Wave Propagation Path Research Articles

Research on the Influence of Cavities in Charge on the Formation and Penetration Performance of Jets

Abstract To investigate the effect of charge cavities on the formation and penetration of shaped charge warheads, we numerically simulate and calculate shaped charge warheads with different cavities using the finite element method. Combining theoretical analysis and numerical simulations, charge structure splitting was performed to obtain jet morphology and properties data for different cavity locations in both axial and radial dimensions. The mechanism of the effect of cavity position on detonation wave propagation, liner propellant, and jet formation processes in shaped charges is analyzed. It has been shown that charge cavities can change the propagation waveform of detonation waves. The effect of different cavity positions on the jet formation and penetration properties varies; Based on the detonation drive theory, a geometrical model was developed to investigate the effect of the charge cavity position on the jet velocity; The shaped charge warhead structure designed using this model can change the propagation path of the detonation wave, adjust the driving process of the shaped charge cover, and effectively increase the head velocity and penetration capacity of the jet. By studying the influence of the cavity on the driving and jet forming processes of the shaped charge cover, the results indicate that a model established by reasonably adjusting the charge cavity to improve the jet damage performance can provide a certain basis for further optimizing the design of the shaped charge structure.

Прогнозирование помехоустойчивости спутниковой системы Коспас-Сарсат на основе результатов GPS-мониторинга мелкомасштабных возмущений ионосферы

A method for predicting the interference immunity of the satellite system Cospas-Sarsat under conditions of small-scale ionospheric disturbances has been developed based on the results of GPS-monitoring of small-scale fluctuations in the total electron content of the ionosphere. Within this method, the problem of developing a technique for determining the dependence of the probability of erroneous signal reception in satellite communication systems on the choice of the carrier frequency and the signal-to-noise ratio at the receiver input, as well as fluctuations in the total electron content of the ionosphere determined by GPS-monitoring methods on the navigation path of radio wave propagation, has been solved. Based on the obtained dependence, the features of the structure and algorithm of the interference immunity prediction complex of the satellite system Cospas-Sarsat based on GPS-monitoring of small-scale fluctuations in the total electron content are justified. These features include that the inclination angle of the radio wave propagation path in the satellite radio line is initially unknown but can be calculated after solving the problem of determining the coordinates of the radio buoy at the receiver of the spacecraft. Furthermore, a hardware and software implementation of the interference immunity prediction complex of the satellite system Cospas-Sarsat based on the results of GPS ionospheric monitoring has been developed. Experimental results have been obtained regarding the change in the probability of erroneous signal reception in the Cospas-Sarsat satellite system during specified signal-to-noise ratios under conditions of small-scale ionospheric disturbances and increasing levels of ionospheric scintillation. Based on this, estimates of the energy reserve have been obtained to ensure an acceptable error probability in the Cospas-Sarsat satellite system for various values of fluctuations in the total electron content of the ionosphere (0.01 TECU; 0.015 TECU; 0.03 TECU) and levels of scintillation index (0.35; 0.55; 0.85). It has been established that to maintain an acceptable level of signal reception error probability during the formation of small-scale ionospheric disturbances over a 4-minute period, accompanied by an increase in the scintillation index to a moderate level (0.55), an increase in the energy reserve of 4.6 dB will be required. In the event of strong scintillations lasting for 20 seconds (at a level of 0.85), it will be necessary to raise the energy reserve of the Cospas-Sarsat satellite system by 13 dB.

Survey of Whistler‐Mode Wave Amplitudes and Frequency Spectra in Jupiter's Magnetosphere

AbstractWe present statistical distributions of whistler‐mode chorus and hiss waves at frequencies ranging from the local proton gyrofrequency to the equatorial electron gyrofrequency (fce,eq) in Jupiter's magnetosphere based on Juno measurements. The chorus wave power spectral densities usually follow the fce,eq variation with major wave power concentrated in the 0.05fce,eq–fce,eq frequency range. The hiss wave frequencies are less dependent on fce,eq variation than chorus with major power concentrated below 0.05fce,eq, showing a separation from chorus at M < 10. Our survey indicates that chorus waves are mainly observed at 5.5 < M < 13 from the magnetic equator to 20° latitude, consistent with local wave generation near the equator and damping effects. The hiss wave powers extend to 50° latitude, suggesting longer wave propagation paths without attenuation. Our survey also includes the whistler‐mode waves at high latitudes which may originate from the Io footprint, auroral hiss, or propagating hiss waves reflected to high M shells.

