Vertical Particle Displacement Research Articles

Asymmetric mobilization of arching effect in granular materials: the role of fabric

We present an experimental and DEM investigation of the arching effect by focusing on the trapdoor test, with an emphasis on the effect of fabric orientation which is created by depositing particles at various directions. The results indicate that the arching effect is the weakest when the deposition plane is oriented at θ = 30°, which is manifested by the largest arching ratio for the trapdoor part and the most considerable particle movement. The arching effect mobilization is found to be asymmetric, apart from the cases θ = 0° & 90°. The arching ratio for the left bottom part is on average lower than that for the right bottom part, and the vertical particle displacement for the neighbouring zone at the left of trapdoor is comparably larger. The asymmetric mobilization of arching effect, as well as the occurrence of the weakest arching effect at θ = 30°, is also clarified.

Wave Kinematics and Pressure Field of Third-Order Theory for Bichromatic Bi-Directional Waves in Water of Finite Depth

Based on the third-order theory for bichromatic bi-directional waves in water of finite depth, a set of explicit formulas for the state-of-the art quantities of wave kinematics for horizontal and vertical particle displacements, velocities and accelerations, and wave pressure field is developed, and would be much more accurate and realistic in the design of harbor, coastal and offshore structures and their structural members.

Wave‐induced transport and vertical mixing of pelagic eggs and larvae

The transport of pelagic plankton by wind‐driven ocean currents and surface gravity waves is investigated for the example of Northeast Arctic cod eggs and larvae on the coast of northern Norway. Previous studies indicate that the wave‐induced drift (i.e., Stokes drift) is relevant for the transport of particles in the upper ocean. We use an ocean general circulation model together with a numerical wave prediction model and a Lagrangian particle tracking model to calculate trajectories of fish eggs and larvae. Waves are considered not only for particle drift but also for the air—sea momentum flux, its contribution to the Coriolis force, and vertical mixing. The sample species provides the advantage that many of its physical and behavioral properties are well known (e.g., egg buoyancy), allowing investigation of vertical particle displacement by turbulent mixing in response to wind forcing and wave breaking. The approach accounting for particle mixing by breaking waves enhances agreement between observed and modeled egg profiles. Results also show a general shoreward transport of particles by the Stokes drift. This wave drift exhibits a more constant direction than the Eulerian current and hence stabilizes particle diffusion to favor a dominant direction. For the case of Northeast Arctic cod, waves concentrated model eggs and larvae on average 1.5 km closer to shore, which is 20% of their total distance to the coast. This increases the residence time of first‐feeding larvae close to the spawning areas compared to earlier models.

Mechanism of radial redistribution of energetic trapped ions due to m=2/n=1 kink instability in plasmas with an internal transport barrier in the Joint European Torus

Internal radial redistribution of MeV energy ion cyclotron resonance frequency driven hydrogen minority ions was inferred from neutral particle analyzer measurements during large amplitude magnetohydrodynamic activity in plasmas with an internal transport barrier in the Joint European Torus. A theory is developed for energetic ion redistribution during a m=2/n=1 kink mode instability. Plasma motion during the instability or during subsequent magnetic reconnection generates an electric field which can change the energy and radial position of the energetic ions. The magnitude of ion energy change depends on the value of the safety factor at the plasma core, q(0) from which the energetic ions are redistributed. A relation is found for the corresponding change in canonical momentum Pφ, which leads to radial displacement of the ions. The model yields distinctive new features of energetic ion redistribution such as more vertical particle displacement as q(0) increases from 1 to 2. Predicted characteristics of ion redistribution are compared with the measurements, and good correlation is found. Sometimes the energetic ions were further transported to the plasma edge due to interaction with a long-lived magnetic fluctuation (often in the form of a magnetic island) with chirping frequency in the laboratory frame which developed after the m=2/n=1 kink instability. Convection of resonant ions trapped in a radially moving phase-space island is modeled to understand the physics of such events.

The moving-load problem in soil dynamics—the vertical displacement approximation

The moving point load problem in soil dynamics is analyzed in the vertical particle displacement approximation. Prior to its motion, the load is stationary. From the instant at which it is set into motion it moves, with constant speed, along a straight path on the (horizontal) planar surface of a semi-infinite elastic medium. The modified Cagniard method for solving transient wave problems is used to determine closed-form expressions for the vertical component of the particle displacement from the elastodynamic wave equation of which only the vertical component is taken into account. The relevant approximation is standard in soil dynamics. Both the cases of “subsonic” and “supersonic” surface load speeds are considered. Methods to include losses in the model are briefly discussed. The study has been initiated with a view to the application of the results to the analysis of the ground motion generated by high-speed trains traveling on a poorly consolidated soil.

