Mean flow instabilities of two-dimensional convection in strong magnetic fields

The interaction of magnetic fields with convection is of great importance in astrophysics. Two well-known aspects of the interaction are the tendency of convection cells to become narrow in the perpendicular direction when the imposed field is strong, and the occurrence of streaming instabilities involving horizontal shears. Previous studies have found that the latter instability mechanism operates only when the cells are narrow, and so we investigate the occurrence of the streaming instability for large imposed fields, when the cells are naturally narrow near onset. The basic cellular solution can be treated in the asymptotic limit as a nonlinear eigenvalue problem. In the limit of large imposed field, the instability occurs for asymptotically small Prandtl number. The determination of the stability boundary turns out to be surprisingly complicated. At leading order, the linear stability problem is the linearisation of the same nonlinear eigenvalue problem, and as a result, it is necessary to go to higher order to obtain a stability criterion. We establish that the flow can only be unstable to a horizontal mean flow if the Prandtl number is smaller than order , where B0 is the imposed magnetic field, and that the mean flow is concentrated in a horizontal jet of width in the middle of the layer. The result applies to stress-free or no-slip boundary conditions at the top and bottom of the layer

Combustor flame flashback

A stainless steel, two-dimensional (rectangular), center-dump, premixed-prevaporized combustor with quartz window sidewalls for visual access was designed, built, and used to study flashback. A parametric study revealed that the flashback equivalence ratio decreased slightly as the inlet air temperature increased. It also indicated that the average premixer velocity and premixer wall temperature were not governing parameters of flashback. The steady-state velocity balance concept as the flashback mechanism was not supported. From visual observation several stages of burning were identified. High speed photography verified upstream flame propagation with the leading edge of the flame front near the premixer wall. Combustion instabilities (spontaneous pressure oscillations) were discovered during combustion at the dump plane and during flashback. The pressure oscillation frequency ranged from 40 to 80 Hz. The peak-to-peak amplitude (up to 1.4 psi) increased as the fuel/air equivalence ratio was increased attaining a maximum value just before flashback. The amplitude suddenly decreased when the flame stabilized in the premixer. The pressure oscillations were large enough to cause a local flow reversal. A simple test using ceramic fiber tufts indicated flow reversals existed at the premixer exit during flickering. It is suspected that flashback occurs through the premixer wall boundary layer flow reversal caused by combustion instability. A theoretical analysis of periodic flow in the premixing channel has been made. The theory supports the flow reversal mechanism

Chain-wide learning for inclusive agrifood market development : a guide to multi-stakeholder processes for linking small-scale producers to modern markets

This guide provides a set of concepts and analytical tools for finding ways to better link small-scale producers to the modern markets associated with today’s largescale supermarket retail and wholesale operations. It is has been developed through iterative testing with partners in several organisations and countries. The guide is a product of the Regoverning Markets Programme, a multi-agency programme to generate strategic information and anticipatory policy advice on small-scale producers in these fast changing markets

A study of the printed books in French in Bishop Cosin’s library

In Chapter I a brief account is given of the life of John Cosin, with special emphasis on his relations with France. Then follows an assessment of John Cosin as a lover of books and book collecting, including the formation and early history of his library. The Chapter ends with a summary of the history of the Episcopal library from 1672 to the present day. In Chapter II the French historical and literary background to the acquisition and the contents of the Cosin French books is outlined in order to place the collection in its true context. Detailed references to the Collection are provided in footnotes. In Chapter III the theological systems which predominated in France at the time are outlined, namely those of Calvin and the Counter Reformation, since most of the works under consideration are serious religious writings deriving from Calvinist and contemporary Roman Catholic sources. As in Chapter II references to specific items in the library are given in footnotes. In Chapter IV the composition of the French section of the library is surveyed in detail, from the point of view of both its bibliographical and subject interest. Chapter V is concerned with the Protestant theologian Pierre du Moulin, who is the most profusely represented French writer in the Collection under review in this study. A short account of Du Moulin’s position in relation to the library and its founder is given: then bibliographical descriptions are provided of all the French items in the library by and relating to this writer. In Chapter VI bibliographical descriptions are given of a further selection of the French religious writings in Bishop Cosin’s Library

A preliminary study of air-pollution measurement by active remote-sensing techniques

Air pollutants are identified, and the needs for their measurement from satellites and aircraft are discussed. An assessment is made of the properties of these pollutants and of the normal atmosphere, including interactions with light of various wavelengths and the resulting effects on transmission and scattering of optical signals. The possible methods for active remote measurement are described; the relative performance capabilities of double-ended and single-ended systems are compared qualitatively; and the capabilities of the several single-ended or backscattering techniques are compared quantitatively. The differential-absorption lidar (DIAL) technique is shown to be superior to the other backscattering techniques. The lidar system parameters and their relationships to the environmental factors and the properties of pollutants are examined in detail. A computer program that models both the atmosphere (including pollutants) and the lidar system is described. The performance capabilities of present and future lidar components are assessed, and projections are made of prospective measurement capabilities for future lidar systems. Following a discussion of some important operational factors that affect both the design and measurement capabilities of airborne and satellite-based lidar systems, the extensive analytical results obtained through more than 1000 individual cases analyzed with the aid of the computer program are summarized and discussed. The conclusions are presented. Recommendations are also made for additional studies to investigate cases that could not be explored adequately during this study