Solid–liquid separation of tailings slurries in gravity thickeners relies on the efficient mixing of slurry and dilute polymer flocculant solutions within the feedwell. Computational fluid dynamics (CFD) can provide predictions of solids distribution, liquor velocity and shear rate within a feedwell, and with the incorporation of an adsorption model, can also assess the effectiveness of flocculant mixing. This study presents the first use of feedwell CFD to examine the effect of the flocculant inlet direction and velocity on the subsequent distribution and adsorption of flocculant. When flocculant is injected inside the feed stream, a high inlet nozzle velocity will maximise adsorption, with injection preferably vertically upward or towards the feedwell walls. For injection inside the dilution stream (vertical upflow of liquor within the feedwell), the flocculant should be directed either upwards or inwards away from the strong downward flow of the feed stream, with the velocity not critical.At flocculant inlet velocities predicted by CFD to enhance mixing and adsorption, the shear rate experienced within the injection pipe exceeds that in the feedwell, and the duration under higher shear may be greater. Pipe flow studies for several flocculants have confirmed reductions in activity at a solution concentration of 0.025 wt.%; this effect diminishes with greater dilution. Much of this lost activity is recovered on standing, indicating that the applied shear leads to a mixture of chain scission (irreversible) and entanglement (reversible). Minimising the duration of such shear effects on flocculant solution transport to the feedwell is essential, as the potential for increased flocculant demand and reduced flocculation efficiency can easily exceed any benefit from improved feedwell mixing. 相似文献
With sea levels projected to rise as a result of climate change, it is imperative to understand not only long-term average trends, but also the spatial and temporal patterns of extreme sea level. In this study, we use a comprehensive set of 30 tide gauges spanning 1954–2014 to characterize the spatial and temporal variations of extreme sea level around the low-lying and densely populated margins of the South China Sea. We also explore the long-term evolution of extreme sea level by applying a dynamic linear model for the generalized extreme value distribution (DLM-GEV), which can be used for assessing the changes in extreme sea levels with time. Our results show that the sea-level maxima distributions range from ~?90 to 400 cm and occur seasonally across the South China Sea. In general, the sea-level maxima at northern tide gauges are approximately 25–30% higher than those in the south and are highest in summer as tropical cyclone-induced surges dominate the northern signal. In contrast, the smaller signal in the south is dominated by monsoonal winds in the winter. The trends of extreme high percentiles of sea-level values are broadly consistent with the changes in mean sea level. The DLM-GEV model characterizes the interannual variability of extreme sea level, and hence, the 50-year return levels at most tide gauges. We find small but statistically significant correlations between extreme sea level and both the Pacific Decadal Oscillation and El Niño/Southern Oscillation. Our study provides new insight into the dynamic relationships between extreme sea level, mean sea level and the tidal cycle in the South China Sea, which can contribute to preparing for coastal risks at multi-decadal timescales.
The area of coastal rivers with a combination of fluvial, tidal and wave processes is defined as the fluvial to marine transition zone and can extend up to several hundreds of kilometres upstream of the river mouth. The aim of this study is to improve the understanding of sediment distribution and depositional processes along the fluvial to marine transition zone using a comprehensive dataset of channel bed sediment samples collected from the Mekong River delta. Six sediment types were identified and were interpreted to reflect the combined action of fluvial and marine processes. Based on sediment‐type associations, the Mekong fluvial to marine transition zone could be subdivided into an upstream tract and a downstream tract; the boundary between these two tracts is identified 80 to 100 km upstream of the river mouth. The upstream tract is characterized by gravelly sand and sand and occasional heterolithic rhythmites, suggesting bed‐load supply and deposition mainly controlled by fluvial processes with subordinate tidal influence. The downstream tract is characterized by heterolithic rhythmites with subordinate sand and mud, suggesting suspended‐load supply and deposition mainly controlled by tidal processes with subordinate fluvial influence. Sediment distributions during wet and dry seasons suggest significant seasonal changes in sediment dynamic and depositional processes along the fluvial to marine transition zone. The upstream tract shows strong fluvial depositional processes with subordinate tidal influence during the wet season and no deposition with weak fluvial and tidal processes during the dry season. The downstream tract shows strong coexisting fluvial and tidal depositional processes during the wet season and strong tidal depositional processes with negligible fluvial influence during the dry season. Turbidity maxima are present along the downstream tract of the fluvial to marine transition zone during both wet and dry seasons and are driven by a combination of fluvial, tidal and wave processes. 相似文献