Bioturbation in the abyssal Arabian Sea: influence of fauna and food supply

In order to evaluate bioturbation in abyssal Arabian-Sea sediments of the Indus fan profiles of 210 Pb (half-life: 22.3 yr) and 234 Th (half-life: 24.1 d) were measured in cores collected during September and October 1995 and April 1997, respectively. The density and composition of epibenthic megafauna and lebensspuren were determined in vertical seafloor photographs during April 1997. Mean eddy-diffusive mixing coefficients according to the distribution of excess 210 Pb ( 210 Pb - D B) were 0.072±0.028, 0.068±0.055, 0.373±0.119, 0.037±0.009 and 0.079±0.119 cm 2 yr −1 in the northern, western, central, eastern and southern abyssal Arabian sea, respectively. Mean eddy-diffusive mixing coefficients according to the distribution of excess 234 Th ( 234 Th - D B) were 0.53, 1.64 and 0.47 cm 2 yr −1 in the northern, western and central abyssal Arabian Sea, respectively. Mobile epibenthic megafauna at the western, northern, central and southern study sites were dominated by ophiuroids, holothurians, ophiuroids and natant decapods (the respective densities were 100, 82, 29 and 6 individuals 1000 m −2). The northern study site was characterized by a high abundance of spoke traces and fecal casts. The central site showed spoke traces and many tracks. The southern site displayed the highest abundance of spoke traces, whereas at the western site hardly any lebensspuren were observed. There is evidence for at least two functional endmember communities in the Arabian Sea. In the northwestern Arabian Sea (WAST) vertical particle displacement seems to be dominated by macrofauna and primarily eddy-diffusive. In the southern Arabian Sea (SAST) non-local and `incidental’ mixing due to spoke-trace producers might become more important and superimpose reduced eddy-diffusive mixing. With respect to biological data CAST is an intermediate location. Given the biological data, average 210 Pb - D B is higher and decimeter-scale variability of 210 Pb - D B smaller at CAST than expected. These findings indicate that in a mixture of both endmember communities the organisms may interact in way that increases values of biodiffusivity, as reflected by 210 Pb - D B, and reduces decimeter-scale 210 Pb - D B heterogeneity in comparison to the simple sum of the isolated effects of the endmembers. For time scales <100 years there was no evidence for a relationship between food supply (POC flux) and bioturbation intensity, as reflected by 210 Pb - D B and 234 Th - D B. Bioturbation intensity should be controlled primarily by the composition of the benthic fauna, its specific adaptation to the environmental setting, and the abundance of each species of the benthic community. Food supply can have only an indirect influence on bioturbation intensity. In certain parts of the ocean the a priori overall positive relationship between POC flux and biodiffusivity might include restricted intervals displaying no or even negative relations.

Benthic macrofauna and depth of bioturbation in Eckernf�rde Bay, southwestern Baltic Sea

The goal of this study was to relate the distribution and abundance of functional groups of benthic macrofauna to the depth of bioturbation in Eckernforde Bay, Germany. Particle bioturbation was limited to the top 0.5–1.0 cm throughout Eckernforde Bay and is consistent with sedimentological and radiochemical studies. Our results indicate that vertical particle displacement between ingestion and defecation controls the depth of bioturbation and is directly related to the functional group. The benthic community is maintained at a low level of complexity due to a regular disturbance, most likely seasonal hypoxia/anoxia.

Scattering of Sound by Internal Wave Currents: The Relation to Vertical Momentum Flux

Internal waves scatter sound by two related perturbations: 1) those associated with vertical particle displacements ζ(x, y, z, t) in the presence of a vertical gradient of (potential) sound speed δc = ζ∂(_z)c(_p)); and 2) those associated with horizontal particle velocities u(x, y, z, t). The combined fractional perturbation in propagation velocity is δc/c + u/c. The second term, generally neglected, introduces a nonreciprocity when source and receiver are interchanged. Nonreciprocity is expected to be relatively small except for transmission along a deep downward loop. The principal internal wave contribution to nonreciprocity is from inertial frequencies. The sum and difference of reciprocal travel times are a measure of ζ and u, respectively, along the ray path; the quadrature spectrum of reciprocal travel times is related by an integral equation to the spectrum of the momentum flux 〉ζu〈. Precise measurements of nonreciprocity could provide an estimate of the vertical momentum flux in an internal wave field.

Annual Baroclinic Rossby Waves in the Central North Pacific

We analyze XBT data, collected in the central North Pacific under the NORPAX program (TRANSPAC), by means of inverse methods (cross-spectral fit). We separate the data into two parts: a fluctuation with length scales in the order of the dimensions of the North Pacific basin and a wavelike fluctuation with much shorter scales of a few hundred kilometers. At the annual frequency we find the wavelike fluctuation to consist of a random field of first-order baroclinic Rossby waves traveling basically northwest with wavelengths of ∼300 km and phase speeds of ∼1 cm s−1. The group velocity is directed mainly southwest and is of the order of 1 cm s−1. We describe the fields of temperature, pressure, velocity, sea level elevation, and horizontal and vertical particle displacement associated with the annual baroclinic Rossby waves.

Solid flow pattern at the wall of a fluidization column induced by single bubbles

Particle flow induced by single bubbles at the wall of fluidized beds, 170–350-μm alumina catalyst, is observed by a photographic technique. The fluidization column is 350 mm i.d. Bubbles are injected by a vertical 6 mm i.d. nozzle placed at different distances between the column axis and the wall. Bubble diameters, ranging from 5 to 15 cm, are determined when emerging at the bed upper surface. The solid is traced with black particles, whose individual motion is filmed at the wall. Overall vertical and circumferential tangential particle displacements, upwards and downwards, are observed for different sizes of bubble, for different distances of the flow path of the bubble center from the wall and for circumferential distances from the point of the wall nearest to the bubble. Vertical particle displacement at this point is correlated to bubble diameter and position.