Showing papers by "Indian Institute of Technology Bombay published in 1996"

Materialized view maintenance and integrity constraint checking: trading space for time

Abstract: We investigate the problem of incremental maintenance of an SQL view in the face of database updates, and show that it is possible to reduce the total time cost of view maintenance by materializing (and maintaining) additional views. We formulate the problem of determining the optimal set of additional views to materialize as an optimization problem over the space of possible view sets (which includes the empty set). The optimization problem is harder than query optimization since it has to deal with multiple view sets, updates of multiple relations, and multiple ways of maintaining each view set for each updated relation.We develop a memoing solution for the problem; the solution can be implemented using the expression DAG representation used in rule-based optimizers such as Volcano. We demonstrate that global optimization cannot, in general, be achieved by locally optimizing each materialized subview, because common subexpressions between different materialized subviews can allow nonoptimal local plans to be combined into an optimal global plan. We identify conditions on materialized subviews in the expression DAG when local optimization is possible. Finally, we suggest heuristics that can be used to efficiently determine a useful set of additional views to materialize.Our results are particularly important for the efficient checking of assertions (complex integrity constraints) in the SQL-92 standard, since the incremental checking of such integrity constraints is known to be essentially equivalent to the view maintenance problem.

Cost-based optimization for magic: algebra and implementation

Abstract: Magic sets rewriting is a well-known optimization heuristic for complex decision-support queries. There can be many variants of this rewriting even for a single query, which differ greatly in execution performance. We propose cost-based techniques for selecting an efficient variant from the many choices.Our first contribution is a practical scheme that models magic sets rewriting as a special join method that can be added to any cost-based query optimizer. We derive cost formulas that allow an optimizer to choose the best variant of the rewriting and to decide whether it is beneficial. The order of complexity of the optimization process is preserved by limiting the search space in a reasonable manner. We have implemented this technique in IBM's DB2 C/S V2 database system. Our performance measurements demonstrate that the cost-based magic optimization technique performs well, and that without it, several poor decisions could be made.Our second contribution is a formal algebraic model of magic sets rewriting, based on an extension of the multiset relational algebra, which cleanly defines the search space and can be used in a rule-based optimizer. We introduce the multiset θ-semijoin operator, and derive equivalence rules involving this operator. We demonstrate that magic sets rewriting for non-recursive SQL queries can be modeled as a sequential composition of these equivalence rules.

Novel grid-connected photovoltaic inverter

Abstract: Detailed analysis and simulation results of a novel solar photovoltaic inverter configuration interconnected to the grid are presented. From the simulation results it is confirmed that the harmonic distortion of the output current waveform of the inverter fed to the grid is within the stipulated limits laid down by the utility companies. Typical hardware aspects are also discussed in detail and the applicability of the design is verified.

Improved coupling for aperture coupled microstrip antennas

Abstract: Aperture shape and size are the crucial parameters for the aperture coupled microstrip antennas (ACMSA). A systematic comparision of various aperture shapes for the aperture coupled rectangular microstrip antenna has been carried out. An optimum "hour glass"-shaped aperture configuration has been proposed for maximum coupling.

Ultrafine Metal Particle Formation in Reverse Micellar Systems: Effects of Intermicellar Exchange on the Formation of Particles

Abstract: A modeling approach is developed to account for the important effects of intermicellar exchange on the ultrafine particle formation in reverse micelles. A set of fusion−fission schemes is specifically proposed and used for modeling intermicellar metal exchange. The dynamic effects largely differing in their time scales are decoupled using a two-staged approach. Growth of metal particles is simulated to occur through intramicellar attachment of free metal atoms from an instantaneous reduction reaction and as well as by transfer and attachment through intermicellar exchange. Simulation results predict increase in particle size with aqueous core size and total aqueous phase volume, increase in particle number density with surfactant concentration at fixed aqueous phase to surfactant molar ratio, a minimum in average particle size as a function of salt concentration, and increase in particle size polydispersity with salt concentration and aqueous core size. Low values of the order of 10-1 are found for the nu...