A simplified procedure for evaluation of damage-depth in concrete exposed to high temperature using the impact-echo method

Concrete is widely recognized as a material capable of withstanding the intrusion of high temperatures during fires. However, under different high-temperature conditions, concrete can still experience strength reduction, cracking, or spalling, which can significantly impact the safety and durability of concrete structures. Conventionally, the wave refraction technique was used to detect the depth of this damage layer. However, the wave refraction technique is a time-consuming point-by-point detection method. In order to increase detection efficiency, this paper proposes a simplified method based on a single-point test. Numerical analysis of the thermal conduction of a concrete slab exposed to elevated temperature was performed first to investigate the temperature distribution within the concrete slab. Subsequently, the wave refraction technique was numerically simulated to evaluate the damage depth of the concrete slab. According to the refracted wave propagation path, a simplified procedure is proposed for the detection of the damage depth of concrete under high temperature. In the simplified procedure, a receiver is placed at an adequate distance from the impact source so that the first arrival wave at the receiver will be a wave refracted from the interface between the damaged layer and the sound layer inside the concrete. To verify the applicability of the proposed simplified procedure, concrete slab specimens subjected to an elevated temperature of 600 °C were tested in this study. The experimental results indicate that the simplified method proposed in this paper can indeed be used to detect the depth of high-temperature damage in concrete. In addition, the experimental results show that under the same high-temperature exposure conditions, the depth of fire damage increases with a decrease in the water-cement ratio. This can be attributed to the higher thermal conductivity coefficient of concrete with a lower water-cement ratio.

A robust numerical method for the generation and propagation of periodic finite-amplitude internal waves in natural waters using high-accuracy simulations

Abstract. The design and implementation of boundary conditions for the robust generation and simulation of periodic finite-amplitude internal waves is examined in a quasi two-layer continuous stratification using a spectral-element-method-based incompressible flow solver. The commonly used Eulerian approach develops spurious, and potentially catastrophic small-scale numerical features near the wave-generating boundary in a non-linear stratification when the parameter A/(δc) is sufficiently larger than unity; A and δ are measures of the maximum wave-induced vertical velocity and pycnocline thickness, respectively, and c is the linear wave propagation speed. To this end, an Euler–Lagrange approach is developed and implemented to generate robust high-amplitude periodic deep-water internal waves. Central to this approach is to take into account the wave-induced (isopycnal) displacement of the pycnocline in both the vertical and (effectively) upstream directions. With amplitudes not restricted by the limits of linear theory, the Euler–Lagrange-generated waves maintain their structural integrity as they propagate away from the source. The advantages of the high-accuracy numerical method, whose minimal numerical dissipation cannot damp the above near-source spurious numerical features of the purely Eulerian case, can still be preserved and leveraged further along the wave propagation path through the robust reproduction of the non-linear adjustments of the waveform. The near- and far-source robustness of the optimized Euler–Lagrange approach is demonstrated for finite-amplitude waves in a sharp quasi two-layer continuous stratification representative of seasonally stratified lakes. The findings of this study provide an enabling framework for two-dimensional simulations of internal swash zones driven by well-developed non-linear internal waves and, ultimately, the accompanying turbulence-resolving three-dimensional simulations.

