TEMPORAL CHARACTERISTICS OF ACOUSTIC RAY PROPAGATION THROUGH "INFINITE" AND "BOX MODEL" TURBULENCE

1995 ◽  
Vol 03 (03) ◽  
pp. 203-218 ◽  
Author(s):  
MICHAEL KARWEIT ◽  
PHILIPPE BLANC-BENON
Keyword(s):  
Random Fields ◽  
Arrival Time ◽  
Scalar Fields ◽  
Acoustic Propagation ◽  
Box Model ◽  
Temperature Fields ◽  
Wave Number ◽  
Arrival Times ◽  
Temporal Characteristics ◽  
Wave Number Region

In this work, we investigate the temporal characteristics of acoustic ray propagation through simulated, weakly turbulent temperature fields. In a first set of experiments, we generate ensembles of random scalar fields from randomly oriented Fourier temperature modes. Then, by integrating the ray trace equations, we estimate the distribution of arrival times for rays propagating a distance R through them. We demonstrate that these arrival time distributions are Gaussian for both axial and 3-D propagation and are primarily determined by the lower wave numbers of the 1-D fluctuation spectrum. In a second set of experiments, we generate random fields comprised of Fourier modes prescribed on a lattice, as in "box model" turbulence. In these simulations, we find that acoustic travel times are significantly affected both by the periodicity of the fields and by the direction of acoustic propagation with respect to the orientation of the box. Both effects can ultimately be attributable to an inadequate representation of the low wave number region of the 1-D spectrum. We suggest that these artifacts of simulated periodic fields may preclude their use for acoustic propagation studies.

scholarly journals Two-Dimensional Convection Induced by Gravity and Centrifugal Forces in a Rotating Porous Layer Far Away from the Axis of Rotation

1998 ◽  
Vol 4 (2) ◽  
pp. 73-90 ◽  
Author(s):  
Peter Vadasz ◽  
Saneshan Govender
Keyword(s):  
Rayleigh Number ◽  
Porous Layer ◽  
Temperature Fields ◽  
Wave Number ◽  
Marginal Stability ◽  
Two Dimensional ◽  
Wide Range ◽  
Gravity Driven ◽  
Gravity Driven Flow ◽  
The Stability

The stability and onset of two-dimensional convection in a rotating fluid saturated porous layer subject to gravity and centrifugal body forces is investigated analytically. The problem corresponding to a layer placed far away from the centre of rotation was identified as a distinct case and therefore justifying special attention. The stability of a basic gravity driven convection is analysed. The marginal stability criterion is established in terms of a critical centrifugal Rayleigh number and a critical wave number for different values of the gravity related Rayleigh number. For any given value of the gravity related Rayleigh number there is a transitional value of the wave number, beyond which the basic gravity driven flow is stable. The results provide the stability map for a wide range of values of the gravity related Rayleigh number, as well as the corresponding flow and temperature fields.

scholarly journals Detection of specific areas and densities for ultrasound tomography

2019 ◽  
pp. 121-127
Author(s):  
Victoria Erofeeva ◽  
Vasilisa Galyamina ◽  
Kseniya Gonta ◽  
Anna Leonova ◽  
Oleg Granichin ◽  
...  
Keyword(s):  
Image Reconstruction ◽  
Ultrasound Imaging ◽  
Arrival Time ◽  
Detection Accuracy ◽  
Problem Solution ◽  
Ultrasound Tomography ◽  
Arrival Times ◽  
Whole Process ◽  
Imaging Reconstruction ◽  
Tomography Data

In this paper we consider the problem of ultrasound tomography. Recently, an increased interest in ultrasound tomography has been caused by non-invasiveness of the method and increased detection accuracy (as compared to radiation tomography), and also ultrasound tomography does not put at risk human health. We study possibilities of detection of specific areas and determining their density using ultrasound tomography data. The process of image reconstruction based on ultrasound data is computationally complex and time consuming. It contains the following parts: calculation of the time-of-flight (TOF) of a signal, detection of specific areas, calculation of density of specific areas. The calculation of the arrival time of a signal is a very important part, because the errors in the calculation of quantities strongly influence the total problem solution. We offer ultrasound imaging reconstruction technology that can be easily parallelized. The whole process is described: from extracting the arrival times of signals raw data feeding from physical receivers to obtaining the desired results.

