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21.
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
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Jarle Brinchmann † Max Pettini Stéphane Charlot 《Monthly notices of the Royal Astronomical Society》2008,385(2):769-782
We have used extensive libraries of model and empirical galaxy spectra [assembled, respectively, from the population synthesis code of Bruzual and Charlot and the fourth data release of the Sloan Digital Sky Survey (SDSS)] to interpret some puzzling features seen in the spectra of high-redshift star-forming galaxies. We show that a stellar He ii λ1640 emission line, produced in the expanding atmospheres of Of and Wolf–Rayet stars, should be detectable with an equivalent width of 0.5–1.5 Å in the integrated spectra of star-forming galaxies, provided the metallicity is greater than about half solar. Our models reproduce the strength of the He ii λ1640 line measured in the spectra of Lyman-break galaxies for established values of their metallicities. With better empirical calibrations in local galaxies, this spectral feature has the potential of becoming a useful diagnostic of massive star winds at high, as well as low redshifts.
We also uncover a relationship in SDSS galaxies between their location in the [O iii ]/Hβ versus [N ii ]/Hα diagnostic diagram (the BPT diagram) and their excess specific star formation rate relative to galaxies of similar mass. We infer that an elevated ionization parameter U is at the root of this effect, and propose that this is also the cause of the offset of high-redshift star-forming galaxies in the BPT diagram compared to local ones. We further speculate that higher electron densities and escape fractions of hydrogen ionizing photons may be the factors responsible for the systematically higher values of U in the H ii regions of high-redshift galaxies. The impact of such differences on abundance determinations from strong nebular lines are considered and found to be relatively minor. 相似文献
We also uncover a relationship in SDSS galaxies between their location in the [O iii ]/Hβ versus [N ii ]/Hα diagnostic diagram (the BPT diagram) and their excess specific star formation rate relative to galaxies of similar mass. We infer that an elevated ionization parameter U is at the root of this effect, and propose that this is also the cause of the offset of high-redshift star-forming galaxies in the BPT diagram compared to local ones. We further speculate that higher electron densities and escape fractions of hydrogen ionizing photons may be the factors responsible for the systematically higher values of U in the H ii regions of high-redshift galaxies. The impact of such differences on abundance determinations from strong nebular lines are considered and found to be relatively minor. 相似文献
25.
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
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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
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Jarle Berntsen Lie-Yauw Oey Tal Ezer Richard Greatbatch Huijie Xue Yasumasa Miyazawa 《Ocean Dynamics》2014,64(10):1531-1534
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Per Jarle Sætre 《Norsk geografisk tidsskrift. Norwegian journal of geography》2013,67(3):120-127
At the end of the 19th century geography became a mandatory subject in primary and secondary schools in Norway. Geography was seen as a useful subject that contributed to the modernization of society, but also as a subject that strengthened the national ideology. A need for better education in geography arose and consequently some authors of geography textbooks wrote about teaching geography. In order to strengthen teachers’ education, geography was taught at university level, and to develop the subject it became necessary to relate it to other subjects, especially geology. The author shows that there are several similarities between the content of geography taught in Norwegian schools in the late 19th century and geography taught in schools today. 相似文献
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Estimation of the internal pressure gradient in σ-coordinate ocean models: comparison of second-, fourth-, and sixth-order schemes 总被引:1,自引:1,他引:0
Sigma-coordinate ocean models are attractive because of their abilities to resolve bottom and surface boundary layers. However,
these models can have large internal pressure gradient (IPG) errors. In this paper, two classes of methods for the estimation
of the IPGs are assessed. The first is based on the integral approach used in the Princeton Ocean Model (POM). The second
is suggested by Shchepetkin and McWilliams (2003) based on Green’s theorem; thus, area integrals of the pressure forces are transformed into line integrals. Numerical tests
on the seamount problem, as well as on a northwestern Atlantic grid using both classes of methods, are presented. For each
class, second-, fourth-, and sixth-order approximations are tested. Results produced with a fourth-order compact method and
with cubic spline methods are also given. The results show that the methods based on the POM approach in general give smaller
errors than the corresponding methods given in Shchepetkin and McWilliams (2003). The POM approach also is more robust when noise is added to the topography. In particular, the IPG errors may be substantially
reduced by using the computationally simple fourth-order method from McCalpin (1994). 相似文献