Sharp Velocity Gradients Research Articles

Structural Setting of Strong Earthquakes in the Huabei Area of China

— Using P and S arrival times, which occurred within the Huabei seismic network, we carried out a tomographic inversion and compared results with the earthquake catalogue of the last 1000 years in the area. The results are as follows:¶1) The hypocenters of most of the strong shocks are distributed in the transitional zones between high- and low-velocity areas in the crust, especially at edges of high-velocity blocks.¶2) Strong shocks predominantly lie above low-velocity blocks, or in transitional zones between low- and high-velocity areas, in the lower crust.¶3) The tectonic settings for the Tangshan and the Sanhe-Pingu earthquakes are similar. Both are not known near large fault belts, and in zones with a sharp lateral velocity gradient.¶4) The Ninghe, Tangshan and Luanxia earthquakes are located in high-velocity blocks that differ in size and depth. This difference can explain the focal depth distribution of the Tangshan earthquake sequence, i.e., earthquakes are shallower in the northeastern Luanxian area but deeper in the southwestern Ninghe area.

On the boundary conditions of the k– ε model in 3D coarse-mesh models for containment analysis

In CFD-type codes like the containment analysis code GOTHIC, one of the options that can be used for modelling of turbulence is the k– ε model. Though, in contrast to commercial general purpose CFD codes which are tailored for making detailed CFD calculations with a large number of spatial meshes, in codes like GOTHIC which are primarily aiming at calculating transients in reactor containments, for practical reasons, one generally uses coarse meshes. The solution of the two parabolic k– ε model equations requires the definition of boundary conditions at physical boundaries and this, in turn, requires very small spatial meshes near these boundaries. Hence, while in CFD codes these boundary conditions are properly defined, in codes like GOTHIC, this is done in an indirect and non-rigorous fashion, exactly due to the fact that except near sharp velocity gradients (e.g. near the walls), one uses large spatial meshes; this can have catastrophic consequences during the calculation of a transient and in this work, we shall give some examples of this and outline one aspect of a general approach which would aim at bypassing this problem. The problems reported here were addressed by the code developers in the later code versions (after the GOTHIC6.0(QA)) in a way different to that addressed here.

Critical comparisons of the measured forward scattering from a clay bottom and a sand bottom

Low grazing angle, medium frequency (3 to 15 kHz) forward scattering from a clay bottom off the coast of Gulf Port, Mississippi was measured using a 32-channel bilinear array and a broadband acoustic source. The clay bottom has a sharp velocity gradient with the sound speed at the interface slightly less than that of sound speed in water. The forward scattering was also measured at a sand-bottom site on the Florida Atlantic coastal shelf. The sound speed of the sand bed was roughly 1700 m/s. The measured bottom scattering-strength patterns are very similar both in magnitudes (−60 to −20 dB) and the distributions (roughly elliptical with the source and receiver being the foci). The apparent similarity, however, results from two different physical mechanisms. The waveform and time analyses show that acoustic energy penetrates deep into the clay bottom and scattered within the sediment volume, whereas acoustic energy is scattered at the surface of the sand bottom. In the clay bed, the sound waves are upward refracted due to the velocity gradient, showing a caustic-like phase shift by 0 to 90 deg. [Work supported by ONR Ocean Acoustics Program.]

Continuity-based and discontinuity-based segmentation in transparent and spatially segregated global motion

