Shock Wave Propagation Research Articles

Evolution of underwater shock wave in far-field regime of electrical wire explosion system over wide range of electric pulse energy

The parametric relationships between underwater shock waves (SWs) in far-field regime and electrically exploded wire under different charging voltages of a generator are investigated. Contributions of the vaporization and current restrike to the resulting SWs have been examined by means of electro-physical diagnostics, free-field pressure probe, and high-speed photography. In the experiment, a 6-μF pulse capacitor is adopted to provide a microsecond timescale pulsed current, exploding a 90/185-μm-diameter, 2-cm-long copper wire with the charging voltage ranging from 7.1 kV (stored energy ∼ 150 J) to 18.3 kV (∼1000 J). Experimental results indicate that as the increase of the charging voltage, the relationship between electro-physical and shock wave parameters shows a rather complex situation. When the charging voltage ranging from 7.1 kV (∼150 J) to 11.6 kV (∼400 J), the peak pressure of SWs (300 mm away from the explosion source) increases almost linearly from 1.16 ± 0.03 to 2.24 ± 0.05 MPa (90-μm wire), and 1.67 ± 0.17 to 2.87 ± 0.12 MPa (185-μm wire). Nevertheless, at a higher charging voltage between 11.6 kV (400 J) to 14.1 kV (600 J), the peak pressure stabilizes in a strange “plateau”. Until the residual energy after the explosion is sufficiently large (>600 J in this study) to induce a strong current restrike, the development of the plasma channel could further enhance the expansion of explosion products, as well as the intensity of measured SWs. In addition, high-speed photography has been applied for those explosions, providing the information of plasma channel expansion and initial SWs propagation.

Review of shock wave pressure reconstruction methods in explosion field

Explosion shock wave pressure is one of the main damage parameters produced in the process of ammunition explosion, and it is also an important technical index to evaluate the damage power of ammunition. However, in the actual testing process, only a limited amount of shockwave pressure data at specific measuring points can be obtained, which cannot accurately reflect the distribution laws of the shockwave pressure propagation after the ammunition explosion. Therefore, it is particularly important to conduct reconstruction of the shockwave pressure field distribution laws based on limited measuring point data. This paper reviews the research work and related achievements obtained by researchers at home and abroad on the reconstruction of shockwave pressure distribution laws in explosion fields. The paper also elucidates the relevant application of computer tomography imaging techniques, shockwave propagation attenuation model reconstruction, and various interpolation algorithms in the reconstruction process of the shockwave pressure field. Given the current research situation both domestically and internationally, we have pinpointed the major issues that still exist in the current stage of research and proposed the key areas of focus that need attention in future research.

Selection of Material and Thickness of the Protective Wall in the Conditions of a Hydrogen Explosion of Various Power

The main purpose of this study is a numerical assessment of the consequences of an explosion of a hydrogen-air cloud on the personnel of a hydrogen fueling station and the strength of a protective solid wall of certain dimensions. An explosive gas mixture is formed as a result of the destruction of high-pressure cylinders, the number of which determines the size of the cloud, the power of the explosion, and the scale of the consequences of environmental impact. To obtain the spatio-temporal distribution of the maximum overpressure and the impulse of the shock wave compression phase, a mathematical model of the dispersion of an active gaseous admixture is used, taking into account the chemical interaction with air oxygen. The probable consequences of the shock-impulse impact on the personnel at the control point are carried out using probit analysis. The values of the maximum bending moment and stress at the base of the protective wall, which result from the impact of the blast wave, are used to deterministically estimate the minimum wall thickness necessary for the safe operation of the protective device. The mathematical model takes into account the complex terrain and the three-dimensional non-stationary nature of the shock wave propagation process, and it is a source of data necessary to solve the problem of the strength of solid objects located in the area of baric perturbation of the gaseous medium. The developed methodology makes it possible to carry out a comparative analysis of the effectiveness of protective structures in relation to the power of the explosion.

