Wave Reflection Process Research Articles

Wave transformations near a coronal magnetic null point

Context. Null points are often invoked in studies of quasi-periodic coronal jets and in connection with periodic signals preceding actual reconnection events. Although the periodicity of these events spans a wide range of periods, most show a 2- to 5-min periodicity compatible with the global p-modes. Aims We investigate whether magnetohydrodynamics (MHD) waves, in particular, acoustic p-modes, can cause strong current accumulation at the null points. This can in turn drive localized periodic heating in the solar corona. Methods. To do this, we began with a three-dimensional numerical setup incorporating a gravitationally stratified solar atmosphere and an axially symmetric magnetic field including a coronal magnetic null point. To excite waves, we employed wave drivers mimicking global p-modes. Using our recently developed wave-mode decomposition technique, we investigated the process of mode conversion, mode transmission, and wave reflection at various important layers of the solar atmosphere, such as the Alfvén acoustic equipartition layer and transition region. We examined the energy flux distribution in various MHD modes or in acoustic and magnetic components, as the waves propagate and interact with a magnetic field of null topology. We also examined current accumulation in the surroundings of the null point. Results. We found that most of the vertical velocity is transmitted through the Alfvén acoustic equipartition layer and maintains an acoustic nature, while a small fraction generates fast waves via the mode conversion process. The fast waves undergo almost total reflection in the transition region due to sharp gradients in density and Alfvén speed. There are only weak signatures of Alfvén wave generation near the transition region through the fast-to-Alfvén mode conversion. Because the slow waves propagate with the local sound speed, they are not much affected by the density gradients in the transition region and undergo secondary mode conversion and transmission at the Alfvén-acoustic equipartition layer surrounding the null point. This leads to fast-wave focusing at the null point. These fast waves have associated perturbations in current density and show oscillatory signatures that are compatible with the second harmonic of the driving frequency. This might result in resistive heating and in an enhanced intensity in the presence of finite resistivity. Conclusions. We conclude that MHD waves are a potential source for oscillatory current dissipation around the magnetic null point. We conjecture that in addition to oscillatory magnetic reconnection, global p-modes could lead to the formation of various quasi-periodic energetic events.

The effect of flexible obstacles with different thicknesses on explosion propagation of premixed methane-air in a confined duct

The effect of flexible obstacles with varying thicknesses on the explosion characteristics of combustible gas in a simulated confined duct (cross section 80 mm × 80 mm, length 3 m) was experimentally investigated, aiming to reduce the huge losses caused by gas explosion accidents in the process industries and mining industries. In this paper, plant fiber membranes with an opening area of 0 and thicknesses of 0.105 mm, 0.210 mm, 0.315 mm, 0.420 mm, 0.525 mm, and 0.630 mm were selected as flexible obstacles. The thickness of the flexible obstacle determines the strength of its compressive resistance. The characteristics of overpressure and flame during methane explosions are analyzed and conclusions are drawn. Results indicate that several shock wave reflection processes occur before the diaphragm ruptures, resulting in turbulent flames. In addition, the explosion wave generated numerous shock reflections during the rupture process of the diaphragm, which was gradually discharged downstream of the pipe by ejection as the pressure wave accumulated in front of the diaphragm. It should be noted that the thickness of the flexible obstacle determines the pressure accumulation in front of the membrane. Generally, the thinner the flexible obstacle, the less intense the turbulent flame is induced by the flexible obstacle, decreasing the contact area between the unignited gas downstream of the pipeline and the turbulent flame area. In conclusion, with an increase in the thickness of the flexible barrier, it exhibits a mechanism of initially suppressing and subsequently enhancing the impact on methane explosions. The increase of the thickness of the flexible obstacle motivates the flame propagation speed, which leads to an increase of turbulence intensity and explosion intensity.

