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The rotating detonation characteristics of high-temperature hydrogen-rich gas were studied. Hydrogen-rich gas was generated by the pre-combustion of hydrogen, and a rotating detonation experiment of hydrogen-rich gas and air was subsequently performed. The auto-initiation of high-temperature hydrogen-rich gas was observed in the experiment, and the influence of pre-detonation tube ignition on the steady propagation of the detonation wave was analyzed. The results show that high-temperature hydrogen-rich gas and air have the ability to spontaneously form rotating detonation waves. The operation of the pre-detonation tube has a significant influence on the propagation mode and propagation velocity of the continuous rotating detonation wave after auto-initiation. The rotational detonation wave formed by the auto-initiation of hydrogen-rich gas and air has a short instability in the propagation process. The propagation velocity of the detonation wave before and after the unstable state is 1345.4 m/s and 1425.3 m/s, respectively, the unstable state is 1345.4 m/s and 1425.3 m/s, respectively.

In integrated PNT systems, due to defects in satellite signals and long-wave signals, VLF signals can be an essential supplement. However, there is currently a lack of VLF timing systems in the world, and it is impossible to evaluate the impact of the propagation delay of these signals. Based on the theory of very-low-frequency propagation, this paper determines the waveguide mode propagation at ultra-long distances as the main research direction, establishes a signal phase change model, gives a theoretical formula for the phase velocity of VLF signals, and analyzes the main factors affecting the phase velocity of VLF signal propagation. Finally, combined with historical observation data, the phase change is predicted, compared, and analyzed. The results show that the theoretical calculation is consistent with the measured data. The average error of the delay prediction is 0.015 microseconds per 100 km, and the maximum error of the delay prediction is 0.152 microseconds per 100 km.

BackgroundInterests on low-loss terahertz (THz) waveguides are increasing due to their remarkable applications in various fields. Since most the materials are highly absorbent to THz waves therefore it is an ongoing challenge to obtain a low-loss waveguide. This paper presents a novel porous-core square lattice photonic crystal fiber (PCF) for efficient transmission of THz waves.MethodsThe guiding properties of the proposed fiber are characterized by using finite element method (FEM) with circular perfectly matched layer (PML) boundary conditions.ResultsIt is demonstrated that the designed PCF shows very low effective material loss (EML) of 0.076 cm-1 at 1.0 THz that indicates about 62 % reduction of bulk absorption loss of the background material. In addition to this, the proposed fiber exhibits low confinement loss of 8.96 × 10-3 dB/cm and low flattened dispersion of 0.96 ± 0.086 ps/THz/cm for the optimal design parameters. Other important propagation characteristics such as single mode propagation, power fraction, and bending loss are also investigated thoroughly.ConclusionsA porous-core PCF is an efficient mechanism for the transmission of THz waves. The proposed low-loss and low-dispersion PCF can find numerous applications in THz regime.

The fracture evolution and the strength characteristics of a jointed rock mass under hydro-mechanical coupling are key issues that affect the safety and stability of underground engineering. In this study, a kind of transparent rock-like resin was adopted to investigate the crack initiation and propagation modes of the 3D flaw under hydro-mechanical coupling. The influences of the water pressure and the flaw dip angle on the fracture modes of the 3D flaw and the strength properties of the specimen were analyzed. The experiment results indicated that under the initiation and propagation modes, the 3D flaw presented two types of modes: the low-water-pressure type and the high-water-pressure type. The increase in the water pressure had a significant promoting effect on the crack initiation and propagation, which changed the overall failure mode of the specimen. With the increase in the flaw dip angle, the critical growth length of the wing crack decreased and the initiation moment of the fin-like crack showed a hysteretic tendency. The influences of the water pressure on the crack initiation stress and failure strength had thresholds. When lower than the threshold, the crack initiation stress increased slightly and the failure strength decreased gradually with the increase in the water pressure. Once the threshold was exceeded, both the crack initiation stress and the failure strength decreased significantly with the increase in the water pressure. With the increase in the flaw dip angle, both the crack initiation stress and the failure strength showed a first decreasing and then increasing tendency. The lowest crack initiation stress and the failure strength were found for the specimen containing the 45° flaw, while the highest were found for the specimen containing the 75° flaw. This study helps to deepen the understanding of the fracture mechanism of the engineering rock mass under hydro-mechanical coupling and has certain theoretical and applied value in engineering design and construction safety.

In this research, a photonic crystal fiber (PCF)-based sulfuric acid detector is proposed and investigated to identify the exact concentration of sulfuric acid in a mixture with water. In order to calculate the sensing and propagation characteristics, a finite element method (FEM) based on COMSOL Multiphysics software is employed. The extensive simulation results verified that the proposed optical detector could achieve an ultra-high sensitivity of around 97.8% at optimum structural and operating conditions. Furthermore, the proposed sensor exhibited negligible loss with suitable numerical aperture and single-mode propagation at fixed operating conditions. In addition, the circular air holes in the core and cladding reduce fabrication complexity and can be easily produced using the current technology. Therefore, we strongly believe that the proposed detector will soon find its use in numerous industrial applications.

