Change In Propagation Speed Research Articles

Application of optical fiber distributed acoustic sensing system in ultrasonic detection

Abstract The optical fiber distributed acoustic sensing system can continuously detect acoustic signals along the optical fiber, with the characteristics of wide response frequency band, large capacity, anti-electromagnetic interference and so on. An ultra-sensitive distributed optical fiber acoustic sensing system (uDAS) was produced with a high response frequency up to 100 kHz and thousands of sensing points. The arrayed ultrasonic detection can be realized by the distributed characteristics of the system. For verifying its ultrasonic detection ability, it is applied to ultrasonic nondestructive testing of structural defects. 20 optical fiber ultrasonic detection units were constructed. Each one is a fiber ring with a diameter of about 1.5 cm and a length of 2 meters. Two cubic cement structures with a side length of 30 cm were manufactured for experiment, one of which had an artificial internal defect. The optical fiber array was attached to the surface of the cement structure, and the ultrasonic signal with a frequency of 40 kHz was excited at a single point on the opposite side. The ultrasonic propagation speed changes on account of the defects such as holes and loose texture in the propagation path. The defects in the structure are identified and located by analyzing the difference of arrival time of waves. It is expected that such sensing system could found important applications in structure defect detection.

The change in propagation speed of natural shear waves by altering preload might differentiate patients with increased myocardial stiffness

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Research Foundation Flanders - FWO. Background Shear wave (SW) elastography (SWE) is a novel ultrasound-based technique for the assessment of myocardial stiffness. SWE tracks the propagation of SWs travelling through the myocardium, induced by e.g. mitral valve closure (MVC). The speed of SWs is related to myocardial stiffness. As myocardial stiffness is defined by a non-linear stress-strain relation, it is dependent on the operating stress levels and thus left ventricular (LV) loading. As a result, elevated SW speed values could be caused by both increased LV preload or structural remodelling of the myocardium. In this way, performing SWE measurements at different loading states could potentially provide additional information aiding with the interpretation of these measurements. Purpose to investigate if performing SWE before and after a loading change could provide more information on tissue properties of the myocardium. Methods Twelve patients [age: 76(41–82)] receiving intermittent hemodialysis (HD) and 12 age-matched healthy volunteers [age: 67(47–71); p = 0.19] were included. Patients were scanned before and 1 hour after the start of HD. Parasternal long-axis views were acquired using an experimental ultrasound scanner with an average frame rate of 1086±185 Hz. Patients were divided into 2 equal groups based on the grade of diastolic dysfunction (DD), determined using conventional echo parameters (group 1: DD grade 0–1 and group 2: DD grade 2−3). SWs after MVC were visualized on M-mode maps along the septum which were colour coded for tissue acceleration (Figure A). The speed was semi-automatically calculated by measuring the slope of the SW. Results The average ultra-filtrated volume after 1h of HD was 0.64±.27 L, which did not significantly differ between patient groups (p = 0.44). Patients in group 2 had significantly higher indexed LV wall mass compared to group 1 (128±39 g/m² vs. 72±24 g/m²; p = 0.02), indicative of LV structural remodelling. SW speed after MVC was significantly different between healthy volunteers and the patient groups at baseline (p<0.01) (Figure B). After 1h of HD, SW speed decreased in both groups, with the largest decrease in group 2 (Figure B). Furthermore, SW speed values of patient group 1 decreased to values similar to healthy volunteers, while patient group 2 still had significantly increased SW propagation speed. Conclusions Overall, myocardial stiffness, assessed using SW propagation speed, of patients with end-stage kidney disease was increased compared to healthy volunteers. This could result from increased LV preload or structural remodelling. Our data indicate that in addition to single measurements, assessing the change in SW speed by altering the loading state might aid in differentiating these causal mechanisms underlying increased myocardial stiffness. Therefore, adding a loading challenge, e.g. leg lifting, when doing SWE could provide additional information. Further research needs to confirm these initial findings.

Fatigue state characterisation of steel pipes using ultrasonic shear waves.