A small-scale pile has been developed in the laboratory to investigate the thermo-mechanical behavior of energy piles subjected to a significant number of thermal cycles. The pile (20 mm external diameter), installed in dry sand, was initially loaded at its head to 0, 20, 40 and 60% of its ultimate bearing capacity (500 N). At the end of each loading step, 30 heating/cooling cycles were applied. The long-term behavior of the pile was observed in terms of head settlement, axial force profile, soil and pile temperature, and stress in soil. The results evidence the irreversible settlement of the pile head induced by thermal cycles under constant load head. In addition, the incremental irreversible settlement that accumulates after each thermal cycle decreases when the number of cycles increases. The evolution of irreversible pile head settlement versus number of cycles can be reasonably predicted by an asymptotic equation. 相似文献
In this paper, a comparative modelling exercise from the DECOVALEX-2015 project is presented. The exercise is based on in situ experiments, performed at the Tournemire Underground Research Laboratory (URL), run by the IRSN (Institut de Radioprotection et de Sûreté Nucléaire), in France. These experiments aim at identifying conditions (e.g. technical specifications, design, construction, and defects) that will affect the long-term performance of swelling clay-based sealing systems, which is of key importance for the safety of underground nuclear waste disposal facilities. A number of materials are being considered as seals; the current work focusses on a 70/30 MX80 bentonite–sand mixture initially compacted at a dry density of 1.94 Mg/m3. The performance of the sealing plug involves at least three different important components, which are the hydro-mechanical behaviour of the bentonite–sand core, the overall permeability of the surrounding argillite, and the influence of the technological gap between the core and the argillite. Two particular tests have been selected for a comparative modelling exercise: the WT-1 test, which was designed to study the rock mass permeability, and the PT-A1 test, which aimed at quantifying the evolution of the hydro-mechanical field within the bentonite–sand core. A number of independent teams have worked towards modelling these experiments, using different codes and input parameters calibrated on additional small-scale laboratory experiments. Their results are compared and discussed. 相似文献
The application of high resolution seismic data using boomer sound source has revealed a wide distribution of large-scale
bedforms (sandwaves) on the Southeast Vietnam continental shelf. Bedforms that are a few meters high in wave height and hundreds
of meters long in wavelength are primarily developed in the inner shelf (20–40 m) and considered to be formed under the present-day
marine hydrodynamic conditions. Those bedforms developed in the deeper water (120 m) of the northernmost part of the continent
can be interpreted as the relict morphological features formed during the latest sea-level lowstand of the late Pleistocene
period. Two sediment transport paths have been identified on the basis of the bedform’s leeward orientation: northeast-southwest
(along-shore) and north-south (cross-shore). A quantitative bottom current map is constructed from sandwave dimensions, surface
sediments and measurement data. The strongest current velocities that gradually decrease toward the southwest are indicated
by large sandwaves in the north (field B). Water depth, surficial sediment composition and bottom current are three factors
that control the development of bedforms. 相似文献
The doses of gamma radiation, concentrations of radium isotopes in water and sediments, radon concentration and concentration
of alpha potential energy of radon decay products in the copper ore mine and in the mining region in the vicinity of Lubin
town in Poland are presented. These data served as a basis for the assessment of radiological hazard to the mine workers and
general public. The results of this assessment indicate that radiological hazard in the region does not differ substantially
from typical values associated with natural radiation background. The calculated average annual effective dose for copper
miners is 1.48 mSv. In general, copper ore mines can be regarded as radiologically safe workplaces. 相似文献
The line-fitting methods such as the Hvorslev method and the Bouwer and Rice method provide a rapid and simple means to analyze slug test data for estimating in situ hydraulic conductivity ( k ) of geologic materials. However, when analyzing a slug test in a relatively compressible geologic formation, these conventional methods may have difficulties fitting a straight line to the semilogarithmic plot of the test data. Data from relatively compressible geologic formations frequently show a concave-upward curvature because of the effect of the compressibility or specific storage ( S s). To take into account the compressibility of geologic formations, a modified line-fitting method is introduced, which expands on Chirlin's (1989) approach to the case of a partially penetrating well with the basic-time-lag fitting method. A case study for a compressible till is made to verify the proposed method by comparing the results from the proposed methods with those obtained using a type-curve method (Kansas Geological Survey method [ Hyder et al. 1994 ]). 相似文献
This paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed. 相似文献