In quasi-brittle materials such as concrete, numerical methods are frequently used to simulate the crack propagation for monotonic loading. However, further research and action are required to better understand the fracture properties under cyclic loading. For this purpose, in this study, we present numerical simulations of mixed-mode crack propagation in concrete using the scaled boundary finite element method (SBFEM). The crack propagation is developed based on a cohesive crack approach combined with the thermodynamic framework of a constitutive concrete model. For validation, two benchmark crack-mode examples are modelled under monotonic and cyclic loading conditions. The numerical results are compared against the results from available publications. Our approach revealed good consistency compared to the test measurements from the literature. The damage accumulation parameter was the most influential variable on the load-displacement results. The proposed method can provide a further investigation of crack growth propagation and damage accumulation for cyclic loading within the SBFEM framework.

We present measurements of the evolution of the core and cladding diameters in tapered silica microfibers by LC-OCT. The results will help to refine the models of propagation of modes in tapers.

This paper discusses novel approaches to the numerical integration of the coupled nonlinear Schrödinger equations system for few-mode wave propagation. The wave propagation assumes the propagation of up to nine modes of light in an optical fiber. In this case, the light propagation is described by the non-linear coupled Schrödinger equation system, where propagation of each mode is described by own Schrödinger equation with other modes’ interactions. In this case, the coupled nonlinear Schrödinger equation system (CNSES) solving becomes increasingly complex, because each mode affects the propagation of other modes. The suggested solution is based on the direct numerical integration approach, which is based on a finite-difference integration scheme. The well-known explicit finite-difference integration scheme approach fails due to the non-stability of the computing scheme. Owing to this, here we use the combined explicit/implicit finite-difference integration scheme, which is based on the implicit Crank–Nicolson finite-difference scheme. It ensures the stability of the computing scheme. Moreover, this approach allows separating the whole equation system on the independent equation system for each wave mode at each integration step. Additionally, the algorithm of numerical solution refining at each step and the integration method with automatic integration step selection are used. The suggested approach has a higher performance (resolution)—up to three times or more in comparison with the split-step Fourier method—since there is no need to produce direct and inverse Fourier transforms at each integration step. The key advantage of the developed approach is the calculation of any number of modes propagated in the fiber.

Phonon polaritons in van der Waals materials can strongly enhance light–matter interactions at mid-infrared frequencies, owing to their extreme field confinement and long lifetimes1–7. Phonon polaritons thus bear potential for vibrational strong coupling with molecules. Although the onset of vibrational strong coupling was observed spectroscopically with phonon-polariton nanoresonators8, no experiments have resolved vibrational strong coupling in real space and with propagating modes. Here we demonstrate by nanoimaging that vibrational strong coupling can be achieved between propagating phonon polaritons in thin van der Waals crystals (hexagonal boron nitride) and molecular vibrations in adjacent thin molecular layers. We performed near-field polariton interferometry, showing that vibrational strong coupling leads to the formation of a propagating hybrid mode with a pronounced anti-crossing region in its dispersion, in which propagation with negative group velocity is found. Numerical calculations predict vibrational strong coupling for nanometre-thin molecular layers and phonon polaritons in few-layer van der Waals materials, which could make propagating phonon polaritons a promising platform for ultrasensitive on-chip spectroscopy and strong-coupling experiments.Real-space mid-infrared nanoimaging reveals vibrational strong coupling between molecules and propagating phonon polaritons in unstructured, thin hexagonal boron nitride layers, which could provide a platform for testing strong coupling and local control of chemical properties.

A cohesive element able to connect and simulate crack growth between independently modeled finite element subdomains with non-matching meshes is proposed and validated. The approach is based on penalty constraints and has several advantages over conventional FE techniques in disconnecting two regions of a model during crack growth. The most important is the ability to release portion of the interface that are smaller than the local finite element length. Thus, the growth of delamination is not limited to advancing by releasing nodes of the FE model, which is a limitation common to the methods found in the literature. Furthermore, it is possible to vary the penalty parameter within the cohesive element, allowing to apply the damage model to a chosen fraction of the interface between the two meshes. A novel approach for modeling the crack growth in mixed mode I + II conditions has been developed. This formulation leads to a very efficient computational approach that is completely compatible with existing commercial software. In order to investigate the accuracy and to validate the proposed methodology, the growth of the delamination is simulated for the DCB, ENF and MMB tests and the results are compared with the experimental data.