Vibrational dynamics of solid poly(ethylene oxide)

Molecular dynamics (MD) simulations of crystalline poly(ethylene oxide) (PEO) have been carried out in order to study its vibrational properties. The vibrational density of states has been calculated using a normal mode analysis (NMA) and also through the velocity autocorrelation function of the atoms. Results agree well with experimental spectroscopic data. System size effects in the crystalline state, studied through a comparison between results for 16 unit cells and that for one unit cell has shown important differences in the features below 100 cm^-1. Effects of interchain interactions are examined by a comparison of the spectra in the condensed state to that obtained for an isolated oligomer of ethylene oxide. Calculations of the local character of the modes indicate the presence of collective excitations for frequencies lower than 100 cm^-1, in which around 8 to 12 successive atoms of the polymer backbone participate. The backbone twisting of helical chains about their long axes is dominant in these low frequency modes.Comment: 19 pages, 7 figures (Phys.Rev.B submitted on 28.11.2002) Revised versio

Area/latency optimized early output asynchronous full adders and relative-timed ripple carry adders

This article presents two area/latency optimized gate level asynchronous full adder designs which correspond to early output logic. The proposed full adders are constructed using the delay-insensitive dual-rail code and adhere to the four-phase return-to-zero handshaking. For an asynchronous ripple carry adder (RCA) constructed using the proposed early output full adders, the relative-timing assumption becomes necessary and the inherent advantages of the relative-timed RCA are: (1) computation with valid inputs, i.e., forward latency is data-dependent, and (2) computation with spacer inputs involves a bare minimum constant reverse latency of just one full adder delay, thus resulting in the optimal cycle time. With respect to different 32-bit RCA implementations, and in comparison with the optimized strong-indication, weak-indication, and early output full adder designs, one of the proposed early output full adders achieves respective reductions in latency by 67.8, 12.3 and 6.1 %, while the other proposed early output full adder achieves corresponding reductions in area by 32.6, 24.6 and 6.9 %, with practically no power penalty. Further, the proposed early output full adders based asynchronous RCAs enable minimum reductions in cycle time by 83.4, 15, and 8.8 % when considering carry-propagation over the entire RCA width of 32-bits, and maximum reductions in cycle time by 97.5, 27.4, and 22.4 % for the consideration of a typical carry chain length of 4 full adder stages, when compared to the least of the cycle time estimates of various strong-indication, weak-indication, and early output asynchronous RCAs of similar size. All the asynchronous full adders and RCAs were realized using standard cells in a semi-custom design fashion based on a 32/28 nm CMOS process technology

A hierarchical framework of barriers to green supply chain management in the construction sector

The research paper presents a hierarchical sustainability framework for evaluating the barriers to the adoption of green supply chain management (GSCM) in the United Arab Emirates (UAE) construction sector. A total of 32 barriers to the adoption of GSCM are identified through extensive literature review and expert interviews with academics and industry professionals. The barriers are grouped on the basis of literature and expert opinion to form 12 criteria. Since the nature of the identified criteria is complex and interdependent; an Interpretive Structural Modeling (ISM) technique is applied to develop a structural model. Driving and dependence power analysis (DDPA) is used to classify and identify the critical barriers. The developed sustainability framework offers a strong and efficient evaluation technique in decision making for policy makers and stakeholders by means of identifying and prioritizing the critical barriers. The barriers identified are also classified as external and internal barriers to the organization and will help policy makers to focus on specific barriers which are important to the adoption of GSCM in the UAE construction sector. The framework has the potential to be applied to other countries across industries