A new method and application of groundwater prediction along the direction of tunnel excavation in karst strata

Abstract Tunnel construction in karst strata with abundant water causes changes in the surrounding groundwater environment, which can easily trigger geological disasters such as mud and water inrush. How to accurately predict the groundwater ahead of tunnel excavation face is a highly challenging problem. In order to improve the detection accuracy of groundwater during the construction of a deep buried tunnel, the transmission and reflection process of seismic waves at the interface and the relationship between the reflection coefficient and seismic wave signal have been analyzed on the basis of a two-phase medium theory in this paper. The expression of seismic wave stress–response relationship associated with the instantaneous amplitude and instantaneous phase and frequency is established. Then the relationship between seismic wave attributes and groundwater seepage potential energy is derived by combining the fluid mechanics theory, which is used as the basis for the determination and identification of groundwater volume and classified, and a new technology of an advanced detection of groundwater by seismic waves is established. This method has been applied to the Zhanghuai Railway in China and quantitatively predicted the karst water and caves in the Tianqiaoshan Tunnel before excavation. The engineering test proves the reliability and advancement of this technology.

Picosecond-range switching of high-voltage Si diode due to the delayed impact-ionization breakdown: Experiments vs simulations

The effect of delayed impact ionization breakdown initiated in a high-voltage Si or GaAs p+nn+ diode by a steep voltage ramp leads to 100 ps avalanche transient from the blocking to conducting state. Here, measurements of the voltage and current dependences in the Si diode exhibiting 100-ps kilovolt switching are presented together with simulations with focus on comparison. Device voltage and current are measured simultaneously and independently in a high-quality matched coaxial circuit. In simulations, we account for wave propagation and reflection processes in the coaxial driving/measuring circuit and for the inhomogeneity of the avalanche switching over the device cross section. This makes quantitative comparison with measurements possible. An agreement in switching time and transient characteristics can be achieved only under the assumption that a smaller part of the cross section is avalanching. The 100-ps switching time is formed not during the passage of superfast ionizing front in the “active” part of the device, as it is widely believed, but by the discharge time of the “passive part” over the conducting “active” part. The inner circuital current that flows within the device along the closed loop plays a dominant role in this process. Sources of initial carriers, the temperature dependence of the effect, and the limits of drift-diffusion transport model in describing the phenomenon of delayed breakdown are discussed.

3D CFD Simulation of Phase Resonance in a Pump Turbine with an Acoustic Model

In order to understand the complex nature of the system dynamic phenomena, such as the strong vibration and noise caused by blade passage in the pump turbine, a state-of-the-art three-dimensional (3D) compressible transient simulation would be desirable to study the problem in depth. This study investigated the phase resonance (PR) that occurred during a full-load operation in the turbine mode of a pump turbine on a prototype scale. As a first step, the wave reflection at the boundaries, and the influence of the timestep and sound speeds on the behavior of traveling pressure waves inside a spiral casing, were studied. It was found that nonreflective boundary conditions and an appropriately small timestep are critical to capturing the wave reflection and superposition process inside a spiral casing; a certain kind of direct PR risk was detected in its system design. The detected direct PR differed from the well-known PR with two features: firstly, it was almost independent of the sound speeds, and secondly, the pressure distribution over the spiral circumference varied among the amplitudes. The latter feature was caused by pressure waves at every stator channel induced by a rotor stator interaction (RSI). The 3D flow simulation with an acoustic model, which couples the RSI and PR phenomena, would predict better results for understanding the problem than the simplified one-dimensional (1D) method.

Full-scale experimental study on wave reflection and run-up at stepped revetments

Stepped revetments are especially suitable in urban areas as they effectively limit wave overtopping whilst offering recreational benefits. A deeper understanding of wave-structure interactions facilitates optimised stepped revetment designs. Wave reflection and wave run-up processes also indirectly provide insights in the wave energy dissipation of stepped revetments. Only few studies have focused on wave reflection from stepped revetments, while the wave run-up process on stepped revetments was studied widely, yet only with small-scale wave flume tests and mostly with regular waves. As the wave overtopping rates in small-scale wave flume tests underestimate those measured in full-scale experiments, these studies are likely affected by scale. This paper investigates wave reflection from and wave run-up on stepped revetments by means of full-scale flume experiments. Two cross-sections, each with a slope of 1:3, were studied with uniform step heights of 0.17 m and 0.50 m. It was found that previous small-scale studies underestimate relative run-up heights by 31–51%. Empirical formulae were derived to estimate wave reflection coefficients and wave run-up heights of stepped revetments. Wave reflection and run-up processes are described and provide further insight in the functioning of stepped revetments.

