High Shear Velocity Research Articles

Propriedades Sísmicas Anisotrópicas Derivadas da Orientação Cristalográfica Preferencial de Muscovita-Quartzo Milonitos

Seismic wave propagation in organized matter usually results in azimuthal variations of longitudinal waves (Pwaves), as well as the effect of birefringence in transversal waves (S-waves), which results in two orthogonal shear waves with contrasting velocities. In this paper we present the results of the anisotropic seismic properties of five samples of muscovitequartz mylonites collected in different parts of a fold in the Saas Fee region, Western Internal Alps. The P-wave velocities in these rocks varies from 5.73 to 6.32 km/s, whereas the high-velocity shear wave (S1) varies from 3.82 to 4.22 km/s and the low velocity (S2) from 3.73 to 4.09 km/s. The anisotropy in these rocks is relatively high and reaches values from 9.5% for P-waves, and almost 11% for shear wave splitting. Both anisotropy and propagation directions seem to be related to from the strong preferred orientation of quartz and muscovite but also depend of muscovite modal content within the different specimens. Development of preferred orientation of minerals destroys and disperses the single crystal seismic properties, which causes a decrease of wave velocities and a dispersion of propagation directions, of both compressional and shear waves. Since the preferred orientation of quartz and muscovite can be directly related to the main macroscopic structures in these rocks (foliation, lineation, and pole of foliation) and the anisotropic seismic properties are related to the preferred orientation, it is possible to determine the propagation directions in terms of these structures. Due to the relatively high muscovite content, many of the maximum propagation velocities are parallel/subparallel to the foliation and some parallel to the lineation of the reference frame. On the other hand, directions of minimum propagation cannot be directly related to the foliation pole. The presence of folds in the mid-to lower crust can exert changes in the propagation directions due to the foliation variation around such structures, mainly in the P-waves.

Hydraulic habitat use with respect to body size of aquatic insect larvae: Case of six species from a French Mediterranean type stream

Macroinvertebrates play a key role in lotic ecosystems, as fish prey and processors of organic material. Therefore, their hydraulic preferences have to be integrated in instream habitat models for ecological stream management. This study characterized physical habitat use in terms of shear velocity for the larvae of three Ephemeropteran ( Ephoron virgo, Oligoneuriella rhenana, and Serratella ignita), two Trichopteran ( Cheumatopsyche lepida and Hydropsyche exocellata) and one Dipteran species ( Blepharicera fasciata) in a Mediterranean stream at a relatively low water discharge. O. rhenana, C. lepida, H. exocellata, and B. fasciata larvae were mainly found in high shear velocity conditions, whereas E. virgo and S. ignita larvae were found in low shear velocity conditions. Knowing that habitat preferences should vary during ontogenesis (with respect to changes in biological requirements and/or morphological abilities to withstand high flow, for example), our second objective was to characterize differences in the hydraulic habitat use (in terms of shear velocity) for different size classes of these six species. Larvae of H. exocellata and B. fasciata mainly colonized high shear velocity conditions and numerous individuals of these species also used medium shear velocity conditions, independent of size class. The use of high shear velocity conditions increased with larval size for C. lepida and O. rhenana, whilst the use of low shear velocity conditions increased for larger larvae of E. virgo and S. ignita. Various hypotheses are proposed to explain these different strategies of habitat use during ontogenesis. We point out the lack of knowledge about physical habitat shifts during the larval growth of freshwater invertebrates. These results highlight the interest to consider the respective habitat requirements of different size classes of invertebrates in instream habitat models. Population bottlenecks should be overcome if hydraulic conditions are kept suitable for all size classes by stream managers.

A Langevin dynamics simulation study of the tribology of polymer loop brushes

The tribology of surfaces modified with doubly bound polymer chains (loops) has been investigated in good solvent conditions using Langevin dynamics simulations. The density profiles, brush interpenetration, chain inclination, normal forces, and shear forces for two flat substrates modified by doubly bound bead-necklace polymers and equivalent singly bound polymers (twice as many polymer chains of 12 the molecular weight of the loop chains) were determined and compared as a function of surface separation, grafting density, and shear velocity. The doubly bound polymer layers showed less interpenetration with decreasing separation than the equivalent singly bound layers. Surprisingly, this difference in interpenetration between doubly bound polymer and singly bound polymer did not result in decreased friction at high shear velocity possibly due to the decreased ability of the doubly bound chains to deform in response to the applied shear. However, at lower shear velocity, where deformation of the chains in the flow direction is less pronounced and the difference in interpenetration is greater between the doubly bound and singly bound chains, some reduction in friction was observed.