scholarly journals On optimal periodic dividend and capital injection strategies for spectrally negative Lévy models

2018 ◽  
Vol 55 (4) ◽  
pp. 1272-1286 ◽  
Author(s):  
Kei Noba ◽  
José-Luis Pérez ◽  
Kazutoshi Yamazaki ◽  
Kouji Yano
Keyword(s):  
Arrival Time ◽  
Arrival Times ◽  
Classical Sense ◽  
Capital Injection ◽  
Dividend Payments ◽  
Poisson Arrival ◽  
Optimal Dividend ◽  
Injection Strategies

Abstract De Finetti’s optimal dividend problem has recently been extended to the case when dividend payments can be made only at Poisson arrival times. In this paper we consider the version with bail-outs where the surplus must be nonnegative uniformly in time. For a general spectrally negative Lévy model, we show the optimality of a Parisian-classical reflection strategy that pays the excess above a given barrier at each Poisson arrival time and also reflects from below at 0 in the classical sense.

Joint microseismic event location and anisotropic velocity inversion with the cross double-difference method using downhole microseismic data

Geophysics ◽  
2020 ◽  
Vol 85 (3) ◽  
pp. KS63-KS73
Author(s):  
Yangyang Ma ◽  
Congcong Yuan ◽  
Jie Zhang
Keyword(s):  
Arrival Time ◽  
Velocity Model ◽  
P Wave ◽  
Double Difference ◽  
S Wave ◽  
Arrival Times ◽  
Monitoring Well ◽  
The Cross ◽  
Microseismic Event ◽  
Wave Arrival

We have applied the cross double-difference (CDD) method to simultaneously determine the microseismic event locations and five Thomsen parameters in vertically layered transversely isotropic media using data from a single vertical monitoring well. Different from the double-difference (DD) method, the CDD method uses the cross-traveltime difference between the S-wave arrival time of one event and the P-wave arrival time of another event. The CDD method can improve the accuracy of the absolute locations and maintain the accuracy of the relative locations because it contains more absolute information than the DD method. We calculate the arrival times of the qP, qSV, and SH waves with a horizontal slowness shooting algorithm. The sensitivities of the arrival times with respect to the five Thomsen parameters are derived using the slowness components. The derivations are analytical, without any weak anisotropic approximation. The input data include the cross-differential traveltimes and absolute arrival times, providing better constraints on the anisotropic parameters and event locations. The synthetic example indicates that the method can produce better event locations and anisotropic velocity model. We apply this method to the field data set acquired from a single vertical monitoring well during a hydraulic fracturing process. We further validate the anisotropic velocity model and microseismic event locations by comparing the modeled and observed waveforms. The observed S-wave splitting also supports the inverted anisotropic results.

scholarly journals The Astrometric Possibilities of Very-Long-Baseline Interferometry

1986 ◽  
Vol 109 ◽  
pp. 143-155
Author(s):  
D. S. Robertson
Keyword(s):  
Plane Wave ◽  
Arrival Time ◽  
Very Long Baseline Interferometry ◽  
Radio Sources ◽  
Arrival Times ◽  
Long Baseline ◽  
The Difference ◽  
Different Sources

In the application of Very-Long-Baseline Interferometry (VLBI) to astrometric problems the fundamental observable is the difference in the arrival times of a wavefront at two widely separated receiving stations. Since the radio sources being observed are sufficiently distant that the arriving wavefront can be considered to be a plane wave, the differential arrival time is a measure of the component of the baseline in the direction of the source. Equivalently, if the baseline is known, the differential arrival time is sufficient to determine a circle on the sky containing the source. It is easy to show that a minimum of ten observations distributed among three different sources is sufficient to determine all of the source coordinates and the baseline coordinates simultaneously (Robertson, 1975).

scholarly journals Periodic Changes in Intensity and Arrival Time of Pulses from the Vela Pulsar: Evidence for Free Precession?

