An element-based displacement preconditioner for linear elasticity problems

Finite element analysis of problems in structural and geotechnical engineering results in linear systems where the unknowns are displacements and rotations at nodes. Although the solution of these systems can be carried out using either direct or iterative methods, in practice the matrices involved are usually very large and sparse (particularly for 3D problems) so an iterative approach is often advantageous in terms of both computational time and memory requirements. This memory saving can be further enhanced if the method used does not require assembly of the full coefficient matrix during the solution procedure. One disadvantage of iterative methods is the need to apply preconditioning to improve convergence. In this paper, we review a range of established element-based preconditioning methods for linear elastic problems and compare their performance with a new method based on preconditioning with element displacement components. This new method appears to offer a significant improvement in performance.     

Reliable eye-diagram analysis of data links via device macromodels

This paper addresses the impact of device macromodels on the accuracy of signal integrity and performance predictions for critical digital interconnecting systems. It exploits nonlinear parametric models for both single-ended and differential devices, including the effects of power supply fluctuations and receiver bit detection. The analysis demonstrates that the use of well-designed macromodels dramatically speeds up the simulation as well it preserves timing accuracy even for long bit sequences.     

PowerNap: an efficient power management scheme for mobile devices

We present PowerNap, an OS power management scheme, which can significantly improve the battery life of mobile devices. The key feature of PowerNap is the skipping of the periodic system timer ticks associated with the operating system. On an idle device, this modification increases the time between successive timer interrupts and enables us to put the processor/system into a more efficient low power state. This saves the energy consumed by workless timer interrupts and the excess energy consumed by the processor in less efficient low power states. PowerNap is tightly integrated with the kernel and is designed for optimal control of the latency and energy associated with transitioning in and out of the low power states. We describe an implementation of PowerNap and its impact on system software. Experiments with IBM's WatchPad verify the ability of PowerNap to extend battery life. An analytical model that quantifies the ability of the scheme to reduce power is also presented. The model is in good agreement with experimental results. We apply the model to small form-factor devices which use processors that have a PowerDown state. In such devices, PowerNap may extend battery life by more than 42 percent for small processor workloads and for background power levels below 10 mW.     

Beneficial effects of wettability altering surfactants in oil-wet fractured reservoirs

In fractured reservoirs, an effective matrix-fracture mass transfer is required for oil recovery. Surfactants have long been considered for oil recovery enhancement, mainly in terms of their ability to reduce oil–water interfacial tension. These surfactants are effective when the fractured formations are water-wet, where capillary imbibition of surfactants from the fracture into the matrix contributes to oil recovery. However, another beneficial aspect of surfactants, namely their ability to alter wettability, remains to be explored and exploited. Surfactants capable of altering wettability can be especially beneficial in oil-wet fractured formations, where the surfactant in the fracture diffuses into the matrix and alters the wettability, enabling imbibition of even more surfactant into the matrix. This sequential process of initial diffusion followed by imbibition continues well into the matrix yielding significant enhancements in oil recovery.In order to test this hypothesis of sequential diffusion–imbibition phenomenon, Dual-Drop Dual-Crystal (DDDC) contact angle experiments have been conducted using fractured Yates dolomite reservoir fluids, two types of surfactants (nonionic and anionic) and dolomite rock substrates. A new experimental procedure was developed in which crude oil equilibrated with reservoir brine has been exposed to surfactant to simulate the matrix-fracture interactions in fractured reservoirs. This procedure enables the measurements of dynamic contact angles and oil–water interfacial tensions, in addition to providing the visual observations of the dynamic behavior of crude oil trapped in the rock matrix as it encounters the diffusing surfactant from the fractures. Both the measurements and visual observations indicate wettability alterations of the matrix surface from oil-wet to less oil-wet or intermediate wet by the surfactants. Thus this study is of practical importance to oil-wet fractured formations where surfactant-induced wettability alterations can result in significant oil recovery enhancements. In addition, this study has also identified the need to include contact angle term in the dimensionless Bond number formulations for better quantitative interpretation of rock–fluids interactions.     

Identification of the adverse effect of nitrate on the phosphate release rate and improvement of EBPR process models.

