排序方式： 共有26条查询结果，搜索用时 234 毫秒
1.
The paper briefly discusses the view and actions of the Norwegian Government in relation to environmental protection and development and the minimization of the effects of industry. Governments and supra-national bodies, industry, municipal bodies, science and training bodies and voluntary and non-governmental organizations all have a major and integrated role in protecting the environment. It is emphasized that as many of the problems are transboundary and trans-sectoral their solution requires an intersectoral approach. 相似文献
2.
With increasing resolution in numerical ocean models, nonhydrostatic pressure effects have to be accounted for. In sigma-coordinate
mode split ocean models, this pressure may be regarded as a pressure correction. An elliptic equation must be solved for the
nonhydrostatic pressure, and the gradients are used to correct the provisional hydrostatic velocity components in each time
step. The focus in the present work is on the surface boundary condition for the elliptic equation. In the literature, both
Dirichlet and Neumann boundary conditions are suggested and applied. To investigate the sensitivity of the numerical results
to the choice of boundary condition, three numerical experiments are performed. The first and second experiments are studies
of the propagation and steepening of nonlinear internal waves. The first study is on tank scale and the second experiment
is on ocean scale. In the tank-scale experiment, the density and the flow fields are very robust to the choice of boundary
condition. In the ocean-scale experiment, the waves produced with a Dirichlet boundary condition become more damped than the
waves produced with a Neumann boundary condition. The third study involves a surface buoyant jet. It is shown that well-known
characteristics of the plume front are reproduced with a Neumann boundary condition, but the rotating turbulent core of this
front is lost with a Dirichlet condition. It is accordingly argued that the appropriate surface boundary condition in mode
split nonhydrostatic ocean models is the Neumann condition. 相似文献
3.
A mechanistic study is performed to examine the coupling between the in-line and the cross-flow motion of a cylindrical structure subjected to current forces. The structure represents a free span pipeline but concerns marine risers as well.A time domain model is formulated in which the in-line and cross-flow deflections are coupled through the axial tension which in turn is computed from the pipeline prolongation at any time. This formulation introduces time dependent tensions and non-linearity into the problem.Preliminary validation of the model simulations vs. physical test data are carried out for one specific case to ensure that the sag and the in-line deflection are correctly resolved by the model. Using this as the initial condition a series of calculations are carried out to examine cross-flow induced deflections induced by an in-line prescribed deflection and vice versa. Finally, an idealistic simulation of flow induced vibration is presented.The model simulations demonstrate that the coupling varies with the mode shape and with which component it is initially introduced into. However, it is evident that the coupling effects may be significant and not negligible. 相似文献
4.
Numerical ocean modelling is computationally very demanding. Traditionally, the hydrostatic approximation has been applied to reduce the computational burden. This approximation is valid in large scale studies with coarse grid resolution. With faster computers and gradually smaller grid sizes, we may expect that more studies will be performed with non-hydrostatic ocean models. In recent papers several methods for including non-hydrostatic pressure in σ-coordinate models have been suggested. In this paper the sensitivity of the non-hydrostatic pressure field, the velocity fields, and the density fields to changes in the method for computing non-hydrostatic pressure in σ-coordinate ocean models is addressed.The first test case used involves the propagation and breaking of an internal wave at an incline in a tank. The other test case concerns tidally driven flow over a sill in a stratified fjord. The results from our numerical exercises suggest that the velocity and density fields are very robust to the model choices investigated here. The differences between the model results are of the same order as the uncertainty due to the internal pressure gradient error, and they are smaller than an estimate of the uncertainty due to subgrid scale closure. 相似文献
5.
In this study, a three-dimensional particle tracking model coupled to a terrain following ocean model is used to investigate
the dispersion and the deposition of fish farm particulate matter (uneaten food and fish faeces) on the seabed due to tidal
currents. The particle tracking model uses the computed local flow field for advection of the particles and random movement
to simulate the turbulent diffusion. Each particle is given a settling velocity which may be drawn from a probability distribution
according to settling velocity measurements of faecal and feed pellets. The results show that the maximum concentration of
organic waste for fast sinking particles is found under the fish cage and continue monotonically decreasing away from the
cage area. The maximum can split into two maximum peaks located at both sides of the centre of the fish cage area in the current
direction. This process depends on the sinking time (time needed for a particle to settle at the bottom), the tidal velocity
and the fish cage size. If the sinking time is close to a multiple of the tidal period, the maximum concentration point will
be under the fish cage irrespective of the tide strength. This is due to the nature of the tidal current first propagating
the particles away and then bringing them back when the tide reverses. Increasing the cage size increases the likelihood for
a maximum waste accumulation beneath the fish farm, and larger farms usually means larger biomasses which can make the local
pollution even more severe. The model is validated by using an analytical model which uses an exact harmonic representation
of the tidal current, and the results show an excellent agreement. This study shows that the coupled ocean and particle model
can be used in more realistic applications to help estimating the local environmental impact due to fish farms. 相似文献
6.
