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The dynamics of finite-amplitude bed forms in a tidal channel is studied with the use of an idealized morphodynamic model. The latter is based on depth-averaged equations for the tidal flow over a sandy bottom. The model considers phenomena on spatial scales of the order of the tidal excursion length. Transport of sediment mainly takes place as suspended load. The reference state of this model is characterized by a spatially uniform M2 tidal current over a fixed horizontal bed. The temporal evolution of deviations from this reference state is governed by amplitude equations: these are a set of non-linear equations that describe the temporal evolution of bed forms. These equations are used to obtain new morphodynamic equilibria which may be either static or time-periodic. Several of these bottom profiles show strong similarity with the tidal bars that are observed in natural estuaries. The dependence of the equilibrium solutions on the value of bottom friction and channel width is investigated systematically. For narrow channels (width small compared to the tidal excursion length) stable static equilibria exist if bottom friction is slightly larger than rcr. For channel widths more comparable to the tidal excursion length, multiple stable steady states may exist for bottom friction parameter values below rcr. Regardless of channel width, stable time-periodic equilibria seem to emerge as the bottom friction is increased.Responsible Editor: Jens Kappenberg 相似文献
2.
In this paper we review the possibilities for
magnetohydrodynamic processes to handle the angular momentum transport
in accretion disks. Traditionally the angular momentum transport has
been considered to be the result of turbulent viscosity in the disk,
although the Keplerian flow in accretion disks is linearly stable towards
hydrodynamic perturbations. It is on the other hand linearly unstable
to some magnetohydrodynamic (MHD) instabilities.
The most important instabilities are the Parker and Balbus-Hawley
instabilities that are related to the magnetic buoyancy and the shear
flow, respectively. We discuss these instabilities not only in the
traditional MHD framework, but also in the context of slender flux
tubes, that reduce the complexity of the problem while keeping most of
the stability properties of the complete problem. In the non-linear
regime the instabilities produce turbulence. Recent numerical
simulations describe the generation of magnetic fields by a dynamo in
the resulting turbulent flow. Eventually such a dynamo may generate a
global magnetic field in the disk. The relation of the MHD-turbulence
to observations of accretion disks is still obscure. It is commonly
believed that magnetic fields can be highly efficient in transporting
the angular momentum, but emission lines, short-time scale variability
and non-thermal radiation, which a stellar astronomer would take as
signs of magnetic variability, are more commonly observed during periods
of low accretion rates.
Received October 12, 1995 / Accepted November 16, 1995 相似文献
3.
Thomas Boelens Henk Schuttelaars George Schramkowski Tom De Mulder 《Ocean Dynamics》2018,68(10):1285-1309
A new depth-averaged exploratory model has been developed to investigate the hydrodynamics and the tidally averaged sediment transport in a semi-enclosed tidal basin. This model comprises the two-dimensional (2DH) dynamics in a tidal basin that consists of a channel of arbitrary length, flanked by tidal flats, in which the water motion is being driven by an asymmetric tidal forcing at the seaward side. The equations are discretized in space by means of the finite element method and solved in the frequency domain. In this study, the lateral variations of the tidal asymmetry and the tidally averaged sediment transport are analyzed, as well as their sensitivity to changes in basin geometry and external overtides. The Coriolis force is taken into account. It is found that the length of the tidal basin and, to a lesser extent, the tidal flat area and the convergence length determine the behaviour of the tidally averaged velocity and the overtides and consequently control the strength and the direction of the tidally averaged sediment transport. Furthermore, the externally prescribed overtides can have a major influence on tidal asymmetry in the basin, depending on their amplitude and phase. Finally, for sufficiently wide tidal basins, the Coriolis force generates significant lateral dynamics. 相似文献
4.
Hepkema Tjebbe M. de Swart Huib E. Nnafie Abdel Schramkowski George P. Schuttelaars Henk M. 《Ocean Dynamics》2020,70(12):1505-1513
Ocean Dynamics - The role of the Coriolis effect in the initial formation of bottom patterns in a tidal channel is studied by means of a linear stability analysis. The key finding is that the... 相似文献
5.
Ocean Dynamics - We investigate the hypothesis by Winterwerp and Wang (Ocean Dyn 63:1279–1292, 2013) that channel deepening in the Scheldt River Estuary could lead to a large increase in... 相似文献
6.
Effect of bottom stress formulation on modelled flow and turbidity maxima in cross-sections of tide-dominated estuaries 总被引:1,自引:0,他引:1
A three-dimensional numerical model with a prognostic salinity field is used to investigate the effect of a partial slip bottom boundary condition on lateral flow and sediment distribution in a transect of a tidally dominated channel. The transect has a symmetrical Gaussian cross-channel bottom profile. For a deep, well-mixed, tidally dominated channel, partial slip decreases the relative importance of Coriolis deflection on the generation of cross-channel flow patterns. This has profound implications for the lateral distribution of residual salinity that drives the cross-channel residual circulation pattern. Transverse sediment transport, however, is always found to be governed by a balance between advection of residual sediment concentration by residual lateral flow on the one hand and cross-channel diffusion on the other hand. Hence, the changes in the cross-channel distribution of residual salinity modify the lateral sediment distribution. For no slip, a single turbidity maximum occurs. In contrast, partial slip gives a gradual transition to a symmetrical density distribution with a turbidity maximum near each bank. For a more shallow, partially mixed tidal channel that represents the James River, a single turbidity maximum at the left bank is found irrespective of the near-bed slip condition. In this case, semi-diurnal contributions to sediment distribution and lateral flow play an important role in cross-channel sediment transport. As vertical viscosity and diffusivity are increased, a second maximum at the right bank again exists for partial slip. 相似文献
7.
Karin M. H. Huijts Huib E. de Swart George P. Schramkowski Henk M. Schuttelaars 《Ocean Dynamics》2011,61(8):1067-1091
An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped
estuarine cross-sections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong
mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional
analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model.
The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter
space considered (average tidal velocity amplitude, 0.1–1 m s − 1 and maximum water depth, 10–60 m). Its skills are lower for along-channel residual flows if nonlinearities are moderate to
high (strong tides in deep estuaries) and for transverse flows and residual sediment concentrations if the Ekman number is
small (weak tides in deep estuaries). An important new aspect of the analytical model is the incorporation of tidal variations
in the across-channel density gradient, causing a double circulation pattern in the transverse flow during slack tides. The
gradient also leads to a new tidally rectified residual flow component via net advection of along-channel tidal momentum by
the density-induced transverse tidal flow. The component features landward currents in the channel and seaward currents over
the slopes and is particularly effective in deeper water. It acts jointly with components induced by horizontal density differences,
Coriolis-induced tidal rectification and Stokes discharge, resulting in different along-channel residual flow regimes. The
residual across-channel density gradient is crucial for the residual transverse circulation and for the residual sediment
concentration. The clockwise density-induced circulation traps sediment in the fresher water over the left slope (looking
up-estuary in the northern hemisphere). Model results are largely consistent with available field data of well-mixed estuaries. 相似文献
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