Ducted wind turbines in yawed flow: a numerical study

Ducted wind turbines (DWTs) can be used for energy harvesting in urban areas where non-uniform flows are caused by the presence of buildings or other surface discontinuities. For this reason, the aerodynamic performance of DWTs in yawed-flow conditions must be characterized depending upon their geometric parameters and operating conditions. A numerical study to investigate the characteristics of flow around two DWT configurations using a simplified duct-actuator disc (AD) model is carried out. The analysis shows that the aerodynamic performance of a DWT in yawed flow is dependent on the mutual interactions between the duct and the AD, an interaction that changes with duct geometry. For the two configurations studied, the highly cambered variant of duct configuration returns a gain in performance by approximately 11 % up to a specific yaw angle (α= 17.5∘) when compared to the non-yawed case; thereafter any further increase in yaw angle results in a performance drop. In contrast, performance of less cambered variant duct configuration drops for α>0∘. The gain in the aerodynamic performance is attributed to the additional camber of the duct that acts as a flow-conditioning device and delays duct wall flow separation inside of the duct for a broad range of yaw angles.

How fast the earth surface breathe? Gas transport in high permeability soils and earth surface discontinuities

<p>Gas movement within the earth’s subsurface and its exchange with the atmosphere are some of the principal processes in soil, ecosystem, and atmospheric environments. For a decade, our group has explored the roles played by atmospheric conditions and matrix properties in gas transport at the earth-atmosphere interface, where surface discontinuities, such as fractures, boreholes and aggregated soils, exist and may affect the process.</p><p>The gas transport mechanisms, resulting from the development of a thermal gradient and surface wind, were analyzed both independently and in combination. Two types of experiments were carried out: (1) under field conditions and (2) under highly controlled laboratory conditions. During all studies, temperature and wind conditions across the media and at the media-atmosphere interface were monitored. Results show that the magnitudes of thermal- and wind-induced convection were directly related to the media permeability, given favorable ambient conditions at the media-atmosphere interface. Such ambient conditions included high diurnal temperature amplitude (~± 10 ᵒC) or high surface wind (~2 m/s measured 10 m above ground). In addition, specific results from the field experiment were used to establish an empirical model that predicts gas transport magnitude as a function of wind speed and media permeability.</p><p>With respect to other discontinuities, such as boreholes and fractures, the effect of atmospheric conditions was investigated, namely atmospheric pressure and temperature, on air, CO<sub>2</sub>, and radon transport. Using high-resolution spatiotemporal measurements, it was concluded that diurnal atmospheric pressure oscillations (barometric pumping) and borehole-atmospheric temperature differences (thermal-induced convection) controlled the air transport within the boreholes. For one of the boreholes monitored, the air velocities and CO<sub>2</sub> emissions to the atmosphere were quantified (up to ~6 m/min and ~5 g-CO<sub>2</sub>/min, respectively). This reveals the role of boreholes as a source of greenhouse gas emissions.</p><p>The results and conclusions derived from our studies are expected to improve our understanding of the governing mechanisms controlling gas movement in porous media, fractures, and boreholes, and their functions in gas exchange across the earth-atmosphere interface.</p>

A review on the recent advances concerning the fatigue performance of titanium alloys for orthopedic applications

This article presents a review on recent advances in the fatigue behavior of Ti alloys, especially the main commercial compositions for orthopedic applications. In the case of well‐known Ti–6Al–4V alloy, the major concern is related to the effect of the surface modification necessary to improve the osseointegration. The introduction of surface discontinuities due to the growth of a porous oxide layer, or the roughness development, may severely affect the fatigue performance depending on the level of alteration. In the case of additive manufactured Ti–6Al–4V, the fatigue response is also influenced by inherent defects of as‐built parts. Regarding the recently developed metastable β alloys, information about the fatigue properties is still scarce and mainly related to the effect of second phase precipitates, which are introduced to optimize the mechanical properties. The fatigue behavior of the Ti alloys is complex, as is their microstructure, and should not be neglected when the alloys are being developed or improved to be applied in medical devices.

