental, Fisheries and Aquaculture Science (CEFAS) network of SmartBuoys. Next, the model performance was assessed against seabed lander data from two sites in the Southern North Sea; one in deep water and another shallow water site at Sea Palling, and a third in Liverpool Bay. Both waves and currents are well simulated at the offshore Southern North Sea site. A large storm event was also well captured, though the model tends to underpredict bottom orbital velocity. Poorer results were achieved at the Sea Palling site, thought to be due to an overly deep model water depth, and missing wave-current interactions. In Liverpool Bay tides were well modelled and good correlations (average R–squared=0.89) observed for significant wave height, with acceptable values (average R–squared=0.79) for bottom orbital velocity. Using the full ten-year dataset, return periods can be calculated for extreme waves and currents. Mapping these return periods presents a spatial picture of extreme bed disturbance, highlighting the importance of rare wave disturbances (e.g. with a return period of 1 in 10 years). Annual maximum currents change little in their magnitude and distribution from year to year, with mean speeds around 0.04 ms−1, and maximums exceeding 3 ms−1. Wave conditions however are widely variable throughout the year, depending largely on storm events. Typical significant wave heights (Hs) lie between 0.5–2 m, but storm events in shallow water can bring with them large waves of 5 m and above and up to 18 m in North West Approaches/North West Scotland ( Sterl and Caires, 2005). The benthic disturbance generated by waves and currents is then estimated by calculating the combined force on an idealised object at the bed. The patterns of this disturbance reflect both regular tidal disturbance and rare wave events. Mean forces are typically 0.05–0.1 N, and are seen largely in areas of fast currents (View the MathML source>1ms−1). The pattern of maximum force however is more dependent on water depth and exposure to long-fetches (View the MathML source>1000km) suggesting it is dominated by wave events

Sediment dynamics of a nearshore sandbank: Results from TELEMAC-2D, TOMAWAC and SISYPHE modelling

Water Qualit

Cure kinetics and interfacial phenomena in polymer matrix composites

Polymer matrix composites with textile reinforcement are used in a wide range of aerospace and industrial applications. Continuum mechanical predictions of the composite behaviors have been inaccurate, possibly because of the lack of information with regard to polymer materials properties, especially near the interfaces with the reinforcing fibers. Concurrent micro-Brillouin and Raman light scattering provides sufficiently high spatial resolution to probe the mechanical properties and chemical composition of the interphase regions of the matrix, without interfering with the thermo-mechanical equilibrium of the material. Using this technique, we mapped the elastic properties of epoxy resin in between and within the fiber tows of a composite, revealing that the modulus exhibits a marked spatial inhomogeneity in proximity of fibers, with a decrease of up to 5% compared to that of bulk epoxy resin in the regions of highest fiber density (see Fig. 1).1 We estimate that it would take a deformation of four times the failure strain to cause such a change in modulus based on residual stresses. Hence, the origin must lie elsewhere. Using the same methodology, we then monitored the elastic properties in situ, during epoxy cure under different thermal and chemical conditions. We find that depending on the reaction rate, the elastic modulus evolves differently as a function of the degree of cure: the faster the rate, the more the modulus lags behind of what would be expected from the amount of cross-links that have formed according to the degree of cure. This is because the overall modulus is based on the stiffness resulting from bonded and non-boned network connections, the latter arising the optimization of network packing that ensues after a slow structural relaxation.2 Provided enough time, the same final modulus is reached, unless network formation is impeded by the under-supply of hardener. To interpret and enhance these results, experiments are complemented with molecular dynamics simulations of the interface. Accordingly, the one-sided confinement of polymer adjacent to a fiber surface results in clearly detectable structural features, e.g., layering and densification, as well as changes in the elastic properties within a spatial extent that reaches significantly beyond the region of distinguishable structural features. In conclusion, we attribute the inhomogeneity in mechanical properties to a combination of hardener depletion and an impediment of structural relaxation due to unilateral confinement that lowers the extent and effectiveness of non-bonded interaction

Implementation of a 3D Coupled Hydrodynamic–Biogeochemical Model in Kuwait Bay

Production of farmed fish is increasing worldwide and in areas which have traditionally not had large scale farming, specifically regions of high sea temperature. This research presents a methodology to assess the impacts of these developments on water quality and to manage them in the context of other discharges into the marine environment. Kuwait Bay, in Kuwait, is used as a case study for these types of environments, where the impacts of finfish farms are assessed regarding their location by implementing a 3D coupled hydrodynamic–biogeochemical model. The model was validated against a monthly climatology of field data for hydrodynamics and biogeochemical parameters. Results show that the impact of a farm size with an average historical production is minimal, with a slight increase in nutrient concentrations (0.4%) and in chlorophyll-a and oxygen (less than 1%) compared to the baseline (no farm). When the farm was located outside the bay, at the southern coast, the impact was even smaller. This suggests that the flushing conditions of the location are a prime consideration and can help mitigate the impacts of larger farm sizes

Trends in Real Food Prices in Six Sub-Saharan African Countries

Demand and Price Analysis, Downloads July 2008-July 2009: 12,