Wave Propagation In Air Research Articles

Terahertz electromagnetic signal enhancement in split ring resonators featuring waveguide modes.

To resolve the high attenuation issue in terahertz (THz) wave propagation in air, we propose a split ring resonator (SRR) structure, consisting of a subwavelength slit and a circular cavity in the wavelength size, which can support coupling resonant modes and achieve a remarkable omnidirectional electromagnetic signals gain (∼40 dB) at 0.4 THz. Based on the Bruijn method, we also develop and numerically confirm a new analytic approach which successfully predicts the dependence of field enhancement on key geometric parameters of the SRR. Compared to the typical LC resonance, the enhanced field at the coupling resonance exhibits a high-quality waveguide mode in the circular cavity, paving a way for direct detection and transmission of the enhanced THz signals in future communication systems.

Stepped-plate ultrasonic transducer used as a source of harmonic radiation force optimized by genetic algorithm

Circular stepped-plates are often chosen to generate continuous waves in air due to their transmission efficiency, directive radiation patterns and capability to generate high-intensity airwave emission. Such features can be suitable for an acoustic radiator intended for the generation of harmonic radiation force, if the narrow bandwidth of the piezoelectric power transducer is not an obstacle to the application. This force has been used as the excitation source in noncontact modal analysis of mechanical devices, since it offers some advantages over the traditional impact method and the use of shakers. In this study, a directional stepped-plate ultrasonic device driven by a bolt-clamped Langevin-type transducer is used to excite the first flexural mode of a clamped-free beam by means of harmonic radiation force. The harmonic component was generated using an amplitude-modulated electrical input signal. The radiator design was done by solving a parametric optimization problem of finding a plate with the desired vibrational behavior using the genetic algorithm. Finite element analysis was used to obtain the dynamic behavior of the device and the airwave propagation was modeled using linear acoustic theory. Modal and harmonic analyses were conducted to obtain some of the electroacoustic parameters of the transducer. The acoustic field generated by the prototype is estimated by solving the Rayleigh integral and the obtained results are compared to experimental measurements.

High-accuracy computation of hard X-ray focusing and imaging for refractive optics.

A mathematical apparatus for solving problems of X-ray wave propagation through complex optical systems, when the lens thickness can change with jumps, is developed and presented. The developed method is based on the use of the superposition of oriented Gaussian beams, which satisfy the Helmholtz equation with high accuracy. The wave propagation in air and through kinoform and ordinary lenses is considered. Focusing and imaging properties are compared for both types of X-ray optics. The diffraction effects arising due to thickness jumps in the kinoform lenses and the influence of these jumps on the X-ray focusing and imaging are investigated. The prospect of using the developed theory for X-ray optics applications is discussed.

Optical measurements of acoustic fields using refracto-vibrometry

Refracto-vibrometry (RV) is an interferometric method for optically measuring acoustic fields. The measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer was directed through water or air towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the measurement laser cause variations in the integrated optical path length. By superimposing a large number of scan points, 2-D projections of propagating acoustic fields can be determined. Videos will be shown that enable visualization of acoustic wave propagation in air, water, and other transparent media. It is also possible to determine acoustic field distributions in 3-D through tomographic reconstruction of multiple RV projections through the acoustic field at different angles. It will be shown that there is strong agreement between the acoustic field distributions measured using a conventional needle hydrophone and the optical measurements using tomographic reconstruction of RV measurements. One of the limitations of the RV technique is that these optical measurements have a relatively low signal-to-noise ratio that may require averaging of a large number of ultrasound pulses. Techniques will be presented to minimize this acquisition time, including deep learning and frequency-sweep cross-correlation.

Numerical Investigation of Thin Plate Subjected to Blast Wave Propagation in Air and Water

Numerical investigations on a thin rectangular plate subjected to air blast and underwater using LS DYNA is presented in this paper. For the analysis, the box model setup, air/water medium and target plate are modelled and the explosive is assumed at a distance of 150 mm from the target plate at the normal line passing through the centre of the plate. The target plate is modelled using piece-wise linear plasticity material model, the fluid using Gruinesian equation of state, air using linear polynomial equation of state and explosive using JWL equation of state. Parametric study in terms of charge weight is carried and the results are presented for various shock factors. The free field pressure time history and the permanent deformation are compared with experimental results available in literature. The study indicates that the permanent deformation is about 5.5 times that of air blast in underwater explosion.