A study of explosive-induced fracture in polymethyl methacrylate (PMMA)

The fracture response of geologic materials is of interest for applications, including geothermal energy harnessing and containment of underground explosions. To better understand the explosively induced fracture response of geomaterials, polymethyl methacrylate (PMMA) was used as a transparent rock surrogate to allow imaging of internal shock propagation and fracture growth processes. Experiments were conducted using high-speed shadowgraphy and photon Doppler velocimetry (PDV), which were compared to numerical simulations. Experiments measured fractures produced in 304.8mm×304.8mm×304.8mm PMMA cubes with two simultaneously initiated detonators. The cubes were subjected to varying amounts and directions of externally applied uniaxial stresses, including no stress, 2 MPa stress, and 20 MPa stress. The fracture radius as a function of time was extracted from the high-speed videos. Post-test images of the PMMA cubes aided in the determination of three-dimensional effects not directly imaged by the cameras. The surface velocity history and the shock response captured in PDV and the high-speed videos were compared to the simulated explosive-induced shock response. The simulation results indicate that the shock drives the fracture for the first 20 μs corresponding to a fracture radius of approximately 15 mm in the experiments. The gas-driven fracture extent was estimated analytically using an equilibrium stress distribution calculated after the shock wave propagation through the sample. Reduction in the gas pressure due to the leakage of the explosive products through the crack as a function of time was accounted for. The estimated fracture lengths were in agreement with the experimentally observed fracture lengths.

The Shock-Induced Deformation and Spallation Failure of Bicrystal Copper with a Nanoscale Helium Bubble via Molecular Dynamics Simulations.

Both the nanoscale helium (He) bubble and grain boundaries (GBs) play important roles in the dynamic mechanical behavior of irradiated nanocrystalline materials. Using molecular dynamics simulations, we study the shock-induced deformation and spallation failure of bicrystal copper with a nanoscale He bubble. Two extreme loading directions (perpendicular or parallel to the GB plane) and various impact velocities (0.5-2.5 km/s) are considered. Our results reveal that the He bubble shows hindrance to the propagation of shock waves at lower impact velocities but will accelerate shock wave propagation at higher impact velocities due to the local compression wave generated by the collapse of the He bubble. The parallel loading direction is found to have a greater effect on He bubble deformation during shock compression. The He bubble will slightly reduce the spall strength of the material at lower impact velocities but has a limited effect on the spallation process, which is dominated by the evolution of the GB. At lower impact velocities, the mechanism of spall damage is dominated by the cleavage fracture along the GB plane for the perpendicular loading condition but dominated by the He bubble expansion and void growth for the parallel loading condition. At higher impact velocities, micro-spallation occurs for both loading conditions, and the effects of GBs and He bubbles can be ignored.

Analysis of the influence of corrugated steel thickness on the damage characteristics and explosion resistance of corrugated steel-concrete composite structure

A three-dimensional refined numerical simulation model of corrugated steel-concrete slab composite structure under contact explosion load was established by using FEM-SPH coupling method to explore the anti-explosion performance of corrugated steel reinforced concrete slabs with different thicknesses. The influence of corrugated steel thickness on the dynamic damage evolution characteristics, anti-explosion performance and shock wave propagation mechanism of composite structure was investigated. The results show that the maximum error between the simulation results and the experimental results of the crater diameter of the concrete slab and the mid-span displacement of the corrugated steel in the corrugated steel-concrete slab composite structure is 1.6 % and 2.3 %, respectively, which verifies the effectiveness of the simulation method. The mid-span displacement, peak stress and acceleration at the center point of corrugated steel in corrugated steel-concrete composite structure decrease with the increase of corrugated steel thickness, while the energy absorption value increases with the increase of corrugated steel thickness. The mid-span displacement of 12 mm thick corrugated steel is 83.4 % lower than that of 3 mm thick corrugated steel. The failure mode of the composite structure under the same explosive equivalent is mainly manifested as the crater and penetration failure of the concrete slab, and the damage range gradually decreases with the increase of the thickness of the corrugated steel. The failure volume of concrete slab in the composite structure decreases with the increase of corrugated steel thickness, while the energy absorption value of the composite structure increases with the increase of corrugated steel thickness. The research results can provide a theoretical basis for the application of corrugated steel-concrete slab composite structure in the field of structural anti-explosion protection.

