Aviation decarbonization is one of the greatest challenges in the pursuit of sustainable mobility. While incremental improvements in aerodynamics, structures, and propulsion have led to sensible efficiency gains over the last years, the transition toward zero emission aircraft configurations requested by international guidelines needs disruptive technologies. Liquid hydrogen propulsion systems coupled with fuel cells are one of the most promising solutions to be investigated, since they offer high energy density, clean exhaust, and compatibility with regional aircraft missions. This study presents a novel configuration for a regional aircraft propelled by liquid hydrogen and fuel cells, based on conceptual design. The proposed configuration is a high-wing aircraft with T-tail. Unlike kerosene, liquid hydrogen requires specialized tanks and insulation, which significantly influence aircraft geometry and weight distribution. To address these challenges, the proposed configuration adopts a fuselage integrated cryogenic tank system installed in the aircraft rear cone, to minimize aerodynamic penalties while ensuring safety and operational feasibility. The fuel cell system is distributed to optimize redundancy and thermal management, enabling efficient power delivery to electric propulsors. The study contributes to the growing body of literature on hydrogen aviation by providing a system level configuration tailored to regional aircraft, a segment particularly suited for early adoption of hydrogen technologies due to shorter ranges and frequent operations. The findings underline the technical feasibility of liquid hydrogen–fuel cell systems, offering insights for future certification frameworks, infrastructure development, and industrial implementation. In conclusion, this work presents a feasible configuration of a novel green regional aircraft powered by liquid hydrogen and fuel cells. The results provide a foundation for further experimental validation and pave the way for the next generation of environmentally responsible regional aircraft.
The elements of a reconfigurable intelligent surface (RIS) are commonly modeled either as frequency-selective time-invariant reflectors or as instantaneous time-varying reflection coefficients. In practice, however, time-modulated metasurfaces exhibit both frequency selectivity and periodic time variation. We develop a physically consistent linear periodically time-varying (LPTV) model that jointly captures these effects and characterizes their impact on wideband orthogonal frequency-division multiplexing (OFDM) communications. From a canonical equivalent circuit of a generic RIS element, we derive a single-resonance model whose physically meaningful parameters determine both the frequency-selective reflection coefficient and the effective impulse-response duration, i.e., the finite memory of the element. The periodically switched dispersive responses are then represented through harmonic transfer functions, leading to a closed-form per-subcarrier OFDM input-output relation. The resulting coupling is generally non-diagonal: each received subcarrier collects contributions from multiple transmitted subcarriers through the RIS harmonics, each weighted by the element response at the corresponding absolute input frequency. We further derive a generalized cyclic-prefix (CP) condition requiring the guard interval to accommodate both the propagation-channel delay spread and the RIS memory. Under this condition, intersymbol interference and out-of-grid spectral leakage are suppressed, while deterministic on-grid harmonic coupling remains. Full-wave simulations of an OpenRIS unit cell designed for a 5G NR channel validate the proposed resonant model and reveal appreciable in-band dispersion despite nearly ideal binary phase switching. Simulations over 3GPP tapped-delay-line (TDL) channels confirm the generalized CP condition and show its relevance for high-quality-factor RIS elements.
Nacre, the iridescent inner layer of many mollusk shells, exhibits a remarkable combination of strength and toughness, characterized by a pronounced asymmetry in its tensile and compressive failure behavior. While its hierarchical microstructure is known to contribute to this performance, a complete mechanistic understanding of how this architecture governs the distinct failure modes under different loading regimes has remained elusive. This work presents a comprehensive numerical investigation into the fracture mechanics of nacre, explaining the origins of its mechanical asymmetry. Our simulations reveal that under tension, damage localizes in the organic matrix and at interfaces, leading to progressive tablet sliding and pull-out which defines the tensile strength. In compression, the model predicts a radically different failure mode: the confinement provided by the tablet structure suppresses matrix damage, leading to widespread buckling and kinking of the mineral tablets, which occurs at a significantly higher stress threshold. The proposed model successfully replicates the macroscopic stress-strain response and the observed failure morphologies for both loading conditions. The results provide a fundamental mechanistic explanation for the superior compressive strength of nacre, directly linking its iconic hierarchical architecture to its exceptional and asymmetric mechanical performance.
The paper presents part of the experimental activities carried out within the framework of the NATO AVT-415 activities aimed at characterising the flow behaviour of a bidimensional bevel rectangular nozzle. The optimisation of Unmanned Air Vehicles (UAVs), for both military and civil applications, necessitates conducting advanced computational studies that rigorously analyse aerodynamic and aeroacoustic behaviours, particularly in proximity to the exhaust region of the propulsion system. This investigation focuses on a wing-blended vehicle with a fully integrated propulsion system. The primary objectives are to enhance aerodynamic performance while concurrently minimising radar cross-section, thermal detection and acoustic emissions. The activity foresees investigating the flow behaviour inside a bevel nozzle characterised by an internal S-shape, an internal section variation from elliptical at the nozzle entrance to rectangular at the exit, for three different mass flow rates. Pressure distribution along the nozzle symmetry plane was investigated by 44 pressure taps, inlet Mach distribution was investigated by two static and total pressure probe rakes and the flow evolution downstream the exit nozzle was depicted by multiplane PIV measurement up to 15 nozzle heights. Vertical and horizontal measurement planes were investigated to consider the three-dimensionality of the wing-bleed vehicle and the integrated nozzle. The paper deals with flow behaviour and pressure distribution inside and outside the nozzle. The velocity fields are discussed in terms of flow velocity and flow fluctuations, taking into consideration the highly three-dimensional nature of the flow velocity.