Investigation on flow field structure and aerodynamic load in vacuum tube transportation system

Vacuum Tube Transportation (VTT) system utilizes the low-pressure environment to achieve aerodynamic drag reduction and reduce the energy consumption of high-speed trains. Within this system, even for trains operating at subsonic speed, the flow acceleration around the streamlined train may leads to local supersonic flow. The appearance of the supersonic flow will dramatically alter the flow field structure and aerodynamic load of the high-speed train. Therefore, this study investigates two types of representative flow fields under different blockage ratio through the improved delayed detached eddy simulation (IDDES) method. The initialization, propagation, reflection and intersection process of shock waves in transonic case are captured and analyzed. It is shown that, the boundary layer thickness and width of vortex group in wake has experienced a slight increase with the increase of vacuum level. Compared with the subsonic case, the occurrence of choked flow and shock wave in transonic flow field lead to a decrease of wake region's vortex group width. As a result, the total drag coefficient of the train is increased by approximately one order of magnitude, and the dominant St number of tail car's drag coefficient has shifted from 0.16 to 0.11.

Research on Seismic Input Method of Layered Ground Foundation

The seismic input of layered foundation is an essential part of seismic analysis of structure- foundation system. Most of the traditional seismic input methods only consider the first reflection process of seismic waves at the interface of different media, ignoring the subsequent reflection and refraction propagation process, which can not truly reflect the wave propagation process in the layered foundation. This paper proposes a new seismic input method for layered foundation, considering all reflected and refracted waves. Firstly, the viscous-spring artificial boundary model of the layered foundation is established, which can simulate the absorption of scattered waves and the elastic recovery capacity of semi-infinite foundation at the same time. Then, based on the wave mechanics theory, the equivalent nodal force calculation formula is derived to realize the vibration input of layered ground. Finally, the calculation results of the layered foundation and two-way surge tower engineering are verified. The results show that: compared to the traditional input method, the method proposed in this paper has a significant influence on the seismic response of layered foundation, and the method in this paper can better meet the accuracy requirements of engineering earthquake resistance, which provides an effective and practical calculation method for seismic analysis of layered foundation structure.

Experimental investigation on damage and wave propagation of PVB laminated glazing structures under impact loading

Experimental investigation on the damage and wave propagation characteristics in PVB laminated glass panels subjected to impact loading was conducted. By applying different impact energies, the effect of two projectile configurations, a wooden post and steel ball, was studied. The effect of impact location was also investigated. The dynamic response of the panels was measured at different locations on the test panels using strain gauges. Damage and transient response and wave propagation characteristics of test panels were reported and compared. The results showed that flexural wave was the predominant wave in the response. In addition, in-plane compressive wave was also observed. It was also shown that while the projectile contact area affected the maximum transient response and crack propagation characteristics, it had a limited effect on the perforation threshold. For panels impacted at the corners, coupled strain waves were created by wave reflection processes when the waves reached the panel boundaries. Moreover, the impact velocity had a pronounced effect on peak strain and strain rates of test panels.

Wave Dissipation Characteristics of A Mountain-Type Breakwater

One mountain-type breakwater consisting of two inclined plates and one vertical plate is proposed based on several types of traditional free surface breakwaters, including the horizontal plate, curtain wall, and trapezoidal barriers. The interaction between the regular waves and the fixed free surface mountain-type breakwater is measured in one wave flume (15.0 m×0.6 m×0.7 m). The wave propagation, reflection, and transmission process are simulated using the VOF method and the hybrid SAS/laminar method. The simulated wave profiles are consistent with the experimental observations. For waves with a length smaller than four times width of the mountain-type breakwater, the reflected wave amplitudes are slightly larger than those of the vertical-plate breakwater, while the wave transmission coefficients are all smaller than 0.5, and the wave loss coefficients are larger than 0.7. The wave energy is dissipated by wave breaking on the windward inclined plate, and turbulent flow around the vertical plate and the leeward inclined plate.