Survey of the quality and origins of websites on penile low-intensity shockwave therapy in Canada.

The aim of this study was to establish the quality of patient-facing websites advertising low-intensity shockwave therapy (LISWT) for erectile dysfunction (ED) and Peyronie's disease (PD) patients in Canada. Canadian websites offering LIWST for ED or PD were identified using online web searches. The characteristics of these websites were reviewed, along with examining the presence of HONCode certification, assigning a brief DISCERN score (a tool designed to evaluate health information online) and readability scores. We also examined the LIWST technology advertised, as well as benefits of LIWST cited by the websites. Twenty-five unique websites linked to 46 clinics were identified and reviewed. Twenty-four percent of websites were run by a urologist. Other specialties offering LISWT included general practitioners, anesthesiologists, naturopaths, nurse practitioners, physiotherapists, and registered massage therapists. Twenty-four percent of the websites advertised the use of a focused shockwave generator. Forty percent of the websites had peer-reviewed references. The average brief DISCERN score was 14 (standard deviation 3.4). There was no association between the physician-or non-physician-led websites and the use of peer-reviewed references, readability scores, the number of clinic locations, or higher brief discern scores. LISWT is readily advertised online for ED and PD patients in Canada; however, only a minority use a focused shockwave generator. There is a wide diversity of practitioners offering LISWT. Websites offering LISWT are generally of poor quality and do not provide adequate information for patients to make educated treatment decisions.

Planetary Impacts: Scaling of Crater Depth From Subsonic to Supersonic Conditions

AbstractPlanetary impacts have shaped the surfaces and interiors of planets. They were particularly critical in the last stage of planetary accretion, as they have eventually formed terrestrial planets. During these large supersonic collisions, shock waves melted the impactor and the target, and formed silicate magma oceans. Because the propagation of shock waves and the melting is faster than the excavation of an impact crater, the cratering stage can be considered as a purely hydrodynamic process. Here, we use both laboratory impact experiments in water and numerical simulations to investigate the crater dimensions resulting from the impact of a liquid impactor onto a liquid target. We show that our numerical models reproduce the laboratory experiments at subsonic impact velocities. We then explore the effect of both the Froude number, which is the ratio of the impactor kinetic energy to gravity, and the Mach number, which is the ratio of the impact speed to the sound speed. We vary these two parameters independently in impact simulations, going from subsonic to supersonic conditions. We obtain a new scaling law for the crater dimensions that describes the transition from subsonic to supersonic impacts. Our results indicate that the transition between these two regimes results from a change in the partitioning of the impactor kinetic energy into potential energy in the crater and internal energy. Finally, our scaling suggests that, in the limit of large Mach numbers, the crater depth depends only on the sound velocity and gravity, and is independent of the impact speed.

A self-similar solution for the flow behind an exponential cylindrical shock in a self-gravitating mixture of non-ideal gas and a pseudo-fluid of solid particles in a rotating medium