Comparative analysis of methods for predicting train-induced vibrations

Railway systems cause more severe vibration pollution than road systems, and the vibration pollution caused by railway systems has been increasing annually over the past two decades. Despite this, there is limited investigation into the accuracy of train-induced vibration prediction methods. Therefore, the present study investigated the accuracy of the detailed and general procedures proposed by the US Federal Transit Administration (FTA) for predicting train-induced vibrations. The accuracy of these methods was determined by evaluating their predictions against in-situ measurements of ground vibration levels induced by passing trains. The field experiments conducted in this study involved the measurement of train-induced vibration levels, the estimation of line-source transfer mobility (LSTM) by dropping a weight on rail sleepers, the evaluation of force density levels, and the determination of ground LSTM by dropping a weight on the ground. In addition, graphs were plotted to determine the relationships of train-induced vertical vibration levels with train speed, distance from the track centreline, and train length. The study compared measured and predicted vibration levels to assess the accuracy of the FTA’s detailed and general prediction methods. Results indicate that detailed assessments tend to overestimate vibration levels, while an optimized general vibration assessment curve offers a more precise estimate. Additionally, on the basis of the measurement data, the study explored the impact of train length on ground vibration and demonstrated that track ballast can absorb vibration energy along the wave propagation path. A frequency propagation contour plot was also created to analyse vibration energy across different frequencies. These findings enhance the reliability of train-induced vibration prediction methods and provide insights for improving vibration mitigation strategies in urban railway systems.

3D-printed gradient conductivity and porosity structure for enhanced absorption-dominant electromagnetic interference shielding

The application of 3D printing for the development of electromagnetic interference (EMI) shielding materials is currently constrained by a limited range of material options and design schemes. In this work, we developed conductivity-modulated polylactic acid (PLA)-MXene composite filaments for fused deposition modeling (FDM) 3D printing, demonstrating excellent printability and durability. Utilizing these filaments, we propose a design scheme focused on absorption-dominant EMI shielding materials. Our design features non-conductive PLA with larger pores at the incident surface, transitioning to layers with increasing conductivity and decreasing pore sizes. This gradient structure minimizes reflection by providing impedance matching and enhances absorption by extending the propagation path of EM waves through multiple reflections and scattering within the pores. Additionally, interfacial polarization effects between air, PLA, and MXene nanoflakes strengthen the absorption mechanisms. Both simulation and experimental results confirm that the combined gradient conductivity and pore size structures significantly improve EMI shielding effectiveness (EMI SE) and absorptivity, achieving an EMI SE of 65 dB and an absorptivity of 0.76 in the X-band. Our findings underscore its potential to create adaptable, high-performance EMI shields suitable for complex geometries and reducing secondary interference.

Ultra-thin low-frequency broadband absorber based on layered coiled channel structure

To improve and broaden the performance in absorbing sound waves of metamaterials at the lower and mid-frequency ranges, a layered coiled channel composite structure (LCCS) with deep subwavelengths is proposed in this paper, which extends the acoustic wave propagation paths longitudinally by connecting the Helmholtz resonator channels in each layer and achieves the multistep resonance of the coupling cavities, which results in the cancellation of acoustic energies, reduces the acoustic reflections and scattering, and enhances the absorption performance. A model to simulate theoretical sound absorption and finite elements simulation of the metamaterial were evolved to reveal its potential mechanismfor absorbing sound. Using the single-variable method to study the impact of altering structural parameters on the effectiveness of sound absorption, it shows good tunability in the target frequency range and obtains four fundamental unit LCCSs having distinct frequency absorption peaks, and designs a multi-unit coupling construction having low frequencies wideband sound absorption performance by connecting them in parallel to realize a large-broadband continuous and high-efficiency sound absorption within the scope of 370–1400 Hz, using a mean peak sound absorption value higher than 0.7. The structure was put together by 3D printing, and the impedance tube method test verified the precision of the simulation’s findings. This study offers a practical means to create lightweight, low-frequency broadband acoustic absorbing metamaterials.

