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The aerodynamic noise radiated by the flow past a cylinder in the subcritical regime can be modeled by a quadrupolar sound source placed at the onset position of the vortex-shedding instability that is scattered by the surface with a dipolar directivity. When the cylinder is coated with a porous material, the intensity of the shed vortices is greatly reduced, determining a downstream shift of the instability-outbreak location. Consequently, sound diffraction is less efficient, and noise is mitigated. In this paper, an innovative design approach for a flow-permeable coating based on a further enhancement of such an effect is proposed. The results of phased-microphone-array measurements show that, once the leeward part of the cover is integrated with components that make the flow within the porous medium more streamlined, the quadrupolar source associated with the vortex-shedding onset is displaced more downstream, yielding additional noise attenuation of up to 10 dB with respect to a uniform coating. Furthermore, the same noise-control mechanism based on the weakening of the sound scattering can be exploited when these components are connected to the bare cylinder without the porous cover. In this case, the mitigation of overall sound pressure levels is comparable to that induced by the coated configurations due to the lack of noise increase produced by the inner flow interacting within the pores of the material. Remarkable sound reductions of up to 10 dB and a potential drag-force decrease are achieved with this approach, which paves the way for disruptive and more optimized noise-attenuation solutions., Wind Energy, Group Garcia Espallargas more...

In the present work, we study the effect of quenching and annealing on the ferroelectric and piezoelectric properties at room temperature and elevated temperatures of a new ternary BiFeO3-PbTiO3-Li0.5Bi0.5TiO3 bulk piezo ceramic. While sacrificing part of the maximally obtainable piezoelectric constant value, using an optimal heat treatment, a quasi-stable value for the piezoelectric constant of 65 pC/N was obtained irrespective of the annealing temperature. All experimental results point to the direction of unusual defect behavior in this novel ternary system leading to a well-defined metastable state. The quenching and annealing process are completely reversible and can be used in combination with additional chemical modifications to tailor the properties of this new high-temperature piezoelectric ceramic to the intended use conditions., Novel Aerospace Materials more...

In this work, we investigated the piezoelectric properties of BiFeO3-rich (1 − (y + x)) BiFeO3–y PbTiO3–x SrTiO3 (0.1 ≤ x ≤ 0.35; 0.1 ≤ y ≤ 0.3) bulk piezoceramics, as this system could potentially lead to the development of bulk piezoelectric ceramics that are suitable for high-temperature applications (>200 °C). Samples with various levels of PbTiO3 and SrTiO3 were prepared via a conventional solid-state route. X-ray diffraction confirmed a pure perovskite phase for the compositions, which was explored without secondary phases. It was found that the addition of comparable levels of PbTiO3 and SrTiO3 to the BiFeO3 ceramic resulted in higher piezoelectric properties compared to those of the pure BiFeO3 and binary systems. The Curie temperature was significantly reduced by dual doping, with SrTiO3 and PbTiO3 additions resulting in comparable Curie temperature depressions. The locations of the phase boundaries between the cubic, pseudocubic, and rhombohedral crystal structures were determined. The highest piezoelectric properties, including a d33 value of 250 pC/N at room temperature, were obtained for the samples with the composition x = 0.3, y = 0.25, which was close to the cubic–pseudocubic phase boundary in the phase diagram. The temperature dependence of the piezoelectric properties varied depending on the previous thermal history, yet an appropriate heat treatment resulted in an almost temperature-stable d33 value. The ceramic with the lowest temperature sensitivity and a high Curie temperature of 350 °C was found for x = 0.1, y = 0.2 with a d33 value of 60 pC/N at RT and 71 pC/N at 300 °C (after poling at 60 kV/cm and a stabilizing heat treatment). However, the materials developed were still unsuitable for applications at high temperatures due to a rapidly increasing electrical conductivity with increasing temperature., Novel Aerospace Materials more...

In order to capture and separate CO2 from the air or flue gas streams through nanoporous adsorbents, the influence of the humidity in these streams has to be taken into account as it hampers the capture process in two main ways: (1) water preferentially binds to CO2 adsorption sites and lowers the overall capacity, and (2) water causes hydrolytic degradation and pore collapse of the porous framework. Here, we have used a water-stable polyimide covalent organic framework (COF) in N2/CO2/H2O breakthrough studies and assessed its performance under varying levels of relative humidity (RH). We discovered that at limited relative humidity, the competitive binding of H2O over CO2 is replaced by cooperative adsorption. For some conditions, the CO2 capacity was significantly higher under humid versus dry conditions (e.g., a 25% capacity increase at 343 K and 10% RH). These results in combination with FT-IR studies on equilibrated COFs at controlled RH values allowed us to assign the effect of cooperative adsorption to CO2 being adsorbed on single-site adsorbed water. Additionally, once water cluster formation sets in, loss of CO2 capacity is inevitable. Finally, the polyimide COF used in this research retained performance after a total exposure time of >75 h and temperatures up to 403 K. This research provides insight in how cooperative CO2-H2O can be achieved and as such provides directions for the development of CO2 physisorbents that can function in humid streams., Novel Aerospace Materials, ChemE/O&O groep, ChemE/Catalysis Engineering more...

The current study reports in situ TEM observations of the growth of bainitic ferrite in an Fe-0.3C-3Mn-1.5Si-0.15Mo steel held isothermally at 300 °C with a higher spatio-temporal resolution than in previous studies. Significant variations were found in the lengthening rate, with the highest being in excess of 30,000 nm.s−1 while more common lengthening rates of 10–1000 nm.s−1 provided the highest quality observations. Both sheaves with visible sub units and individual laths were observed during growth with the lengthening behaviour of sheaves found to be discontinuous - in the most favourably oriented sheave this could be linked to sub unit behaviour. The transformation behaviour was comparable to that of HT-LSCM observations of bainitic ferrite growth for the most comparable steel compositions and to ‘textbook’ descriptions of the formation of bainite sheaves. In addition, other relevant phenomena were recorded, including the generation and movement of dislocations in the austenite during transformation, the interaction of laths with twin boundaries and the initially slow growth of bainitic ferrite laths., Team Maria Santofimia Navarro, Novel Aerospace Materials more...