Decrease In Wave Speed Research Articles

Seismic response analysis of long-span suspen-dome under multi-support excitations

A practical suspen-dome project, Changzhou Gym roof, is adopted as an example and its transient analysis based on the multi-support excitations of the earthquake wave is carried out. Compared with the single support excitation, the position and value of the maximum stress under multi-support excitations both change and the amount of elements with obvious changes is large and more than 70% of the total. Moreover, when other terms are not changed, this influence will decrease as the span decreases, but increase as the traveling wave speed decreases. The final result indicates that the influence of the earthquake wave passage effect on the structure system is negative and cannot be ignored.

Numerical Simulation of Thermocapillary Convection in a Shallow Rectangular Cavity Under the Action of Combining Horizontal Temperature Gradient with Vertical Heat Flux

In order to understand the influence of the vertical heat flux on thermocapillary convection, we conducted a series of unsteady two-dimensional numerical simulations of thermocapillary convection in a differently heated shallow rectangular cavity with vertical heat flux on the bottom by means of the finite volume method. The cavity was filled with the 1cSt silicone oil (Prandtl number Pr = 13.9) and aspect ratio is 30. It is found that a small vertical heat flux has slightly influence on the flow pattern of stable or unstable thermocapillary convection. However, the critical Marangoni number increases first, and then decreases with the increase of the heat flux. And the flow pattern of the oscillatory thermocapillary convection transits from a series of the rolls rotating clockwise and moving from the cold wall to the hot wall to the single roll near the hot wall and a series of rolls near the cold wall, further, two series of rolls moving from the hot wall and cold wall towards the hot spot with the maximum temperature. With the increase of the Marangoni number, the period and the wavelength of the oscillatory thermocapillary convection increase, but the wave speed decreases.

Mapping the Micromechanical Properties of Cryo-sectioned Aortic Tissue with Scanning Acoustic Microscopy

Although the gross mechanical properties of ageing tissues have been extensively documented, biological tissues are highly heterogeneous and little is known concerning the variation of micro-mechanical properties within tissues. Here, we use Scanning Acoustic Microscopy (SAM) to map the acoustic wave speed (a measure of stiffness) as a function of distance from the outer adventitial layer of cryo-sectioned ferret aorta. With a 400 MHz lens, the images of the aorta samples matched those obtained following chemical fixation and staining of sections which were viewed with fluorescence microscopy. Quantitative analysis was conducted with a frequency scanning or V(f) technique by imaging the tissue from 960 MHz to 1.1 GHz. Undulating acoustic wave speed (stiffness) distributions corresponded with elastic fibre locations in the tissue; there was a decrease in wave speed of around 40 ms(-1) from the adventitia (outer layer) to the intima (innermost).

The state of the upper mantle beneath southern Africa

We present a new upper mantle seismic model for southern Africa based on the fitting of a large (3622 waveforms) multi-mode surface wave data set with propagation paths significantly shorter (≤ 6000 km) than those in globally-derived surface wave models. The seismic lithosphere beneath the cratonic region of southern Africa in this model is about 175 ± 25 km thick, consistent with other recent surface wave models, but significantly thinner than indicated by teleseismic body-wave tomography. We determine the in situ geotherm from kimberlite nodules from beneath the same region and find the thermal lithosphere model that best fits the nodule data has a mechanical boundary layer thickness of 186 km and a thermal lithosphere thickness of 204 km, in very good agreement with the seismic measurement. The shear wave velocity determined from analyzes of the kimberlite nodule compositions agree with the seismic shear wave velocity to a depth of ∼150 km. However, the shear wave velocity decrease at the base of the lid seen in the seismic model does not correspond to a change in mineralogy. Recent experimental studies of the shear wave velocity in olivine as a function of temperature and period of oscillation demonstrate that this wave speed decrease can result from grain boundary relaxation at high temperatures at the period of seismic waves. This decrease in velocity occurs where the mantle temperature is close to the melting temperature (within ∼100 °C).