Development of Simulation Model for the Propagation of Pressure Wave

Detonation is a specific type of a rapid exothermic reaction, which always involves a detonation wave (in the explosive) and the shock wave (in the environment). Modelling of the pressure wave falls in the fluid flow and interference with obstacles in the flow. The purpose of this paper is to development of simulation model based on the finite element method (FEM) for shock wave propagation in air form the explosion of a spherical charge from TriNitroToluene (TNT) material. The air is the classical ideal gas and explosive material TNT is defined using Jones-Wilkins-Lee (JWL) equation. The computational model is 2D-dimensional model with four node axisymmetric elements. The effect of the explosion on the pressure distribution in the air and rigid surface (ground) is investigated. Numerical solution of the dynamic response was performed using commercial FEM software ADINA.

An assessment of blast modelling techniques for injury biomechanics research.

Blast-induced traumatic brain injury (TBI) has been affecting combatants and civilians. The blast pressure wave is thought to have a significant contribution to blast-related TBI. Due to the limitations and difficulties of conducting blast tests on surrogates, computational modelling has been used as a key method for exploring this field. However, the blast wave modelling methods reported in current literature have drawbacks. They either cannot generate the desirable blast pressure wave history or they are unable to accurately simulate the blast wave/structure interaction. In addition, boundary conditions, which can have significant effects on model predictions, have not been described adequately. Here, we critically assess the commonly used methods for simulating blast wave propagation in air (open-field blast) and its interaction with the human body. We investigate the predicted blast wave time history, blast wave transmission, and the effects of various boundary conditions in three-dimensional (3D) models of blast prediction. We propose a suitable meshing topology, which enables accurate prediction of blast wave propagation and interaction with the human head and significantly decreases the computational cost in 3D simulations. Finally, we predict strain and strain rate in the human brain during blast wave exposure and show the influence of the blast wave modelling methods on the brain response. The findings presented here can serve as guidelines for accurately modelling blast wave generation and interaction with the human body for injury biomechanics studies and design of prevention systems.

Choosing a textbook for the way your students learn

This presentation describes the challenges, opportunities, and lessons learned in the selection and use of textbooks for a 10-week course in acoustics for undergraduate physics majors at the junior or senior level at Central Washington University. This course emphasizes both theoretical and experimental skills; topics include damped harmonic systems, vibration in strings, bars, and membranes, plane and spherical wave propagation in air, and basic geometric acoustics. Two textbooks are employed for this course: the recently published Understanding Acoustics by Steve Garrett and the fourth edition of Fundamentals of Acoustics by Kinsler and Frey, revised and extended by Coppins and Sanders. Although both cover essentially the same topics, they have different attributes that facilitate learning at the undergraduate level. They are supplemented by online materials, interactive simulations, and journal articles. Examples of how these texts are used, and assessments of their efficacy, will be discussed.This presentation describes the challenges, opportunities, and lessons learned in the selection and use of textbooks for a 10-week course in acoustics for undergraduate physics majors at the junior or senior level at Central Washington University. This course emphasizes both theoretical and experimental skills; topics include damped harmonic systems, vibration in strings, bars, and membranes, plane and spherical wave propagation in air, and basic geometric acoustics. Two textbooks are employed for this course: the recently published Understanding Acoustics by Steve Garrett and the fourth edition of Fundamentals of Acoustics by Kinsler and Frey, revised and extended by Coppins and Sanders. Although both cover essentially the same topics, they have different attributes that facilitate learning at the undergraduate level. They are supplemented by online materials, interactive simulations, and journal articles. Examples of how these texts are used, and assessments of their efficacy, will be discussed.

