Calculation of primary and secondary flow and boundary shear stresses in a meandering channel

Turbulent flow in a meandering channel is computed with two Computational Fluid Dynamics (CFD) codes solving the Navier–Stokes equations by employing different turbulence closure approaches. The first CFD code solves the steady Reynolds-Averaged Navier–Stokes equations (RANS) using an isotropic turbulence closure. The second code is based on the concept of Large Eddy Simulation (LES). LES resolves the large-scale turbulence structures in the flow and is known to outperform RANS models in flows in which large-scale structures dominate the statistics. The results obtained from the two codes are compared with experimental data from a physical model study. Both, LES and RANS simulation, predict the primary helical flow pattern in the meander as well as the occurrence of an outer-bank secondary cell. Computed primary as well as secondary flow velocities are in reasonably good agreement with experimental data. Evidence is given that the outer-bank secondary cell in a meander bend is the residual of the main secondary cell of the previous bend. However, the RANS code, regardless of the turbulence model employed, overpredicts the size and strength of the outer-bank secondary cell. Furthermore, only LES is able to uphold the outer-bank second secondary cell beyond the bend apex until the exit of the bend as turbulence anisotropy contributes to its persistence. The presence of multiple secondary cells has important consequences for the distribution of shear stresses along the wetted perimeter of the channel, and thereby the sediment transport in meandering channels. Consequently, even though LES is expected to compute the bed-shear stresses along the wetted perimeter of the channel with a higher degree of accuracy than the RANS model, comparisons between LES and RANS computed wall shear stresses agree well. These findings are useful for practitioners who need to rely on RANS model predictions of the flow in meandering channels at field scale.     

Depth colonization of eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis>) and macroalgae as determined by water transparency in Danish coastal waters

We present a comparative analysis of lower depth limits for growth of eelgrass, large brown algae and other macroalgae measured by SCUBA-diving along 162 transects in 27 Danish fjords and coastal waters, coupled to 1,400 data series of water chemistry (especially nitrogen) and Secchi depth transparency collected between March and October. Danish coastal waters are heavily eutrophied and characterized by high particle concentrations, turbid water and lack of macrophyte growth in deep water. Median values are 3.6 m for Secchi depth and median lower-depth limits are 4.0 m for eelgrass, 5.3 m for brown algae and 5.0 m for other macroalgae. Depth limits for growth of eelgrass and macroalgae increase linearly with transparency in the coastal waters. The relationships are highly significant (p<10⁻⁶) and transparency accounts for about 60% of the variability of depth limits. Eelgrass extends approximately to half the maximum depth of macroalgae, presumably because of greater respiratory costs to maintain the below-ground rhizomes and roots of eelgrass, which often constitutes half the plant weight. As a reflection of the importance of total nitrogen (TN) in controlling phytoplankton biomass and thus Secchi depth in coastal marine waters, we found that TN could explain 48–73% of the variation in depth limits of eelgrass and macroalgae, according to a multiplicative model (Y=aX^b). As with Secchi depth, the relationship to eelgrass showed a lower slope, reflecting the higher respiratory costs of eelgrass. The models show great sensitivity and a profound quantitative response with proportional effects on Secchi depth and depth limits when total-N concentrations are reduced.     

The dogma of matters neutrino-transparency

In this paper the observed underrepresentation of solar neutrinos in the various detector experiments performed is tentatively interpreted as a result of a substantial and often variable obsorption of neutrinos by various layers of the Sun, contrary to the dogma of neutrino-transparency by matter.     

Mesozoic palaeomagnetism in Vestfjella, Dronning Maud Land, East Antarctica

Summary. Thermal and alternating field demagnetization of oriented samples of Mesozoic lava flows (200–230 Myr) and dykes (154–172Myr) collected from seven nunataks within the Vestfjella region, Dronning Maud Land, revealed stable directions of magnetization of normal and reverse polarity. Directional distributions of both polarities define tight groups along the same palaeomagnetic axis. Depending on whether the regional westward dip of the lava flows originated prior to, or after the intrusive igneous phase, two or one, respectively, palaeomagnetic pole(s) can be estimated. Both poles, however, are in general accord with previous Mesozoic poles from East Antarctica.     