Shock wave mitigation using zig-zag structures and cylindrical obstructions

The present study focuses on the mitigation of shock wave using novel geometric passages in the flow field. The strategy is to produce multiple shock reflections and diffractions in the passage with minimum flow obstruction, which in turn is expected to reduce the shock wave strength at the target location. In the present study the interaction of a plane shock front (generated from a shock tube) with various geometric designs such as, 1) zig-zag geometric passage, 2) staggered cylindrical obstructions and 3) zig-zag passage with cylindrical obstructions have been investigated using computational technique. It is seen from the numerical simulation that, among the various designs, the maximum shock attenuation is produced by the zig-zag passage with cylindrical obstructions which is then followed by zig-zag passage and staggered cylindrical obstructions. A comprehensive investigation on the shock wave reflection and diffraction phenomena happening in the proposed complex passages have also been carried out. In the new zig-zag design, the initial shock wave undergoes shock wave reflection and diffraction process which swaps alternatively as the shock front moves from one turn to the other turn. This cyclic shock reflection and diffraction process helps in diffusing the shock wave energy with practically no obstruction to the flow field. It is found that by combining the shock attenuation ability of zig-zag passage (using shock reflection and diffraction) with the shock attenuation ability of cylindrical blocks (by flow obstruction), a drastic attenuation in shock strength can be achieved with moderate level of flow blocking.

Performance Assessment of Three Turbulence Models Validated through an Experimental Wave Flume under Different Scenarios of Wave Generation

This study aimed to adjust the turbulence models to the real behavior of the numerical wave flume (NWF) and the future research that will be carried out on it, according to the turbulence model that best adjusts to each particular case study. The k-ε, k-ω and large-eddy simulation (LES) models, using the volume of fluid (VOF) method, were analyzed and compared respectively. The wavemaker theory was followed to faithfully reproduce the waves, which were measured in an experimental wave flume (EWF) and compared with the theory to validate each turbulence model. Besides, reflection was measured with the Mansard and Funke method, which has shown promising results when studying one of the most critical turbulent behaviors in the wave flume, called the breaking of the waves. The free surface displacement obtained with each turbulence model was compared with the recorded signals located at three points of the experimental wave flume, in the time domain of each run, respectively. Finally, the calculated reflection coefficients and the amplitudes of the reflected waves were compared, aiming to have a better understanding of the wave reflection process at the extinction zone. The research showed good agreement between all the experimental signals and the numerical outcomes for all the turbulence models analyzed.

Detection object application at mirror for smart home with fuzzy logic method using Raspberry PI Microcontroller

During this time, the mirror is used to view objects that are not visible to the eye, but can be used as a supporting tool in providing information, which is to detect the distance of objects that are around it. By using Raspberry PI Mini PC which is equipped with Raspberry Pi camera module, speaker and ping sensor. In displaying information the application is designed using python, PHP, css that runs on a single board operating system, the output is a display of a camera or PHP display. Therefore, the mirror can have different functions by applying the Fuzzy Logic method as a decision making process on the micro. This algorithm is used to translate a quantity expressed using linguistic language. In the application of intelligent systems on smart mirrors, there are three fuzzy logic processes, namely fuzzification, evaluation of rules and defuzzification. From each of these processes will affect the response of the system that is controlled. But there is a shortage, in the ping sensor delay process with Raspberry pi cameras because the ultrasonic wave reflection process is rather slow.