Green supply chain management: an investigation on the construction sector

Environmental pollution and climate change have become one of the greatest challenges of the 21st century, which have forced governments and businesses alike to assess the environmental impacts of their activities. Among the sectors, construction is the single largest contributor of global carbon emissions, resource, water and energy consumption, and landfill waste. With environmental implications expected to be even greater in the future due to increasing urbanisation and the consequent increase in construction activities, curtailing the negative environmental impacts of the sector or greening the construction sector, therefore, has become critical. Unfortunately, any limited efforts to date to address these concerns have been less fruitful as most of these efforts have been largely fragmented and disjoint, addressing issues in an ad-hoc, standalone manner such as green design, green purchasing, green construction or environmental management systems; or management issues such as ‘drivers’ and/or ‘barriers’ affecting these specific green practices; or specific performance implications from these green practices such as environmental and/or financial performance. This lack of holistic orientation also carries the risk that practitioners and policymakers could mistakenly be addressing the wrong issues and neglecting aspects that have more significance in greening the sector. Given the environmental consequences of a construction project are typically dispersed across the different stages in the supply chain, i.e. from design through to end-of-life, and that several stakeholders, each with their own conflicting interests, are involved in the different stages of the construction supply chain, greening the sector, therefore, requires a supply chain wide focus, inclusive of all key stages and stakeholders (Developers, Architects/Consultants, Contractors and Suppliers). Therefore, the application of green supply chain management (GSCM) or incorporating environmental concerns into supply chain management (a systematic and integrated approach) makes perfect sense for greening the construction sector. GSCM contributes to greening by promoting supply chain-wide implementation of efficient and effective green practices by means of managing the ‘drivers’ and ‘barriers’ affecting its implementation to achieve the desired environmental performance along with short-term economic/cost performance and long-term organisational performance. This formed the focus of this study, wherein, it explores the application of GSCM in greening the construction sector. The study also explores the impact of firm size and ownership on GSCM, because, given the inherent complexity of the construction supply chain, i.e. it comprises of hundreds of firms with varying size and ownership, a comprehensive greening of the construction supply chain would not be possible without managing the impact of size and ownership on GSCM. Finally, given that GSCM understanding would be of limited value unless accompanied by general principles (theories) that inform wider application, the study utilises several established and emerging management/organisational theories to underpin the multifaceted reality of GSCM. In short, each of these GSCM aspects, i.e. green practices, green drivers and barriers, and green performance; and their interrelationships; and the impact of firm size and ownership on GSCM are investigated as separate research questions in this thesis. UAE is carefully chosen as the research setting for this GSCM study mainly because it gives an exemplary opportunity to understand the competing actions required from governments and construction sector firms to lessen the environmental impacts associated with the rapid urbanisation and economic modernisation. Specifically, on one side, the UAE construction sector is growing at more than 9% per annum, while on the other side several green initiatives are considered by practitioners and policymakers to reduce its environmental burden on the country. Therefore, the related findings are expected to be more practically relevant to comprehend the challenges and opportunities in the application of GSCM. A pragmatic, multi-methodology, sequential exploratory approach (i.e. the qualitative investigation followed by quantitative investigation) was employed to comprehensively answer the research questions. For the qualitative investigation, both semi-structured interviews (to explore and define each GSCM themes/sub-themes), and focussed, in-depth interviews (to gain operational/implementation level understanding) were employed. For the quantitative investigation, a structured country-wide survey was employed. The findings derived from the multiple methods (interviews and survey), were then combined to develop a comprehensive picture on the various facets of GSCM in relation to greening the construction sector. With regard to the findings, the important/relevant core green practices (or environmental activities/initiatives undertaken across each of the distinct functional stages of the supply chain) identified for greening the construction sector include green design, green purchasing, green transportation, green construction/manufacturing and end of life green practices, whereas the important/relevant facilitating green practices (or activities/initiatives undertaken to build internal environmental resources and capabilities) identified for greening the construction sector include environmental management systems (EMS) and ISO 14001 certification, cross-functional integration, environmental auditing, environmental training and green-related research and development. The extent of implementation of these practices, in general, was found to