Dynamics of Flexible Multibody Systems Using Virtual Work and Linear Graph Theory

By combining linear graph theory with the principle of virtualwork, a dynamic formulation is obtained that extends graph-theoreticmodelling methods to the analysis of flexible multibody systems. Thesystem is represented by a linear graph, in which nodes representreference frames on rigid and flexible bodies, and edges representcomponents that connect these frames. By selecting a spanning tree forthe graph, the analyst can choose the set of coordinates appearing inthe final system of equations. This set can include absolute, joint, orelastic coordinates, or some combination thereof. If desired, allnon-working constraint forces and torques can be automaticallyeliminated from the dynamic equations by exploiting the properties ofvirtual work. The formulation has been implemented in a computerprogram, DynaFlex, that generates the equations of motion in symbolicform. Three examples are presented to demonstrate the application of theformulation, and to validate the symbolic computer implementation.     

Threshold-Based Dynamic Replication in Large-Scale Video-on-Demand Systems

Recent advances in high speed networking technologies and video compression techniques have made Video-on-Demand (VOD) services feasible. A large-scale VOD system imposes a large demand on I/O bandwidth and storage resources, and therefore, parallel disks are typically used for providing VOD service. Although striping of movie data across a large number of disks can balance the utilization among these disks, such a striping technique can exhibit additional complexity, for instance, in data management, such as synchronization among disks during data delivery, as well as in supporting fault tolerant behavior. Therefore, it is more practical to limit the extent of data striping, for example, by arranging the disks in groups (or nodes) and then allowing intra-group (or intra-node) data striping only. With multiple striping groups, however, we may need to assign a movie to multiple nodes so as to satisfy the total demand of requests for that movie. Such an approach gives rise to several design issues, including: (1) what is the right number of copies of each movie we need so as to satisfy the demand and at the same time not waste storage capacity, (2) how to assign these movies to different nodes in the system, and (3) what are efficient approaches to altering the number of copies of each movie (and their placement) when the need for that arises. In this paper, we study an approach to dynamically reconfiguring the VOD system so as to alter the number of copies of each movie maintained on the server as the access demand for these movies fluctuates. We propose various approaches to addressing the above stated issues, which result in a VOD design that is adaptive to the changes in data access patterns. Performance evaluation is carried out to quantify the costs and the performance gains of these techniques.     

Time-Constrained Keyframe Selection Technique

In accessing large collections of digitized videos, it is often difficult to find both the appropriate video file and the portion of the video that is of interest. This paper describes a novel technique for determining keyframes that are different from each other and provide a good representation of the whole video. We use keyframes to distinguish videos from each other, to summarize videos, and to provide access points into them. The technique can determine any number of keyframes by clustering the frames in a video and by selecting a representative frame from each cluster. Temporal constraints are used to filter out some clusters and to determine the representative frame for a cluster. Desirable visual features can be emphasized in the set of keyframes. An application for browsing a collection of videos makes use of the keyframes to support skimming and to provide visual summaries.     

Slicing Software for Model Construction

Applying finite-state verification techniques (e.g., model checking) to software requires that program source code be translated to a finite-state transition system that safely models program behavior. Automatically checking such a transition system for a correctness property is typically very costly, thus it is necessary to reduce the size of the transition system as much as possible. In fact, it is often the case that much of a program's source code is irrelevant for verifying a given correctness property.In this paper, we apply program slicing techniques to remove automatically such irrelevant code and thus reduce the size of the corresponding transition system models. We give a simple extension of the classical slicing definition, and prove its safety with respect to model checking of linear temporal logic (LTL) formulae. We discuss how this slicing strategy fits into a general methodology for deriving effective software models using abstraction-based program specialization.     

Signal and noise theory for a dc SQUID amplifier

A theory is presented for the gain and noise in an amplifier based on a dc SQUID. In the lumped circuit approximation, the total inductance of the input circuitL _T is coupled to the SQUID inductanceL via a mutual inductanceM _ii = _e (LL _T )^1/2 and is in series with a voltage source with a resistanceR _i and a capacitanceC _i . The results are expressed in terms of parameters for a SQUID with reduced inductance (1– _e ² )L. The voltage gain of the amplifier at frequency /2 isM _i V ^r /Z _T ^* (), while the total voltage noise at the output of the SQUID isV _N ^r ()+M _i ² V ^r J _N ^r () (R _i + 1/jC _i )/L _TZ _T ^* (). Here,Z _T ^* ()=Z _T ()–J ^r M _i ² (R _i + 1/jC _i )/L _i , whereZ _T () is the total impedance of the unloaded input circuit,V ^r andJ ^r are the flux-to-voltage and flux-to-circulating current transfer functions of the reduced SQUID, andV _N ^r () andJ _N ^r () are the noise voltage and noise current of the reduced SQUID.     