The adverse effect of nitrate on the phosphate release rate in the anaerobic phase was observed and was hardly explainable with conventional EBPR process models. Four possible mechanisms were proposed including substrate competition, reduced fermentation, parallel reaction and sequential reaction. Batch experiments were designed and conducted to identify the dominant mechanism. Results showed that the sequential reaction was the only possible mechanism where only denitrification occurred if any nitrate existed in the anaerobic phase. Then the phosphate release following after the nitrate was completely removed. Nitrate inhibition effect was added into the PHA storage rate to incorporate the sequential reaction in the conventional ASM3 plus EAWAG bio-P module (ASM3 + P). Nitrate inhibition coefficient, K(I,NO,PAO) was found to be as low as 0.05 mg/L. This correlated well with experimental observation where no also meant that the anaerobic compartment of a continuous flow reactor could be seriously affected by the residual nitrate contained in the sludge recycle flow. This phenomenon caused overestimation of the phosphate uptake rate and consequently underestimation of PO4(3-) -P concentration. This problem was resolved by incorporation of a nitrate inhibition term in the ASM3 + P for more accurate simulation of the EBPR process.     

WWTP dynamic disturbance modelling--an essential module for long-term benchmarking development.

Intensive use of the benchmark simulation model No. 1 (BSM1), a protocol for objective comparison of the effectiveness of control strategies in biological nitrogen removal activated sludge plants, has also revealed a number of limitations. Preliminary definitions of the long-term benchmark simulation model No. 1 (BSM1_LT) and the benchmark simulation model No. 2 (BSM2) have been made to extend BSM1 for evaluation of process monitoring methods and plant-wide control strategies, respectively. Influent-related disturbances for BSM1_LT/BSM2 are to be generated with a model, and this paper provides a general overview of the modelling methods used. Typical influent dynamic phenomena generated with the BSM1_LT/BSM2 influent disturbance model, including diurnal, weekend, seasonal and holiday effects, as well as rainfall, are illustrated with simulation results. As a result of the work described in this paper, a proposed influent model/file has been released to the benchmark developers for evaluation purposes. Pending this evaluation, a final BSM1_LT/BSM2 influent disturbance model definition is foreseen. Preliminary simulations with dynamic influent data generated by the influent disturbance model indicate that default BSM1 activated sludge plant control strategies will need extensions for BSM1_LT/BSM2 to efficiently handle 1 year of influent dynamics.     

Design of a retention tank: comparison of stormwater quality models with various levels of complexity.

Stormwater quality simulation models are useful tools for the design and management of sewer systems. Modelling results are highly sensitive to experimental data used for calibration. This sensitivity is examined for three modelling approaches of various complexities (site mean concentration approach, event mean concentration approach and build-up, washoff and transport modelling approach) applied to a typical case study (design of a dry detention tank), accounting for the variability of calibration data and their effect on simulation results. Calibrated models with different calibration data sets were used to simulate 3 years of rainfall with different retention tank specific volumes. Annual pollutant load interception efficiencies were determined. Simulations results revealed (i) that there is no advantage in using the EMC model compared to the SMC model and (ii) that the BWT model resulted in higher design ratios than those given by the SMC/hydraulic approach. For both EMC and BWT models, using an increasing number n of events for calibration leads to narrower confidence intervals for the design ratios. It is crucial for design ratios to account for successive storm events in chronological order and to account for the maximum allowable flow to be transferred to the downstream WWTP.     

An inventory analysis of rural pollution loads in China.

Rural pollution has attracted increasing attention over the past decade for its important consequences on surface and groundwater quality. This is particularly so in China due to the wide and effective control of industrial and municipal wastewater discharges over the past decade. Based on the discussion of rural activities in China, this paper develops a new method of inventory analysis through the definition of elementary unit (EU). This inventory analysis approach is then applied to estimate the total rural pollution loads of COD, TN and TP, as well as their spatial distributions in China. Further analysis is also conducted to investigate the correlations between rural pollution loads and agricultural production outputs, as well as population. It is found that China's agriculture is developing largely at the cost of environment quality, and a high population, as well as the increasing demand for agricultural products, is one of the dominant driving forces. The constructed EKC, which describes the relationship between rural pollution loads and agricultural income, also reveals that current agricultural development in China is mostly far from de-locked from the environmental problem.     

The Applied Mathematics and Computer Science Schism

Isolating applied mathematics from computer science harms the profession and those who depend on it.     

Comprehensive heat exchange model for a semiconductor laser diode

By measuring the total energy flow from an optical device, we can develop new design strategies for thermal stabilization. Here we present a comprehensive model for heat exchange between a semiconductor laser diode and its environment that includes the mechanisms of conduction, convection, and radiation. We perform quantitative measurements of these processes for several devices, deriving parameters such as a laser's heat transfer coefficient, and then demonstrate the feasibility of thermal probing for the nondestructive wafer-scale characterization of optical devices.