The wave-induced velocity and pressure fields beneath a large amplitude internal solitary wave of depression propagating over
a smooth, flat, horizontal, and rigid boundary in a shallow two-layer fluid are computed numerically. A numerical ocean model
is utilised, the set-up of which is designed and tuned to replicate the previously published experimental results of Carr
and Davies (Phys Fluids 18(1):016,601–1–016,601–10, 2006). Excellent agreement is found between the two data sets and, in particular, the numerical simulation replicates the finding
of a reverse flow along the bed aft of the wave. The numerically computed velocity and pressure gradients confirm that the
occurrence of the reverse flow is a consequence of boundary layer separation in the adverse pressure gradient region. In addition,
vortices associated with the reverse flow are seen to form near the bed. 相似文献
7.
Abstract The Kyoto Protocol has an ambitious reporting and review system to assess Parties' compliance with their emission commitments. It is based on a ‘bottom-up’ approach; that is, each Party is required to submit detailed inventories of emissions and removals. This requires considerable resources and may still not detect all important cases of non-compliance. We consider the case for introducing ‘top-down’ methods; that is, independent inverse modelling methods that calculate probable emissions using measured concentrations of gases in the atmosphere and meteorological models. We argue that the top-down methods are at present too inaccurate, too cumbersome, and politically too problematic to serve as independent alternatives to the reported emission inventories for assessing compliance, although they could be useful in monitoring the global success of the protocol. We conclude that these top-down approaches may supplement the traditional emission inventories, in particular those dealing with fluorinated gases, thereby providing input for improving the emission inventory methods. 相似文献
8.
Hydro carbon fields beyond the shelf break are presently being explored and developed, which has increased the scientific
focus in this area. Measurements from the slopes reveal large variability in temperature and velocity, and some of the observed
events are due to interactions between large-amplitude oscillations of the thermocline and the topography. The present study
focuses on the strong currents that are generated near the seabed during shoaling and breaking of internal waves along shelf
slopes. The parameter regime used is similar to the one for the Nordic Seas. The results show that, during shoaling of large
internal waves along (gentle) slopes, the energy is transferred towards smaller scales and strong velocities (over 1 m s − 1) can be generated. To resolve all scales involved is still not feasible, and therefore, the model results are sensitive to
the grid size and the subgrid scale closure. 相似文献
9.
A three-dimensional non-linear, non-hydrostatic model in cross-sectional form is used to determine the factors influencing
the relative importance of the linear, non-hydrostatic and non-linear contributions to the internal wave energy flux in sill
regions due to tidal forcing. The importance of the free surface elevation term is also considered. Idealised topography representing
the sill at the entrance to Loch Etive, the site of a recent measurement programme, is used. Calculations show that the non-linear
terms in the energy flux become increasingly important as the sill Froude Number (F
s) increases and the sill aspect ratio is increased. The vertical profile of the stratification, in particular its value close
to the sill crest where internal waves are generated, has a significant influence on unsteady lee wave and mixed tidal–lee
wave generation and the non-linear contribution to the energy flux. Calculations show that as F
s increases, the energy flux due to the non-linear and non-hydrostatic terms increases more rapidly than the linear term. The
importance of the non-linear terms in the energy flux also increases as the sill aspect ratio is increased. Increasing the
buoyancy frequency reduces the contribution of the non-hydrostatic and non-linear terms to the total energy flux. Also, as
the buoyancy frequency is increased, this reduces unsteady lee wave and mixed tidal–lee wave generation. In essence, these
calculations show that the energy flux due to the non-hydrostatic and non-linear terms is appreciable in sill regions. 相似文献
10.
Jarle Berntsen Lie-Yauw Oey Tal Ezer Richard Greatbatch Huijie Xue Yasumasa Miyazawa 《Ocean Dynamics》2014,64(10):1531-1534