be the highest among Suppliers, moderate among Architects/Consultants and Contractors, and lowest among Developers. The important/relevant external green drivers (external forces/pressures that coerce firms to implement green practices) identified include government green-related regulation, supply chain stakeholder pressure, competitor pressure and buyer/end-consumer pressure, whereas internal green drivers (internal forces/pressures that motivate firms to implement green practices) identified include environmental commitment of firms, enhance reputation/brand image, to reduce costs and to enter foreign markets. The relevance/importance perceived by stakeholders shows that all stakeholders except Developers are more motivated internally than externally to engage in green practices. On the other hand, the important/relevant external green barriers (external forces that hinder or restricts firms from implementing green practices) identified include shortage of green professionals, shortage of green suppliers, tight and inflexible stakeholder deadlines and lack of stakeholder collaboration, whereas internal green barriers identified include high cost of implementation and lack of knowledge and awareness. The relevance/importance perceived by stakeholders shows that Developers and Suppliers perceive internal barriers more than external, while Architects/Consultants and Contractors were found to perceive external and internal barriers to be more or less the same. The study also identified several important/relevant performance measures to capture environmental, economic/cost and organisational performance that firms could operationalise to capture the benefits of green practices. With regard to actual improvement in these performances, all three performances were found to be relatively lower for Developers, while moderate to high for other stakeholders. With regard to the relationship between GSCM aspects, the extent of the impact of green drivers on green practices was found to far exceed the impact of green barriers on green practices, which self-explains the moderate extent of implementation of green practices across stakeholders. Moreover, it was found that both core and facilitating green practices have a significant and positive impact on three dimensions of performance across stakeholders, and the strength of impact, in general, ranged from moderate to high. Furthermore, facilitating green practices was found to have a strong and positive impact on core green practices. Finally, firm size and ownership was found to have a significant impact on the various GSCM aspects. The study provides practitioners (across all stakeholders) with a potential stock of core and facilitating green practices that they could implement as well as potential performance measures they could operationalise in their respective firms. Also, it helps them to gauge the green drivers and barriers affecting their green practices implementation. The understanding of relationships such as the impact of green drivers and barriers on green practices is important for both policymakers (at the sector level) and practitioners (at the firm level) to devise strategies to effectively maximise/leverage the drivers and minimise/eliminate the barriers to promote efficient and effective green practices implementation. The positive impact of green practices on all three aspects of performance demonstrates the significant “win-win” opportunities that exist for stakeholders, and should therefore provide the impetus for firms to implement green practices. Also, practitioners could use this understanding to prioritise the implementation of those individual facilitating and/or core green practices that deliver the firm’s targeted green performance goals (taking all three performance aspects into consideration). Furthermore, the finding on the impact of facilitating green practices on core green practices shows that facilitating practices is a necessary precursor to the implementation of core green practices and therefore should provide the impetus for firms to make prioritised investment in facilitating practices to improve their core green practices. Also, the findings on the impact of size and ownership are important for all concerned parties to devise actions, strategies and policy interventions to ensure all firms, regardless of their size and ownership, contribute towards greening the sector. Overall, given that most of the underlying issues in construction are similar across countries, the insights from this study can be used as a good starting point for practitioners and policymakers elsewhere in greening the construction sector. The study is arguably the first comprehensive attempt to understand GSCM and its importance/relevance in greening the construction sector. The study also provides several validated first-order constructs, namely external and internal drivers, external and internal barriers, facilitating green practices, environmental, economic/cost and organisational performance, and a second-order construct, core green practices, underlying the following first-order constructs: green design, green purchasing, green transportation, green construction/manufacturing and end of life green practices. This itself is a significant research contribution given that construct development and validation is at the heart of theory building. The study also provides a comprehensive GSCM framework underlying these constructs and their relationships. Future researchers could use/adapt this GSCM framework in their respective settings in construction or other sectors. Also, the application of several established/emerging theories to understand the various GSCM aspects has not been undertaken previously in the construction sector and hence constitutes a novelty