Although the shock wave propagation in the dusty gas medium is widely used in research, astrophysics, military, and industrial applications, the shock wave in a self-gravitating mixture of van der Waals gas and a pseudo-fluid of inert solid particles in a rotating medium, is not well understood. In the present paper, the strong exponential shock wave propagation in a self-gravitating mixture of non-ideal gas and a pseudo-fluid of small solid particles is modelled for unsteady adiabatic and isothermal flows in a rotating medium. In our model, the solid particles are considered as chemically inert pseudo-fluid, and the equilibrium flow conditions hold in the entire flow field region. The similarity solutions are derived for isothermal and adiabatic flows. The vorticity vector components, and isothermal and adiabatic compressibility's are also taken into account. It is shown that the zero-temperature gradient supposition brings the reflective changes in pressure, axial vorticity vector, density, and compressibility distributions in contrast to the adiabatic flow. The shock strength decreases, and the compressibility increases due to the zero-temperature gradient consideration. Also, the effects of the problem parameters on shock waves and the distributions of the flow variables velocity, density, etc., are discussed in detail. It is remarkable to note that due to the consideration of a self-gravitating mixture or with an increase in the ratio of the density of solid particles to the initial density of the gas, the shock strength increases; whereas due to the consideration of non-idealness of the gas or rotating medium the shock strength decreases.

A critical review of experimental analyses performed on animals, post-mortem human subjects, and substitutes to explore primary blast-induced Traumatic Brain Injuries

Although blast-induced Traumatic Brain Injury (bTBI) has become a signature wound of conflict, its cause is not yet fully understood. Regarding primary blast injuries, i.e., those caused by the propagation of shock waves in the body, four direct and two indirect injury mechanisms have been mainly proposed in the literature. Since numerous authors have exposed instrumented animals, Post-Mortem Human Subjects (PMHS), and head substitutes to blast conditions, the aim of this review is to classify them in terms of threat, instrumentation, and investigated mechanisms. In the first part, data are collected from 6 studies on PMHS, 1 on primates, 11 on rodents, and 6 on swine for comparison purposes. Peak amplitudes of reflected pressures, intracranial pressures and cranial strains are extracted and analyzed to establish trends. Despite the small number of comparable studies, several similarities can be highlighted. Indeed, the analyses revealed a dose-response effect for most measurements. The results also depend on the orientation of the subject (forward, backward, and sideways) for the PMHS, primates, and swine. The second goal of this review is to evaluate the behavior of substitutes developed to replace PMHS experiments. Shell strains and internal pressures are thus collected on 19 geometric and anthropomorphic substitutes to assess whether they faithfully represent a human head. The results showed that these substitutes are for the most part not properly designed and therefore cannot yet reliably replace PMHS experimental data.

Blast resistance of ultra high performance concrete-filled steel tube (UHPC-FST) pier against vehicular bombs

Bridge piers will lose axial bearing capacities under accidental or deliberate explosions, which may cause partial or even multi-span collapse of the bridges. Compared with the conventional RC pier, the ultra high performance concrete-filled steel tube (UHPC-FST) pier exhibits superior strength, ductility, and durability, thus having promising applications in the field of blast-resistant bridges. This study aims to experimentally and numerically examine the blast-resistant performance of the UHPC-FST pier under the near-range explosion of Vehicle-Borne Improvised Explosive Devices (VBIED). Firstly, a prototype UHPC-FST pier with earthquake intensity VII was designed following the Chinese specifications, and four 1/4-scale UHPC-FST specimens were scaled and fabricated accordingly. The near-range explosion and succeeding axial compression test were carried out on the specimens with TNT masses of 3 kg, 4 kg, 7 kg, and 10 kg, respectively. Then, full parameters of the Karagozian & Case (K&C) concrete material model applicable to UHPC were systematically calibrated based on the available test data, and the refined finite element (FE) models were established correspondingly based on the explosion and axial compression test. The verification of FE modeling approach was verified by comparing the failure modes and axial force–displacement curves of specimens obtained from the numerical simulation with the test. Finally, two types of VBIED (compact sedan and cargo van) and three standoff distances were selected from the report FEMA-428 and Chinese specifications, and six explosion scenarios were designed correspondingly. The shock wave propagation, shock wave-pier interaction, and the damage evolutions of prototype piers were numerically reproduced. The blast-resistant performance of the prototype UHPC-FST pier was assessed from the aspect of the failure modes and residual axial bearing capacities, which were quantitatively compared with those of conventional RC pier with identical earthquake intensity.