Spatiotemporal Variability of Aeolian Sand Transport in a Coastal Dune Environment

Aeolian sand transport on beaches and in dune environments shows a great spatial and temporal variability that has important implications for modeling and monitoring of coastal systems. Yet there have been few quantifications or statistical characteristics of transport variability in natural environments. Transport variability can result from bed surface control in the form of differentiation in grain size, surface moisture, and microtopography, or can be induced by fluid forcing in the form of gusts, burst-sweep events, and streamwise vortices. A field experiment was conducted on a coastal dune near Guadalupe, California, to quantify transport variability over spatial scales of 0.1–4.0 m and temporal scales of 1–120 seconds. Results show that spanwise (lateral) variability increases with spatial scale and decreases with temporal scale. Minimum transport variability over the smallest distances and longest time scales is on the order of 30%, providing error margins for transportrate measurements and model extrapolations. Variability reaches a maximum level at spatial scales larger than roughly half the boundary-layer height. In relation to shear velocity, greatest variability is found near the transport threshold and smallest variability occurs during periods of high shear velocities.

Hydrodynamic lubrication in nanoscale bearings under high shear velocity

The setting up process in a nanoscale bearing has been modeled by molecular dynamics simulation. Contrary to the prediction from the classical Reynolds' theory, simulation results show that the load capacity of the nanoscale bearing does not increase monotonically with the operation speed. This is attributed to the change of the local shear rate, which will decrease with the shear velocity of the bearing as the shear velocity exceeds a critical value, i.e., the local shear rate has an upper limit. A simple nonlinear dynamic model indicates that the momentum exchange between the liquid and the solid wall is reduced with the shear velocity when the shear velocity is above a critical value. The weak momentum exchange results in a decrease of the local shear rate, which in turn causes a sharp increase of the slip length.

Dissipation of the Perpendicular Turbulent Cascade in the Solar Wind

The core solar wind protons are observed to be heated perpendicularly to the magnetic field. This is taken to be a signature of the cyclotron damping of the turbulent fluctuations, which are thought to be responsible for the heating. At the same time, it is commonly accepted that the turbulent cascade produces mostly highly oblique (quasi-two-dimensional) fluctuations, which cannot be immediately cyclotron resonant with the ions because of their low frequencies and small parallel wavenumbers. To address this problem, we propose a new, indirect mechanism for damping the quasi-two-dimensional fluctuations. The mechanism involves a plasma instability, which excites ion cyclotron resonant waves. As the cascade proceeds to higher wavenumbers, it generates increasingly high velocity shear associated with the turbulent fluctuations. The shear eventually becomes unstable to waves near harmonics of the ion cyclotron frequency. Once the frequency of the waves is upshifted, they can heat ions perpendicularly, extracting the energy from the quasi-two-dimensional fluctuations. The dissipation rates of quasi-two-dimensional fluctuations are incorporated into a model of the energy transfer in the turbulent cascade. Our analysis of the observed spectra shows that the spectral break separating the inertial and dissipation ranges of the turbulence, where the dissipation sets in, corresponds to the same shear under a wide range of plasma conditions, in agreement with the prediction of the theory. The observed turbulence spectra often have power-law dissipation ranges with an average spectral index of -3. We demonstrate that this fact is simply a consequence of a marginal state of the instability in the dissipation range.

Mechanisms and timescales of felsic magma segregation, ascent and emplacement in the Himalaya

Abstract We combine field, petrological, geochemical and experimental observations to evaluate the timescales of compaction-driven and shear-assisted melt extraction and ascent in the Himalaya. The results show that melt migration via compaction and channelling is inescapable and operates on timescales of less than 1 million years and possibly as short as 0.1 million years. Field and petrological data show that such a fast and efficient melt transfer results from a combination of favourable factors, including: (1) low but constant melt viscosity (10 4.5 Pa s) during extraction and ascent; (2) grain size coarsening of the source rocks in response to prolonged heating prior to melting; and (3) high source fertility and thus high melt fraction, owing to elevated modal amounts of muscovite in leucogranite sources. All three factors dramatically increase source permeability. Calculations show that shear-assisted melt extraction had a time interval recurrence in the range 10 000–100 000 years (10–100 ka), leading to sill thicknesses of 1–30 m. Yet melts falling at the low end of the viscosity range when coupled to high shear velocities may lead to veins several hundred metres thick. The deepest structural levels (e.g. central Zanskar Range) show that in-situ melts formed where pure shear compaction was greatest and where simple shear was also operative. Magma extracted from migmatite leucosomes was injected along planes of weakness parallel to the ductile shear fabric, probably by some form of hydraulic fracturing crack propagation mechanism. Large High Himalayan leucogranite (HHL) bodies (e.g. c. 5 km thick sills at Manaslu, Makalu and northern Bhutan) may thus represent inflated laccoliths assembled via dykes that tapped a 100–300 m melt layer produced by compaction of the Greater Himalayan Series (GHS). Thermal simulations show that such melt layers may have incubation times of several million years. Although transport time for magmas associated with the HHL is short, the time for assembly may take several million years for the largest HHL, as geochronological data indicate (up to 5 million years for Manaslu, Shisha Pangma). Transport of leucogranite melt from mid-crustal levels towards the surface was concomitant with active low-angle normal faulting along the South Tibetan Detachment (STD) normal fault, a structure that effectively formed the lid to the extrusion of a partially molten layer of mid-crustal rocks (channel flow). Rapid cooling of the granites emplaced at the top of the GHS implies rapid extrusion and lateral flow of GHS rocks beneath the STD during the period c. 20–17 Ma. Weakening of the crust by partial melting is thus likely to be pulsatory in time, and future thermomechanical models should incorporate such aspects to model tectonic evolution of hot orogens.