Machine learning-based intelligent localization technique for channel classification in massive MIMO

Multiple-input multiple-output (MIMO) technology has been widely adopted in wireless communications, which enables the simultaneous transmission of multiple data streams via multiple transmitting and receiving antennas. In a MIMO system with non-line-of-sight (NLOS), transmitted signals are reflected by various obstacles along the path, reaching the antenna at different angles and times. In 5G networks, the NLOS problem is a major challenge for massive MIMO localization, significantly reducing positioning accuracy. In this work, an intelligent localization technique based on NLOS identification and mitigation is proposed to address this problem. In this solution, a Convolutional Neural Network (CNN) based hybrid Archimedes-based Salp Swarm Algorithm (HASSA) technique is proposed to detect NLOS or the line of sight (LOS) and estimate the location. The accuracy can be analyzed by considering the angle of arrival of signals, threshold-based time of arrival, and time difference of arrival from different antennas. A novel reinforcement learning-based optimization approach is used for the mitigation of NLOS in the radio wave propagation path, which in turn reduces the computational complexity. We use the Ensemble Deep Deterministic Policy Gradient-Based Approach (EDDPG)-based Honey Badger algorithm (HBA) for the aforementioned process. The simulation of this approach assesses diverse scenarios and considers different parameters, and the approach is compared with various state-of-the-art works. From the simulation results, our proposed approach can be used for the identification and detection of LOS and NLOS components and can precisely enhance the localization compared with other approaches.

Study on Nondestructive Detection Imaging Method of Log Knot Based on Judging the Shortest Path of Stress Wave Propagation

This paper presents an iterative stress wave log beam imaging algorithm based on external shape to predict the size, shape, and position of internal features in logs. The algorithm’s accuracy is analyzed to establish a theoretical and scientific basis for the prediction and evaluation of internal knots in standing trees. Six sample logs with natural knots were selected for study, and cross-sectional stress wave propagation tests were conducted using FAKOPP to collect data. Using the shortest propagation path method, the algorithm iteratively produced fault images of the log cross-sections. While the algorithm can roughly predict the location of internal features, discrepancies between predicted and actual shapes and sizes result in relative errors ranging from 15.66% to 52.08%. Except for sample log 6, the relative error for other logs is mostly within 31%, with logs 1, 2, and 5 showing errors under 20%. However, the imaging accuracy and effectiveness need improvement. Further experimental studies and algorithm enhancements are necessary to improve fault imaging and prediction accuracy, particularly in terms of shape and size precision.

Influence of input parameters on the measurement accuracy of external clamp-on ultrasonic flowmeters: An experimental study

Abstract This study comprehensively evaluated the performance of clamp-on ultrasonic gas flowmeters at a natural gas field. The research focused on the influence of key input parameters on flow measurement accuracy during practical application of the instrument and proposed specific measures to improve measurement results. It was found that errors in outer diameter and wall thickness not only lead to flow calculation inaccuracies but also directly impact the propagation path of ultrasonic waves. Therefore, accurate input parameters are crucial to ensure precise flow results. Additionally, the study revealed that changes in installation distance have minimal effects on flow measurement results and adjustments can be made as needed to obtain better signal parameters. Variations in input temperature and pressure mainly affect the conversion coefficient from operating conditions to standard conditions, with minor impacts on flow velocity and operating flow rate. Thus, the velocity ratio method can be considered during on-site calibration to mitigate the effects of temperature and pressure changes.

Physical analysis of high refractive index metamaterial-based radiation aggregation engineering of planar dipole antenna for gain enhancement of mm-wave applications

A metamaterial-incorporated high-gain and broadband dipole antenna is proposed for 5G mm-wave applications. The designed high refractive index metamaterial (HRIM) properties are presented in detail with supporting results. The proposed dipole antenna achieved broadband features, and the antenna gain increased significantly by incorporating HRIM in the electromagnetic (EM) wave propagation path. Besides, the EM wave aggregation engineering of utilizing the HRIM on the dipole antenna is analyzed using the electric field, magnetic field, and power flow distributions. Both conventional and HRIM-based dipole antenna (HRIMDA) were fabricated on thin (0.254 mm) Rogers RT5880 substrate material with a low dielectric constant of 2.2, where the noticeable gain enhancement by HRIM is observed. The measured results show the operational bandwidth (BW) from 23 to 38 GHz frequency, and the highest gain of 9.5 dBi is accomplished at 35 GHz frequency. Finally, a four-element MIMO configuration is numerically and experimentally analyzed where the isolation of the two conjugated MIMO elements is < − 20 dB. The MIMO parameters like envelop correlation coefficient (ECC) < 1 × 10–3 and diversity gain (DG) value of > 9.9 were also achieved. Hence, the proposed HRIM base dipole antenna is a potential candidate for 5G mm-wave applications.