Mechanism of the discharge behavior in electromagnetic pulse welding: Combination of electron emission and electric field

The analysis of transient phenomena in electromagnetic pulse welding is of great interest for unraveling the underlying principles. This research systematically investigates the mechanism of the discharge phenomenon by integrating finite element modeling with surface morphology analysis. The results revealed that the discharge area is discretely distributed on the surface, primarily concentrated around the welding area where the severe collision occurs. In contrast, when the collision is eliminated, the discharge is confined only to the edge. Given a fixed input energy, the discharge ablation on the surface is more pronounced for materials with a low work function. The research further establishes that both electron emission and the intense electric field contribute to the discharge behavior. This discharge behavior subsequently leads to high temperatures in the discharge area, facilitating element diffusion and shock wave generation. However, intermetallic compounds are not observed due to the brief duration of the discharge. This study illuminates the adverse effects of the collision and provides profound insights into the dynamic behaviors during welding. The findings offer valuable theoretical guidelines for the development and optimization of welding processes.

“Litchi-like” metastable Al/Ti/CuO micro-nano composites with enhanced combustion reaction and their energy characteristics

To design and prepare multifunctional metastable intermixed composites (MICs) with excellent properties, the enhanced “litchi-like” core-shell structure Al/Ti/CuO MICs were prepared by high-energy ball milling (Al/Ti/CuO-B). The morphology, structure, composition, thermal reactivity, energy release and combustion performance of the prepared MICs were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and synchronous thermal analysis (DSC-TG), closed system constant volume combustion experiment, open system CO2 laser ignition experiment and high-energy laser-induced ablation experiment. This study reveals that, the obtained Al/Ti/CuO-B composites have a “litchi-like” core-shell structure, the components are evenly distributed and closely contacted. The exothermic reaction of Al/Ti/CuO-B should be divided into two stages. Due to the introduction of Ti and the special structure of litchi, Al/Ti/CuO-B has superior reactivity (Ea=178.15 kJ/mol), energy release (Q = 6443 J/g, Pmax =164.90KPa), shock wave propagation (v = 1028.33 m/s) and combustion performance than Al/Ti/CuO-P (simple physical mixing). The thrown Al/Ti/CuO composite particles will undergo secondary micro-reactions during the combustion, and a continuous multi-point “micro-explosion” phenomenon will occur, then self-sustainable combustion can be achieved through multi-level energy superposition release, which will enhance its reaction, combustion and energy release. Al/Ti/CuO-B undergoes an ultrafast reaction to generate plasma under high-energy laser induction, which expands and pushes the surrounding air to form a shock wave. The application of Al/Ti/CuO-B shows that the density, heat of detonation, shock wave overpressure, fireball diameter and temperature of composite structural explosive charge containing Al/Ti/CuO ring are all significantly improved.

Numerical Simulation to Investigate Interactions of Generated Underwater Micro Shock Waves and Micro Bubbles by Focusing Femtosecond Pulse Laser

The purpose of this study is to elucidate the mechanism of propagation of the laser-induced micro shock waves under condition where the micro bubbles are generated. In this paper, effects of generated micro bubbles on propagation of the laser-induced micro shock waves were investigated by CFD (computational fluid dynamics). Firstly, the two models (1-D model and 1-D spherical symmetric model) were computed for comparison of the peak pressure variation of the shock waves with propagation. As for governing equations for the propagation of the shock waves, continuity equation, Euler’s momentum equation and Tait’s state equation are used. From the computation, it is confirmed that attenuation of pressure of the 1-D spherical symmetric model was earlier than the 1-D model. In addition, the attenuation of the 1-D spherical symmetric model agreed with the laser-induced shock waves obtained experimentally. However, the peak pressure and duration time of the shock wave was not the same as the experimental result. Then, the bubble behavior was included in the computation of the shock wave propagation. As for the bubble behavior, Rayleigh-Plesset equation is used. From this computation, pressure wave was obtained which superposed the pressure of the shock wave on the internal pressure of the bubble. Although the duration time of the pressure wave was close to the experimental result, the value of the pressure was almost the same as atmospheric pressure. It is suggested that there is a possibility that phenomenon other than the bubbles is generated such as plasma when the shock wave is generated by focusing the femtosecond pulse laser

