Modelling of fluid–solid interactions using an adaptive mesh fluid model coupled with a combined finite–discrete element model

Fluid–structure interactions are modelled by coupling the finite element fluid/ocean model ‘Fluidity-ICOM’ with a combined finite–discrete element solid model ‘Y3D’. Because separate meshes are used for the fluids and solids, the present method is flexible in terms of discretisation schemes used for each material. Also, it can tackle multiple solids impacting on one another, without having ill-posed problems in the resolution of the fluid’s equations. Importantly, the proposed approach ensures that Newton’s third law is satisfied at the discrete level. This is done by first computing the action–reaction force on a supermesh, i.e. a function superspace of the fluid and solid meshes, and then projecting it to both meshes to use it as a source term in the fluid and solid equations. This paper demonstrates the properties of spatial conservation and accuracy of the method for a sphere immersed in a fluid, with prescribed fluid and solid velocities. While spatial conservation is shown to be independent of the mesh resolutions, accuracy requires fine resolutions in both fluid and solid meshes. It is further highlighted that unstructured meshes adapted to the solid concentration field reduce the numerical errors, in comparison with uniformly structured meshes with the same number of elements. The method is verified on flow past a falling sphere. Its potential for ocean applications is further shown through the simulation of vortex-induced vibrations of two cylinders and the flow past two flexible fibres.

     

Negative Bias and Increased Variability in VOC Concentrations Using the HydraSleeve in Monitoring Wells

The HydraSleeve is a sampling device for collecting groundwater from the screened interval of a monitoring well without purging that uses a check valve to take in water over the first 3 to 5 feet of an upward pulling motion. If the check valve does not perform as expected, then the HydraSleeve has the potential to collect water from an incorrect depth interval, possibly above the screened interval of the well. We have evaluated volatile organic chemical (VOC) results from groundwater samples collected with the HydraSleeve sampler compared to other methods for sampling monitoring wells at three sites. At all three sites, lower VOC concentration results were observed for samples collected using the HydraSleeve. At two of these three sites, the low concentration sample results were most strongly associated with monitoring wells with more than 10 feet of water above the monitoring well‐screened interval. At the site with the largest dataset, the median bias for samples collected with HydraSleeve was ?20% (p < 0.001). At this site, a bias of ?26% (p < 0.001) was observed for the subset of monitoring wells with greater than 10 feet of water above the screened interval compared to a bias of ?7% (p = 0.21) for wells screened across the top of the water table. In addition to lower VOC concentrations, the monitoring records obtained using the HydraSleeve were more variable compared to monitoring records obtained using purge sampling methods, a characteristic that would make it more difficult to determine the long‐term concentration trend in the well.     

Evaluation of Long‐Term Performance and Sustained Treatment at Enhanced Anaerobic Bioremediation Sites

This study evaluated the long‐term performance of enhanced anaerobic bioremediation (EAB) at chlorinated solvent sites to determine if sustained treatment processes were helping to prevent concentration rebound. A database of groundwater concentration versus time records was compiled for 34 sites, with at least 3 years of posttreatment monitoring data (median = 4.7 years, range = 3.0 to 11.7 years). Long‐term performance was evaluated based on order‐of‐magnitude (OoM) changes in parent compound concentrations during various monitoring periods. Results indicate that, relative to the pretreatment concentration, a median concentration reduction for all 34 sites of 1.0 OoM (90% reduction) was achieved by the end of the posttreatment monitoring period. No rebound was observed at 65% of the sites between the first year of posttreatment monitoring and the final year. During this posttreatment period, Mann‐Kendall trend analysis indicated that the concentration was stable or decreasing at 89% of the sites where a trend could be established (n = 27; 33% decreasing, 56% stable, 11% increasing). Statistical analysis indicates there is no evidence that the distribution of median concentration reductions after the first year of posttreatment monitoring was different than the distribution of median reductions 2 to 11 years later at the end of the monitoring period (p = 0.67). Similarly, statistical analysis indicates that there is no evidence that the distribution of median reductions for a larger set of sites (n = 84) with less than 3 years of posttreatment monitoring data (1.1 OoM; 92% reduction) was different than the distribution of median OoM reductions for the 34‐site dataset with longer monitoring periods (p = 0.80). This suggests that, at a typical site, a 3‐year monitoring period should be sufficient for evaluating performance. The results of this study indicate that, in the long term, after the end of active treatment, sustained treatment processes contribute to relatively modest concentration reductions but do mitigate rebound at the majority of EAB sites.     

Testing the reliability of pollen-based diversity estimates

Rarefaction analysis is a common tool for estimating pollen richness. Using modern and fossil pollen data from the Canadian Arctic and Greenland, we examine the effects of pollen concentration (grains/cc) and evenness (the distribution of species abundances) on palynological richness. Our results show that pollen richness and concentration have a strong negative correlation at low pollen concentrations. There is a positive correlation between pollen evenness and richness, although the strength of this relationship is difficult to determine. Rarefaction analysis on samples of low concentration or high evenness is likely to lead to pollen richness being less underestimated than on samples of high concentration or low evenness. These findings corroborate theoretical research on these issues.     