Prestack reservoir prediction method in ray parameter domain based on wide azimuth ocean bottom node seismic data

AbstractWide azimuth seismic data play an important role in deep reservoir prediction. According to the Paleogene clastic rock reservoir prediction, the high‐density and wide azimuth 3D seismic data acquisition of ocean bottom nodes was first carried out in a Chinese offshore oilfield in 2019. After high precision amplitude‐preserving processing, we obtained the high‐quality wide azimuth gathers. However, the research on anisotropy and reservoir prediction using wide azimuth seismic data mainly focuses on carbonate and bedrock intervals, which is not suitable for clastic rock reservoir prediction. Therefore, this paper innovatively proposes a clastic rock reservoir prediction method, which studies prestack reservoir prediction in the ray parameter domain based on wide azimuth ocean bottom node seismic data. Based on the azimuth gathers, we can obtain elastic parameters through prestack amplitude versus offset inversion, which is used to characterize the reservoir, so it is significant to obtain high precision elastic parameters in order to get highly reliable reservoir prediction results. In this paper, we develop an amplitude versus offset inversion method based on the Bayesian theory in ray parameter domain, the output of which is density, P‐wave impedance and Vp/Vs. These elastic parameters have high precision, and density data are valuable input for reservoir characterization because they are sensitive to the lithology of clastic rock reservoir at different orientations. In ray parameter domain inversion, the ray path of seismic wave propagation is considered polyline, which is more consistent with the actual situation; thus, extracted amplitudes of P gathers used in inversion are more accurate. In addition, the reflection coefficient formula in ray parameter domain has higher precision when the incident angle is large. The inversion based on the Bayesian theory can improve the stability of the inversion. Test on the actual data shows that the result of ray parameter domain inversion with a Bayesian scheme is more accurate, stable and reliable. Based on the above high precision density inversion results, an innovative wide azimuth data reservoir prediction technology based on elliptical short‐axis fitting was proposed. The actual prediction of the deep reservoir in the Bohai oilfield shows that sand thickness fitting prediction results in the short axis can best match the actual drilling sandstone thickness. The coincidence rate is 86% and the short‐axis fitting results are more in agreement with geological laws. Theoretical research and practical applications have shown that this method is feasible and effective, with high prediction accuracy, computational efficiency and strong application value.

A Software Tool for the True Height Analysis of Ionograms Using the Iterative Gradient Correction (IGC) Method

AbstractDeriving the precise true height electron density profile from the measured ionosonde virtual heights is quite a challenging problem. Recently, Ankita and Tulasi Ram (2023, https://doi.org/10.1029/2023RS007808) presented a new method, Iterative Gradient Correction (IGC) method, for true height analysis that uses HF radio wave propagation path computations to reconstruct the true height profile. Through iterative corrections on electron density gradients between successive points, the IGC method minimizes errors below a specified tolerance at each point and reconstructs a complete electron density profile. The derived profiles from the IGC method are found to be accurate when compared with Incoherent Scatter Radar and Global Navigation Satellite System—Radio Occultation observations. To facilitate true height analysis by IGC method for a wider user community, a MATLAB‐based software has been developed and is outlined in this report. The software can be installed on any Windows platform and is designed with a user‐friendly interface for easy and efficient application by the users. It can analyze multiple scaled ionograms in a single run and outputs the real height profiles in ASCII format. Further, the software also captures important ionospheric parameters such as the base altitudes and peak frequencies of E‐ and F‐layers (e.g., hE, hF, foE, and foF2) etc., from the computed true height profiles and tabulates in a separate output file for the ready use. The software also provides the option for extrapolation of true height profile into top‐side ionosphere up to a user‐specified height and reconstructs the complete vertical electron density profile.

