State-space forms for higher-order discrete-time models

This paper presents a fundamental study of the connection between continuous- and discrete-time systems. Provided is a definition for discrete-time models, that is discrete-time systems with a continuous-time counterpart, whose order can be higher than that of the continuous-time system. This definition is based on a comparison in a certain sense on the time responses of continuous- and discrete-time systems. A theorem is presented for relating the higher-order discrete-time models to their continuous-time counterparts, which is an extension of a previous theorem for models with order equal to that of the continuous-time system. State-space forms are derived for models obtained through the use of a certain class of hold elements and through the use of mapping models, and these discrete-time systems are shown to be valid according to the definition. Special cases are models obtained using first-order and slewer hold devices, whose convergence to a continuous-time counterpart has not been shown mathematically before, and mapping models corresponding to two-step linear multi-step methods, which have not previously presented in the state-space form. The derived state-space forms provide a convenient way to implement these models for purposes of analysis, design, and implementation of discrete-time systems and finds applications in such areas as digital signal processing, digital simulation, and digital control.     

Hitting the wall: errors in developing and code inspecting a `simple' spreadsheet model

Field audits and experiments have found substantial error rates when students and professionals have built spreadsheet models. In this study, 102 undergraduate MIS majors and 50 MBA students developed a model from a word problem that was relatively simple and free of domain knowledge. Even so, 35% of their 152 models were incorrect. There was no significant difference in errors per model between undergraduates and MBAs. Even among the 17 MBAs with 250 h or more of experience, 24% of the models contained errors. The cell error rate (CER)—the percentage of cells with errors—was 2.0%. When 23 undergraduates attempted to audit their models through code inspection, only three with incorrect spreadsheets (15%) produced clean spreadsheets when they finished the audit.     

Oscillation and convergence behaviours exhibited in an ‘unstable’ second‐order digital filter with saturation‐type non‐linearity

This letter explains the oscillatory behaviours exhibited in a second‐order digital filter with saturation‐type non‐linearity via the Hopf bifurcation theorem. It is shown that depending on the bifurcation parameter, the state variables may converge to zero even when the eigenvalues of the system matrix are outside the unit circle. Copyright © 2004 John Wiley & Sons, Ltd. abstract     

Decoupling climate-policy objectives and mechanisms to reduce fragmentation

ABSTRACT

The efficacy of climate-change mitigation policy within the building sector is examined in terms of how fragmentation can limit the extent of mitigation actions that can be achieved in a timely manner. The policy and regulatory context for the building industry is examined in relation to the policy context for solutions and recommendations that will work for all parties. Based on this analysis, two substantive recommendations are made for improved policy design. Firstly, a decoupling of policy objectives and policy mechanisms is needed so that the policy-taking stakeholders (in design, development and construction) can reduce energy use in buildings more effectively. Secondly, policy-taking stakeholders need an explicit and diverse system in order to advocate for policy objectives. The major aspect of this work is the development of a new conceptual framework that ties together these recommendations into a continuous process of policy-making and policy-taking. This framework demonstrates an idealized system that operates simultaneously top down and bottom up, and the development of policy objectives is influenced by stakeholders of all kinds to further the goals of an energy-efficient, low-carbon built environment.     

Nonclassical dynamics with time- and intensity-dependent coupling

Nonclassical light and collapse-revival dynamics are consequences of dynamical quantum interference in transient photon-atom interaction. We study the time evolution of atom and photons in a high quality cavity for time-dependent atom-field coupling, with different initial field states and initial atomic states. The inversion for initial superposed atomic state seems to be independent of initial classical fields but can be stimulated by the Schrodinger cat field. Interesting effects of the transient coupling are found through analysis of the collapse-revival in population inversion and the features in the Wigner function. Oscillatory coupling coefficient can prolong the occurrence of collapse, in analogy to the Zeno effect. The intensity atom-field coupling duration is an important parameter for controlling atomic inversion and producing frozen nonclassical light in the cavity after the atom-field coupling ceases.     

Natural rubber (Hevea Brasiliensis)-based quasi-solid electrolyte as a potential candidate for arresting recombination and improving performance in aqueous dye-sensitized solar cells

Journal of Materials Science: Materials in Electronics - We successfully fabricated dye sensitized solar cells (DSSCs) employing a quasi-solid state electrolyte based on pristine insulator natural...     