A structural analysis of green supply chain management enablers in the UAE construction sector

The aim of the research is to develop a structural analysis of the enablers of green supply chain management (GSCM) in the UAE construction sector. An Interpretive structural modeling (ISM) approach is used to identify the contextual relationship of the enablers and to develop their hierarchical structure. Further the enablers are classified into visual quadrants on a graph using dependence-driving power analysis (DDPA). The hierarchical structure and graph will provide useful insights to corporates, government bodies and supply chain managers to understand and prioritize the key enablers of GSCM and the organizational strategies adopted by firms in the UAE. The study will contribute significantly to the first wave of empirical investigation in the region and will provide useful insights into GSCM in the UAE. A structural analysis of GSCM enablers as well as industry specific research of GSCM in construction sector is not previously developed in the UAE

Spacetime and the Holographic Renormalization Group

Anti-de Sitter (AdS) space can be foliated by a family of nested surfaces homeomorphic to the boundary of the space. We propose a holographic correspondence between theories living on each surface in the foliation and quantum gravity in the enclosed volume. The flow of observables between our ``interior'' theories is described by a renormalization group equation. The dependence of these flows on the foliation of space encodes bulk geometry.Comment: 12 page

Drag reduction effects in turbulent boundary layers over wavy walls

Two dimensional incompressible flow over wavy surfaces are analyzed numerically by spectral methods. Algorithms for periodic flows (Fourier modes in the periodic flow direction and Chebycheff modes in the normal direction), and inflow-outflow boundary conditions (Chebycheff modes used in both directions) are described. Results obtained using both codes are reported for laminar flows. Comparisons with known theoretical and experimental results are made

Numerical studies of laminar and turbulent drag reduction

Two-dimensional incompressible flow over wavy surfaces is studied numerically by spectral methods. Turbulence effects are modeled. Results for symmetric and asymmetric wave forms are presented. Effect of propagating surface waves on drag reduction is studied. Comparisons between computer simulations and experimental results are made

Spatial navigation and multiscale representation by hippocampal place cells

Hippocampal lesions are known to impair success in navigation tasks. While such tasks could be solved by memorizing complete paths from a starting location to the goal, animals still perform successfully when placed in a novel starting position. We propose a navigation algorithm to solve the latter problem by exploiting two facts about hippocampal organization: (1) The size of the place fields of hippocampal place cells varies systematically along the dorsoventral axis, with dorsal place cells having smaller place fields than ventral (Kjelstrup et. al. 2008); and (2) the theta oscillation propagates as a traveling wave from dorsal to ventral hippocampus (Lubenov and Siapas, 2009). Taken together, these observations imply that the hippocampal representation of space progresses from fine- to coarse-grained within every theta cycle. 

The algorithm assumes that place cells can be activated by the animal's imagining a goal location, in addition to physically standing in the appropriate location. In the proposed algorithm, place cell activation propagates from small scale to large scale until place cells are found which respond strongly to both the physical location and the goal location. These place fields have their centers aligned roughly in the direction of the goal, providing a crude estimate of which direction the animal should step to approach the goal. Fine-grained directional information is contained in the smaller scale place fields within these large ones. Our algorithm therefore identifies a sequence of place cells, one from each scale, whose centers lie roughly along the line to the goal. 

Simulations reveal successful navigation to the goal, even around obstacles. By minimizing the number of steps the animal takes to reach the goal, we predict the organization of the optimal place field "map"; specifically the fraction of place cells which should be allocated to each spatial scale. This prediction is, in principle, experimentally testable.

The set of place fields with centers lying along a line to the goal is used to compute a step direction by maximizing the probability that those cells will be active in the next time step, given that a particular step direction is chosen.

The proposed algorithm handles navigation around obstacles by including "border cells" (Solstad et. al. 2008) which inhibit place cells in proportion to the degree of overlap between the place field and the obstacle. Furthermore, including firing rate adaptation of place cells prevents the animal from getting stuck in one spot