Finite volume methods for numerical simulation of the discharge motion described by different physical models

This paper deals with the numerical solution of an ionization wave propagation in air, described by a coupled set of convection-diffusion-reaction equations and a Poisson equation. The standard three-species and more complex eleven-species models with simple chemistry are formulated. The PDEs are solved by a finite volume method that is theoretically second order in space and time on an unstructured adaptive grid. The upwind scheme and the diamond scheme are used for the discretization of the convective and diffusive fluxes, respectively. The Poisson equation is also discretized by the diamond scheme. The results of both models are compared in details for a test case. The influence of physically pertinent boundary conditions at electrodes is also presented. Finally, we deal with numerical accuracy study of implicit scheme in two variants for simplified standard model. It allows us in the future to compute simultaneously and efficiently a process consisting of short time discharge propagation and long-term after-discharge phase or repetitively pulsed discharge.

Numerical Simulation of Blast Action on Civil Structures in Urban Environment

Nowadays, a lot of industrial accidents accompanied by explosions are happening throughout the world. Also, increase in the number of terrorist acts committed by means of explosions is observed. For improving safety of buildings and structures it is necessary to raise their resistance to explosive effects, as well as to be able to predict degree of potential damage upon explosive loads of various intensities. One of the principal goals in designing the structure resistant to explosive effects is to determine the dynamic response of structures to the impact of the blast wave. To this end, the transient pressure loads on the walls of the civil engineering structures are to be determined. The simulation of explosion is highly complicated, involving an explosion causing the shock wave propagation in air and then interaction with a structure. The engineering-level techniques permit one to estimate an explosive shock impact only for isolated buildings. The complexity of the building, the presence of nearby structures and the surrounding environment cannot be taken into account. Advanced computer aid engineering (CAE) software techniques combined with the latest methods of discrete three-dimensional city modelling permits one to simulate and analyse the effects of explosions in urban areas with a precision which previously was not possible. In the paper, the simulation results are presented of shock wave forming due to a spherical explosive charge and its propagation in the vicinity of geometrical configuration imitating an urban environment. The numerical simulation of a flow in the vicinity of prisms of different cross-sections and heights located on a flat plate was performed. The calculations are carried out in a three-dimensional non-viscous formulation using ANSYS software. On a basis of simulation results, a complex wave structures were analysed, and all the peculiarities of flows and pressure history records on building walls were described and explained. The possibility of a correct description of the non-stationary wave flow in the vicinity of the complex of obstacles is demonstrated. The results are compared with the experimental data on the pressure distribution in gauges located on the prism walls. The estimation of shock wave exposure intensity was performed to different objects.

Theoretical and Experimental Studies on Blast Wave Propagation in Air

AbstractHigh pressure and temperature are produced when high explosives are detonated in open air. The heat of detonation of the explosive compound, peak pressure, and temperature of the blast wave are important blast parameters. A blast wave generated due to explosion propagates into the air medium at supersonic speed until the pressure in the blast zone is released completely. The intensity of the impact by the blast wave on any intervening solid object depends on the blast parameters and the speed of propagation of the blast wave. A theoretical analysis is carried out to predict the pressure produced in the expanding blast zone as function of distance and time by analytically solving the governing equations. The initial peak pressure and temperature of blast wave, which are required in the theoretical analysis, were calculated making use of the blast wave theory. For comparison, experiments were conducted by detonating different weights of high explosives, and pressures were recorded at various distances from the blast point. The high explosives used in the experiments were TNT (0.045, 0.5, 1, 15, and 40 kg) and Composition B (0.045, 0.5, 1, and 15 kg). The theoretical results are validated by comparison with the experimental data and empirical equations available in literature.