The mechanisms underlying the parsing of a spatial distribution of velocity vectors into two adjacent (spatially segregated) or overlapping (transparent) motion surfaces were examined using random dot kinematograms. Parsing might occur using either of two principles. Surfaces might be defined on the basis of similarity of motion vectors and then sharp perceptual boundaries drawn between different surfaces (continuity-based segmentation). Alternatively, detection of a high gradient of direction or speed separating the motion surfaces might drive the process (discontinuity-based segmentation). To establish which method is used, we examined the effect of blurring the motion direction gradient. In the case of a sharp direction gradient, each dot had one of two directions differing by 135°. With a shallow gradient, most dots had one of two directions but the directions of the remainder spanned the range between one motion-defined surface and the other. In the spatial segregation case the gradient defined a central boundary separating two regions. In the transparent version the dots were randomly positioned. In both cases all dots moved with the same speed and existed for only two frames before being randomly replaced. The ability of observers to parse the motion distribution was measured in terms of their ability to discriminate the direction of one of the two surfaces. Performance was hardly affected by spreading the gradient over at least 25% of the dots (corresponding to a 1° strip in the segregation case). We conclude that detection of sharp velocity gradients is not necessary for distinguishing different motion surfaces.

The flow induced by yawing motion in unidirectionally solidifying alloys

As the cooling tank is yawing, there occurs an induced flow in the binary solution cooled from below. The induced flow may stabilize the potentially unstable solution during directional solidification and suppress the formation of freckles in the cast alloys. We investigate this induced flow by an analytical approach. Results show that the induced flow in the melt region consists of three paralleling shear flows of different length scales. The one in the vicinity above the melt/mush interface has the largest velocity gradient. The amplitude of the induced velocity in the melt is much larger than that in the dendritic mushy region, and the former is virtually inversely proportional to the yawing frequency while the latter is completely independent of the yawing frequency. The amplitudes of the induced velocities in both the melt and the mush are proportional to the yawing amplitude. The phase of induced flow in the melt lags by about 90° behind the yawing motion while the induced flow in the mush is of the same phase with yawing. The sharp velocity gradient of the induced flow in the shallow layer above the melt/mush interface may play a crucial role in stabilizing the potentially very unstable fluid in the solute boundary layer above the interface. This suggests that the yawing motion may be an efficient scheme for the prohibition of the occurrence of the plume convection and for the suppression of the formation of freckles as well.

CO Distribution and Kinematics along the Bar in the Strongly Barred Spiral NGC 7479

We report on the 2.5 arcsec (400 pc) resolution CO (J = 1 -> 0) observations covering the whole length of the bar in the strongly barred late-type spiral galaxy NGC 7479. CO emission is detected only along a dust lane that traverses the whole length of the bar, including the nucleus. The emission is strongest in the nucleus. The distribution of emission is clumpy along the bar outside the nucleus, and consists of gas complexes that are unlikely to be gravitationally bound. The CO kinematics within the bar consist of two separate components. A kinematically distinct circumnuclear disk, < 500 pc in diameter, is undergoing predominantly circular motion with a maximum rotational velocity of 245 km/s at a radius of 1 arcsec (160 pc). The CO-emitting gas in the bar outside the circumnuclear disk has substantial noncircular motions which are consistent with a large radial velocity component, directed inwards. The CO emission has a large velocity gradient across the bar dust lane, ranging from 0.5 to 1.9 km/s/pc after correcting for inclination, and the projected velocity change across the dust lane is as high as 200 km/s. This sharp velocity gradient is consistent with a shock front at the location of the bar dust lane. A comparison of H-alpha and CO kinematics across the dust lane shows that although the H-alpha emission is often observed both upstream and downstream from the dust lane, the CO emission is observed only where the velocity gradient is large. We also compare the observations with hydrodynamic models and discuss star formation along the bar.

Heliospheric tomography using interplanetary scintillation observations: 3. Correlation between speed and electron density fluctuations in the solar wind

We have examined the relationship between solar wind speed and electron density fluctuations on scale sizes around 100 km in the heliocentric distance range of 0.3 to 0.8 AU using interplanetary scintillation (IPS) data obtained at the Solar‐Terrestrial Environment Laboratory. The solar wind properties derived from the IPS data are biased by line of sight integration through a three‐dimensional structured solar wind. Therefore we have applied a computer‐assisted tomography (CAT) method to deconvolve the line of sight integration and reconstruct the solar wind structure. The analysis was made for the solar wind speed V and electron density fluctuations δNe in the solar activity minimum phase when high‐speed regions are separated from an equatorial low‐speed region by a sharp velocity gradient. From results of the CAT analysis we derived the best fit power law relation of δNe ∝ V−γ with γ = 0.5 ± 0.15, indicating that fractional density fluctuations δNe/Ne in the high‐speed wind are larger than those in the low‐speed wind. Combining this relation with results of previous workers [Coles et al., 1995; Manoharan, 1993; Celnikier et al., 1987; Jackson et al., this issue], we suggest that the fractional density fluctuation level of the high‐speed wind evolves with heliocentric distance.

