58,791 research outputs found
Mosses of Mt. Trus Madi, Sabah, Malaysia
A species list of mosses collected in Mt. Trus Madi, Sabah, between 10 September and 10 October 1996 is presented. A total of 153 taxa were collected, of which 11 are new to Borneo and five new to Sabah
Statistical Mechanics of Vibration-Induced Compaction of Powders
We propose a theory which describes the density relaxation of loosely packed,
cohesionless granular material under mechanical tapping. Using the compactivity
concept we develope a formalism of statistical mechanics which allows us to
calculate the density of a powder as a function of time and compactivity. A
simple fluctuation-dissipation relation which relates compactivity to the
amplitude and frequency of a tapping is proposed. Experimental data of
E.R.Nowak et al. [{\it Powder Technology} 94, 79 (1997) ] show how density of
initially deposited in a fluffy state powder evolves under carefully controlled
tapping towards a random close packing (RCP) density. Ramping the vibration
amplitude repeatedly up and back down again reveals the existence of reversible
and irreversible branches in the response. In the framework of our approach the
reversible branch (along which the RCP density is obtained) corresponds to the
steady state solution of the Fokker-Planck equation whereas the irreversible
one is represented by a superposition of "excited states" eigenfunctions. These
two regimes of response are analyzed theoretically and a qualitative
explanation of the hysteresis curve is offered.Comment: 11 pages, 2 figures, Latex. Revised tex
Exploring roles and relationships in the production of the built environment
Given the number of different agencies and the complexity of institutional and professional relationships in the production, management and regulation of the built environment, many students entering built environment professions leave university education to take up work placements or employment without a sufficient understanding of the different actors and the formal and informal interactions and social relationships between them. Furthermore, destructive stereotypes may form during the educational process as students construct their own professional identity, in part learnt from their teachers and peers, and naturalised by the academic and professional institutions that form the context of their education – a process of enculturation termed ‘professional socialization’ by social scientists (Cuff, 1991: 118). These stereotypes may lead ultimately to inter-professional tensions and hostilities. Innovations in practice often involve challenges to established roles or joined-up thinking which breaches institutional structures, for all of which graduates may be ill-prepared
Dynamic mechanical response of polymer networks
The dynamic-mechanical response of flexible polymer networks is studied in
the framework of tube model, in the limit of small affine deformations, using
the approach based on Rayleighian dissipation function. The dynamic complex
modulus G* is calculated from the analysis of a network strand relaxation to
the new equilibrium conformation around the distorted primitive path. Chain
equilibration is achieved via a sliding motion of polymer segments along the
tube, eliminating the inhomogeneity of the polymer density caused by the
deformation. The characteristic relaxation time of this motion separates
the low-frequency limit of the complex modulus from the high-frequency one,
where the main role is played by chain entanglements, analogous to the rubber
plateau in melts. The dependence of storage and loss moduli, G' and G'', on
crosslink and entanglement densities gives an interpolation between polymer
melts and crosslinked networks. We discuss the experimental implications of the
rather short relaxation time and the slow square-root variation of the moduli
and the loss factor tan at higher frequencies.Comment: Journal of Chemical Physics (Oct-2000); Lates, 4 EPS figures include
Near-Term Options for a Nuclear Thermal Propulsion Flight Demonstrator
The Appropriations Bill passed by the US Congress in February 2019 instructed NASA to direct not less than 70,000,000 shall be for the design of a flight demonstration by 2024 for which a multi-year plan is required by both the House and the Senate within 180 days of enactment of this agreement." As part of NASAs response to this direction, the Advanced Concepts Office (ACO) at the Marshall Space Flight Center (MSFC) was tasked with leading a study to develop a nuclear thermal propulsion (NTP) flight demonstration (FD) concept and evaluate its feasibility with respect to the near-term schedule goal. During formulation for the NTP FD study, two perspectives emerged with regards to FD concept design. The first seeks to strictly observe the immediate near-term schedule goal, embracing a completely off-the-shelf, high-TRL approach to subsystem design and component selection. The downside to this approach is that the propulsion performance to be expected from such a design is significantly lower than what NTP promises for operational systems, and the value of the flight demo is potentially reduced due to a lack of traceability. The second approach advocates for an FD concept that shows increased traceability to the projected designs of operational systems, providing risk reduction for future NTP-enabled missions. This option comes at the cost of schedule and development risks, as it requires some new investments in nuclear reactor fuels and design. In order to understand the implications and differences between these two approaches, the ACO team elected to perform a concept design of each type, labeling the immediate near-term concept Flight Demo 1 (FD1), and the higher traceability concept Flight Demo 2 (FD2). This paper will present a summary of the mission profiles and system designs for both FD1 and FD2, identifying key drivers and challenges for each design
Mechanism for the failure of the Edwards hypothesis in the SK spin glass
The dynamics of the SK model at T=0 starting from random spin configurations
is considered. The metastable states reached by such dynamics are atypical of
such states as a whole, in that the probability density of site energies,
, is small at . Since virtually all metastable states
have a much larger , this behavior demonstrates a qualitative failure of
the Edwards hypothesis. We look for its origins by modelling the changes in the
site energies during the dynamics as a Markov process. We show how the small
arises from features of the Markov process that have a clear physical
basis in the spin-glass, and hence explain the failure of the Edwards
hypothesis.Comment: 5 pages, new title, modified text, additional reference
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