Flow resistance of a liquid droplet confined between two hydrophobic surfaces

Flow resistance of a liquid droplet confined between two hydrophobic surfaces has been investigated experimentally and factors contributing to the flow resistance have been studied. A water droplet has been sheared between two hydrophobic surfaces and shear resistance has been measured. The experimental results show that the shear resistance at low shear velocities is primarily caused by asymmetrical surface tension due to the contact angle hysteresis. A droplet on a rough hydrophobic surface remains almost symmetrical under shear and exhibits extremely low friction. The shear resistance at high shear velocities is affected by viscous force. Furthermore, sliding angles of water droplets on micropatterned hydrophobic surfaces have been measured to clarify the effects of surface topography on flow resistance. Surfaces with many prismatic structures raised out of the plane tend to exhibit low sliding angles.

Finite frequency tomography of D″ shear velocity heterogeneity beneath the Caribbean

The shear velocity structure in the lowermost 500 km of the mantle beneath the Caribbean and surrounding areas is determined by seismic tomography applied to a suite of Sd‐SKS, ScS‐S, (Scd + Sbc)‐S, and ScS‐(Scd + Sbc) differential times, where (Scd + Sbc) is a pair of overlapping triplication arrivals produced by shear wave interaction with an abrupt velocity increase at the top of the D″ region. The inclusion of the triplication arrivals in the inversion, a first for a deep mantle tomographic model, is possible because of the widespread presence of a D″ velocity discontinuity in the region. The improved ray path sampling provided by the triplication arrivals yields improved vertical resolution of velocity heterogeneity within and above the D″ region. The reference velocity model, taken from a prior study of waveforms in the region, has a 2.9% shear velocity discontinuity 250 km above the core‐mantle boundary (CMB). Effects of aspherical structure in the mantle at shallower depths than the inversion volume are suppressed by applying corrections for several different long‐wavelength shear velocity tomography models. Born‐Fréchet kernels are used to characterize how the finite frequency data sample the structure for all of the differential arrival time combinations; inversions are performed with and without the kernels. The use of three‐dimensional kernels stabilizes the tomographic inversion relative to a ray theory parameterization, and a final model with 60‐ and 50‐km correlation lengths in the lateral and radial dimensions, respectively, is retrieved. The resolution of the model is higher than that of prior inversions, with 3–4% velocity fluctuations being resolved within what is commonly described as a circum‐Pacific ring of high velocities. A broad zone of relatively high shear velocity material extends throughout the lower mantle volume beneath the Gulf of Mexico, with several percent lower shear velocities being found beneath northern South America. Concentrated low‐velocity regions extend through the D″ layer under the Caribbean, Colombia, and Ecuador, suggestive of small‐scale plumes in the boundary layer and possible lateral variations of the D″ discontinuity. One scenario consistent with the imaged features involves subducted Farallon plate ponding at the base of the mantle and laterally displacing hot boundary layer material that piles up and destabilizes on its margins.