Improved Electromagnetic Interference Shielding Efficiency of PVDF/rGO/AgNW Composites via Low-Pressure Compression Molding and AgNW-Backfilling Strategy.

Silver nanowires (AgNWs) have excellent electrical conductivity and nano-sized effects and have been widely used as a high-performance electromagnetic shielding material. However, silver nanowires have poor mechanical properties and are prone to fracture during the preparation of composite materials. In this study, PVDF/rGO/AgNW composites with a segregated structure were prepared using low-pressure compression molding and the AgNW-backfilling process. The low-pressure compression of the composite significantly improves its electromagnetic shielding performance because the low-pressure process can maintain the AgNWs' integrity. The backfilled AgNWs played a vital role in increasing the path of electromagnetic wave propagation and the absorption of electromagnetic waves. The backfilled amount of AgNWs was only 1 wt%, which increased the composite material's conductivity by one order of magnitude. The total electromagnetic interference shielding (SET) of the composite materials increased by 23.3% from 24.88 dB to 30.67 dB. The absorption contribution (SEA/SET) increased from 84.2% to 92.8%, significantly improving the electromagnetic interference shielding and the absorption contribution of the AgNWs in the composites. This was attributed to the backfilling of the porous structure by the AgNWs, which promoted multiple reflections and enhanced the absorption contribution.

Fault slip and seismic source response characteristics under induced stress wave

As coal mining depth and structural complexity increase, the failure severity of mining-induced fault activation leading to rock bursts escalates. Fault fracture zones, consisting of various secondary structures, are essential for understanding tectonic evolution and seismic wave propagation. The interaction between mining-induced stress waves and in-situ stress complicates the dynamics and activation markers of fault fracture zones. This paper aims to clarify the patterns of stick–slip instability and stress evolution in these zones under coal mining-induced stress waves. Using theoretical models and continuous-discrete coupling simulations, the study explores the dynamic response of fault fracture zones in the meta-instability stage, the micro-scale fracture force chain structure, and the distribution and synergistic instability of fault plane seismic sources. The research shows that fault fracture zones changes stress wave propagation path and energy redistribution due to its heterogeneity leads to multiple times decomposition of wave field. Minor delamination between the hanging wall and footwall can trigger unstable slip. Tensional seismic sources with extensive failure drive fault slip. Dynamic stress waves disrupt force chains near seismic sources, increasing hazard levels. Static in-situ stress affects shear seismic sources by influencing porosity and crack angles. Seismic sources along the fault strike transition from divergent to concentrated, trending horizontally. In the meta-instability stage, the initial fracture area locks, with strong structures causing cohesive fractures in the middle of the fault zone, releasing strain energy and forming a sudden increase in slip intensity. These findings provide crucial insights into fault fracture-development-activation markers, fault region stress deformation evolution mechanisms, and risk control of fault-induced rock bursts.

Bifunctional acoustic lossy coupler for broadband power splitting and absorption

In this work, we propose a bifunctional acoustic lossy coupler for broadband power splitting and absorption. Following the three-level system in quantum physics, the three-waveguide (WG) acoustic system is proposed to obtain efficient energy transfer behaviors. Given the agreement in form between Schr o¨ dinger equation and coupled mode equation, the time-dependent external fields are mapped into space-varying coupling actions between adjacent WGs. The propagation paths of the incident waves can be predicted by Schr o¨ dinger-like equation, which are verified by numerical simulations and experimental measurements. Owing to the different responses to lossy medium in WG B for WG A incidence and WG C incidence, the acoustic coupler can be considered as a broadband power splitter and muffler by selecting different input ports. Meanwhile, according to the reversibility of acoustic path, a switchable acoustic logic gate with functions of “OR” and “XOR” is constructed as well by taking advantage of the lossless adiabatic coupler, and the function can be controlled conveniently by reversing the phase of input waves from WG A or C. Our work provides a new scheme for the design of bifunctional acoustic coupler, which may have a profound impact on the development of non-resonance acoustic high-performance devices.