Evidence for Long-term Retrograde Motions of Sunspot Patterns and Indications of Coupled g-mode Rotation Rates

Solar g-modes are global oscillations that would exist primarily in the radiative zone (RZ) and would be excited by either convective overshoot or nuclear burning in the core. Wolff and O’Donovan (Astrophys. J. 661, 568, 2007) proposed a non-linear coupling of g-modes into groups that share the same harmonic degree ℓ. Each group (denoted set(ℓ)) exhibits a unique retrograde rotation rate with respect to the RZ that depends mainly on ℓ. The coupling yields a standing wave (nearly stationary in longitude) that has two angularly defined hot spots offset from the equator on opposite sides of the Sun that would deposit energy asymmetrically in the lower convective envelope (CE). It is anticipated that when two or more groups overlap in longitude, an increase in local heating would influence the distribution of sunspots. In this paper, we scanned a multitude of rotational reference frames for sunspot clustering to test for frames that are concordant with the rotation of these g-modes sets. To achieve this, spherical harmonic filtering of sunspot synoptic maps was used to extract patterns consistent with coalesced g-modes. The latitude band, with minimal differential rotation, was sampled from each filtered synoptic map and layered into a stackplot. This was progressively shifted, line-by-line, into different rotational reference frames. We have detected long-lived longitudinal alignments, spanning 90 years of solar cycles, which are consistent with the rotation rate of the deep solar interior as well as other rotational frames predicted by the coupled g-mode model. Their sidereal rotation rates of 370.0, 398.8, 412.7, 418.3, 421.0, 424.2 and 430.0 nHz correspond, respectively, to coupled g-modes for ℓ = 2 through 7 and G, where G is a set with high ℓ values or a group of such sets (unresolved) that rotate almost as fast as the RZ. While the clustering in these reference frames offers new approaches for studying the longitudinal behavior of solar activity, it tentatively leads to the more profound conclusion that a portion of the driving force for sunspot occurrence is linked to energy extracted from the solar core and deposited at the top of the RZ by solar g-modes.     

Observational properties of accretion discs: Precessing and warped?

Over the last 40 years, X-ray astronomy missions have revealed long-term, superorbital periods in a variety of X-ray binaries. These modulations can provide significant constraints on the physical properties of accretion discs. Some of these modulations are Her X-1-like and are interpreted as irradiation-driven, tilted, precessing accretion discs. Others show more complex light curves, with the period changing on timescales >1000 d, and are interpreted in terms of the Ogilvie and Dubus [Ogilvie, G.I., Dubus, G., 2001, MNRAS 320, 485 (OD01)] disc stability criteria. We suggest a categorisation of superorbital periods into six different types, based on their observed characteristics.     

Global warming: a review of this mostly settled issue

Global warming and attendant climate change have been controversial for at least a decade. This is largely because of its societal implications since the science is largely straightforward. With the recent publication of the Fourth Assessment Report of the United Nations’ Intergovernmental Panel on Climate Change (Working Group 1) there has been renewed interest and controversy about how certain the scientific community is of its conclusions: that humans are influencing the climate and that global temperatures will continue to rise rapidly in this century. This review attempts to update what is known and in particular what advances have been made in the past 5 years or so. It does not attempt to be comprehensive. Rather it focuses on the most controversial issues, which are actually few in number. They are:

• Is the surface temperature record accurate or is it biased by heat from cities, etc.?
• Is that record significantly different from past warmings such as the Medieval Warming Period?
• Are human greenhouse gases changing the climate more than the sun?
• Can we model climate and predict its future, or is it just too complex and chaotic?
• Are there any other changes in climate other than warming, and can they be attributed to the warming?

Finally there is a very brief discussion of the societal policy response to the scientific message. Note that much of the introductory material in each section is essentially the same as that which appears in Keller 2003 (hereafter referred to as OR = original review) and its update (Keller 2007). Despite continued uncertainties, the review finds an affirmative answer to these questions. Of particular interest are advances that seem to explain why satellites do not see as much warming as surface instruments, how we are getting a good idea of recent paleo-climates, and why the twentieth century temperature record was so complex. It makes the point that in each area new information could come to light that would change our thinking on the quantitative magnitude and timing of anthropogenic warming, but it is unlikely to alter the basic conclusions.     

