Defence Research Agency

About: Defence Research Agency is a based out in . It is known for research contribution in the topics: Synthetic aperture radar & Radar. The organization has 1211 authors who have published 1109 publications receiving 31542 citations.

Fibre-reinforced intermetallic compounds by physical vapour deposition

Abstract: This paper describes work on SiC fibre-reinforced γ-Tial and MoSi 2 intermetallic matrix composites (IMCs). Electron beam evaporation and vapour mixing were used to matrix-coat SiC fibre with a thick layer of intermetallic compound, and these coated fibres were successfully fabricated into IMCs. Intermetallic compounds have attractive properties for high-temperature structural applications and have been the subject of much research effort in the last 10 years. Progress has been made in improving the ductility of these rather brittle materials, particularly with the titanium aluminides Ti 3 Al and TiAl, by alloying and thermomechanical processing. However, it is unlikely that monolithic intermetallics will have the right combination of ambient temperature ductility and high-temperature strength to allow widespread commercial application. Further progress depends on the development of engineered materials which exploit the combined potential of ceramics, intermetallics and metals to give an attractive balance of properties.

Partitioned mixture distribution: an adaptive Bayesian network for low-level image processing

Abstract: Bayesian methods are used to analyse the problem of training a model to make predictions about the probability distribution of data that has yet to be received. Mixture distributions emerge naturally from this framework, but are not ideally matched to the density estimation problems that arise in image processing. An extension, called a partitioned mixture distribution is presented, which is essentially a set of overlapping mixture distributions. An expectation maximisation training algorithm is derived for optimising partitioned mixture distributions according to the maximum likelihood description. Finally, the results of some numerical simulations are presented, which demonstrate that lateral inhibition arises naturally in partitioned mixture distributions, and that the nodes in a partitioned mixture distribution network co-operate in such a way that each mixture distribution in the partitioned mixture distribution receives its necessary complement of computing machinery.

The Time-Domain Dbem for Rapidly Growing Cracks

Abstract: The time-domain Dual Boundary Element Method (DBEM) is developed to analyse rapidly growing cracks in structures subjected to dynamic loads. Two-dimensional problems, where the velocity of the crack growth is constant and the path is not predefined are studied. The present method uses the dual boundary formulation, i.e. the displacement and the traction boundary integral equations to obtain the solution by discretizing the boundary of the body and the crack surfaces only. The crack growth is modelled by adding new elements ahead of the crack tip. It is assumed that the direction of the increment is perpendicular to the direction of maximum circumferential stress. The method is used to analyse growing cracks in infinite sheet and finite plates, and the results are compared with other reported solutions, showing good agreement. © 1997 by John Wiley & Sons, Ltd.

Comparison of calculated and measured peroxide data collected in marine air to investigate prominent features of the annual cycle of ozone in the troposphere

Abstract: Large amounts of data on the concentration of peroxides have been collected in vertical profiles over the North Atlantic Ocean by a Hercules aircraft. The measured peroxide concentrations have been compared with concentrations calculated by a simple algorithm derived assuming that the standing peroxide concentration is in equilibrium with its production and loss processes. In clean air where the peroxide and ozone concentrations are anticorrelated throughout the profile measured and calculated peroxide concentrations coincide, whereas in layers of polluted air within the profile, as determined from positive ozone peroxide correlations, calculated peroxide concentrations are greatly in excess of measured values. Using the degree of correlation between measured and calculated peroxide concentrations as a diagnostic, it is possible to show that many aspects of the seasonal cycle of ozone are caused by in situ tropospheric chemistry. Thus the summer minimum in the seasonal cycle of ozone, observed at clean marine ground-based sites such as Mace Head, is due to photochemical destruction, and elevated levels of ozone are associated with the transport of polluted air, on occasion over thousands of kilometers. Of particular interest if our analysis is correct, the broad maximum of ozone occurring between March and May at ground-based sites has a large contribution from ozone formed by tropospheric as well as stratospheric chemistry.

Systolic array for recursive least squares by inverse updating

Abstract: A novel systolic array is described for recursive least squares estimation based on the method of ‘inverse updating’. It achieves an O(n0) throughput rate with O(n2) parallelism.