1,522 research outputs found
A correction to the enhanced bottom drag parameterisation of tidal turbines
Hydrodynamic modelling is an important tool for the development of tidal
stream energy projects. Many hydrodynamic models incorporate the effect of
tidal turbines through an enhanced bottom drag. In this paper we show that
although for coarse grid resolutions (kilometre scale) the resulting force
exerted on the flow agrees well with the theoretical value, the force starts
decreasing with decreasing grid sizes when these become smaller than the length
scale of the wake recovery. This is because the assumption that the upstream
velocity can be approximated by the local model velocity, is no longer valid.
Using linear momentum actuator disc theory however, we derive a relationship
between these two velocities and formulate a correction to the enhanced bottom
drag formulation that consistently applies a force that remains closed to the
theoretical value, for all grid sizes down to the turbine scale. In addition, a
better understanding of the relation between the model, upstream, and actual
turbine velocity, as predicted by actuator disc theory, leads to an improved
estimate of the usefully extractable energy. We show how the corrections can be
applied (demonstrated here for the models MIKE 21 and Fluidity) by a simple
modification of the drag coefficient
On spinodal decomposition in alnico---a transmission electron microscopy and atom probe tomography study
Alnico is a prime example of a finely tuned nanostructure whose magnetic
properties are intimately connected to magnetic annealing (MA) during spinodal
transformation and subsequent lower temperature annealing (draw) cycles. Using
a combination of transmission electron microscopy and atom probe tomography, we
show how these critical processing steps affect the local composition and
nanostructure evolution with impact on magnetic properties. The nearly 2-fold
increase of intrinsic coercivity () during the draw cycle is not
adequately explained by chemical refinement of the spinodal phases. Instead,
increased Fe-Co phase () isolation, development of Cu-rich
spheres/rods/blades and additional rod precipitation that occurs
during the MA and draw, likely play a key role in enhancement.
Chemical ordering of the Al-Ni-phase () and formation of Ni-rich
() may also contribute. Unraveling of the subtle effect of these
nano-scaled features is crucial to understanding on how to improve shape
anisotropy in alnico magnets
Simulation of alnico coercivity
Micromagnetic simulations of alnico show substantial deviations from
Stoner-Wohlfarth behavior due to the unique size and spatial distribution of
the rod-like Fe-Co phase formed during spinodal decomposition in an external
magnetic field. The maximum coercivity is limited by single-rod effects,
especially deviations from ellipsoidal shape, and by interactions between the
rods. Both the exchange interaction between connected rods and magnetostatic
interaction between rods are considered, and the results of our calculations
show good agreement with recent experiments. Unlike systems dominated by
magnetocrystalline anisotropy, coercivity in alnico is highly dependent on
size, shape, and geometric distribution of the Fe-Co phase, all factors that
can be tuned with appropriate chemistry and thermal-magnetic annealing
Investigation of Cyclic Liquefaction with Discrete Element Simulations
A discrete-element method (DEM) assembly of virtual particles is calibrated to approximate the behavior of a natural sand in undrained loading. The particles are octahedral, bumpy clusters of spheres that are compacted into assemblies of different densities. The contact modelis a Jäger generalization of the Hertz contact, which yields a small-strain shear modulus that is proportional to the square root of confining stress. Simulations made of triaxial extension and compression loading conditions and of simple shear produce behaviors that are similar to sand. Undrained cyclic shearing simulations are performed with nonuniform amplitudes of shearing pulses and with 24 irregular seismic shearing sequences. A methodology is proposed for quantifying the severities of such irregular shearing records, allowing the 24 sequences to be ranked in severity. The relative severities of the 24 seismic sequences show an anomalous dependence on sampling density. Four scalar measures are proposed for predicting the severity of a particular loading sequence. A stress-based scalar measure shows superior efficiency in predicting initial liquefaction and pore pressure rise
Efficient unstructured mesh generation for marine renewable energy applications
Renewable energy is the cornerstone of preventing dangerous climate change whilst maintaining a robust energy supply. Tidal energy will arguably play a critical role in the renewable energy portfolio as it is both predictable and reliable, and can be put in place across the globe. However, installation may impact the local and regional ecology via changes in tidal dynamics, sediment transport pathways or bathymetric changes. In order to mitigate these effects, tidal energy devices need to be modelled in order to predict hydrodynamic changes. Robust mesh generation is a fundamental component required for developing simulations with high accuracy. However, mesh generation for coastal domains can be an elaborate procedure. Here, we describe an approach combining mesh generators with Geographical Information Systems. We demonstrate robustness and efficiency by constructing a mesh with which to examine the potential environmental impact of a tidal turbine farm installation in the Orkney Islands. The mesh is then used with two well-validated ocean models, to compare their flow predictions with and without a turbine array. The results demonstrate that it is possible to create an easy-to-use tool to generate high-quality meshes for combined coastal engineering, here tidal turbines, and coastal ocean simulations
Vaccination against Allergy: A Paradigm Shift?
Since the discovery that IgE antibodies mediate allergy, decades of research have unraveled complex mechanisms associated with conventional immunotherapy and the vital protagonists that shape 'immune tolerance' to allergens. Debate exists on what should constitute the dominant effector mechanism in driving rational drug designs for next-generation immunotherapies. As vaccine technology continues to advance, the development of novel vaccines in this area of continued medical need might stand on a threshold of breakthrough inspired by experiments by Dunbar on the passive vaccination of allergic animals more than 100 years ago. In this opinion article, we discuss both novel insights into IgG antibodies as the principle effector modality induced by specific immunotherapy and advances in antigen-carrier design that may catapult allergy treatment into our modern world
Improving tidal turbine array performance through the optimisation of layout and yaw angles
Tidal stream currents change in magnitude and direction during flood and ebb tides. Setting the most appropriate yaw angles for a tidal turbine is not only important to account for the performance of a single turbine, but can also be significant for the interactions between the turbines within an array. In this paper, a partial differentiation equation (PDE) constrained optimisation approach is established based on the Thetis coastal ocean modelling framework. The PDE constraint takes the form here of the two-dimensional, depth-averaged shallow water equations which are used to simulate tidal elevations and currents in the presence of tidal stream turbine arrays. The Sequential Least Squares Programming (SLSQP) algorithm is applied with a gradient obtained via the adjoint method in order to perform array design optimisation. An idealised rectangular channel test case is studied to demonstrate this optimisation framework. Located in the centre of the computational domain, arrays comprised of 12 turbines are tested in aligned and staggered layouts. The setups are initially optimised based on their yaw angles alone. In turn, turbine coordinates and yaw angles are also optimized simultaneously. Results indicate that for an aligned turbine array case under steady state conditions, the energy output can be increased by approximately 80\% when considering yaw angle optimisation alone. For the staggered turbine array, the increase is approximately 30\%. The yaw optimised staggered array is able to outperform the yaw optimised aligned array by approximately 8\%. If both layout and the yaw angles of the turbines are considered within the optimisation then the increase is more significant compared with optimising yaw angle alone
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