Adiabatic Effectiveness, Thermal Fields, and Velocity Fields for Film Cooling With Large Angle Injection

The film cooling performance and velocity field were investigated for discrete round holes inclined at an injection angle of 55 deg. Results are compared to typical round film cooling holes, with an injection angle of 35 deg. All experiments in this study were performed at a density ratio of DR = 1.6, using cryogenic cooling of the injected air. Centerline and lateral distributions of effectiveness were obtained for a range of momentum flux ratios. Thermal field and two component mean velocity and turbulence intensity measurements were made at a momentum flux ratio that was within the range of maximum spatially averaged effectiveness. Compared to round holes with 35 deg injection angle, the 55 deg holes showed only a slight degradation in centerline effectiveness for low momentum flux ratios, while a significant reduction in effectiveness was seen at high momentum flux ratios. The thermal field for the 55 deg round holes indicated a faster decay of cooling capacity for the 55 deg round holes. The high turbulence levels for the 55 deg round hole coincided with the sharp velocity gradients between the jet and free stream, and the decay of turbulence levels with downstream distance was found to be similar to those for a 35 deg hole.

Effects of Free-Stream Turbulence Intensity on a Boundary Layer Recovering From Concave Curvature Effects

Experiments are conducted on a flat recovery wall downstream of sustained concave curvature in the presence of high free-stream turbulence (TI ∼ 8%). This flow simulates some of the features of the flow on the latter parts of the pressure surface of a gas turbine airfoil. The combined effects of concave curvature and TI, both present in the flow over a turbine airfoil, have so far been little studied. Computation of such flows with standard turbulence closure models has not been particularly successful. This experiment attempts to characterize the turbulence characteristics of this flow. In the present study, a turbulent boundary layer grows from the leading edge of a concave wall, then passes onto a downstream flat wall. Results show that turbulence intensities increase profoundly in the outer region of the boundary layer over the recovery wall. Near-wall turbulent eddies appear to lift off the recovery wall and a “stabilized” region forms near the wall. In contrast to a low-free-stream turbulence intensity flow, turbulent eddies penetrate the outer parts of the “stabilized” region where sharp velocity and temperature gradients exist. These eddies can more readily transfer momentum and heat. As a result, skin friction coefficients and Stanton numbers on the recovery wall are 20 and 10 percent, respectively, above their values in the low-free-stream turbulence intensity case. Stanton numbers do not undershoot flat-wall expectations at the same Reδ2 values as seen in the low-TI case. Remarkably, the velocity distribution in the core of the flow over the recovery wall exhibits a negative gradient normal to the wall under high-free-stream turbulence intensity conditions. This velocity distribution appears to be the result of two effects: (1) cross transport of kinetic energy by boundary work in the upstream curved flow and (2) readjustment of static pressure profiles in response to the removal of concave curvature.

Integral Field Spectrography of the double AGN merging system Markarian 463

AbstractMarkarian 463, a system of 2 interacting/merging galaxies (E and W) has been observed with SILFID at CFHT in sub-arcsec resolution conditions. We found that both nuclei are AGNs, with the W component being a Sy 2 or LINER and the E component, hitherto known to be a “hidden” Seyfert 1, having essentially Sy 2 characteristics. The bipolar collimated emission originating from the E nucleus is a conspicuous feature in the [OIII]5007 and Hα reconstructed images, as well as in the V – R color map. In velocity maps some previously undetected kinematical features are detected, most notably a sharp velocity gradient (more than 300 km s−1) within 2″ NE of the E nucleus. In addition, in the direct imaging mode a bright optical jet extending ~ 1.2″ S from the E nucleus and a fuzzy blue component 0.8″ W of the W nucleus were seen. Both were also discovered on HST images. Detailed ionization maps reveal individual regions with distinct physical conditions, which constrain the mechanism(s) responsible for the ionization of the gas.