Multi-month sedimentological characterization of the backshore of an artificial coarse-clastic beach in Italy

In this paper, a sedimentologic characterization of an artificial coarse-clastic beach at Marina di Pisa (Tuscany, Italy) is carried out within a 5 month timespan (October 2018–February 2019). The beach was built in 2007 as a form of coastal protection due to severe erosion processes occurred along this sector of the coast in the past 150 years. The grain-size characterization was performed using pyDGS, a digital grain-size analysis software developed by Buscombe (2013). It is an open-source Python framework that uses a continuous wavelet transform method to calculate grain-size parameters (e.g., mean grain size, sorting) from images. By matching grain-size data with the topographic evolution of the beach provided by DGPS surveys of the backshore, we pointed out a seaward shift of coarser pebbles after medium-energy events (max significant wave height about 3 m). This trend suggests that wave reflection processes onto the seawall are generated during those events, producing a significant cross-shore transport of pebbles along the backshore. Such reflection processes had already been reported in the scientific literature during higher energy wave states though, which means that the artificial beach had decreased the protection level over time. These observations might be useful for coastal managers to optimize future coarse-clastic beach fill interventions. Digital grain-size analysis proved to be reliable and less time-consuming compared to traditional techniques, such as dry-sieving grain-size analysis and caliper measurements. Further improvements are still required specially to extend the survey in the underwater environment.

A new method to estimate the overtopping and overflow discharge at over-washed and breached dikes

Most of the experience collected on wave overtopping is concentrated on emerged or zero-freeboard structures, and only few experiments are available on over-washed dikes. The experience on submerged crest conditions is principally related to rubble mound structures and limited to the wave reflection and transmission processes. The existing tools for the prediction of the average wave overtopping discharge (q) at dikes are therefore targeted to represent overtopped structures, leading to cautious estimates at over-washed dikes and unrealistic overestimations in case of catastrophic scenarios such as breached dikes. This paper proposes a new conceptual and practical method for the prediction of q that is valid for both over-washed and fully breached dikes. It consists of three physically-based formulae whose coefficients are fitted on a new numerical database of smooth structures under variable wave attack. The predictions are compared with the new numerical results and the data and the tools available from the literature, i.e. experimental data, formulae and neural network. The new method gives a more realistic representation of the overtopping and overflow processes, reducing significantly the overestimation in case of breaching and being at least as accurate as the existing tools in case of over-washing.

The influence of radial entrance width of the circular cavity on incident shock wave focusing

Despite the achievement of shock wave focusing with certain reflectors, the influence of the radial entrance width of a circular cavity on the flow field has yet to be addressed. In this study, we systematically investigated the effects of the shock wave focusing process in a cavity based on the radial entrance widths. An experimental system was installed to research the evolution of the flow field under conditions with different radial entrance widths of 3.0, 11.1, 19.5, and 33.0 mm. A schlieren system was used to photograph the structures of the flow field in the cavity, and a data acquisition system was used to record the dynamic pressure histories of different points. A numerical simulation was carried out to investigate greater details of the shock wave focusing process. A third-order strong stability-preserving Runge-Kutta method, third-order weighed essential non-oscillation scheme, and an adaptive mesh refinement algorithm were adopted to simulate the shock wave reflection, diffraction, and focus process. Good agreement between the experimental and numerical results was observed. By comparing the evolution process of the flow field under the conditions of four different entrance cavity widths, we found that when the entrance width was 19.5 mm, there was the stronger intensity of the shock wave focusing in the focal region, and the larger pressure value at the apex of the cavity than the other three entrance widths, occur. This study improves our understanding of shock wave focusing.

Selection of internal wave beam directions by a geometric constraint provided by topography.