Integral representation for energies in linear elasticity with surface discontinuities

In this paper we prove an integral representation formula for a general class of energies defined on the space of generalized special functions of bounded deformation ({\mathrm{GSBD}^{p}}) in arbitrary space dimensions. Functionals of this type naturally arise in the modeling of linear elastic solids with surface discontinuities including phenomena as fracture, damage, surface tension between different elastic phases, or material voids. Our approach is based on the global method for relaxation devised in [G. Bouchitté, I. Fonseca and L. Mascarenhas, A global method for relaxation, Arch. Ration. Mech. Anal. 145 1998, 1, 51–98] and a recent Korn-type inequality in {\mathrm{GSBD}^{p}}, cf. [F. Cagnetti, A. Chambolle and L. Scardia, Korn and Poincaré–Korn inequalities for functions with a small jump set, preprint 2020]. Our general strategy also allows to generalize integral representation results in {\mathrm{SBD}^{p}}, obtained in dimension two [S. Conti, M. Focardi and F. Iurlano, Integral representation for functionals defined on \mathrm{SBD}^{p} in dimension two, Arch. Ration. Mech. Anal. 223 2017, 3, 1337–1374], to higher dimensions, and to revisit results in the framework of generalized special functions of bounded variation ({\mathrm{GSBV}^{p}}).

Failures of Flexible Diaphragm Couplings of Power Take Off (PTO) Shafts of an Aircraft by Surface Discontinuity, Controlled by Stress Concentration or Stress Intensity Factor

Failures of two power take off (PTO) shafts of an aircraft have been analysed. Two shafts, one each developed by two different manufacturers failed separately during power run endurance test conducted at room temperature and ambient normal atmosphere. In both the cases, cracks were observed on the outer diaphragm disc. One shaft showed cracking in the engine side, while the other one exhibited cracks in the aircraft mounting accessory gearbox (AMAGB) side. Chemical analysis, microstructure and hardness evaluation indicate that the diaphragm material of the shafts is Ti-6Al-4V alloy used in solution treated and aged condition, as per the desired specification AMS 4928. Microstructural in-homogeneity, possibly a result of improper forging, was observed in diaphragm material of both the shafts. Additionally, surface discontinuities induced by forging and subsequent insufficient machining were noticed on the diaphragms. The diaphragms failed by fatigue with cracks possibly nucleating at surface discontinuities. Discontinuities with lower availability in one shaft led to somewhat increased life (466 million cycles) as compared to the life (104 million cycles) of the other shaft. Another possible factor contributing to lower life in the later shaft is the presence of higher quantity of nitrogen rich inclusions. Controlling factor triggering the failure of diaphragm of shaft with lower life seems to be the available high stress level along the rim periphery, while that for shaft with higher life is presence of few localized sharp surface discontinuities.

Hydraulic behavior of subsidence-induced surface discontinuities in the hydrogeology of the Aguascalientes valley

The impact of faults and fissures (discontinuities) on the groundwater flow has become important in several parts of the world because the heterogeneous and anisotropic distribution of permeability in fault zones is difficult to characterize. Based on this, we propose an analysis of patterns of parameters measured in groundwater, under the premise that the observed anomalies can be indicators of the hydraulic behavior of the flow in the direction perpendicular to the fault plane. In this context, if the discontinuities are sealed, they behave as hydraulic barriers, causing variation in the continuity of the parameters across the fault plane. Conversely, when faults are a conduit, they appear to have a small or null variation in the distribution of the parameter measurements. The impact of discontinuities in groundwater flow in a zone with a large number of faults and fissures such as that of the Aguascalientes valley is being studied using a graphical-correlation analysis with the revision of 230 wells, through the measurement of parameters such as temperature and static levels across discontinuities, in order to determine the hydraulic behavior of the faults. This investigation considered values over 4 ∘C for geothermal variations and 10 m for hydraulic-head changes to define fault behavior. Results show three zones through mapping analysis, where the fault presents barrier behavior and where the hanging block represents high values; these anomalies are much higher than the average across the valley and indicate the propensity for the fault to restrict horizontal flow. In conclusion, the Oriente fault presents complex behavior of a barrier–conduct system along the fault. This analysis gives a robust way to describe fault behavior without referring to elaborate and invasive hydrological investigations.