CFD investigation on the aerodynamic interferences between medium-solidity Darrieus Vertical Axis Wind Turbines

The present study contributes to understand physical mechanisms involved in an achievable power enhancement by setting vertical axis wind turbines in close proximity. The turbines are straight-bladed Darrieus micro-turbines characterized by medium-high solidity and therefore low tip-speed ratio. Preliminary CFD simulations of the isolated turbine explain the reasons why it has a low power output, namely which are laminar flow and laminar separation bubbles on the blades. This fact is expected also considering the low Reynolds number. Subsequently a campaign of CFD simulations has been performed to analyse the aerodynamic interferences in two-rotor configurations. The behaviour of counter-rotating and co-rotating arrangements is analysed at different distances between rotor axes. The simulations show an increasing of power production of about 10% compared to results for the isolated turbine, independently of the sense of rotation. In order to verify wheter vortex shedding suppression might be the cause of the enhanced performance interactions has been simulated between two closely spaced Magnus spinning cylinders with the same tip-speed ratio of the turbines. These last results don’t show reasonable analogies with VAWT wake structures and interactions. Our main conclusion is that accelerated free-stream flow between the turbines is the principle cause of the power extraction enhancement by means of contraction and re-energisation of the turbine wakes. CFD predictions of a four-rotor configuration confirm our hypothesis, nevertheless the wind direction strongly affects the overall efficacy

Ultra-thin 3D silicon sensors for neutron detection

We present a novel neutron detector based on an ultra-thin 3D silicon sensor with a sensitive volume only 10 µm thick. This ultra-thin active volume allows a high gamma-ray rejection, a key requirement in order to discriminate the signal coming from the neutrons in a mixed neutron-gamma ray environment. The device upper-side is covered with a novel boron-based compound that detects neutrons by means of the 10B(n,α)7Li nuclear reaction. The performance of test devices has been investigated first with a gamma-ray source to evaluate the gamma-ray rejection factor, and then with an 241AmBe neutron source to assess the neutron-gamma ray discrimination properties.Peer reviewe

The coherent dynamics of photoexcited green fluorescent proteins

The coherent dynamics of vibronic wave packets in the green fluorescent protein is reported. At room temperature the non-stationary dynamics following impulsive photoexcitation displays an oscillating optical transmissivity pattern with components at 67 fs (497 cm-1) and 59 fs (593 cm-1). Our results are complemented by ab initio calculations of the vibrational spectrum of the chromophore. This analysis shows the interplay between the dynamics of the aminoacidic structure and the electronic excitation in the primary optical events of green fluorescent proteins.Comment: accepted for publication in Physical Review Letter

Ultra Thin 3D Silicon Detector for Plasma Diagnostics at the ITER Tokamak

An ultra thin silicon detector called U3DTHIN[1,2] has been designed and built to be used in detection systems for Neutral particle analyzers (NPA). The main purpose of this detector is to provide a state-of-the-art solution for NPAs at ITER experimental reactor. In the past the NPAs were using very thin scintillators read out by photomultiplier tubes[3,4], and their main drawbacks were poor energy resolution, intrinsic scintillation non-linearity, relative low count rate capability and finally poor signal-to-background separation for the low energy channels. The proposed U3DTHIN detector is based on very thin sensitive substrate which will provide nearly 100% detection efficiency for ions and at the same time very low sensitivity for the neutron and gamma radiation background. To achieve a very fast charge collection of the carriers generated by the ions a 3D electrode structure[5] has been introduced in the sensitive volume of the detector. One of the most innovative features of these detectors has been the optimal combination of the thin entrance window and the sensitive substrate thickness, to accommodate very large dynamic range primary ions energy. With an entrance window of tens of nanometers; together with a sensitive substrate thickness of less than 5 μm we used them for the low energetic channels and lest than 20 μm for the medium and high energetic ones. In order to find the optimal Signal to background ratio simulations with GEANT4 and TCAD has been performed. The first results obtained during characterization of such U3DTHIN detectors will be presented.Non peer reviewe

Performance of neutron-irradiated 4H-Silicon Carbide diodes subjected to Alpha radiation

The unique electrical and material properties of 4H-silicon-carbide (4H-SiC) make it a promising candidate material for high rate particle detectors. In contrast to the ubiquitously used silicon (Si), 4H-SiC offers a higher carrier saturation velocity and larger breakdown voltage, enabling a high intrinsic time resolution and mitigating pile-up effects. Additionally, as radiation hardness requirements grow more demanding, wide-bandgap materials such as 4H-SiC could offer better performance. In this work, the detector performance of 50 micron thick 4H-SiC p-in-n planar pad sensors was investigated at room temperature, using an 241Am alpha source at reverse biases of up to 1100 V. Samples subjected to neutron irradiation with fluences of up to 1e16/cm^2 were included in the study in order to quantify the radiation hardness properties of 4H-SiC. The obtained results are compared to previously performed UV-TCT studies. Samples exhibit a drop in charge collection efficiency (CCE) with increasing irradiation fluence, partially compensated at high reverse bias voltages far above full depletion voltage. A plateau of the collected charges is observed in accordance with the depletion of the volume the alpha particles penetrate for an unirradiated reference detector. For the neutron-irradiated samples, such a plateau only becomes apparent at higher reverse bias. For the highest investigated fluence, CCE behaves almost linearly with increasing reverse bias. Compared to UV-TCT measurements, the reverse bias required to deplete a sensitive volume covering full energy deposition is lower, due to the small penetration depth of the alpha particles. At the highest reverse bias, the measured CCE values agree well with earlier UV-TCT studies, with discrepancies between 1% and 5%.Comment: 10 pages (8 without references), 6 figures, 1 table, to be published in the Proceedings Section of Journal of Instrumentation (JINST) as a proceeding of iWoRiD202