Implications on flame stabilization in a precessing jet flame from near field velocity measurements

The near field of the three-dimensional velocity field in a turbulent precessing jet diffusion flame is investigated. In the present case the precessing jet flame is generated by mechanically rotating a round jet of propane inclined at 45° relative to the axis of rotation. Three-dimensional laser Doppler anemometry enables the fully turbulent, three-dimensional velocity field of a precessing jet flame to be measured. This provides insight into the mechanisms by which the flame is stabilized and supports previous explanations of why NOx emissions are reduced and fuel efficiency is increased in industrial precessing jet gas burners. Velocity data are presented, time-averaged, and phase-averaged at the frequency of precession. The conditional phase-averaging technique enables phase-averaged velocity contours and vectors to be obtained that reveal flow patterns and structures within the flame. Time-averaging of the velocity data also reveals the presence of a reverse flow (recirculation) region between the emerging fuel jet and its spinning axis. The recirculation and high shear is associated with a rapid decay of the mean velocity, which together act to stabilize the flame in the high velocity and high shear region near to the nozzle exit. The characteristics of the velocity field of a precessing jet flame found here are compared with previous cold-flow investigations of the same flow and with other investigations of turbulent jets.

Coherent Structure of Zonal Flow and Nonlinear Saturation

The nonlinear structure of the zonal flow in toroidal plasma is investigated in the case that the autocorrelation times of drift waves are much shorter than the autocorrelation time of zonal flow. It is found that the turbulent drive of the zonal flow starts to decrease at a high velocity shear of the zonal flow. By this mechanism, the zonal flow evolves into a stable stationary structure in turbulent plasmas. Flow velocity and radial wavelength are obtained.

Hydrodynamic model with wave–current interaction in coastal regions

The effects of wave–current interaction on the hydrodynamics in coastal regions are investigated. A modified Grant–Madsen analytical model is incorporated into the Princeton Ocean Model (POM) to describe the wave–current interaction. The model is applied to the Singapore Straits for the prediction of tide and wind driven circulation. The simulation results confirm that high shear velocity within the wave bottom boundary layer produces high levels of turbulence intensities. The strong turbulence intensities within the thin wave bottom boundary layer in turn affect the currents through increased bottom resistance.

Stress‐induced seismic velocity anisotropy and physical properties in the SAFOD Pilot Hole in Parkfield, CA

A comprehensive suite of geophysical logs was collected in the SAFOD Pilot Hole from a depth of 775 m to 2150 m in highly fractured Salinian granite. The Pilot Hole intersected numerous macroscopic fractures and faults with extremely varied orientations. Despite the highly variable orientation of the fractures and faults, the fast polarization direction of the shear waves is very consistent with the direction of maximum horizontal compression determined from wellbore breakouts and drilling induced tensile fractures. At least three major shear zones were intersected by the borehole that are characterized by anomalously low velocity and resistivity, anomalously high shear velocity anisotropy and an absence of stress‐induced wellbore breakouts (which suggests anomalously low differential stress). We argue that the physical mechanism responsible for the seismic velocity anisotropy observed in the Pilot Hole is the preferential closure of fractures in response to an anisotropic stress state.

Clinical trial of an educational intervention to achieve recommended cholesterol levels in patients with coronary artery disease

We present a shear velocity model of the crust and upper mantle beneath central Eurasia by simultaneous inversion of broadband group and phase velocity maps of fundamental-mode Love and Rayleigh waves. The model is parameterized in terms of velocity depth profiles on a discrete 2°×2° grid. The model is isotropic for the crust and for the upper mantle below 220 km but, to fit simultaneously long period Love and Rayleigh waves, the model is transversely isotropic in the uppermost mantle, from the Moho discontinuity to 220 km depth. We have used newly available a priori models for the crust and sedimentary cover as starting models for the inversion. Therefore, the crustal part of the estimated model shows good correlation with known surface features such as sedimentary basins and mountain ranges. The velocity anomalies in the upper mantle are related to differences between tectonic and stable regions. Old, stable regions such as the East European, Siberian, and Indian cratons are characterized by high upper-mantle shear velocities. Other large high velocity anomalies occur beneath the Persian Gulf and the Tarim block. Slow shear velocity anomalies are related to regions of current extension (Red Sea and Andaman ridges) and are also found beneath the Tibetan and Turkish–Iranian Plateaus, structures originated by continent–continent collision. A large low velocity anomaly beneath western Mongolia corresponds to the location of a hypothesized mantle plume. A clear low velocity zone in vSH between Moho and 220 km exists across most of Eurasia, but is absent for vSV. The character and magnitude of anisotropy in the model is on average similar to PREM, with the most prominent anisotropic region occurring beneath the Tibetan Plateau.

