Increase In Wave Speed Research Articles

Seismostratigraphy and Thermal Structure of Earth's Core-Mantle Boundary Region

We used three-dimensional inverse scattering of core-reflected shear waves for large-scale, high-resolution exploration of Earth's deep interior (D'') and detected multiple, piecewise continuous interfaces in the lowermost layer (D'') beneath Central and North America. With thermodynamic properties of phase transitions in mantle silicates, we interpret the images and estimate in situ temperatures. A widespread wave-speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite. Internal D'' stratification may be due to multiple phase-boundary crossings, and a deep wave-speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick. The core-mantle boundary temperature is estimated at 3950 +/- 200 kelvin. Beneath Central America, a site of deep subduction, the D'' is relatively cold (DeltaT = 700 +/- 100 kelvin). Accounting for a factor-of-two uncertainty in thermal conductivity, core heat flux is 80 to 160 milliwatts per square meter (mW m(-2)) into the coldest D'' region and 35 to 70 mW m(-2) away from it. Combined with estimates from the central Pacific, this suggests a global average of 50 to 100 mW m(-2) and a total heat loss of 7.5 to 15 terawatts.

The role of the lateral line and vision on body kinematics and hydrodynamic preference of rainbow trout in turbulent flow

The ability to detect water flow using the hair cells of the lateral line system is a unique feature found in anamniotic aquatic vertebrates. Fishes use their lateral line to locate prey, escape from predators and form cohesive schooling patterns. Despite the prevalence of complex flows in nature, almost nothing is known about the function of the lateral line and its relationship to other sensory modalities for freely swimming fishes in turbulent flows. Past studies indicate that under certain conditions the lateral line is not needed to swim steadily in uniform flow. This paper examines how the lateral line and vision affect body kinematics and hydrodynamic habitat selection of rainbow trout (Oncorhynchus mykiss) exposed to vortices generated behind a cylinder. Trout Kármán gaiting (i.e. exploiting vortices to hold station in a vortex street) with a pharmacologically blocked lateral line display altered kinematics; body wavelength and wave speed increase compared to control animals. When visual cues are withheld by performing experiments in the dark, almost all Kármán gait kinematics measured for fish with and without a functional lateral line are the same. The lateral line, rather than vision, plays a larger role in affecting body kinematics when trout hold station in a vortex street. Trout show a preference to Kármán gait in the light but not in the dark, which may be attributed to physiological state rather than hydrodynamic or sensorimotor reasons. In the dark, trout both with and without a functional lateral line hold station near the downstream suction region of the cylinder wake (i.e. entraining) and avoid the vortex street. Vision therefore plays a larger role in the preference to associate with a turbulent vortex street. Trout in the light with a blocked lateral line show individual variation in their preference to Kármán gait or entrain. In the dark, entraining trout with an intact lateral line will alternate between right and left sides of the cylinder throughout the experiment, showing an ability to explore their environment. By contrast, when the lateral line is blocked these fish display a strong fidelity to one side of the cylinder and are not inclined to explore other regions of the flow tank. Both entraining and Kármán gaiting probably represent energetically favorable strategies for holding station relative to the earth frame of reference in fast flows. The ability to decipher how organisms collect and process sensory input from their environment has great potential in revealing the mechanistic basis of how locomotor behaviors are produced as well as how habitat selection is modulated.

The function of parapodial setae in a nereidid polychaete moving on two different substrata

Active polychaetes of the family Nereididae possess a large number of compound setae originating in seta-bundles at the distal end of the parapodium. Here we investigate the functional role of the setae in Nereis diversicolor by comparing the locomotor performance of neuropodial seta-ablated worms with normal (control, non-ablated) worms moving on sand and mud. No differences between worms walking on sand and mud were found, except that they were more submerged in the latter substratum. On both substrata, a similar decrease in forward speed and increase in wave speed and slip were found in seta-ablated worms. However, this was not associated with any changes in gait, since both amplitude and wavelength were similar for seta-ablated and control worms. This suggests that the setae are not adapted to a specific habitat. The same parameters were compared between seta-ablated and control worms swimming over the substratum, but no significant differences were found, indicating that the setae are not as important during swimming as previously thought.