Finite integration technique for coupled acoustic and elastic wave simulation and its application to noncontact ultrasonic testing

To better understand wave propagation and scattering phenomena in noncontact ultrasonic testing (UT), we have developed a time domain simulation tool to model wave propagation in air and solids. The tool is based on the finite integration technique (FIT) and an image-based modeling approach. The image-based FIT can treat the calculation of acoustic, elastic, and coupled waves in a unified grid form. In this paper, we first investigate the accuracy of the interface condition in FIT modeling by comparing numerical results with analytical solutions of transmission and reflection coefficients. Then we validate the FIT using measured signals in guided wave testing with air-coupled transducers. To simulate noncontact UT, a fine mesh is required in the FIT because the wavelength in air is much smaller than that in a solid. To solve the numerical problem with the finer mesh at high speed, we introduce a parallel computation technique with general-purpose computing on graphics processing units (GPUs). It is shown that the noncontact UT simulation can be executed within a reasonable time and with reasonable accuracy using multiple GPUs.

Wave propagation in air from a vertical magnetic dipole located in three rough-layered structures

In this paper, the field scattered in the upper medium (air) by a system composed of three homogeneous media (air–sea–ground) that are separated by two rough interfaces is derived from a perturbation technique. The source is located in the inner medium (seawater), which is a vertical magnetic dipole. Then, by applying the boundary conditions at each interface, the scattered field in each medium is derived analytically and expressed using the Hankel transformation (integral over the radial distance r). This research assumes that the modulus of the sea permittivity () is much larger than that of the air and that the parameter (far field assumption), closed-form expressions of the involved integrals are found for the scattered field in the air by using an approximate method. No pole is considered and therefore the formulation is consistent with previous published results. Numerical results are given and represented graphically. In addition, this research presents a comparison of the results obtained here with those of one rough surface and two planar surfaces. This study will be useful for remote sensing of the ocean surface, especially when the transmitter is close to the surface.

Theory of wideband instability at combination frequencies of a plane electromagnetic wave propagating in a nonlinear medium

We study the propagation of electromagnetic waves in nonlinear media with the polynomial dependence of the dielectric permittivity on the intensity of the electric field. The instability of a plane wave with respect to noncollinear perturbations is studied in a wide band of combination frequencies. The perturbation growth rate and their dependencies on angles and frequencies are found. It is shown that waves with frequencies close to the second harmonic of the frequency of the propagating wave can increase faster than the perturbations in the case of the well-known modulation instability. We discuss application of the developed theory for the description of the light wave propagation in air, for which a representation of the nonlinear dielectric permittivity in the form of the electric field polynomial is known. The results of the work can be useful in explaining the properties of optical rogue waves.

Lower atmospheric anomalies following the 2008 Wenchuan Earthquake observed by GPS measurements

The Mw=8.0 Wenchuan Earthquake occurred on May 12, 2008 at the Longmen Shan fault, the western Sichuan Basin, China, killing more than ten thousand people in several cities and causing large economic losses. Global Positioning System (GPS) observations have provided unique insights on this event, including co-seismic ionospheric disturbances, co-/post-seismic crustal deformations and fault slip distributions. However, the processes and the driving mechanisms are still not clear, particularly possible seismo-lower atmospheric–ionospheric coupling behaviors. In this paper, the lower atmospheric (tropospheric) variations are investigated using the total zenith tropospheric delay (ZTD) from GPS measurements around this event. It has the first found co-seismic tropospheric anomalies during the mainshock with an increase and then a decrease, mainly in the zenith hydrostatic delay component (ZHD), while it is also supported by the same pattern of surface-observed atmospheric pressure changes at co-located GPS site that are driven by the ground-coupled air waves from ground vertical motion of seismic waves propagation. Therefore, the co-seismic tropospheric disturbances (CTD) indicate again the acoustic coupling effect of the atmosphere and the solid-Earth with air wave propagation from the ground to the top atmosphere.

Prediction of the air wave due to blasting inside tunnels: Approximation to a ‘phonometric curve’

A review of the results of a previous research, has pointed out that only the magnitude of the blasting air wave at the tunnel portal depends on the tunnel and blasting design parameters (section, length, distance to the portal, maximum charge per delay…). For a given sound level at the tunnel portal, the magnitude of the air wave outside the tunnel depends on the attenuation due to natural or artificial barriers, similar to how a lamp focuses the light more along a given direction than along others. In this work, the air wave propagation outside the tunnel is briefly explained and a phonometric and iso-attenuation curves are proposed in order to represent the phenomenon and to synthesize the solution for a given case. Finally, a charge–distance curve is proposed in order to solve the problem in an easy way.