Effect of theta-alumina formation on the growth kinetics of alumina-forming superalloys

Abstract: Alumina-forming ODS superalloys are excellent oxidation-resistant materials. Their resistance relies upon the establishment of a stable, slow-growing, and adherent α-alumina. In the present investigation, these alloys exhibited unstable and relatively less adherent θ-alumina phase, which increased the oxidation rate in the transient stage and converted into α-alumina in the later part of the exposure. The oxide-growth process was found to depend upon various parameters such as temperature, time, and presence of an active elecment in the superalloy. Characterization carried out by XRD, SEM/EDAX, and AES on oxidized ODS and non-ODS alloys demonstrated a significant influence of the active element, Y, on the transformation of θ- to α-alumina. SIMS analysis of two-stage oxidation at 900°C for two different durations evidently showed that the change in the transport process is due to θ-to-α-alumina transformation. On the basis of these results, a new and consistent mechanism is proposed to explain the influence of θ-alumina and its transformation on growth kinetics and the effect of yttrium on the transformation leading to good scale adherence and oxidation resistance.

Monitoring of desertification process in Karnataka state of India using multi-temporal remote sensing and ancillary information using GIS

Abstract: This study evaluates in detail the indicators of desertification process in semi-arid lands by making use of temporal satellite information along with the surface and statistical data with the aid of a GIS. The indicators were correlated to the surface information to establish the severity of desertification and factors helping the desertification process to continue. The results show that the process is a natural phenomenon but is aggravated by human activity.

An O(log*n) approximation algorithm for the asymmetric p-center problem

Abstract: The input to the asymmetricp-center problem consists of an integerpand ann×ndistance matrixDdefined on a vertex setVof sizen, wheredijgives the distance fromitoj. The distances are assumed to obey the triangle inequality. For a subsetS?Vthe radius ofSis the minimum distanceRsuch that every point inVis at a distance at mostRfrom some point inS. Thep-center problem consists of picking a setS?Vof sizepto minimize the radius. This problem is known to be NP-complete.For the symmetric case, whendij=dji, approximation algorithms that deliver a solution to within 2 of the optimal are known. David Shmoys, in his article 11, mentions that nothing was known about the asymmetric case. We present an algorithm that achieves a ratio ofO(log*n).

Image interpretation using Bayesian networks

Abstract: The problem of image interpretation is one of inference with the help of domain knowledge. In this paper, we formulate the problem as the maximum a posteriori (MAP) estimate of a properly defined probability distribution function (PDF). We show that a Bayesian network can be used to represent this PDF as well as the domain knowledge needed for interpretation. The Bayesian network may be relaxed to obtain the set of optimum interpretations.

Microencapsulation in polyurea shell: Kinetics and film structure

Abstract: Interfacial polycondensation is an important polymerization technique that encapsulates a variety of active principles. Mechanisms governing the reaction and interplay of physical and chemical rate processes need to be understood for both rational design of reaction equipment and the process control to produce capsules with desired characteristics. A theoretical and experimental study of the process is reported here. Kinetic data were obtained over a range of concentrations, monomer mole ratios and polymer film thicknesses, using a technique that relies on the on-line measurement of pH as a function of time. To understand the mechanisms the reaction was slowed down by reducing the interfacial area. A minimum thickness of the polymer was observed to be necessary so that capsules preserve their integrity and do not break up. The theoretical model developed considers ionic equilibria in the aqueous phase and the resistance due to external mass transfer, diffusion through the polymer, and interfacial reaction. Under the conditions chosen, the resistance due to the chemical reaction is generally more dominant. Values of rate parameters were determined by fitting the model to the experimental data. Observed variations in the diffusivity between different experiments were rationalized through a study of the crystalline structure of polymers.

Seismic response of sliding structures to bidirectional earthquake excitation

Abstract: Seismic response of a one-storey structure with sliding support to bidirectional (ie two horizontal components) earthquake ground motion is investigated Frictional forces, which are mobilized at the sliding support, are assumed to have ideal Coulomb-friction characteristics Coupling effects due to circular interaction between the frictional forces are incorporated in the governing equations of motion Effects of bidirectional interaction of frictional forces on the response are investigated by comparing the response to two-component excitation with the corresponding response produced by the application of single-component excitations in each direction independently It is observed that the response of the sliding structure is influenced significantly by the bidirectional interaction of frictional forces Further, it is shown that the design sliding displacement may be underestimated if the bidirectional interaction of frictional forces is neglected and the sliding structures are designed merely on the basis of single-component excitation

Complete pressure correction algorithm for solution of incompressible Navier-Stokes equations on a nonstaggered grid.