Investigation of shock-induced reaction behavior of as-blended and ball-milled Ni+Ti powder mixtures using time-resolved stress measurements

The shock-compression response of as-blended and ball-milled elemental Ni and Ti powder mixtures is investigated in this study. An 80-mm diameter single-stage gas-gun was employed to perform time-resolved measurements using piezoelectric stress gauges to monitor the stress wave profiles at the front (input) and back (propagated) surfaces of the ∼50% dense Ni+Ti powder mixture compacts. Shock-compression experiments performed on as-blended Ni+Ti powder mixtures in the range of 522m∕sto1046m∕s impact velocities, showed characteristics of powder densification at measured input stress of 1.12GPa, and shock-induced chemical reaction indicated by volume expansion and wave speed increase at measured input stress of 3.2GPa. Ball-milled powder mixtures also showed similar evidence of shock-induced chemical reaction at stresses exceeding 3GPa, with the degree of expansion depending on the energy release associated with the reaction in the powder mixtures ball-milled for various times. The measured high-pressure expanded state was observed to correspond to that calculated using a thermodynamic “ballotechnic” model for shock-induced formation of reaction products in as-blended and ball-milled powder mixtures. The results of instrumented experiments provide clear evidence of shock-induced chemical reactions occurring in Ni+Ti powder mixtures (following densification at stresses exceeding the crush strength) as evidenced by increased shock velocity and generation of an expanded state of products resulting from the associated heat of reaction.

Traveling waves of calcium in pancreatic acinar cells: model construction and bifurcation analysis

We construct and study a model for intracellular calcium wave propagation, with particular attention to pancreatic acinar cells. The model is based on a model of the inositol trisphosphate (IP 3) receptor, which assumes that calcium modulates the binding affinity of IP 3 to the receptor. Two versions of the model, one simpler than the other, are studied numerically. In both versions, solitary waves in the excitable regime arise via homoclinic bifurcations in the traveling wave equations. As the background concentration of IP 3 is increased, the wave speed increases, and for some values of the IP 3 concentration, the initial pulse gives rise to secondary pulses that travel in both directions. This can give rise to irregular spatio-temporal behavior, or to trains of pulses. In the simpler model, these secondary waves are related to the presence of a T-point, a heteroclinic cycle, and an associated spiral of homoclinic orbits, which terminate the branch of homoclinic orbits.

On tide propagation in convergent estuaries

We revisit the problem of one‐dimensional tide propagation in convergent estuaries considering four limiting cases defined by the relative intensity of dissipation versus local inertia in the momentum equation and by the role of channel convergence in the mass balance. In weakly dissipative estuaries, tide propagation is essentially a weakly nonlinear phenomenon where overtides are generated in a cascade process such that higher harmonics have increasingly smaller amplitudes. Furthermore, nonlinearity gives rise to a seaward directed residual current. As channel convergence increases, the distortion of the tidal wave is enhanced and both tidal wave speed and wave lenght increase. The solution loses its wavy character when the estuary reaches its “critical convergence”; above such convergence the weakly dissipative limit becomes meaningless. Finally, when channel convergence is strong or moderate, weakly dissipative estuaries turn out to be ebb dominated. In strongly dissipative estuaries, tide propagation becomes a strongly nonlinear phenomenon that displays peaking and sharp distortion of the current profile, and that invariably leads to flood dominance. As the role of channel convergence is increasingly counteracted by the diffusive effect of spatial variations of the current velocity on flow continuity, tidal amplitude experiences a progressively decreasing amplification while tidal wave speed increases. We develop a nonlinear parabolic approximation of the full de Saint Venant equations able to describe this behaviour. Finally, strongly convergent and moderately dissipative estuaries enhance wave peaking as the effect of local inertia is increased. The full de Saint Venant equations are the appropriate model to treat this case.