The phenomenon of the reduction in the propagation speed of an ultrasonic wave when it travels through a fatigue zone has been well studied in the literature. Additionally, it has been established that shear waves are more severely affected by the presence of such a zone, compared with longitudinal waves. Our study utilises these phenomena to develop a method able to characterise the fatigue state of steel pipes. Initially, the existing theory regarding the increased sensitivity of shear waves to the presence of fatigue is validated through measuring and comparing the change in propagation speed of both longitudinal and bulk shear waves on flat geometries, at different fatigue states. The comparison is achieved with the aid of ultrasonic speed C-scans of both longitudinal and shear waves, with the latter now being obtainable through our implementation of advances in Electromagnetic Acoustic Transducers (EMAT) technology. EMATs have not been traditionally used for producing C-scans, and their ability do to so with adequate repeatability is demonstrated here; we show that shear wave scanning with EMATs now provides a possibility for inspection of fatigue damage on the inner surface of pressure-containing components in the nuclear power industry. We find that the change in ultrasonic wave speed is amplified when shear waves are used, with the magnitude of this amplification agreeing well with the theory. Following the verification of the theory, the use of EMATs allowed us to tailor the shear wave scanning method to pipe geometries, where C-scans with conventional piezoelectric transducers would not have been possible, with the results successfully revealing the presence of fatigue zones.

Transit Time of Lamb Wave-Based Ultrasonic Flow Meters and the Effect of Temperature.

Transit-time flow meters need to compensate for cross-sensitivity to temperature. We show that Lamb wave-based setups are less affected by temperature. An optimality criterion is derived that allows to tune the meter into a zero local sensitivity to temperature. For this end, the flow-induced change in ultrasonic transit time is revisited first. While wetted piston transducer meters are directly sensitive to the change in propagation speed, the change in time of flight of Lamb wave-based systems is due to the beam displacement. Second, the effect of temperature is incorporated analytically. It is found that the temperature-dependent radiation angle of Lamb waves is able to compensate for changes in the speed of sound, leading to an (almost) unaffected overall time of flight. This effect is achievable with any fluid and in a wide temperature range. As an example, we discuss a water meter in the range from 0°C to 100°C. The model is validated against temperature and flow rate-dependent measurements obtained on a prototype. The measured data fits well to the developed model and confirms the reduced cross-sensitivity to temperature. Although an in-line meter is considered here, the results extend to clamp-on devices.

Wave Activity for Large-Amplitude Disturbances Described by the Primitive Equations on the Sphere

Abstract Pseudomomentum and pseudoenergy are both measures of wave activity for disturbances in a fluid, relative to a notional background state. Together they give information on the propagation, growth, and decay of disturbances. Wave activity conservation laws are most readily derived for the primitive equations on the sphere by using isentropic coordinates. However, the intersection of isentropic surfaces with the ground (and associated potential temperature anomalies) is a crucial aspect of baroclinic wave evolution. A new expression is derived for pseudoenergy that is valid for large-amplitude disturbances spanning isentropic layers that may intersect the ground. The pseudoenergy of small-amplitude disturbances is also obtained by linearizing about a zonally symmetric background state. The new expression generalizes previous pseudoenergy results for quasigeostrophic disturbances on the β plane and complements existing large-amplitude results for pseudomomentum. The pseudomomentum and pseudoenergy diagnostics are applied to an extended winter from the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis data. The time series identify distinct phenomena such as a baroclinic wave life cycle where the wave activity in boundary potential temperature saturates nonlinearly almost two days before the peak in wave activity near the tropopause. The coherent zonal propagation speed of disturbances at tropopause level, including distinct eastward, westward, and stationary phases, is shown to be dictated by the ratio of total hemispheric pseudoenergy to pseudomomentum. Variations in the lower-boundary contribution to pseudoenergy dominate changes in propagation speed; phases of westward progression are associated with stronger boundary potential temperature perturbations.

