Diffusing particles with electrostatic repulsion

Summary. We study a diffusion model of an interacting particles system with general drift and diffusion coefficients, and electrostatic inter-particles repulsion. More precisely, the finite particle system is shown to be well defined thanks to recent results on multivalued stochastic differential equations (see [2]), and then we consider the behaviour of this system when the number of particles goes to infinity (through the empirical measure process). In the particular case of affine drift and constant diffusion coefficient, we prove that a limiting measure-valued process exists and is the unique solution of a deterministic PDE. Our treatment of the convergence problem (as ) is partly similar to that of T. Chan [3] and L.C.G. Rogers - Z. Shi [5], except we consider here a more general case allowing collisions between particles, which leads to a second-order limiting PDE. Received: 5 August 1996 / In revised form: 17 October 1996     

Differentiability of the Value Function of Nonclassical Optimal Growth Models

We consider an optimal growth (multi-sector) model with nonconvex technology. Using the Clarke results on generalized gradients, we prove that the value function has left and right derivatives with respect to the initial capital stock, without requiring supermodularity assumptions.     

Photodiffraction in InGaAs/InGaAsP multiple quantum wells enclosed in a microcavity

We report new results on the diffraction properties of photoinduced gratings in InGaAs/InGaAsP MQW structures. The original feature of this device is that the QWs are enclosed in an asymmetric Fabry–Perot microcavity in order to increase the diffraction efficiency. We observe oscillations in the diffraction efficiency due to resonant effects in the microcavity. The experimental spectra are compared with theory. Diffraction efficiency at 1.55 μm attains a maximum value of 2.7% at a write beam fluence of 260 μ J cm⁻², and then decreases at higher fluences. We explain this phenomenon by an absorption saturation at high excitation.     

Success and survival of operational research groups—A review

Operational Research groups were established in private and public sector organizations following on from their success supporting the military in the Second World War. In the early years their numbers grew rapidly but by the mid-1970s evidence started to accumulate that they were being viewed less favourably. Many organizations chose not to set up OR groups. Established groups were closed down or dispersed to operating units in organizations, particularly during the recessionary periods. In 1993 following on the closures of some major OR groups in the UK, the Operational Research Society decided to fund research to examine those factors influencing a group's success and survival. This review, which forms part of the research, first examines the history of OR groups in the UK and North America, arguing that in practice they were one of a number of internal organizational consultancies. As such they were judged by their competitive performance. Despite the intent of the founding fathers of OR many groups found their scope was limited to mainly tactical problems using analytical model based approaches although there were exceptions that were successful in making major contributions at the strategic level also. Within their chosen fields of activity however many groups flourished whilst some failed. This review presents a comprehensive list of influences on success and survival. They include changes in the external environment, together with shifts in managerial style and culture, organizational factors, and the OR group itself. The review concludes that, while OR groups can do much to protect themselves from closure, organizational changes within a hostile managerial culture demand a creative response that acknowledges new requirements. Areas where an OR group has greatest influence over its effectiveness include its leadership and personnel, project balance and management. In addition, to be successful OR groups need to positively develop and market their services in an increasingly competitive managerial climate.     

Double-diffusive convection for a heated cylinder submerged in a salt-stratified fluid layer

Double-diffusive convection due to a cylindrical source submerged in a salt-stratified solution is numerically investigated in this study. For proper simulation of the vortex generated around the cylinder, a computational domain with irregular shape is employed. Flow conditions depend strongly on the thermal Rayleigh number, Ra _T , and the buoyancy ratio, R _ρ. There are two types of onset of instability existing in the flow field. Both types are due to either the interaction of the upward temperature gradient and downward salinity gradient or the interaction of the lateral temperature gradient and downward salinity gradient. The onset of layer instability due to plume convection is due to the former, whereas, the onset of layer instability of layers around the cylinder is due to the latter. Both types can be found in the flow field. The transport mechanism of layers at the top of the basic plume belongs to former while that due to basic plume and layer around the cylinder are the latter. The increase in Ra _T reinforces the plume convection and reduces the layer numbers generated around the cylinder for the same buoyancy ratio. For the same Ra _T , the increase of R _ρ suppresses the plume convection but reinforces the layers generated around the cylinder. The profiles of local Nusselt number reflects the heat transfer characteristics of plume convection and layered structure. The profiles of averaged Nusselt number are between the pure conduction and natural convection modes and the variation is due to the evolution of layers. Received on 13 September 1996