Construction and experimental verification of wave velocity model for source location in goaf overlying rock strata

The geological structure of the goaf overlying rock is complex, a consequence of coal mining that has modified the original stratified structure of the sedimentary strata. To enhance the accuracy of microseismic source location in such intricate geological formations, a wave velocity model for the “three zones” goaf was constructed based on natural divisions within the strata using Snell's law and assuming a homogeneous medium. The model took into account the effects of rock deformation and fracture development, enabling the derivation of formulas for microseismic wave propagation path and travel time calculation. Additionally, the concept of equivalent wave velocity was defined. An indoor simulation test using similar materials was conducted to establish a geological model of the goaf. By comparing the errors between the theoretical and measured values of equivalent wave velocity, assessing the locating effects before and after implementing the wave velocity model of the goaf, and verifying the feasibility of the model, it was demonstrated that establishing a wave velocity model based on the characteristics of the strata structure was crucial for improving the accuracy of the microseismic source location. Notably, as the propagation path of microseismic waves in the goaf increased, the equivalent wave velocity decreased. The wave velocity structure in the goaf exhibited nonuniformity, with the relative error between the theoretical and measured values of equivalent wave velocity being limited to 10 %. The incorporation of this established wave velocity model into the location method resulted in a substantial 58.57 % increase in locating accuracy.

Infrasound associated with the eruption of the Hunga volcano.

On 13-15 January 2022, the Hunga Tonga-Hunga Ha'apai underwater volcano erupted. This powerful eruption generated infrasonic waves with amplitudes of thousands of Pascals in the near field. The ground infrasonic stations in China, located approximately 10 000 km from the Hunga volcano, also received waves with frequencies from 0.01 to 0.05 Hz. However, the amplitude reached 17 Pa, which is higher than the predicted amplitude using the absorption model without considering the dispersion effect in the thin thermosphere. At high altitudes, dispersion exists and the sound speed depends on the ratio of the molecular mean collision ratio to sound frequency, which is proportional to the ratio (frequency/pressure). And attenuation coefficients are complex to model. We simulate dispersive sound speeds and attenuation coefficients at different frequencies according to theory and our experimental data. In the thermosphere, the dispersion effect causes noticeable changes of sound speed and then affects wave propagation paths in the far field. The abnormal attenuation coefficient has a smaller impact on thermospheric returns than that of the dispersive sound speed, but it is also not negligible. It explains the large amplitude of thermospheric signals received in our infrasound stations. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

Oscillation mechanism and predictive model of explosion load for natural gas in confined tube

Gas explosion in confined space often leads to significant pressure oscillation. It is widely recognized that structural damage can be severe when the oscillation frequency of the load resonates with the natural vibration frequency of the structure. To reveal the oscillation mechanism of gas explosion load, the experiment of gas explosion was conducted in a large-scale confined tube with the length of 30 m, and the explosion process was numerically analyzed using FLACS. The results show that the essential cause of oscillation effect is the reflection of the pressure wave. In addition, due to the difference in the propagation path of the pressure wave, the load oscillation frequency at the middle position of the tunnel is twice that at the end position. The average sound velocity can be used to calculate the oscillation frequency of overpressure accurately, and the error is less than 15%. The instability of the flame surface and the increase of flame turbulence caused by the interaction between the pressure wave and the flame surface are the main contributors to the increase in overpressure and amplitude. The overpressure peaks calculated by the existing flame instability model and turbulence disturbance model are 31.7% and 34.7% lower than the numerical results, respectively. The turbulence factor model established in this work can describe the turbulence enhancement effect caused by flame instability and oscillatory load, and the difference between the theoretical and numerical results is only 4.6%. In the theoretical derivation of the overpressure model, an improved model of dynamic turbulence factor is established, which can describe the enhancement effect of turbulence factor caused by flame instability and self-turbulence. Based on the one-dimensional propagation theory of pressure wave, the oscillatory effect of the load is derived to calculate the frequency and amplitude of pressure oscillation. The average error of amplitude and frequency is less than 20%.