Colour and ultrasound propagation speed changes by different ageing of freezing/thawing and cooling/heating in granitic materials

Abstract In the present work we determined the chromatic coordinates (L*, a*, b*) and ultrasound propagation speeds on the three spatial planes (Vx, Vy, Vz) of three ornamental granites (Aqueduct of Segovia, Spain) before, during, and after being subjected to 70 cycles of two types of accelerated ageing (typical of cold regions): a) freezing/thawing and cooling/heating (T1), and b) freezing/thawing and cooling/heating + salt crystallization (T2). A multivariate technique (Canonical Biplot) was applied to the data obtained, with the observation of significant variations between the two types of accelerated artificial ageing as compared with those obtained in quarry rock in the three chromatic coordinates (L*, a*, b*). With regard to the ultrasound propagation speed, we only detected differences in the results of the T2 artificial ageing treatment with respect to those of quarry rock. This fact is confirmed by the estimated data of resistance to compression.

Exploratory studies of modeling approaches for hydrogen triple flames

A review of triple flame modeling is first presented, which demonstrates the need for additional work in this area. Building on previous methods described in the literature, a hybrid model that uses a weighted average of one-dimensional premixed and diffusion flamelet reaction rates has been proposed and evaluated for a hydrogen triple flame. Results indicated that some type of progress variable is needed for application of the diffusion flamelet contribution. Weighting the premixed flamelet reaction rate contribution at 100%, it is shown that peak temperatures between the model and a case employing detailed chemistry vary 7.5%, while heat release rate, flame speed, and mass fraction contours agree well. A second model, based on a library of reaction rates built from numerical studies which directly resolve the propagating triple flame has also been tested. Computational time for the baseline case is shown to be reduced by a factor of 3 ½ in comparison to use of detailed chemistry. The role of scalar dissipation rate as a necessary independent variable to the library has also been investigated using simulations with variable mixing layer thicknesses. Overall, it is found that large changes in local mixture fraction gradient cause rather small changes in propagation speed and total heat release rate of the hydrogen triple flame. This implies that such a model may be useful for CFD simulations that do not employ spatial resolution capable of resolving the triple flame itself.

Wideband synthetic aperture beamforming for through-the-wall imaging [Lecture Notes

Through-the-wall radar imaging (TWRI) and sensing is emerging as an important area of research and development. A TWRI system provides enhanced situational awareness in operational environments for a variety of civilian and military applications. In particular, a TWRI system facilitates realtime information gathering and intelligent decision-making about the contents of the indoor scene. Compared to other radar applications, TWRI faces unique challenges due to signal propagation through walls. The composition and thickness of the wall, its dielectric constant, and the angle of incidence affect the strength and characteristics of the propagating signal. The change in propagation speed and wave refraction must be taken into account for effective and accurate imaging. In this article, we present a synthetic aperture beamforming approach that uses ray perturbation theory to account for the effects of transmission through a single uniform wall.

Space Weather and the Global Positioning System

Space Weather and the Global Positioning System

Three-Dimensional Wideband Beamforming for Imaging Through a Single Wall

Through-the-wall imaging and urban sensing is an emerging area of research and development. The incorporation of the effects of signal propagation through wall material in producing an indoor image is important for reliable through-the-wall mission operations. We have previously analyzed wall effects, such as refraction and change in propagation speed, and designed a wideband beamformer for 2D imaging using line arrays. In this letter, we extend the analysis to 3D imaging via delay-and-sum beamforming in the presence of a single uniform wall. The third dimension provides valuable information on target heights that can be used for enhancing target discrimination/identification. Supporting simulation results are provided.

Change in propagation constant and propagation velocity in a dielectric: A microscopic approach

The problem of a source atom radiating into a medium of dielectric atoms is considered using a microscopic approach. The source atom is excited by a pulsed, classical driving field, and the average field amplitude and intensity radiated by the source atom is calculated to first order in the dielectric density. It is shown how both the change in propagation vector and change in propagation speed of the radiation in the medium can be traced to scattering events in the medium.