High-resolution radar on Ice Stream B2, Antarctica: measurements of electromagnetic wave speed in firn and strain history from buried crevasses

Two types of experiments were performed near Upstream B Camp on Ice Stream B2 using a high-resolution ground-penetrating radar system. In the first type of experiment, tracking a metal drill tip through the upper 85 m of the ice column indicated an approximately linear decrease of wave speed with depth down to 50 m, with a constant speed of 170 ± 4 m µs−1 below. We believe the (unexpected) linearity may have been caused by one or more buried crevasses. In the second experiment, a survey of a 250 km2 grid showed a population of buried crevasses at about 30 m depth that have an estimated burial age of 210 ± 25 years, which is taken to indicate that this ice has been exhibiting streaming behavior for at least that length of time. One 3 km segment along the edge of the survey area nearest the center of the stream showed virtually flat, undisrupted stratigraphy down to the maximum depth of our measurements. The fact that this ice was accelerated from near-zero speed to its present 440 m a−1 without cracking or folding suggests that this ice may have been incorporated into the ice stream as a solid block.

High-resolution radar on Ice Stream B2, Antarctica: measurements of electromagnetic wave speed in firn and strain history from buried crevasses

Two types of experiments were performed near Upstream B Camp on Ice Stream B2 using a high-resolution ground-penetrating radar system. In the first type of experiment, tracking a metal drill tip through the upper 85 m of the ice column indicated an approximately linear decrease of wave speed with depth down to 50 m, with a constant speed of 170 ± 4 m µs−1 below. We believe the (unexpected) linearity may have been caused by one or more buried crevasses. In the second experiment, a survey of a 250 km2 grid showed a population of buried crevasses at about 30 m depth that have an estimated burial age of 210 ± 25 years, which is taken to indicate that this ice has been exhibiting streaming behavior for at least that length of time. One 3 km segment along the edge of the survey area nearest the center of the stream showed virtually flat, undisrupted stratigraphy down to the maximum depth of our measurements. The fact that this ice was accelerated from near-zero speed to its present 440 m a−1 without cracking or folding suggests that this ice may have been incorporated into the ice stream as a solid block.

High-resolution radar on Ice Stream B2, Antarctica: measurements of electromagnetic wave speed in firn and strain history from buried crevasses

Two types of experiments were performed near Upstream B Camp on Ice Stream B2 using a high-resolution ground-penetrating radar system. In the first type of experiment, tracking a metal drill tip through the upper 85 m of the ice column indicated an approximately linear decrease of wave speed with depth down to 50 m, with a constant speed of 170 ± 4 m µs−1 below. We believe the (unexpected) linearity may have been caused by one or more buried crevasses. In the second experiment, a survey of a 250 km2 grid showed a population of buried crevasses at about 30 m depth that have an estimated burial age of 210 ± 25 years, which is taken to indicate that this ice has been exhibiting streaming behavior for at least that length of time. One 3 km segment along the edge of the survey area nearest the center of the stream showed virtually flat, undisrupted stratigraphy down to the maximum depth of our measurements. The fact that this ice was accelerated from near-zero speed to its present 440 m a−1 without cracking or folding suggests that this ice may have been incorporated into the ice stream as a solid block.

Erratum: Drift, shape, and intrinsic destabilization of pulses of traveling-wave convection

I describe experiments on ``pulses'' of convective traveling waves in an annular geometry, using ethanol-water mixtures with moderate negative separation ratio. In a sufficiently uniform cell at constant Rayleigh number, pulses drift in the direction of propagation of the underlying traveling waves, with no long-term change in velocity. The drift velocity increases with increasing distance \ensuremath{\epsilon} above onset. In contrast with previous observations of motionless pulses, this result is qualitatively consistent with theories based on a subcritical Ginzburg-Landau equation. The pulse shape is also described in detail. The pulses exhibit a noticeable asymmetry, which decreases as \ensuremath{\epsilon} is increased. I also describe experiments at high \ensuremath{\epsilon} in which pulse destruction by convective amplification of traveling-wave fluctuations is suppressed by the existence of multiple pulses. In this case, destabilization takes place by an intrinsic mechanism: Above a certain threshold, the pulse simply expands into the rest of the system, accompanied by a large decrease in wave speed.