1996 ◽  
Vol 160 ◽  
pp. 101-102
Author(s):  
A.A. Deshpande ◽  
P.M. McCulloch
Keyword(s):  
Arrival Time ◽  
Time Of Arrival ◽  
Free Precession ◽  
Dual Frequency ◽  
Arrival Times ◽  
Routine Monitoring ◽  
Vela Pulsar ◽  
Observed Behaviour ◽  
Pulse Arrival ◽  
Periodic Changes

We present dual-frequency measurements on the Vela pulsar with a view to study the slow variations in the pulsed flux and the apparent differences in the pulse arrival times. We examine the data for correlated variations between the pulse intensities and arrival times at the two frequencies and discuss two main possibilities in order to explain the observed behaviour.The data presented here consists of a) Pulse intensities, S635& S950, at S635& S950MHz respectively and b) the ‘residual’ differences in the time of arrival of the pulse at the lower frequency, ΔTOA, with respect to that at the higher frequency. These data, over a span of ~1300 days (during 1988-92), were obtained as a part of the routine monitoring of the Vela pulsar from Mt. Pleasant Observatory of University of Tasmania, Hobart (see McCullochet al. 1990).

Reverse Marangoni surfing

2016 ◽  
Vol 811 ◽  
pp. 612-621 ◽  
Author(s):  
Vahid Vandadi ◽  
Saeed Jafari Kang ◽  
Hassan Masoud
Keyword(s):  
Surface Tension ◽  
Liquid Layer ◽  
Scalar Fields ◽  
Finite Depth ◽  
Lower Surface ◽  
Temperature Fields ◽  
Spherical Particles ◽  
Surface Tension Gradient ◽  
Oblate Spheroidal ◽  
Active Particles

We theoretically study the surfing motion of chemically and thermally active particles located at a flat liquid–gas interface that sits above a liquid layer of finite depth. The particles’ activity creates and maintains a surface tension gradient resulting in the auto-surfing. It is intuitively perceived that Marangoni surfers propel towards the direction with a higher surface tension. Remarkably, we find that the surfers may propel in the lower surface tension direction depending on their geometry and proximity to the bottom of the liquid layer. In particular, our analytical calculations for Stokes flow and diffusion-dominated scalar fields (i.e. chemical concentration and temperature fields) indicate that spherical particles undergo reverse Marangoni propulsion under confinement whereas disk-shaped surfers always move in the expected direction. We extend our results by proposing an approximate formula for the propulsion speed of oblate spheroidal particles based on the speeds of spheres and disks.

scholarly journals Scalar gradients in stirred mixtures and the deconstruction of random fields

2017 ◽  
Vol 812 ◽  
pp. 578-610 ◽  
Author(s):  
T. Le Borgne ◽  
P. D. Huck ◽  
M. Dentz ◽  
E. Villermaux
Keyword(s):  
Turbulent Flows ◽  
Random Fields ◽  
Scalar Fields ◽  
Spatial Correlations ◽  
Concentration Gradients ◽  
Lamellar Structures ◽  
History Of ◽  
Media Flows ◽  
Spatial Locations ◽  
Analytical Expressions

A general theory for predicting the distribution of scalar gradients (or concentration differences) in heterogeneous flows is proposed. The evolution of scalar fields is quantified from the analysis of the evolution of elementary lamellar structures, which naturally form under the stretching action of the flows. Spatial correlations in scalar fields, and concentration gradients, hence develop through diffusive aggregation of stretched lamellae. Concentration levels at neighbouring spatial locations result from a history of lamella aggregation, which is partly common to the two locations. Concentration differences eliminate this common part, and thus depend only on lamellae that have aggregated independently. Using this principle, we propose a theory which envisions concentration increments as the result of a deconstruction of the basic lamella assemblage. This framework provides analytical expressions for concentration increment probability density functions (PDFs) over any spatial increments for a range of flow systems, including turbulent flows and low-Reynolds-number porous media flows, for confined and dispersing mixtures. Through this deconstruction principle, scalar increment distributions reveal the elementary stretching and aggregation mechanisms building scalar fields.

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