The reactivity of iron oxides towards reductive dissolution with ascorbic acid in a shallow sandy aquifer (Rømø, Denmark)

The pool of iron oxides, available in sediments for reductive dissolution, is usually estimated by wet chemical extraction methods. Such methods are basically empirically defined and calibrated against various synthetic iron oxides. However, in natural sediments, iron oxides are present as part of a complex mixture of iron oxides with variable crystallinity, clays and organics etc. Such a mixture is more accurately described by a reactive continuum covering a range from highly reactive iron oxides to non-reactive iron oxide. The reactivity of the pool of iron oxides in sediment can be determined by reductive dissolution in 10 mM ascorbic acid at pH 3. Parallel dissolution experiments in HCl at pH 3 reveal the release of Fe(II) by proton assisted dissolution. The difference in Fe(II)-release between the two experiments is attributed to reductive dissolution of iron oxides and can be quantified using the rate equation J/m₀ = k′(m/m₀)^γ, where J is the overall rate of dissolution (mol s⁻¹), m₀ the initial amount of iron oxide, k′ a rate constant (s⁻¹), m/m₀ the proportion of undissolved mineral and γ a parameter describing the change in reaction rate over time. In the Rømø aquifer, Denmark, the reduction of iron oxides is an important electron accepting process for organic matter degradation and is reflected by the steep increase in aqueous Fe²⁺ over depth. Sediment from the Rømø aquifer was used for reductive dissolution experiments with ascorbic acid. The rate parameters describing the reactivity of iron oxides in the sediment are in the range k′ = 7·10⁻⁶ to 1·10⁻³ s⁻¹ and γ = 1 to 2.4. These values are intermediate between a synthetic 2-line ferrihydrite and a goethite. The rate constant increases by two orders of magnitude over depth suggesting an increase in iron oxide reactivity with depth. This increase was not captured by traditional oxalate and dithionite extractions.     

A summary of the PRUDENCE model projections of changes in European climate by the end of this century

An overview of the PRUDENCE fine resolution climate model experiments for Europe is presented in terms of their climate change signals, in particular 2-meter temperature and precipitation. A comparison is made with regard to the seasonal variation in climate change response of the different models participating in the project. In particular, it will be possible to check how representative a particular PRUDENCE regional experiment is of the overall set in terms of seasonal values of temperature and precipitation. This is of relevance for such further studies and impact models that for practical reasons cannot use all the PRUDENCE regional experiments. This paper also provides some guidelines for how to select subsets of the PRUDENCE regional experiments according to such main sources of uncertainty in regional climate simulations as the choice of the emission scenario and of the driving global climate model.     

Complete remagnetization of some Permian dykes from western Norway induced during burial/uplift

Stable paleomagnetic directions in four basaltic dykes and in some associated Caledonian metamorphic rocks define high-latitude, Mesozoic paleomagnetic pole positions which are not compatible with the K---Ar age of 250 My determined for the dykes. A monotonic increase of 40% occurs in the potassium content of samples taken across a dyke 32 cm wide. This is accompanied by only a 1–2% variation in the K---Ar age of the samples, suggesting the absence of any significant level of initial argon. Titanomagnetite grains having bulk compositions around x = 0.6 have suffered extensive low-temperature alteration, forming assemblages of ferri-rutile granules in a matrix of pure magnetite. The complete remagnetisation of both the dykes and the associated country rocks is probably an expression of a VRM acquired at elevated temperatures (150–500°C) at the emplacement depth of the dykes. A stable remanent magnetisation was locked-in during uplift of the area, probably related to the Kimmerian basin development in the adjacent North Sea and the epeirogenic uplift of western Fennoscandia.     

中国黄土磁学与古气候(英文)

近二十年来中国黄土环境磁学的研究基本上是沿着两条主线进行的 ,即磁性地层学和古气候研究两个方面。对早先确定的典型黄土剖面的磁性地层研究表明风成黄土的堆积开始于Matuyama的下界 ,约 2 .6Ma。而近年来对黄土古土壤序列之下的红粘土的进一步研究显示风成堆积的开始比原先认为的要早的多 ,至少可追溯到约 7Ma。尽管目前在古土壤磁化率的增强机制和磁化率如何记录古气候变化等问题上还没有达成广泛的一致 ,而黄土古土壤系列磁化率与深海氧同位素的良好对比可证实中国黄土确是记录全球气候变化非常理想的信息载体。近年来许多学者尝试性提出的其它古气候替代性指标 ,如全岩粒度、稳定同位素组成、FeD/FeT指标、Rb/Sr比值和CaCO3 含量等必将为重建古季风变化和古气候提供重要信息。