Simultaneous travel time inversion for earthquake location and subduction zone structure in the Central Aleutian Islands

A combined location and velocity inversion technique is applied to travel time data from 151 well‐recorded central Aleutian earthquakes. The data include P and S wave arrivals at stations of a local network and P, pP, and sP wave arrivals at teleseismic stations. After correction for crustal structure at the reflection points, the depth phases pP and sP provide important constraints on subduction zone structure not ordinarily resolved by other types of data. The structure is assumed to vary only across the arc and in depth; it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section. To avoid ray tracing, it is assumed that the velocity part of the problem is linear. Preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab, in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab. A slab thickness of 80–100 km and a downdip length of about 400 km, well below the deepest seismic activity, are indicated. The slab is characterized by seismic velocities as much as 11% higher than the surrounding mantle in its upper portions and 4–6% higher at depth. A sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab. The slab anomaly appears to spread out and fall off very slowly with depth; these are probably not real effects but consequences of the omission of three‐dimensional ray tracing.

FLOW COMPUTATIONS NEARBY A STORM SURGE BARRIER UNDER CONSTRUCTION WITH TWO-DIMENSIONAL NUMERICAL MODELS

This paper discusses the application of two-dimensional tidal models to the hydraulic research for the storm surge barrier in the Eastern Scheldt in the Netherlands. At the site of the barrier local energy losses dominate the flow. Three methods are discussed for dealing with these energy losses in a numerical model based on the long wave equations. The construction of the storm surge barrier provided extensive field data for various phases of the construction of the barrier and these field data are used as a test case for the computation at methods developed. One method is preferred since it gives good agreement between computations and field data. The two-dimensional flow patterns, the discharge and the head-difference agree well,, The results of scale model tests were also available for comparison. This comparison demonstrated that depth-averaged velocities, computed by a two-dimensional numerical model, are as accurate as values obtained from a large physical scale model. Even compicated flow patterns with local energy losses and sharp velocity gradients compared well.

Anomalous Love-wave phase velocities in the Pacific: sequential pure-path and spherical harmonic inversion

Summary. We have determined the lateral distribution of Love-wave phase velocities in the Pacific for the periods 40, 67, 91 and 125 s. Application of the pure-path and spherical harmonic representation methods indicates that the velocities are primarily a function of the age of the seafloor. A comparison of the results from these two techniques indicates inherent modelling constraints in both methods. The pure-path method is limited by its a priori nature while the spherical harmonic approach is unsuitable in describing sharp lateral velocity gradients. To circumvent these limitations, we propose the sequential application of the pure-path and spherical harmonic methods. The sequential inversion separates the velocity distribution into two separate components; velocity as a function of the age of the oceanic plate and variations superimposed on this relationship. Application of this method demonstrates the presence of velocity anomalies which cannot be modelled by an agevelocity relationshp. These anomalies are tentatively correlated with regions of anomalous seafloor depths and/or the presence of active hot-spots. In the central south Pacific, an area with numerous active hot-spots coincides roughly with a region of anomalously slow Love wave velocities. A method for determining the errors associated with the slowness distributions calculated by the spherical harmonic method is presented and provides a means for determining the resolvability of these features.

Turbulent blood flow in the ascending aorta of dogs.

