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.

Effect of cure temperature on the structure and water absorption of epoxy/amine thermosets

Abstract: Dielectric and rheological measurements are reported on the effect of cure temperature on the water absorption of tris[(2,3-epoxypropoxy)phenyl] methane cured with a 1 : 1 stoichiometric ratio of 4,4′-diaminodiphenylsulphone. Analysis of the water absorption characteristics of these materials using a combination of dielectric and gravimetric measurements has indicated that water molecules can be found in two distinctly different types of environments. There are water molecules which are strongly interacting with polar groups and water molecules clustered together into sub-micro-scale cavities within the matrix structure. Changes in the final cure temperature have the effect of changing both the extent and distribution of the types of water molecules present in the matrix. Validation of the diffusion coefficients obtained from the dielectric analysis is based on a comparison with gravimetric data and the implications are discussed. Differences observed between these two different types of measurement are related to peculiarity in the dielectric method and its extreme sensitivity to interfacial phenomena.

Adjoint-based optimization of steady suction for disturbance control in incompressible flows

Abstract: The optimal distribution of steady suction needed to control the growth of single or multiple disturbances in quasi-three-dimensional incompressible boundary layers on a flat plate is investigated. The evolution of disturbances is analysed in the framework of the parabolized stability equations (PSE). A gradient-based optimization procedure is used and the gradients are evaluated using the adjoint of the parabolized stability equations (APSE) and the adjoint of the boundary layer equations (ABLE). The accuracy of the gradient is increased by introducing a stabilization procedure for the PSE. Results show that a suction peak appears in the upstream part of the suction region for optimal control of Tollmien–Schlichting (T–S) waves, steady streamwise streaks in a two-dimensional boundary layer and oblique waves in a quasi-three-dimensional boundary layer subject to an adverse pressure gradient. The mean flow modifications due to suction are shown to have a stabilizing effect similar to that of a favourable pressure gradient. It is also shown that the optimal suction distribution for the disturbance of interest reduces the growth rate of other perturbations. Results for control of a steady cross-flow mode in a three-dimensional boundary layer subject to a favourable pressure gradient show that not even large amounts of suction can completely stabilize the disturbance.

Superexchange in the cuprates.

Abstract: It is shown that the usual three-center cation-anion-cation model or fourth-order pertrubation expression (in the Cu-O hopping parameter ${\mathit{t}}_{\mathit{p}\mathit{d}}$) does not give a useful (even qualitative) estimate for the superexchange J, even though, by coincidence, it predicts a magnitude that is reasonable. There are two additional contributions, both due to the oxygen-oxygen hopping ${\mathit{t}}_{\mathit{p}\mathit{p}}$, which are estimated to be responsible for about 2/3 of the total exchange interaction. The first causes a strong enhancement of the usual fourth-order J and is conventional in the sense that it goes like 1/U for a large on-site Coulomb interaction U on copper and oxygen, and is always antiferromagnetic (AFM). Its importance is due to a large prefactor in the perturbation series in ${\mathit{t}}_{\mathit{p}\mathit{p}}$/\ensuremath{\Delta}, where \ensuremath{\Delta} is the charge-transfer energy. The second AFM term is of a topological nature in the sense that its sign is determined by the signs of the different hopping parameters and the arrangement of the Cu and O sites in the ${\mathrm{CuO}}_{2}$ planes. This term does not involve doubly occupied sites, and is a consequence of the extra degrees of freedom formed by the oxygen 2p orbitals. Starting from the three-band model, these two effects are examined by means of fifth-order perturbation theory and perturbation theory that involves oxygen bands explicitly. The results are compared with numerical estimates of J from finite-size clusters.