Drift, shape, and intrinsic destabilization of pulses of traveling-wave convection.

I describe experiments on ``pulses'' of convective traveling waves in an annular geometry, using ethanol-water mixtures with moderate negative separation ratio. In a sufficiently uniform cell at constant Rayleigh number, pulses drift in the direction of propagation of the underlying traveling waves, with no long-term change in velocity. The drift velocity increases with increasing distance \ensuremath{\epsilon} above onset. In contrast with previous observations of motionless pulses, this result is qualitatively consistent with theories based on a subcritical Ginzburg-Landau equation. The pulse shape is also described in detail. The pulses exhibit a noticeable asymmetry, which decreases as \ensuremath{\epsilon} is increased. I also describe experiments at high \ensuremath{\epsilon} in which pulse destruction by convective amplification of traveling-wave fluctuations is suppressed by the existence of multiple pulses. In this case, destabilization takes place by an intrinsic mechanism: Above a certain threshold, the pulse simply expands into the rest of the system, accompanied by a large decrease in wave speed.

Nonlinear, periodic, internal waves

It is shown how to calculate large-amplitude, periodic, internal waves in a channel filled with continuously stratified fluid by using a Stokes-type amplitude expansion along the channel and a modal expansion across the channel. We obtain explicit solutions for the case where the density increases exponentially with depth. It is found that periodic waves in exponentially stratified fluid are waves of depression: as the wave increases in amplitude the wave speed decreases and the mean density of the fluid at a given height decreases. The waves are limited in height by the formation of an eddy of fluid on the upper boundary above the trough of the wave. This is consistent with the description of waves of depression given by Amick and Toland (1984). There is no backflow before the eddy forms. By contrast, Amick and Toland (1984) have shown that solitary waves in the same system are waves of elevation, limited in height by a cusp in the streamlines at some interior point. A relationship is found between the mass flux and the mass displacement. The expressions necessary to calculate the potential and kinetic energies are given. A simple analytic solution for the internal wave is presented for a fluid with a weak exponential stratification, that is when the Boussinesq approximation is appropriate. For an Nth mode wave the limiting amplitude of the perturbation streamfunction is shown to be 1/Nπ. This corresponds to a maximum streamline displacement of 2z/Nπ, where z (=0.7391) is the solution to z = cos z.

Elastic moduli of precompressed pyrophyllite used in ultrahigh-pressure research

The propagation of ultrasonic pulses in pyrophyllite specimens was studied to investigate the effect of specimen precompression on the measured elastic moduli. Measurements were made at room pressure and, for the precompressed specimens, to pressures of 3 kbar. We find pyrophyllite to be elastically anisotropic, apparently the result of the fabric present in our material. The room-pressure adiabatic bulk modulus as measured on specimens made of isostatically compacted powdered pyrophyllite was determined to be 96.1 kbar. We found the wave speeds of ultrasonic pulses in pyrophyllite to decrease with increasing specimen precompression. A limiting value of precompression was found, above which no further decrease in wave speed is observed. For the shear-wave speeds this occurs at 10 kbar, while for the longitudinal wave at 25 kbar. In the limit, the shear waves propagate 20% slower than in the unprecompressed samples; for the longitudinal wave the difference is 30%. The change in bulk modulus resulting from increasing pressure shows a behavior which can be approximated by a bilinear curve with decreasing slope, the discontinuity occurring between 1.6 and 1.8 kbar. In the second portion of the curve, the bulk modulus derivative is determined to be 22.1. We believe that this value indicates the presence of voids which are closing under pressure. A regression analysis of the data shows that a P1/2 equation appears to give the best fit. The resulting empirical law for the bulk modulus derivative at pressure P becomes: dB/dP=[CL0(0.251P−0.147)−1/2 −CS0(2.76P−2.56)−1/2]×10−1, where P is in kbar and in the present measurements CL0, the longitudinal modulus, is 204 kbar and CS0, the average shear modulus, is 113 kbar.