On the effects of viscosity and anisotropic resistivity on the damping of Alfvén waves

The equations of magnetohydrodynamics (MHD) are written for non-uniform viscosity and resistivity – the latter in the cases of Ohmic and anisotropic resistivity. In the case of Ohmic (anisotropic) diffusivity, there is (are) one (two) transverse components of the velocity and magnetic field perturbation(s), leading to a second-order (fourth-order) dissipative Alfvén- wave equation. In the more general case of dissipative Alfvén waves with isotropic viscosity and anisotropic resistivity, the fourth-order wave equation may be replaced by two decoupled second-order equations for right- and left-polarized waves, whose dispersion relations show that the first resistive diffusivity causes dissipation like the viscosity, whereas the second resistive diffusivity causes a change in propagation speed. The second resistive diffusivity invalidates the equipartition of kinetic and magnetic energy, modifies the energy flux through the propagation speed, and also changes the ratio of viscous to resistive dissipation. If the directions of propagation and polarization are equal (i.e. for right-polarized upward-propagating or left-polarized downward-propagating waves), the magnetic energy increases relative to the kinetic energy, the resistive dissipation increases relative to the viscous dissipation, and the total energy density and flux increase relative to the case of isotropic resistivity; the reverse is the case for opposite directions of propagation, i.e. upward-propagating left-polarized waves and downward-propagating right-polarized waves, which can lead to the existence of a critical layer. The role of the viscosity and first and second resistive diffusiveness on the dissipation of Alfvén waves is discussed with reference to the solar atmosphere.

On the Hall effect on vertical Alfvén waves in an isothermal atmosphere

The Alfvén wave equation in an atmosphere in the presence of Ohmic and Hall currents is obtained. It is shown that, for vertical propagation and vertical external magnetic field, there is no coupling to gravity or compressibility, because the magnetic field and velocity perturbation are transverse, and hence horizontal. The wave equation is valid for Alfvén speed, external magnetic field, gyrofrequency, and Ohmic diffusivity arbitrary functions of altitude; under these conditions the transverse wave perturbation is shown to decouple into right- and left-polarized waves. The wave field in an isothermal atmosphere, under a uniform magnetic field, is affected by the Hall current only through a change of propagation speed. The neglect of Ohmic compared to Hall current is justified for low-density plasma, e.g., in the solar wind. It is shown that the Hall effect changes the eigenfrequencies and eigenfunctions of standing atmospheric waves. It does not change the scaling of asymptotic wave fields at high altitude, but it breaks equipartition of energy at low altitude, e.g., for propagating waves, even when the ray approximation holds. Possible extensions of this problem are discussed in connection with Alfvén waves in the solar wind.

Implications of solar p-mode frequency shifts

We relate entropy and magnetic field perturbations to variations of solar p-mode eigenfrequencies. The frequency variations result from changes in path length and propagation speed. These produce shifts of opposite sign. Path length changes dominate for entropy perturbations, and propagation speed changes dominate for most types of magnetic field perturbations. The p-mode frequencies increased along with solar activity between 1986 and 1989. The frequency shifts exhibit a rapid rise with increasing frequency followed by a precipitous drop. The positive component signals a strengthening of the photospheric magnetic field to an rms value of order 200 G. The sudden drop at high frequency is due to a combination of a resonance and an increase in temperature in the chromospheric cavity. The magnetic stress perturbation decays above the top of the convection zone on a length scale comparable to the pressure scale height and grows gradually with depth below. The former characteristic implies that the stress is mainly due to small magnetic elements of the enhanced network, a conclusion supported by our analysis of Kitt Peak magnetograms. The latter property suggests that the flux tubes which pierce the photosphere strengthen with depth, at least to a pressure level of 10^8 dynes cm^(-2). The presence of a resonance in the chromospheric cavity means that the transition layer maintains enough coherence to partially reflect acoustic waves even near cycle maximum. The fractional chromospheric temperature rise implies a much larger fractional increase in the rate of mechanical heating, as indicated by the variation of the Ca II H and K lines.

Near‐field surface motions excited by radiation from a slip zone of arbitrary shape

A ray method is used to calculate the near‐field ground acceleration excited by sliding on a fault plane. The sliding action is modeled by a propagating mode II crack. The crack is planar and it has an edge of arbitrary but smooth curvature. Attention is focused upon the case in which the expanding crack abruptly changes its rate of growth to a slower one. This change in propagation speed causes a burst of wave motion to be emitted. The interaction of the high‐frequency (or wave front) part of the emitted acceleration pulse with the free surface is studied. The near field, where body waves dominate the wave motion at the surface, is considered in some detail.