Spatiotemporal clustering for the impact region caused by a traffic incident: an improved fuzzy C-means approach with guaranteed consistency

Traffic incidents disrupt the normal flow of vehicles and induce nonrecurrent traffic congestion. It has been well accepted that the shape of the spatiotemporal region impacted by a traffic incident should be consistent with the propagation of shockwaves. Although there has been a variety of approaches that attempt to estimate the impact region of traffic incidents, most of them are not capable of producing results with guaranteed consistency. In this research, we propose an improved fuzzy clustering approach that integrates the domain knowledge of shockwave theory for freeway incidents to address this issue, which is new to the literature. Compared to the general clustering approaches, our improved fuzzy clustering approach takes control of the clustering process by leveraging the directional propagation of shockwaves in the form of constraints, which can guarantee the consistency. In addition, unlike existing studies that employ discrete variables to distinguish traffic status in case of traffic incidents, the fuzzy clustering approach uses the continuous variable to indicate the incident impact on vehicle speed. This can help to reduce the information loss and estimate the impact region more accurately. Numerical experiments are conducted to evaluate the performance of our approach using both simulation and real data. Results show that our approach is able to guarantee that the shape of the impact region is consistent with the propagation of shockwaves and achieve higher accuracy of the estimated delay induced by the incident than the current state-of-the-art approach.

Experimental investigation on shock wave propagation and spontaneous ignition of high-pressure hydrogen release through a sho (ϸ)-shaped extension tube into the atmosphere

The geometry of the extension tube has a great influence on the shock wave propagation and spontaneous ignition of high-pressure hydrogen release in this paper, piezoelectric pressure transducers, photodiodes and a high-speed camera are used to study the shock wave propagation and spontaneous ignition of high-pressure hydrogen released to the downstream sho (ϸ)-shaped extension tube. Experimental results show that the intensity of the leading shock wave is weakened due to the presence of branches when it propagates in the straight section of the sho-shaped tube, and a strong reflected shock wave is generated in the bend section. Spontaneous ignition is more prone to occur in the bend section than in the straight section, with a large optical signal value and long flame duration. However, due to the complex structure of the extension tube, there are still cases of the hydrogen flame extinguishing inside the tube even under high burst pressure. The flame detection time after the leading shock wave at the measured positions decreases with increasing burst pressure, and the time in the bend section is greater than that of the straight section. Radial flame expansion, flame separation, downstream flame extinction and upstream flame development, primary jet fire and secondary jet fire are successively observed outside the tube.

A robust high-resolution discrete-equations method for compressible multi-phase flow with accurate interface capturing

The discrete-equations method (DEM) [1] provides a universal approach to solve multi-phase-flow equations as it combines the solutions of pairwise Riemann problems. Although very robust, the original DEM with piecewise-constant volume fractions suffers from strong diffusion preventing accurate interface capturing. High-order interface reconstruction, however, introduces a restrictive time-step limit.This paper presents RDEMIC, a robust extension of DEM for accurate interface capturing on Cartesian meshes. By a modified partitioning of the Riemann solutions and a specific combination of fluxes and non-conservative terms, the time-step restriction is effectively prevented, which is critical for making the method practically applicable. Moreover, the accuracy of interface and shock-wave propagation is maintained. RDEMIC is not limited to two-phase flow but defined for an arbitrary number of phases.The method is combined with a THINC scheme [2] to reconstruct volume fractions. The reconstruction is enhanced by a positivity-preserving averaging procedure, which is consistent with the underlying multi-stage Runge–Kutta scheme of the flow solver.The resulting scheme consisting of RDEMIC and the positivity-preserving THINC reconstruction is very robust and captures the interface with high accuracy. We demonstrate its performance for various cases of shock-interface interactions, which show very good agreement with reference results from literature.