Abstract: When Navier-Stokes equations for incompressible flow are solved on a nonstaggered grid, the problem of checkerboard prediction of pressure is encountered. So far, this problem has been cured either by evaluating the cell face velocities by the momentum interpolated principle III or by evaluating an effective pressure gradient in the nodal momentum equations [2] In this article it is shown that not only are these practices unnecessary, they can lead to spurious results when the true pressure variation departs considerably from linearity. What is required instead is afresh derivation of the pressure correction equation appropriate for a nonstaggered grid. The pressure correction determined from this equation comprises two components: a mass-conserving component and a smoothing component. The former corresponds to the pressure correction predicted by a staggered grid procedure, whereas the latter simply accounts for the difference between the point value of the pressure and the cell-averaged value o...

Casting solidification analysis by modulus vector method

Abstract: This paper briefly reviews various geometry-driven methods for analysing the solidification process of a casting, which offer the advantage of being faster and less dependent on material properties...

Tracking of Occluded Vehicles in Traffic Scenes

Abstract: Vehicles on downtown roads can be occluded by other vehicles or by stationary scene components such as traffic lights or road signs. After having recorded such a scene by a video camera, we noticed that the occlusion may disturb the detection and tracking of vehicles by previous versions of our computer vision approach. In this contribution we demonstrate how our image sequence analysis system can be improved by an explicit model-based recognition of 3D occlusion situations. Results obtained from real world image sequences recording gas station traffic as well as inner-city intersection traffic are presented.

Distributed multi-level recovery in main-memory databases

Abstract: The authors present two schemes for concurrency control and recovery in distributed main-memory databases. In the client-server scheme, clients ship log records to the server, which applies the updates to its database copy. In the shared disk scheme, each site broadcasts its updates to other sites. The above enable the schemes to support concurrent updates to the same page at different sites. Both schemes support an explicit multi-level recovery abstraction for high concurrency, reduced disk I/O by writing only redo log records to disk during normal processing, and use of per-transaction redo and undo logs to reduce contention. Further, they use a fuzzy checkpointing scheme that writes only dirty pages to disk, yet minimally interferes with normal processing, not requiring updaters to even acquire a latch before updating a page.

Processing and properties of SiC particulate reinforced Al6.2Zn2.5Mg1.7Cu alloy (7010) matrix composites prepared by mechanical alloying

Abstract: SiC particulate reinforced aluminium alloy (7010) matrix composites have been processed by mechanical alloying route starting from elemental powders. Proper post-consolidation by hot pressing or hot extrusion results in a fully dense composite compact that retains the fine microstructure developed during mechanical alloying. Addition of SiC was found to result in lower strengths at room temperature but higher strength at temperatures above 200°C. There was nearly a 64% increase in the yield strength at 350°C, brought about by addition of 20 wt.% SiC. The beneficial effects of SiC additions on mechanical strength are best realized at elevated temperatures. Fine precipitates of various intermetallics (CuAl2, Mg2Zn11, etc.) are found to be present in the solutionised and aged samples in addition to fine dispersoids of Al2O3, MgAl2O4, etc. inherited from the as-milled powders and/or formed during subsequent degassing and consolidation.

Quasirandom spanning tree model for the early river network.

Abstract: We consider a model for the formation of a river network in which the erosion process plays a role only at the initial stage. Once a global connectivity is achieved, no further evolution takes place. In spite of this, the network reproduces approximately most of the empirical statistical results of the natural river network. It is observed that the resulting network is a spanning tree graph and therefore this process could be looked upon as a new algorithm for the generation of spanning tree graphs in which different configurations occur quasirandomly. A new loopless percolation model is also defined at an intermediate stage of evolution of the river network.

Fractionation of heavy metals and phosphorus in suspended sediments of the Yamuna river, India.

Abstract: The Yamuna river is the largest tributary of the Ganges river system. It originates in the Himalayas and flows through a varied geological terrain encompassing a large basin area. Metals Fe, Mn, Pb, Zn, Cu in different chemical fractions of suspended sediments such as exchangeable, carbonates, Fe−Mn oxides, organics and residual fractions were studied. Phosphorus associated with different chemical forms are discussed. The metals are mostly associated with residual fractions in the sediments followed by organics, Fe−Mn oxides, exhangeable and carbonates. Intensive use of chemical fertilizers and pesticides in agriculture in the basin affects the high inorganic phosphorus content in sediments.