Variation Of Average Velocity Research Articles

Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia

SUMMARY We use the recently deployed ScanArray network of broad-band stations covering most of Norway and Sweden as well as parts of Finland to analyse the propagation of Rayleigh waves in Scandinavia. Applying an array beamforming technique to teleseismic records from ScanArray and permanent stations in the study region, in total 159 stations with a typical station distance of about 70 km, we obtain phase velocities for three subregions, which collectively cover most of Scandinavia (excluding southern Norway). The average phase dispersion curves are similar for all three subregions. They resemble the dispersion previously observed for the South Baltic craton and are about 1 per cent slower than the North Baltic shield phase velocities for periods between 40 and 80 s. However, a remarkable sin(1θ) phase velocity variation with azimuth is observed for periods >35 s with a 5 per cent deviation between the maximum and minimum velocities, more than the overall lateral variation in average velocity. Such a variation, which is incompatible with seismic anisotropy, occurs in northern Scandinavia and southern Norway/Sweden but not in the central study area. The maximum and minimum velocities were measured for backazimuths of 120° and 300°, respectively. These directions are perpendicular to a step in the lithosphere–asthenosphere boundary (LAB) inferred by previous studies in southern Norway/Sweden, suggesting a relation to large lithospheric heterogeneity. In order to test this hypothesis, we carried out 2-D full-waveform modeling of Rayleigh wave propagation in synthetic models which incorporate a steep gradient in the LAB in combination with a pronounced reduction in the shear velocity below the LAB. This setup reproduces the observations qualitatively, and results in higher phase velocities for propagation in the direction of shallowing LAB, and lower ones for propagation in the direction of deepening LAB, probably due to the interference of forward scattered and reflected surface wave energy with the fundamental mode. Therefore, the reduction in lithospheric thickness towards southern Norway in the south, and towards the Atlantic ocean in the north provide a plausible explanation for the observed azimuthal variations.

Computational fluid dynamic (CFD) simulation of a cuboid packed-bed chromatography device

Flow non-uniformity is a major problem with process chromatography columns which typically have small bed height to diameter ratios. In recent publications, we have proposed box-shaped or cuboid packed-bed devices as efficient alternative to process columns for carrying out high-resolution chromatographic separations. Cuboid packed-bed devices show superior performance in terms of number of theoretical plates, peak width, and peak resolution in multicomponent separations. This paper attempts to explain this based on computational fluid dynamic (CFD) simulations of a cuboid packed-bed device and its equivalent column, i.e. having the same bed height and area of cross-section, and packed with the same chromatographic media. The radial velocity in the column headers decreased very rapidly from the axis to the periphery resulting in significant variation in average velocity along the different flow paths within the column. By contrast, the velocity decreased linearly in the top channel and increased linearly in the bottom channel of a cuboid packed-bed device, resulting in significantly lower variation in the average velocity. Simulated flow-through peaks obtained with the cuboid packed-bed device using dextran as a macromolecular and sodium chloride as a low molecular weight tracer were significantly sharper and more symmetric. Experimental results were in good agreement with those obtained by simulation. Overall, the superior performance of the cuboid packed-bed device could be primarily attributed to the narrower solute residence time distribution resulting from greater flow uniformity.

Seismicity in colombian llanos foothills: Characterization, relocation and local seismic tomography

An earthquake relocation by seismic simultaneous inversion and double differences methods were done in the Colombian Llanos Foothills from 3° to 5°N and from 73° to 75°W. The data used in this work take account 483 earthquakes registered by The Colombia National Seismological Network (RSNC) between 1993 and 2012. For the events relocation the root mean square (RMS) was reduced and several earthquake clusters were identified. The new locations shows principally at southwestern zone are related with the Eastern Frontal Fault System with faults as the Servitá-Santa Maria fault and the Algeciras Fault. In addition this work shows a 3D velocity model indicating an anomaly in the wave behavior related mainly to the low velocity zone under the Eastern Cordillera and minimum variations in average velocity toward southeast zone related with the Amazon Craton. Finally in southwest region where located the faults shows a Vp high velocity anomaly.

Pore‐scale modeling and continuous time random walk analysis of dispersion in porous media

We provide a physically based explanation for the complex macroscopic behavior of dispersion in porous media as a function of Peclet number, Pe, using a pore‐scale network model that accurately predicts the experimental dependence of the longitudinal dispersion coefficient, DL, on Pe. The asymptotic dispersion coefficient is only reached after the solute has traveled through a large number of pores at high Pe. This implies that preasymptotic dispersion is the norm, even in experiments in statistically homogeneous media. Interpreting transport as a continuous time random walk, we show that (1) the power law dispersion regime is controlled by the variation in average velocity between throats (the distribution of local Pe), giving DL ∼ Peδ with δ = 3 − β ≈ 1.2, where β is an exponent characterizing the distribution of transit times between pores, (2) the crossover to a linear regime DL ∼ Pe for Pe > Pecrit ≈ 400 is due to a transition from a diffusion‐controlled late time cutoff to transport governed by advective movement, and (3) the transverse dispersion coefficient DT ∼ Pe for all Pe ≫ 1.