Optimal system of solution using group invariance technique for shock wave in a non-ideal self-gravitating gas in rotating medium in presence of magnetic field

Abstract This work demonstrates the study of the optimal system of solutions for shock wave propagation in a non-ideal self-gravitating gas in rotating medium with magnetic field (axial or azimuthal) for the adiabatic flow in cylindrical geometry by applying the group invariance technique. Using the group invariance technique, we have obtained the one-dimensional (1-D) optimal system of sub-algebra for the basic governing equations. The infinitesimal group optimal classes are obtained and the similarity solution in four possible cases (two cases for perfect gas and two cases for non-ideal gas) with exponential law shock path are discussed. The numerical solution by using the Runge Kutta 4th order method is obtained and the distribution of physical variables are shown via graph. The impact of the rotational parameter, non-idealness parameter, shock Cowling number, similarity exponent and gravitational parameter on the strength of the shock and flow variables are investigated. With an increase in the shock Cowling number, non-idealness and rotational parameters, the shock strength decreases, i.e., they have decaying impact on shock wave; whereas the shock strength increases with gravitational parameter and similarity exponent. Also, the strength of the shock is reduced by considering the magnetic field to be axial instead of azimuthal.

Геомагнитные эффекты взрывов сибирских метеоров

The geomagnetic effects of falling meteors have been known for a long time, but so far they have not received a complete theoretical explanation. The paper considers the response of the geomagnetic field to the destruction of meteors in Yakutia (March 4, 2014, 21:29 UT) and Khakassia (December 6, 2016) according to the Irkutsk magnetic observatory data of the Institute of Solar-Terrestrial Physics SB RAS. The analysis of variations in the geomagnetic field components showed that the passages and explosions of meteors were accompanied by the appearance of several groups of geomagnetic disturbances in the area of the Irkutsk magnetic observatory. Their appearance can be caused by the propagation of shock, internal gravity and magnetohydrodynamic waves formed as a result of the passages and destructions of meteors. The periods of these wave disturbances and the velocities of their propagation from the generation regions are estimated. It is noted that noticeable wave manifestations in the geomagnetic field give waves with a period of more than 10 minutes, i.e. having gravity modes.

The importance of the metal reinforcement of low voltage cables in the process of identifying defects

Regarding the identification of defect in the electrical energy cables, in exploatation, usual or high-performance methods are used, based in particular on the phenomenon of reflection and refraction of the impulse transmitted in the cable, phenomenon generated by the impedance change at the defect location. In order to identify the defect, an important role is played by the existence of a current path on the cable route between the ends of the electrical cable and the place where the defect occurred. In the case of low voltage cables, there are situations where, following the sleeve process the continuity of the mechanical protection armature of the steel strip or the copper screen has not been restored. To measure the distance to the defect location, location equipment uses low and high voltage pulses which moves to the defect site with a certain speed and which is repeated after a selected time interval. These impulses are transmitted to the location of the defect at one end of the cable, through the circuit generated by the short circuit between the faulted conductor and the metal armature, which is usually found at ground potential. At the same time, there is situations where the defect is difficult to identify precisely because of the continuity of the reinforcement or screen and their connection to ground potential. This paper describes the stages of fault identification in a low-voltage power cable and presents a case study that highlights the importance of the metal reinforcement in the case of the high-voltage impulse method. Ways to detect defects using the arc reflected method (ARM) using the shock wave generator as well as the inductive method are also briefly presented.