Special Issue on Computer Aided Engineering in Honor of Professor Steven J. Fenves

Engineering with Computers -     

A multiscale model for polymer crystallization. I: Growth of individual spherulites

In this two-part paper, we present a model for the solidification of semicrystalline thermoplastic polymers that links the microscopic and macroscopic length scales. The model accounts for the important physical phenomena occurring during spherulitic cyrstallization and predicts microstructural evolution. In this first part, we concentrate on the growth of individual spherulites. We present a fast and accurate model that predicts the shape of a spherulite as well as the path of its constitutive lamellae in any thermal situation. In part II of this paper we shall couple this new model for spherulite growth with a nucleation law and a macroscopic heat flow computation, thus enabling us to model realistic crystallization conditions.     

Magnetic and transport properties of Laves phase Dy1−xMmxCo2 (x = 0–0.5) alloys

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy_1?xMm_x)Co₂ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity (ρ(T)), ac susceptibility (χ(T)) and thermopower (S(T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature (T_C) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in T_C and sharpness of discontinuities in ρ(T) and S(T) with increasing Mm substitution has been discussed.     

Content-based assembly search: A step towards assembly reuse

The increased use of CAD systems by product development organizations has resulted in the creation of large databases of assemblies. This explosion of assembly data is likely to continue in the future. In many situations, a text-based search alone may not be sufficient to search for assemblies and it may be desirable to search for assemblies based on the content of the assembly models. The ability to perform content-based searches on these databases is expected to help the designers in the following two ways. First, it can facilitate the reuse of existing assembly designs, thereby reducing the design time. Second, a lot of useful designs for manufacturing, and assembly knowledge are implicitly embedded in existing assemblies. Therefore a capability to locate existing assemblies and examine them can be used as a learning tool by designers to learn from the existing assembly designs. This paper describes a system for performing content-based searches on assembly databases. We identify templates for comprehensive search definitions and describe algorithms to perform content-based searches for mechanical assemblies. We also illustrate the capabilities of our system through several examples.     

Thermal decomposition of doped calcium hydroxide for chemical energy storage

Thermal decomposition of Ca(OH)₂ with and without additives has been experimentally investigated for its application as a thermochemical energy storage system. The homogeneous reaction model gives a satisfactory fit for the kinetic data on pure and Ni(OH)₂---, Zn(OH)₂--- and Al(OH)₃---doped Ca(OH)₂ and the order of reaction is 0.76 in all cases except for the Al(OH)₃-doped sample for which the decomposition is zero order. These additives are shown not only to enhance the reaction rate but also to reduce the decomposition temperature significantly. Some models for solid decomposition reactions, and possible mechanisms in the decomposition of solids containing additives, are also discussed.     

Drug release rate method for a liposome preparation

An in vitro method was developed to determine the rate of drug release from a liposome formulation. Liposome formulation containing metaproterenol sulfate was evaluated for release of the drug over a 24-hour period in an end-over-end tumbler device. Drug release appeared to be a function of all the operating parameters, such as rotational speed of the end-over-end tumbler, temperature of suspension, and the geometry of sample container that is placed in the end-over-end tumbler device.     

Traffic smoothing effects of bit dropping in a packet voicemultiplexer

The performance of a packet voice multiplexer queue in which the less significant bits of voiced packets are dropped during states of congestion in the multiplexer is examined. Using the results of simulation and analytical modeling, it is illustrated that bit dropping of voice packets significantly smooth the burstiness of superposition packet voice traffic by speeding up the packet service rate during critical periods of congestion in the queue. The smoothing effect renders it possible to approximate the superposition by a Poisson process for modeling a packet voice multiplexer with bit dropping. By comparison with a simulation, an analytical model based on the Poisson assumption is shown to produce quite accurate performance predictions. The results indicate that significant capacity and performance advantages are gained in the multiplexer as a result of the bit-dropping scheme     

End-to-end performance models for variable bit rate voice overtandem links in packet networks

Analytical models are presented for computing the end-to-end voice call performance in a packet network that drops the less significant bits in voice packets during periods of congestion. These models provide information about the end-to-end quality likely to be experienced in future packet-switched integrated services networks. An existing single-node bit-dropping model is modified to include the situation resulting when the overall arrival process at an internal node consists of a mix of packets of different sizes due to bit dropping at previous nodes. A detailed model to capture bit-dropping effects in a tandem connection of nodes is presented. The model includes the effect of load fluctuations at each node, and also takes into account the dependencies in bit dropping experienced by a voice packet at successive nodes in a tandem connection. The model also incorporates the internodal dependence when reductions in packet service times occur at intermediate nodes due to bit dropping at previous nodes. Two approximation procedures are discussed that serve as upper and lower bounds. In particular, the upper bound is shown to be very tight for a practical range of loads, and hence serves as a good approximation with significant computational simplicity     

Protection switching for optical bursts using segmentation and deflection routing

Burst segmentation in OBS networks can significantly reduce the amount of data that is lost due to contention events by dropping or deflecting only the portion of a burst that overlaps another contending burst. In this letter, we demonstrate how segmentation combined with deflection routing can be used to reduce the amount of data that is lost when network elements fail. By enabling an OBS switch to deflect the tail-end segments of bursts that are in transmission as soon as it becomes aware of a downstream link failure, the retransmission of lost data can be reduced.     

Crystallization of Zircon in Stoneware Glazes

High levels of zircon are required to impart opacity in glazes. Zircon additions of <3 wt% dissolve into the glassy phase; additions of >12% cause all the zircon to crystallize from the glaze. Dynamic high-temperature X-ray diffraction has shown that, during heating, a portion of the zircon dissolves and subsequently recrystallizes, using undissolved zircon crystals as seeds. This recrystallization does not occur at lesser zircon levels, because of a lack of seed crystals. When zirconia is added to the glaze, zircon is formed as elongated crystals at the glaze surface. Lesser zircon levels have been observed in the glazes that have zirconia additions.