Two-phase modeling of DDT: Structure of the velocity-relaxation zone

The structure of the velocity relaxation zone in a hyperbolic, nonconservative, two-phase model is examined in the limit of large drag, and in the context of the problem of deflagration-to-detonation transition in a granular explosive. The primary motivation for the study is the desire to relate the end states across the relaxation zone, which can then be treated as a discontinuity in a reduced, equivelocity model, that is computationally more efficient than its parent. In contrast to a conservative system, where end states across thin zones of rapid variation are determined principally by algebraic statements of conservation, the nonconservative character of the present system requires an explicit consideration of the structure. Starting with the minimum admissible wave speed, the structure is mapped out as the wave speed increases. Several critical wave speeds corresponding to changes in the structure are identified. The archetypal structure is partly dispersed, monotonic, and involves conventional hydrodynamic shocks in one or both phases. The picture is reminiscent of, but more complex than, what is observed in such (simpler) two-phase media as a dusty gas.

A theoretical study of cytosolic calcium waves in xenopus oocytes

Calcium waves have been observed in Xenopusoocytes through the use of confocal fluorescence microscopy. The waves assume many patterns including plane waves, target patterns and spirals. Modification of the membrane model (Jafri et al.,1992) to include diffusion terms in two dimensions reproduces planar and circular propagating waves. A one-dimensional model is used to study the properties of these waves. The model predicts that increasing the buffering capacity of the cytosol decreases the wave amplitude and speed. A decrease in the wave speed is also observed when the net calcium entry rate into the cytosol decreases. The model also predicts that as the diffusion constant is increased, the wave speed increases.

Transmission of pressure waves in partially saturated soils

A split-Hopkinson pressure bar (SHPB) was used to experimentally determine stress-transmission coefficients and wave speeds for dry and moist 50/80 silica soil specimens. Results show that for a constant input pressure the transmitted pressure and wave speed increase to a maximum at approximately five- to ten-percent moisture content. Then, both wave speed and transmission ratio decrease with increasing moisture content down to the approximate values of dry soil. Preliminary analysis indicates that these trends can be explained by the effects that saturation and soil capillarity have on effective stress in the soil.

Higher-order modulation effects on solitary wave envelopes in deep water

The envelope equation of Dysthe (1979), which provides an extension of the nonlinear Schrodinger equation (NLS) to fourth order in wave steepness, is used to discuss higher-order modulation effects on the long-time evolution of solitary wave envelopes in deep water. The Dysthe equation admits solitary-wave solutions, similar to those of the NLS. Using perturbation methods, it is shown that an initial disturbance in the form of a solitary wave group of the NLS evolves to a solitary wave of the Dysthe equation having lower peak amplitude and moving with higher speed than the original wave; the increase in wave speed is caused by a downshift in wavefrequency. Asymptotic expressions are derived for this amplitude decrease and frequency downshift, which are consistent with numerical and experimental results.

Crustal structure beneath the inner trench slope of the Japan trench

A seismic refraction profiling experiment, using an airgun and dynamite as sources and ocean-bottom seismometers (OBSs) as receivers to delineate the structure down to the uppermost mantle, was carried out at the lower inner trench slope of the Japan Trench region in 1982. The results from airgun profiles are presented in this paper. Two airgun profiles were shot along the 4500 m isobath over five OBSs spaced at intervals of about 50 km and three 60 km long profiles were shot perpendicular to the trench axis across three OBSs.The sedimentary layer is 5–6 km thick with P wave speeds 2–3 km/s. Beneath the sedimentary layer lies a layer about 5 km thick with a wavespeed of 6.3 km/s. A layer indicative of subducting oceanic Layer 2 could not be substantiated.Moho-reflected phases were recorded which could constrain the Moho depth. Taking into account the wavespeed increase inside the 6.3 km/s layer required from the dynamite data, the Moho depth is about 15 km, which is about 2 km shallower than previously reported.The data obtained from profiles shot in an E-W direction could be explained moderately well by a model characterized by a thick sediment in between the trench axis and the lower inner trench slope, and a gradually subducting oceanic Layer 3. The model was constructed to accord with previous results because of large lateral variability in structure.

Propagation of Stress Waves in Alpine Snow

AbstractThe propagation of pressure waves in low-density snow is investigated analytically to determine the variation of wave pressure and wave speed with density and frequency. The results show that, for pressure waves that produce finite volumetric deformations, both pressure jump across the wave and wave-speed increase with initial density and final density. The pressure jump was also found to increase with the wave frequency if other parameters were held constant, although the dependence on frequency is not as strong as the dependence on the initial and final densities. The relationship between pressure jump and frequency implies that high-frequency waves would tend to dissipate more quickly than lower-frequency waves, although like pressure, the attenuation rate would not be strongly frequency dependent.