Direct numerical simulations are performed to investigate the generation of internal waves in a linearly stratified fluid by oscillating barotropic flows over a model continental shelf-slope topography. The presence of a third wave-beam emitted from an abrupt shelf break and transverse to the topography, which has not been adequately interpreted, is now explained in terms of a geometric constraint provided by the topography. This explanation applies to wave beam selection at any abrupt topographic junction point, no matter whether it is convex or concave, or its nearby slope is subcritical or supercritical. One exception is that, at an abrupt concave point with a nearby supercritical slope, the blocking effect leads to the presence of "dead water" (i.e., no flow) and thus no wave beam is emitted. On a critical slope, two beams with opposite directions are emitted from an amphidromic point that has a distinct distance from the shelf break. In addition to the internal wave dispersion relation that restricts possible wave beam directions to form an X-pattern, the geometric constraint proposed in the present work serves as a second selection mechanism that further restricts wave beam directions. The reflective direction of a wave beam incident onto a slope can also be explained by this geometric constraint. The present work provides an updated explanation of internal wave beams emitted at abrupt topographic junction points and unifies the explanation of the wave beam direction for both wave generation and reflection processes.

Giant Terahertz-Wave Absorption by Monolayer Graphene in a Total Internal Reflection Geometry

We experimentally demonstrated significant enhancement of terahertz-wave absorption in monolayer graphene by simply sandwiching monolayer graphene between two dielectric media in a total internal reflection geometry. In going through this structure, the evanescent wave of the incident terahertz beam interacts with the sandwiched graphene layer multiple (up to four) times at varying incidence angles. We observed extremely large attenuation (up to ∼70% per reflection), especially for s-polarized radiation. The experimental results are quantitatively consistent with our calculations, where we modeled the experiment as an electromagnetic wave reflection process in monolayer graphene. We also derived analytical expressions for the absorptance, showing that the absorptance is proportional to the amount of Joule heating on the graphene surface induced by the terahertz radiation.

Experimental Study on the Current Transmission Efficiency for the Transition Structure of Vacuum Transmission Line MITL on Flash-II Accelerator

An experimental platform based on a coaxial cylinder magnetically insulated transmission line (MITL) was established on the basis of Flash-II accelerator. An experimental study was conducted on the current loss characteristics of the transition region between the vacuum transmission line and the MITL under different impedance match conditions by adjusting the loading gap of an e-beam diode. The results showed that under shorted load, the whole vacuum transmission region suffers no current loss. Furthermore, the voltage waveform continuously decreases when approaching the load region. When the load impedance is smaller than the working impedance of the transmission line, the MITL becomes undermatched, and the current transmission efficiency of the transition region reaches 98% without the evident current loss. When the load impedance increases to make MITL overmatched, an MITL operates under a self-limited flow accompanied with huge current loss in the transition region. The current transmission efficiency decreases to 87%, and the current loss process significantly affects the working process of the probe on an anodic differential coil. On the basis of the working mechanism of an MITL and the wave reflection process, a qualitative analysis was conducted, and the experimental results were explained.

Predictability of the stratospheric polar vortex breakdown: An ensemble reforecast experiment for the splitting event in January 2009

AbstractA series of ensemble reforecast experiments is conducted to investigate the predictability and the occurrence mechanism of a stratospheric sudden warming occurred in late January 2009, which is a typical polar vortex splitting event. To fully examine the rapid vortex splitting evolution and predictability variation, ensemble forecasts are carried out every day during January 2009. The vortex splitting event is reliably predicted by forecasts initialized after 6 days prior to the vortex breakup. It is also found that the propagating property of planetary waves within the stratosphere is a key to the successful prediction for the vortex splitting event. Planetary waves incoming from the troposphere are reflected back into the troposphere for failed forecasts, whereas they are absorbed within the stratosphere for succeeded forecasts. Composite analysis reveals the following reflection process of planetary waves for the failed forecast: Upward propagation of planetary wave activity from a tropospheric blocking over Alaska is weaker during initial prediction periods; then, the deceleration of the zonal wind in the upper stratosphere becomes weaker over Europe, which produces a preferable condition for the wave reflection; hence, subsequently incoming wave activity from the troposphere over Europe is reflected back over the Siberia inducing the eastward phase tilt of planetary waves, which shuts down the further upward propagation of planetary waves leading to the vortex splitting. Thus, this study shows that the stratospheric condition would be another important control factor for the occurrence of the vortex splitting event, besides anomalous tropospheric circulations enforcing upward propagation of planetary waves.