Kalman filter-based algorithms for monitoring the ionosphere and plasmasphere with GPS in near-real time

Data collected from a GPS receiver located at low latitudes in the American sector are used to investigate the performance of the WinTEC algorithm [Anghel et al., 2008a, Kalman filter-based algorithm for near realtime monitoring of the ionosphere using dual frequency GPS data. GPS Solutions, accepted for publication; for different ionospheric modeling techniques: the single-shell linear, quadratic, and cubic approaches, and the multi-shell linear approach. Our results indicate that the quadratic and cubic approaches perform much better than the single-shell and multi-shell linear approaches in terms of post-fit residuals. The performance of the algorithm for the cubic approach is then further tested by comparing the vertical TEC predicted by WinTEC and USTEC [Spencer et al., 2004. Ionospheric data assimilation methods for geodetic applications. In: Proceedings of IEEE PLANS, Monterey, CA, 26–29 April, pp. 510–517] at five North American stations. In addition, since the GPS-derived total electron content (TEC) contains contributions from both ionospheric and plasmaspheric sections of the GPS ray paths, in an effort to improve the accuracy of the TEC retrievals, a new data assimilation module that uses background information from an empirical plasmaspheric model [Gallagher et al., 1988. An empirical model of the Earth's plasmasphere. Advances in Space Research 8, (8)15–(8)24] has been incorporated into the WinTEC algorithm. The new Kalman filter-based algorithm estimates both the ionospheric and plasmaspheric electron contents, the combined satellite and receiver biases, and the estimation error covariance matrix, in a single-site or network solution. To evaluate the effect of the plasmaspheric component on the estimated biases and total TEC and to assess the performance of the newly developed algorithm, we compare the WinTEC results, with and without the plasmaspheric term included, at three GPS receivers located at different latitudes in the American sector, during a solar minimum period characterized by quiet and moderate geomagnetic conditions. We also investigate the consistency of our plasmaspheric results by taking advantage of the specific donut-shaped geometry of the plasmasphere and applying the technique at 12 stations distributed roughly over four geomagnetic latitudes and three longitude sectors.     

Arctic microbial ecosystems and impacts of extreme warming during the International Polar Year

As a contribution to the International Polar Year program MERGE (Microbiological and Ecological Responses to Global Environmental change in polar regions), studies were conducted on the terrestrial and aquatic microbial ecosystems of northern Canada (details at: http://www.cen.ulaval.ca/merge/). The habitats included permafrost soils, saline coldwater springs, supraglacial lakes on ice shelves, epishelf lakes in fjords, deep meromictic lakes, and shallow lakes, ponds and streams. Microbiological samples from each habitat were analysed by HPLC pigment assays, light and fluorescence microscopy, and DNA sequencing. The results show a remarkably diverse microflora of viruses, Archaea (including ammonium oxidisers and methanotrophs), Bacteria (including filamentous sulfur-oxidisers in a saline spring and benthic mats of Cyanobacteria in many waterbodies), and protists (including microbial eukaryotes in snowbanks and ciliates in ice-dammed lakes). In summer 2008, we recorded extreme warming at Ward Hunt Island and vicinity, the northern limit of the Canadian high Arctic, with air temperatures up to 20.5 °C. This was accompanied by pronounced changes in microbial habitats: deepening of the permafrost active layer; loss of perennial lake ice and sea ice; loss of ice-dammed freshwater lakes; and 23% loss of total ice shelf area, including complete break-up and loss of the Markham Ice Shelf cryo-ecosystem. These observations underscore the vulnerability of Arctic microbial ecosystems to ongoing climate change.     

Evolution of overland flow connectivity in bare agricultural plots

Soil surface roughness not only delays overland flow generation but also strongly affects the spatial distribution and concentration of overland flow. Previous studies generally aimed at predicting the delay in overland flow generation by means of a single parameter characterizing soil roughness. However, little work has been done to find a link between soil roughness and overland flow dynamics. This is made difficult because soil roughness and hence overland flow characteristics evolve differently depending on whether diffuse or concentrated erosion dominates. The present study examined whether the concept of connectivity can be used to link roughness characteristics to overland flow dynamics. For this purpose, soil roughness of three 30‐m² tilled plots exposed to natural rainfall was monitored for two years. Soil micro‐topography was characterized by means of photogrammetry on a monthly basis. Soil roughness was characterized by the variogram, the surface stream network was characterized by network‐based indices and overland flow connectivity was characterized by Relative Surface Connection function (RSCf) functional connectivity indicator. Overland flow hydrographs were generated by means of a physically‐based overland flow model based on 1‐cm resolution digital elevation models. The development of eroded flow paths at the soil surface not only reduced the delay in overland flow generation but also resulted in a higher continuity of high flow velocity paths, an increase in erosive energy and a higher rate of increase of the overland flow hydrograph. Overland flow dynamics were found to be highly correlated to the RSCf characteristic points. By providing information regarding overland flow dynamics, the RSCf may thus serve as a quantitative link between soil roughness and overland flow generation in order to improve the overland flow hydrograph prediction. Copyright © 2016 John Wiley & Sons, Ltd.