Flow Structure of Polymer Film Confined between Walls (1st Report, Shear Velocity Dependence of Flow Structure and Orientation of Molecule)

Lubrication by an extremely thin liquid film has become very important in micro machines, magnetic storage devices and so on. When the thickness of lubricant film is thinned to several nanometers, conventional theories of lubrication cannot be used any more. Molecular dynamics simulation was carried out in the present study to investigate the characteristics of the thin lubricant film confined between solid walls. The model used in this simulation is composed of two solid walls and fluorocarbon polymer lubricant confined between them. One of the walls is moving at constant velocity to shear the film. As a result, the lubricant film forms the adsorbed layers near thewalls and the quasi-fluid layer between them. Under a high shear velocity, the slip occuers between the adsorbed layer and the wall and between the adsorbed layer and the quasi-fluid layer. The orientation of molecule influences the occurrence of this slip.

Splash–Saltation of Sand due to Wind‐Driven Rain

Although transport of sediment under wind‐driven rains is generally not accounted for in equations for sediment transport by wind, the contribution of this rainsplash–saltation process can be substantial. Wind‐tunnel experiments, in which vertical deposition fluxes were measured at 23 distances from a sand tray, were conducted to study sediment transport under wind‐driven rain and rainless wind conditions. It was shown that the vertical deposition flux could be described by a double exponential equation. By integration of the vertical deposition flux over the distance of deposition, the sediment transport rate was computed. A power‐law function including both the normal component of the kinetic energy or momentum of the raindrops and the wind shear velocity was presented. However, including the wind shear velocity in the equation increased the model performance only slightly. When comparing the sediment transport rates as determined under the wind‐driven rain events with those that were observed when rain was absent, it was shown that in the latter case, the transport rate is much higher at high wind shear velocities. However, at low wind shear velocities and moisture conditions where no motion is predicted by aeolian equations, saltation due to rainsplash is likely to occur and can be predicted with the presented model.

Stability of the Landau resonance for drift modes in rotating tokamak plasma

The linear stability of drift waves in a poloidally rotating tokamak plasma is considered. The derived dispersion relation features a peaking of the diamagnetic frequency which gives the drift modes an irreducible two-dimensional character. We then show that inverse Landau damping can be suppressed and even stabilized, if the flow's shear is strong. Even though the instability, excited by the Landau resonance, is stronger at a high velocity shear for positive rotation velocities, effects due to the rotation of the plasma can reverse the sign and induce damping of the two-dimensional drift modes. This stabilizing mechanism works only for positive rotation velocities. For negative rotation velocities, we show that only modes with high poloidal mode numbers are unstable.

Degassing of molten polymers

Degassing is a key-step in polymer processing. Low-molecular-weight components are removed from a polymeric system. The transport of these components takes place by diffusion to the polymer–vapour interface. This interface can be formed by free surfaces of single-phase polymer melts or by bubbles. In this study, the transport with and without bubble nucleation is investigated independently from each other in a special designed apparatus similar to a degassing extruder. The mass transport in thin films and in rotating pools with surface renewal is measured. High surface renewal rates and thick films enhance the mass transfer for single phase flow and bubbly flow. Dimensionless mass transfer coefficients are given as a function of the surface renewal rate, the area of the free surface and the total mass of the polymer. The conditions for bubble nucleation and foam formation are investigated. The bubble nucleation is observed in the rotating pool in the area of high shear velocity.

Fractural characteristic evaluation of a microcapsule suspension using a rotational shear stressor.

We are developing microcapsule suspensions to evaluate the absolute hemolytic properties of centrifugal blood pumps. Four types of microcapsule suspensions, with maximum diameters of 100 microm or 10 microm, and membranes of polyurethane or melamine resin, were exposed to fluid dynamic shear up to 15,000 s(-1) with a rotating shear stressor developed in our laboratory. As a result, destruction was observed of only the 100 microm microcapsules. The microcapsules with polyurethane membranes broke at a high shear velocity range over 11,250 s(-1), a tendency similar to that observed using bovine blood. The microcapsules with melamine resin ruptured at approximately 7,500 s(-1). These observations could lead to determination of absolute hemolysis from shear stress occurring in centrifugal blood pumps.

Calibration of Seismic Reflectors Inferred from Observations at the Chinese Continental Drilling Area

AbstractGeophysical investigations have been carried out for the Chinese Continental Scientific Drilling Project (CCSDP), including 2D seismic reflection, MT, brevity and magnetic surveys. Seismic surveys show that many reflectors, such as dipping reflectors and arch like reflectors, exist in the upper crust of the drilling area. Based on cores and well‐logging data from a CCSDP pre‐pilot hole, VSP data and synthetic computations, this paper demonstrates that the dipping reflectors can be caused by high‐velocity eclogite bodies, fractures and ductile shear zones. However, the arch like reflectors may come from steep eclogites plus low‐velocity fractures. As the upper crust usually contains metamorphic rocks, the interpretation proposed in this study should be useful for calibration of deep reflection signals.