Predictive Reactive Transport Modeling at a Proposed Uranium In Situ Recovery Site with a General Data Collection Guide

Restoration of uranium in situ recovery (ISR) sites to predevelopment conditions is often very difficult. Future downgradient groundwater geochemistry can be evaluated using reactive transport modeling coupled with appropriate data collection. U.S. regulatory requirements specify that the geochemistry at the aquifer exemption boundary should never be affected, but compliance with this regulation has not been monitored at previous ISR sites. At the Dewey Burdock site near Edgemont, SD, USA, a change in groundwater flow direction created a scenario in which the oxidized side of a U roll-front deposit is downgradient of the ore zone. This increases the potential for future U transport, since conventional understanding of U geochemistry is that the reduced side provides more natural attenuation. Reactive transport modeling using U sorption parameters from batch sorption tests provides a predictive tool for future U transport. Prediction variations were tested using two different samples, considering different reaction assumptions and possible pH measurement errors. The results indicate a large range in U transport predictions, with high sensitivity to sorption parameters due to sample heterogeneity, pH, and the presence or absence of calcite. While the sample data set for these initial predictions was limited, the results highlight the need for additional calibration points and a thorough understanding of rock/water interactions in the downgradient zone. We provide a general data collection guide for steps in evaluating downgradient transport at future U ISR sites. These steps include core sampling in the downgradient and restored zones, along with batch sorption and column testing with restored and background groundwater in contact with the restored zone solid phase. Final reactive transport modeling will rely on high-quality calibration data from batch and column testing (plus any available field testing), but thorough site evaluation will also require appropriate long-term monitoring.     

Effect of Fiber Length on Carbon Nanotube-Induced Fibrogenesis

Given their extremely small size and light weight, carbon nanotubes (CNTs) can be readily inhaled by human lungs resulting in increased rates of pulmonary disorders, particularly fibrosis. Although the fibrogenic potential of CNTs is well established, there is a lack of consensus regarding the contribution of physicochemical attributes of CNTs on the underlying fibrotic outcome. We designed an experimentally validated in vitro fibroblast culture model aimed at investigating the effect of fiber length on single-walled CNT (SWCNT)-induced pulmonary fibrosis. The fibrogenic response to short and long SWCNTs was assessed via oxidative stress generation, collagen expression and transforming growth factor-beta (TGF-β) production as potential fibrosis biomarkers. Long SWCNTs were significantly more potent than short SWCNTs in terms of reactive oxygen species (ROS) response, collagen production and TGF-β release. Furthermore, our finding on the length-dependent in vitro fibrogenic response was validated by the in vivo lung fibrosis outcome, thus supporting the predictive value of the in vitro model. Our results also demonstrated the key role of ROS in SWCNT-induced collagen expression and TGF-β activation, indicating the potential mechanisms of length-dependent SWCNT-induced fibrosis. Together, our study provides new evidence for the role of fiber length in SWCNT-induced lung fibrosis and offers a rapid cell-based assay for fibrogenicity testing of nanomaterials with the ability to predict pulmonary fibrogenic response in vivo.     

Global modeling of hydrogen using GFDL-AM4.1: Sensitivity of soil removal and radiative forcing

Hydrogen (H₂) has been proposed as an alternative energy carrier to reduce the carbon footprint and associated radiative forcing of the current energy system. Here, we describe the representation of H₂ in the GFDL-AM4.1 model including updated emission inventories and improved representation of H₂ soil removal, the dominant sink of H₂. The model best captures the overall distribution of surface H₂, including regional contrasts between climate zones, when v_d(H₂) is modulated by soil moisture, temperature, and soil carbon content. We estimate that the soil removal of H₂ increases with warming (2–4% per K), with large uncertainties stemming from different regional response of soil moisture and soil carbon. We estimate that H₂ causes an indirect radiative forcing of 0.84 mW m^?2/(Tg(H₂)yr^?1) or 0.13 mW m^?2 ppbv^?1, primarily due to increasing CH₄ lifetime and stratospheric water vapor production.