A conical hot film anemometer probe was used to measure instantaneous velocities in the ascending aorta of anaesthetised, open-chest dogs. The probe was mounted on a saddle which allowed traversal of the aorta in 1 mm increments 4 cm above the aortic valve. From these point measurements, the radial distribution of velocity was obtained by averaging ten cardiac cycles. The contractile state of the heart was increased by sequential intravenous infusions of isoprenaline. The absolute peak centreline velocity in the baseline state ranged from 28 to 56 cm x s-1 and, in 20 micrograms x min-1 isoprenaline infusion, from 39 to 112 cm x s-1. Two major effects of isoprenaline on blood flow were noted: 1) isoprenaline dramatically increased peak centreline velocity, and 2) disturbances resembling turbulence appeared as peak velocity increased. With isoprenaline infusion disturbances existed throughout the deceleration portion of the aortic blood flow. Analysis of the frequency components of the velocity wave was performed, and significantly higher frequency components up to 100 Hertz were found in the turbulent cases compared to the laminar ones. Turbulent flow or disturbed flow is found when the ratio of Reynolds number to Womersley number is above 200. In general the hot film measurements showed that both laminar and disturbed velocity profiles tended to be flat throughout the cardiac cycle, with the sharp velocity gradient confined to the region of the wall. Turbulent normal stress during the deceleration portion of aortic blood flow were found in the orders of 15 to 30 dynes x cm-2 and the wall shear stresses were found to be from 10 dynes x cm-2 at the baseline condition to 50 dynes x cm-2 during the 20 micrograms x min-1 isoprenaline infusion.

Displaced Antarctic diatoms in the Amirante Passage

Displaced Antarctic-source diatoms are found in surface sediments below 4100 in the Amirante Passage area but not above this depth. Their distribution tends to support the conclusion that a sharp velocity gradient exists within the Antarctic Bottom Water (AABW) in this region. Below about 4100 m flow velocities are sufficient to resuspend and transport sediment while above this level the AABW is incapable of transporting sediment in suspension.

Sedimentary fabric: A tool to delineate a high-velocity zone within a deep western Indian Ocean bottom current

Magnetic particle alignment and silt mean particle size data have been determined for sediment samples taken from the tops of piston cores recovered from the Amirante Passage in the western Indian Ocean. Our results suggest that the deep western boundary current (DWBC) is flowing through the passage fastest in the deeper portions of the passage, while lower velocities are inferred for depths of < 4100 m, possibly indicating the presence of a sharp velocity gradient within the DWBC. Sediment waves occur at the margin of the Amirante Passage where near-bottom current velocities may be below the minimum critical value for sediment resuspension.

Fermi acceleration and particle pitch angle scattering

We suggest that sharp velocity gradients will exist in fluid-like turbulence in nearly collisionless plasma. This implies effective quenching of Fermi acceleration of thermal particles, but the Fermi acceleration coefficient for relativistic particles remains essentially unchanged.

Evidence of multiplicity in the P-travel time curve beyond 30 degrees

Slowness measurements on first and later arrivals from earthquakes in the Philippine and Taiwan regions recorded at the Warramunga array in Australia indicate abrupt decreases in slowness of the first arrival as well as triplications in the travel time curve at epicentral distances of about 38 and 43°. These results imply the presence of regions of rapid or discontinuous velocity increase at depths of about 900 and 1050 km, respectively. Between these regions of sharp velocity increases the d T/d Δ measurements indicate that the velocity gradients are lower than those determined by previous investigators. The observed extensions of the 650- and 770-km branches out to 50° can be explained in terms of the triplications if small negative velocity gradients of the order of 0.1 km/s per 100 km exist between 650–770 and 770–900 km depths. An alternative explanation of these observed extensions may be provided in terms of underside reflections from the bottom of the velocity discontinuities. Either of the two explanations require sharp velocity gradients at the depth of the velocity discontinuities. These observations are at variance with earth models where the P-wave velocity increases continuously with depth below a depth of 650 km.