Nonlinear acoustic wave propagation in inhomogeneous moving media.

Extensive theoretical analysis, numerical studies, and both large-scale and laboratory-scale experiments have been dedicated to the problem of shock wave propagation in air during recent years. The current interest is motivated by supersonic civil transport which is necessarily affected by problems of sonic boom propagation in the atmosphere. The high-amplitude shock wave generated by a supersonic aircraft propagates through the atmosphere toward the ground and generates an acoustic field with non-uniform pressure distribution. Temporal characteristics and spatial structure of the sonic boom are influenced by aircraft trajectory, nonlinear effects, and diffraction and scattering by inhomogeneities. We review recent results from various teams based on a generalized KZK-type equation that includes the effects of a moving inhomogeneous media. Statistical analysis of the numerical solutions is performed, and the results are compared to experimental data obtained in the controlled laboratory-scale experiments conducted in the Ecole Centrale de Lyon anechoic wind tunnel.

Two-Dimensional Simulation of Nonlinear Acoustic Wave Propagation Using Constrained Interpolation Profile Method

In this study, a numerical simulation of the nonlinear sound wave propagation in the time domain is demonstrated by constrained interpolation profile/cubic-interpolated pseudoparticle (CIP) method. The CIP method is a novel numerical scheme which was proposed by Yabe et al. It is a method of characteristics (MOC) and is a low-dispersion and stable scheme. The CIP method is suitable for analysis of nonlinear wave propagation including weak shock formation because the rapid pressure change, such as that caused by a shock front, easily causes numerical dispersion error in a conventional numerical scheme. Some of the authors have reported application of the CIP method to a one-dimensional nonlinear sound wave propagation in air. Then, we demonstrate a numerical calculation of the nonlinear sound wave propagation for a two-dimensional acoustic field.

Single transducer parametric sound source

The idea of using the nonlinear effects of the propagation of sound waves with finite amplitude to build up highly directed sound sources is well known. In underwater acoustics, such parametric arrays are widely used. There are also attempts to build up parametric arrays in air to generate highly directed air-borne sound. To be able to use nonlinear effects in air as a carrier medium, one needs high amplitudes of the propagating sound wave. Consequently, a superposition of single sound sources (arrays) is often reported in the literature to increase the size, and so the directivity, of the sound source as well as the acheived sound pressure level. A major disadvantage of using arrays is the problem of slightly varying resonance frequencies and phase characteristics of the single source components as well as a complicated superposition of single directivity patterns. It is difficult to investigate basic properties of wave propagation and the nature of distortion in such a way. In this work, a single, high-power ultrasound transducer was used to radiate sound waves with finite amplitude, able to generate extraneous frequency components in air. Using only a single transducer, it was possible to investigate basic properties of nonlinear wave propagation in air.

Numerical simulation of finite amplitude wave propagation in air using a realistic atmospheric absorption model

A nonlinear system of fluid dynamic equations is modeled that accounts for the effects of classical absorption, nitrogen and oxygen molecular relaxation, and relative humidity. Total variables rather than acoustic variables are used, which allow for the inclusion of frequency-independent terms. This system of equations is then solved in two dimensions using a fourth-order Runge-Kutta scheme in time and a dispersion-relation-preserving scheme in space. It is shown that the model accurately simulates wave steepening for propagation up to one shock formation distance. For a source amplitude of 157dB re 20μPa, the Fourier component amplitudes of the analytical and computed waveforms differ by 0.21% at most for the first harmonic in a lossless medium and 0.16% at most for the first harmonic in a medium that includes thermoviscous losses. It is shown that the absorption due to classical effects and molecular relaxation demonstrated by the model is within 1% of the analytical model and the computed dispersion due to molecular relaxation of nitrogen and oxygen is within 0.7% over a large frequency range. Furthermore, it is shown that the model can predict the amplification factor at a rigid boundary, which in a nonlinear system will be greater than two, to within 0.015%.