Propagation of Stress Waves in Alpine Snow

AbstractThe propagation of pressure waves in low-density snow is investigated analytically to determine the variation of wave pressure and wave speed with density and frequency. The results show that, for pressure waves that produce finite volumetric deformations, both pressure jump across the wave and wave-speed increase with initial density and final density. The pressure jump was also found to increase with the wave frequency if other parameters were held constant, although the dependence on frequency is not as strong as the dependence on the initial and final densities. The relationship between pressure jump and frequency implies that high-frequency waves would tend to dissipate more quickly than lower-frequency waves, although like pressure, the attenuation rate would not be strongly frequency dependent.

Solid-particle motion in two-dimensional peristaltic flows

Some insight into the mechanism of solid-particle transport by peristalsis is sought experimentally through a two-dimensional model study (§ 2). The peristaltic wave is characterized by a single bolus sweeping by the particle, resulting in oscillatory motion of the particle. Because of fluid-particle interaction and the significant curvature in the wall wave, the peristaltic flow is highly nonlinear and time dependent.For a neutrally buoyant particle propelled along the axis of the channel by a single bolus, the net particle displacement can be either positive or negative. The instantaneous force acting upon the particle and the resultant particle trajectory are sensitive to the Reynolds number of the flow (§ 3 and 4). The net forward movement of the particle increases slightly with the particle size but decreases rapidly as the gap width of the bolus increases. The combined dynamic effects of the gap width and Reynolds number on the particle displacement are studied (§ 5). Changes in both the amplitude and the form of the wave have significant effects on particle motion. A decrease in wave amplitude along with an increase in wave speed may lead to a net retrograde particle motion (§ 6). For a non-neutrally buoyant particle, the gravitational effects on particle transport are modelled according to the ratio of the Froude number to the Reynolds number. The interaction of the particle with the wall for this case is also explored (§ 7).When the centre of the particle is off the longitudinal axis, the particle will undergo rotation as well as translation. Lateral migration of the particle is found to occur in the curvilinear flow region of the bolus, leading to a reduction in the net longitudinal transport (§ 8). The interaction of the curvilinear flow field with the particle is further discussed through comparison of flow patterns around a particle with the corresponding cases without a particle (§ 9).

Turbulent velocity waves and kinematic dynamo activity

It is demonstrated that a turbulent distribution of small amplitude velocity waves gives rise to kinematic dynamo activity on its own in an infinite medium. As the wave speed increases, the intensity of the waves has to increase in order to support dynamo regeneration. The theory given is statisticallyexact, but in view of the complexity of the resulting equations, the answers presented are for the long-wavelength (i.e. large-scale) magnetic field. In view of the fact that most astrophysical objects apparently contain turbulent wave motions, the present calculation is indicative of the extent to which turbulent dynamo activity may be physically importat in such objects.

Dispersion and attenuation of small artificial pressure waves in the canine aorta.

A method was developed to determine the elastic behavior of large blood vessels in terms of their transmission characteristics for small sinusoidal pressure signals. The method is new insofar as it utilizes transient signals of the form of finite trains of sine waves that are superimposed on the naturally occurring pressure fluctuations and are generated by an electrically driven impactor or by a pump. Its application to the thoracic aortas of 18 mature mongrel dogs anesthetized with pentobarbital has shown that dispersion and attenuation data for frequencies between 40 and 200 cps can be obtained without requiring either Fourier transform computations or resolution of reflection interference. For the frequency range considered, the descending aorta is only mildly dispersive but exhibits strong attenuation that must be attributed primarily to dissipative mechanisms in the vessel wall. At normal blood pressure levels, the wave speed during diastole can have a value between 4 and 6 m/sec. For all frequencies tested the amplitude ratio of the waves exhibits the same exponential decay pattern with distance measured in wavelengths. A marked increase in wave speed observed from diastole to systole can be associated with an increase in mean flow and with a stiffening of the aortic wall due to the rise in pressure. This phenomenon implies that the aortas of anesthetized dogs should exhibit nonlinear properties with respect to large amplitude pulse waves such as those generated by the heart.