The Crust and Upper Mantle of Central Mexico

Summary Six strain and inertial seismographs at Queen Creek, Arizona (QC-AZ), recorded six modes of surface waves from a suite of 18 earthquakes near Chiapas, Mexico. The six group-velocity dispersion curves were used in a least-squares inversion to estimate the shear velocity structure of the upper 380km of crust and mantle in central Mexico. Multiple filter analysis of 108 seismograms produced group velocity dispersion curves slower than average continental paths. The range of the average deviation from the mean dispersion curve for the fundamental modes was 0.025 to 0-160 kni s-' for Rayleigh waves and 0.025-0.281 km s-l for Love waves. The inversion models have low velocities that correspond with a representative geotherm and petrologic P-Tdiagrams to indicate partial melting. The 4layer crust is 30 km thick with a high-temperature gradient LVZ in the granitic layer and with a LVZ in the lower 8 km of a basaltic layer resulting from a high geothermal gradient or from partial melting of water saturated rock. The mantle has a 4-8 km thick solid lid and a shallow low velocity zone. The lowest velocities correspond to 10-20 per cent partial melting. A sharp velocity gradient at 70-80 km probably results from both the phase change to garnet pyrolite and the lower extreme of partial water pressure with the disappearance of amphiboles from the host pyrolite. Based on the velocities, 5 per cent anhydrous melting extends to 260 km. From 300 to 380 km temperature gradients and crystal lattice instabilities prior to the olivine-spinel phase change produce another LVZ. A hypothesis is presented that the large volume of low density magma produces a regional vertical force that creates high Aat plateaus as found in central Mexico and the Colorado Plateau. The inversion models have a 7.5s S-wave residual relative to the Canadian Shield model CANSD in agreement with observed US station anomalies.

Crustal structure of the superior and Grenville provinces of the Northeastern Canadian Shield

abstract A large scale seismic refraction experiment was mounted in the Superior and Grenville provinces of the NE Canadian Shield during the summer of 1968. The purpose was to investigate possible differences in their crustal structure and to study the nature of the large gravity anomaly which is centered along the boundary or front between them. We tentatively interpret the confusing P n data as indicating a weak anisotropic velocity behavior in the material at the top of the upper mantle, coupled with considerable topographic relief on the Mohorovicic transition and a complex crustal structure above. The preferred upper mantle velocity is 8.06±0.06 km/sec with a maximum deviation caused by the anisotropy of 0.17 km/sec. We are not able to discern a marked difference in the upper mantle velocities beneath the two provinces, but we cannot be definite on this point. The average velocity of the crust is 6.57±0.03 km/sec and appears to be the same over the whole region. Crustal thickness varies from 48 to 29 km over the survey area with the average thickness in the Superior being 34 km compared to 39 km in the Grenville to the south. The crust is thickest along the Grenville Front and has an average value of 45 km along our profile. We have made extensive use of synthetic seismograms to interpret the later events on our three seismic profiles. Our preferred models each show a family resemblance. The upper crust has a sharp velocity gradient at the surface with velocities increasing from 5.0 to approximately 6.2 km/sec. From a depth of 1 or 2 km down to the Conrad discontinuity the upper crust is essentially homogeneous except for a low-velocity channel, the depth of whose bottom varies from 6 to 15 km. The channel is thickest, about 5 km, and deepest at the east end of the central profile along the Grenville Front and the center of the gravity anomaly. The channel appears to rise and to become thinner to the north and south. The Conrad discontinuity lies at a depth of approximately 14 km in the Superior, 21 km in the Grenville and 24 km along the Front. The material below the Conrad has a velocity from 6.5 to 6.8 km/sec and a small velocity gradient of roughly 0.02 sec −1 . Approximately 12 km above the Mohorovicic boundary, some of the profiles indicate another low-velocity channel, but in any case, at this depth or below the channel, all of the profiles require a model with a strong velocity gradient down to the crust-upper mantle transition. Here, the velocity rises sharply but more or less continuously and, if there is a discontinuity at all, it is probably a jump of less than 0.02 km/sec. The thickening of the low-velocity channel and the thickening of the upper crust along the Front together are able to explain much of the strong local negative Bouguer anomaly there. The strong regional trend across the area is not simply explained by our models.