Velocity gradient

About: Velocity gradient is a research topic. Over the lifetime, 3013 publications have been published within this topic receiving 77120 citations.

The seismic response of two-dimensional sedimentary deposits with large vertical velocity gradients

Abstract: An extension of the Aki-Larner technique to vertically inhomogeneous media is presented, so that vertical velocity gradients may be taken into account in two-dimensional models. Only SH waves are considered here, but the theory is valid for P and SV waves as well. This method is applied to investigate the seismic response of two-dimensional sedimentary deposits with large velocity gradients. Three different cases are considered: a shallow, high-contrast valley, a deep, high-contrast valley, and a deep, low-contrast valley. In each case, a comparison is performed with the results of a two-dimensional model taking into account only homogeneous sediments and a one-dimensional model taking into account the vertical inhomogeneity of sediments. The presence of a large velocity gradient does not change a lot the qualitative behavior of a two-dimensional deposit: local surface waves and/or two-dimensional resonance patterns are observed as in the case of homogeneous sediments. Nevertheless, the surface waves are much more dispersive with lower phase and group velocities and larger amplitudes. Only very deep valleys give rise to the development of two-dimensional resonance. On the other hand, the amplifications obtained in such deposits may reach much larger values than those predicted with either two-dimensional, homogeneous models or one-dimensional, inhomogeneous models. Since these results have been obtained for realistic values of valley geometrical and mechanical considerations, they should find some application in earthquake engineering or seismic microzonation studies.

Momentum extraction with saltation: implications for experimental evaluation of wind profile parameters

Abstract: In wind tunnel studies of aeolian transport, the number and position of pitot tubes are decided by the researcher, so that there are important variations in the computation ofU* between studies. Velocity measurements seldom are made very close to mobile surfaces because the tubes become blocked by drifting sand grains. This practice is fortuitous as demonstrated by recent selfregulatory models of saltation which indicate that fluid and grain-borne shear stress vary substantially within the lowest 0.01 m and application of the logarithmic law is therefore unsound. This study reports detailed velocity measurements which further suggest that no single logarithmic expression, based on fixed values of κ and τ, adequately represents the full wind profile which includes the inner saltation cloud above 0.01 m and the outer grain-free region of the boundary layer. A much improved fit over the logarithmic wind profile model is achieved with a square root relation, although there is no known physical basis for this specific form of power model. Relatively shallow boundary-layer development in wind tunnels forces the velocity gradient above the region of momentum extraction to attain exceptionally large values, uncommon in natural settings.

A hybrid RANS/LES simulation of turbulent channel flow

Abstract: Hybrid models combining large eddy simulation (LES) with Reynolds-averaged Navier–Stokes (RANS) simulation are expected to be useful for wall modeling in the LES of high Reynolds number flows. Some hybrid simulations of turbulent channel flow have a common defect; the mean velocity profile has a mismatch between the RANS and LES regions due to a steep velocity gradient at the interface. This mismatch is reproduced and examined using a simple hybrid model; the Smagorinsky model is switched to a RANS model increasing the filter width. It is suggested that a rapid spatial variation in the eddy viscosity is responsible for an underestimate of the grid-scale shear stress and for the steep velocity gradient. To reduce the mean velocity mismatch a new scheme is proposed; additional filtering is introduced to define two kinds of velocity components at the interface between the two regions. The two components are used to remove inconsistency in the velocity equations due to a rapid variation in the filter width. Using the new scheme, simulations of channel flow are carried out with the simple hybrid model. It is shown that the grid-scale shear stress becomes large enough and most of the mean velocity mismatch is removed. Simulations for higher Reynolds numbers are carried out with the k–e model and the one-equation subgrid-scale model. Although it is necessary to improve the turbulence models and the treatment of the buffer region, the new scheme is shown to be effective for reducing the mismatch and to be useful for developing better hybrid simulations.

Electrorheology of a nematic poly(n‐hexyl isocyanate) solution

Abstract: The shear flow behavior of a unidomain nematic solution of poly(n‐hexyl isocyanate) (PHIC) subjected to an electric field was studied experimentally in a parallel plate rotational rheometer. At a shear rate of 0.4 s‐1, the Miesowicz viscosity ηc (with the director oriented along the velocity gradient) was found to be 35 times greater than the steady shear viscosity in the absence of an electric field. The mechanism for this electrorheological (ER) effect is the orientation of the permanent dipole moment of the PHIC molecules; the ER effect is an order of magnitude larger than that for low molecular weight liquid crystals. Director tumbling is postulated to occur at a shear rate of 0.4 s−1 for the PHIC solution if the applied dc electric field is lower than approximately 0.4 MV/m. At higher electric fields, flow alignment is regained. In this case, the transient stress undershoot is suppressed. Steady state viscosity vs electric field data were fit to Carlsson and Skarp’s two‐dimensional approximation of t...

Massive molecular gas flows in the Abell 1664 brightest cluster galaxy

Abstract: We report ALMA Early Science CO(1-0) and CO(3-2) observations of the brightest cluster galaxy (BCG) in Abell 1664. The BCG contains 1.1x10^{10} solar masses of molecular gas divided roughly equally between two distinct velocity systems: one from -250 to +250 km/s centred on the BCG's systemic velocity and a high velocity system blueshifted by 570 km/s with respect to the systemic velocity. The BCG's systemic component shows a smooth velocity gradient across the BCG center with velocity proportional to radius suggestive of solid body rotation about the nucleus. However, the mass and velocity structure are highly asymmetric and there is little star formation coincident with a putative disk. It may be an inflow of gas that will settle into a disk over several 10^8 yr. The high velocity system consists of two gas clumps, each ~2 kpc across, located to the north and southeast of the nucleus. Each has a line of sight velocity spread of 250-300 km/s. The velocity of the gas in the high velocity system tends to increase towards the BCG center and could signify a massive high velocity flow onto the nucleus. However, the velocity gradient is not smooth and these structures are also coincident with low optical-UV surface brightness regions, which could indicate dust extinction associated with each clump. If so, the high velocity gas would be projected in front of the BCG and moving toward us along the line of sight in a massive outflow most likely driven by the AGN. A merger origin is unlikely but cannot be ruled out.