The different capillary flow regimes of entangled polydimethylsiloxane polymers: macroscopic slip at the wall, hysteresis and cork flow

Polymers, melted at room temperature, have been extruded through capillary dies using either a controlled pressure system or an automatic capillary rheometer in which the rate of flow is controlled. When the polymer is highly entangled, macroscopic slip is observed at the wall. However, flow curves differ: for controlled pressure conditions, slip appears simultaneously with a sudden increase in the rate of flow, an dflow curves exhibit a hysteretic regime. For controlled flow conditions, slip is accompanied by oscillations of the rate of flow and pressure head around a mean value, as a result of polymer compressibility. The succession and evolution of the different flow defects are clearly identified. As the flow regime increases, scratches appear first. Then, beyond a critical value of wall shear stress in the exit region, cracks are formed just at the exit of the capillary die. These cracks are accompanied by the formation of “rings”, which are more easily observed as the molecular weight increases. At high flow rates, when macroscopic slip appears at the wall, the aspect of the extrudate depends on the system used. For controlled pressure conditions, the polymer is ejected in the form of an opaque, irregular jet, where swelling is quasi-non-existent. For controlled flow conditions, cork flow is observed. At higher flow regimes the extrudate becomes chaotic. The existence of a master curve was also shown for variations in average velocity vs. wall stress, during flow with macroscopic slip at the wall. It is important to note that this curve represents the sliding friction properties of the polymer under consideration.

Hydrodynamic hot-electron transport model with Monte Carlo-generated transport parameters

A unique hot-electron transport model suitable for studying submicron GaAs device structures is presented. The model is based on the semiclassical “hydrodynamic” conservation equations for the average electron density, momentum, and energy. The model includes electron relaxation times, momentum relaxation times, energy relaxation times, electron temperature tensors and heat flow vectors as a function of average electron energy for the Γ, X and L valleys of GaAs. The relaxation times represent rates of exchange of electrons between valleys and rates of loss of average momentum and average energy between and within the individual valleys. The electron temperature tensor and heat flow vector depend on the electron velocity distribution about the average electron velocity and ultimately affect transport when spatial variations in average velocity and average energy exist. Transport parameters are calculated using the Monte Carlo method and the ergodic principle applied directly to the integral definitions for the parameters. Therefore, the model includes nonequilibrium transport effects such as velocity overshoot and nonuniform average electron energy. The new model should prove instrumental in optimizing electron transport through submicron structures for high-speed device applications.

Depth Conversion of Velocity Data Using Velocity Analyses

Routinely produced velocity analyses are used for a number of purposes in processing and interpretation of seismic data. These analyses have errors due to limitations in techniques and due to a number of assumptions made in their production. The random errors are much greater than the errors in reflection times. These random variations can be separated from the real velocity variations by averaging or smoothing a number of velocity analyses. However, if this is done by simply smoothing average velocity, there is a risk of introducing error by smoothing the variation in average velocity with depth. This can be avoided by smoothing instead the velocity Anomaly. This is the difference at a given time between the measured velocity and the standard velocity taken from a time-velocity curve adopted for the area. An example of the method in the Gippsland Basin shows a marked improvement in accuracy of ties to wells.

The spectrum of ρ Cassiopeiae.

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The spectrum of ρ Cassiopeiae. Beardsley, Wallace R. Abstract The unprecedented decrease in light of the F8 supergiant p Cassiopeiae in 1946 was accompanied by marked changes in its spectrum. One striking feature was the appearance of many lines of o-volt excitation potential not previously present in the spectrum, which are, in fact, normally observed only in M-type supergiant stars. Radial velocity measures and intensity estimates for a number of these lines have been made for a series of 30 plates with a dispersion of 26 A/mm at H~ taken at the McDonald Observatory at intervals since the star's return to normal brightness in 1947. The spectra studied were obtained by a number of members of the Yerkes and McDonald staffs. Radial velocity results for sharp, unblended lines are as follows: No. of No. of Lines Measures Av. Vel. Ioniz. Pot. Sci 2 25 -69.6km/sec 6.7 volts Vi 6 8o -63.4 6.7 Tii 6 69 -6o.o 6.8 Coi 3 63 -5i.8 7.8 Fei 6 100 -55.2 7.9 A correlation between velocity and ionization potential is indicated which corresponds to that which Keen an found between Ti and Fe in the infrared spectrum of p Cas.1 No marked variation in average velocity per plate for these lines has been found for the interval between minimum light and the present time. In addition, a plate taken during minimum light gave the same average velocity. Eye estimates of line intensity reveal, however, considerable changes. The lines retained the minimum phase intensity through 1948. In 1949 and 1950 a general decrease in intensity occurred and the lines disappeared or became very weak. This was followed in 1951 by a moderate increase in strength of many lines which have then remained to the present time. Probable emission H~, H~, and X4226 of Ca I also appeared during the post-minimum phase. The continued existence to the present time of many of these unusual lines in the spectrum of p Cas, and the apparent dependence of the radial velocity of these lines upon ionization potential would appear to support the hypothesis that these lines are formed in a low-excitation shell rather than by the absorption of light by an otherwise invisible eclipsing star. I.Ap. J. 106, 295, 1947. Yerkes Observatory, Williams Bay, Wis. Publication: The Astronomical Journal Pub Date: March 1953 DOI: 10.1086/106807 Bibcode: 1953AJ.....58...34B full text sources ADS | data products SIMBAD (1)