Characterizations of reduction classes modulo oracle conditions

It is known that nondeterministic polynomial time truth-table reducibility is exactly the same as nondeterministic polynomial time Turing reducibility. Here we study the standard nondeterministic reducibilities (conjunctive, bounded truth-table, bounded positive truth-table, and many-one) and show that each is a restriction of nondeterministic polynomial time Turing reducibility corresponding to acceptance modulo a set of oracle conditions. Then we show that the reduction classes of these reducibilities are classes of formal languages and as such have language theoretic characterization theorems. The same program is carried out for polynomial space.This research was supported in part by the National Science Foundation under Grants MCS77-23493, MSC80-11979, MCS81-20263, and MCS83-12472. The work of the second author was also supported by the United States-Israel Educational Foundation (Fulbright Award).     

Polynomial enumeration of multidimensional lattices

Denoting the nonnegative (resp. signed) integers byN (resp.Z) and the real numbers byR, letK R ^m andf: R ^m R. Thenf is astoring function (resp.packing function) onK wheneverf|(Z ^m K) is an injection into (resp. bijection onto)N. Unit translations g_m of some P. Chowla [1961] polynomials are packing functions on the correspondingN ^m , and all compositions of these polynomials yield further packing functions on variousN ^r . We study this accessible family of packing functions, using standard properties ofordered trees to classify all those compositions, up to a simple equivalence, which define polynomial packing functions on eachN ^m . The numberc(m) of equivalence classes is an exponentially growing function for largem, whence the uniqueness conjecture of our prior two-dimensional work has no counterpart for largerm. We obtain the admissible degrees for composition polynomials inm variables; we describe the tre structures for all such polynomials with extremal degrees. Them-variable polynomials of least degree form a rather irregular numbera(m) of equivalence classes. Density considerations give some degree constraints on ageneral polynomial packing function whose domainK is the topological closure of a nonvoid open cone.     

The role of inorganic polymer technology in the development of ‘green concrete’

The potential position of and drivers for inorganic polymers (“geopolymers”) as an element of the push for a sustainable concrete industry are discussed. These materials are alkali-activated aluminosilicates, with a much smaller CO₂ footprint than traditional Portland cements, and display very good strength and chemical resistance properties as well as a variety of other potentially valuable characteristics. It is widely known that the widespread uptake of geopolymer technology is hindered by a number of factors, in particular issues to do with a lack of long-term (20+ years) durability data in this relatively young research field. There are also difficulties in compliance with some regulatory standards in Europe and North America, specifically those defining minimum clinker content levels or chemical compositions in cements. Work on resolving these issues is ongoing, with accelerated durability testing showing highly promising results with regard to salt scaling and freeze–thaw cycling. Geopolymer concrete compliance with performance-based standards is comparable to that of most other high-strength concretes. Issues to do with the distinction between geopolymers synthesised for cement replacement applications and those tailored for niche ceramic applications are also discussed. Particular attention is paid to the role of free alkali and silicate in poorly-formulated systems and its deleterious effects on concrete performance, which necessitates a more complete understanding of the chemistry of geopolymerisation for the technology to be successfully applied. The relationship between CO₂ footprint and composition in comparison with Portland-based cements is quantified.     

Electron-phonon scattering rates in noble metals from magnetoacoustic experiments

The temperature dependence of the carrier scattering rate in copper, silver, and gold at low temperatures has been determined from magnetoacoustic data. Both the (100) central belly orbit and the (110) dogs-bone orbit exhibit a T ³ dependence in all three metals. For copper and silver the coefficients of the corresponding term in the scattering rate agree well with those obtained from radiofrequency size-effect experiments. Previously reported discrepancies between the results for copper obtained by the two techniques are discussed and tentatively resolved. No effects due to electron-electron scattering are observed. Differences in the scattering rates between the noble metals appear to be primarily correlated with changes in the phonon density of states.Work supported by National Science Foundation.