Field test of multiple 1/10 scale tidal turbines in steady flows

Queen's University Belfast and Wave Barrier Ltd have developed a tidal testing platform to test hydrokinetic turbines at medium scale. Multiple turbines can be pushed through still water conditions, in steady-state pushing tests. Experiments were conducted to evaluate the interactions between two identical, mono-strut, horizontal axis tidal turbines (HATTs) of 1.5 m diameter (D) rotor. Their relative performance when located individually, in-plane and in-line are investigated. The data shows a high consistency in the power curves at different flow speeds, which indicates high repeatability in this Reynolds range. For an individual turbine, there is no performance difference when the rotor is mounted either upstream or downstream of the supporting structure. When placed in-plane, the turbines have no adverse effect on one another. When spaced in-line with 2D separation, there is a 63% reduction in the performance of the downstream turbine. At 6D downstream this performance reduction is still 59%, indicating some wake recovery between 2D and 6D, though the influence from the upstream rotor persists to at least 6D downstream of the first device. In contrast the performance of the downstream turbine when placed at 1.5D offset of the upstream device at 6D downstream is approximately recovered to the individual turbine performance.

A spatio-temporal contrast of the predatory impact of an invasive freshwater crustacean

Aim: Impacts of invasive species may vary across invasion gradients, owing to trait-based sorting of individuals through dispersal: those aggregating at invasion fronts may be more aggressive and voracious. We examine, in the field and laboratory, variation in the predatory impacts of an invasive Ponto-Caspian crustacean Hemimysis anomala G.O. Sars, 1907 at two sites along a spatio-temporal gradient of invasion.

Location: Republic of Ireland.

Methods: We used reciprocal transplant field-deployed mesocosms to compare predation rates of invasion front and well-established H. anomala on natural zooplankton assemblages. In the laboratory, we measured the functional response (relationship between predation rate and prey supply) of H. anomala from both sites, for a per capita mechanistic comparison of predation efficiency. We also assessed prey selectivity of H. anomala in the mesocosm experiments to further compare feeding behaviour. Finally, we used a correlative approach to assess the community impact of H. anomala across sites, including a nearby uninvaded site, by comparing zooplankton diversities and densities.

Results: Invasion front H. anomala had higher predation rates than well-established H. anomala at high in situ zooplankton densities. Invasion front H. anomala also had higher functional responses - in particular showing higher 'attack rates' - indicating a heightened ability to locate and capture prey. Prey selectivity was consistent across the spatio-temporal contrast, with positive selection for cladocerans. Zooplankton diversity and density declined with time since H. anomala invasion, both being maximal at the uninvaded site.

Main conclusions: Our study, for the first time, (1) reveals differences in predatory per capita effects and associated behavioural traits between two sites along a spatio-temporal invasion gradient and (2) shows a negative community-level impact of the invasive H. anomala in natural water bodies. Further spatio-temporal comparisons of predatory per capita effects of invaders are needed to assess the generality of these results.

Resonantly loaded apertures for high-resolution near field surface imaging

A novel type of microwave probes based on the loaded aperture geometry has been proposed and experimentally evaluated for dielectrics characterisation and high-resolution near-field imaging. Experimental results demonstrate the possibility of very accurate microwave spectroscopic characterisation of thin lossy dielectric samples and biological materials containing water. High-resolution images of the subwavelength lossy dielectric strips and wet and dry leaves have been obtained with amplitude contrast around 10-20 dB and spatial resolution better than one-tenth of a wavelength in the near-field zone. A microwave imaging scenario for the early-stage skin cancer identification based on the artificial dielectric model has also been explored. This model study shows that the typical resolution of an artificial malignant tumour with a characteristic size of one-tenth of a wavelength can be discriminated with at least 6 dB amplitude and 50° phase contrast from the artificial healthy skin and with more than 3 dB contrast from a benign lesion of the same size. It has also been demonstrated that the proposed device can efficiently deliver microwave energy to very small, subwavelength, focal areas which is highly sought in the microwave hyperthermia applications.

Dynamics and structure of self-generated magnetics fields on solids following high contrast, high intensity laser irradiation

The dynamics of self-generated magnetic B-fields produced following the interaction of a high contrast, high intensity (I > 10¹⁹W cm^-2) laser beam with thin (3 μm thick) solid (Al or Au) targets is investigated experimentally and numerically. Two main sources drive the growth of B-fields on the target surfaces. B-fields are first driven by laser-generated hot electron currents that relax over ∼10-20 ps. Over longer timescales, the hydrodynamic expansion of the bulk of the target into vacuum also generates B-field induced by non-collinear gradients of density and temperature. The laser irradiation of the target front side strongly localizes the energy deposition at the target front, in contrast to the target rear side, which is heated by fast electrons over a much larger area. This induces an asymmetry in the hydrodynamic expansion between the front and rear target surfaces, and consequently the associated B-fields are found strongly asymmetric. The sole long-lasting (>30 ps) B-fields are the ones growing on the target front surface, where they remain of extremely high strength (∼8-10 MG). These B-fields have been recently put by us in practical use for focusing laser-accelerated protons [B. Albertazzi et al., Rev. Sci. Instrum. 86, 043502 (2015)]; here we analyze in detail their dynamics and structure.

High-Resolution Microwave Near-Field Imaging Using Resonance Probes

A novel microwave high-resolution near-field imaging technique is proposed and experimentally evaluated in reflectometry imaging scenarios involving planar metal-dielectric structures. Two types of resonance near field probes-a small helix antenna and a loaded subwavelength slot aperture are studied in this paper. These probes enable very tight spatial field localization with the full width at half maximum around one tenth of a wavelength, λ, at λ/100-λ/10 standoff distance. Importantly, the proposed probes permit resonance electromagnetic coupling to dielectric or printed conductive patterns, which leads to the possibility of very high raw image resolution with imaged feature-to-background contrast greater than 10-dB amplitude and 50° phase. In addition, high-resolution characterization of target geometries based on the cross correlation image processing technique is proposed and assessed using experimental data. It is shown that printed elements features with subwavelength size ~λ/15 or smaller can be characterized with at least 10-dB resolution contrast.