A critical evaluation of two-equation models for near wall turbulence

A variety of two-equation turbulence models,including several versions of the K-epsilon model as well as the K-omega model, are analyzed critically for near wall turbulent flows from a theoretical and computational standpoint. It is shown that the K-epsilon model has two major problems associated with it: the lack of natural boundary conditions for the dissipation rate and the appearance of higher-order correlations in the balance of terms for the dissipation rate at the wall. In so far as the former problem is concerned, either physically inconsistent boundary conditions have been used or the boundary conditions for the dissipation rate have been tied to higher-order derivatives of the turbulent kinetic energy which leads to numerical stiffness. The K-omega model can alleviate these problems since the asymptotic behavior of omega is known in more detail and since its near wall balance involves only exact viscous terms. However, the modeled form of the omega equation that is used in the literature is incomplete-an exact viscous term is missing which causes the model to behave in an asymptotically inconsistent manner. By including this viscous term and by introducing new wall damping functions with improved asymptotic behavior, a new K-tau model (where tau is identical with 1/omega is turbulent time scale) is developed. It is demonstrated that this new model is computationally robust and yields improved predictions for turbulent boundary layers

A Differential pH Requirement for Extraction of Stable Polyribosomes from Winter Barley in Relation to RNase Activity of the Tissue

The development of a systematic procedure to fractionate cells into functional components is important to many current problems in biological research. Polyribosomes are of considerable interest since it is this cell fraction which is the site of protein synthesis. They are unique structures in that they are reported to contain the transcribed genetic information in the form of messenger RNA (mRNA) which functions as the binding site for ribosomes in the decoding process. A satisfactory isolation of polyribosomes provides an opportunity to analyze the nature of this transcribed genetic message and also determine the extent to which protein synthesis is controlled by the translational process. Considerable insight into current theory of the genetic code problem has come about as a result of in vitro synthesis systems that have involved isolated ribosomes. Most of these advances have been made using single cell organisms. Efforts to duplicate these results with higher plants have been sketchy. A major obstacle has been an inability to inhibit endogenous enzymes that a.re responsible for degrading both synthetic and naturally occurring messenger RNA (mRNA). Extraction of stable polyribosomes provides reasonable assurance that single-stranded mRNA has been protected. From such preparations one would also stand the best, chance of recovering undegraded transfer RNA (tRNA) which is vital in protein synthesis but vulnerable to RNase attack. Progress toward reducing the influence of nucleases during extraction of plant tissue has been made by the use of diethyl pyrocarbonate. Weeks and Harcus have shown that it is an unstable compound although an effective one in inhibiting RNase. Should there be an interest in the isolation of immediate products resulting from the translation of the genetic message, it again becorr.es important to isolate stable polyribosomes containing nascent peptides. Petermann and Pavlovec (21) have stressed the importance of eliminating the basic cytoplasmic proteins which can be randomly bound to the ribosomal particles. They found that high salt levels during extraction were essential to obtaining stable polyribosomes from muscle tissue. Thus it becomes important to not only retain the identity of the polyribosomes, but also to achieve some compartmentalization of other cell components

Hierarchies, Networks and Local Government in Viet Nam

This article explores how decentralization supports the policy commitments made by the Viet Nam government to increase citizen participation and accountability, and to reduce poverty and regional disparities. The article includes a review of basic definitions of decentralization that places the case study in an international context, a brief look at the unique, historical context in Viet Nam and its relevance, a brief comparison of policy intention vs implementation reality, a preliminary analysis of the impact of decentralization during the period, and a concluding section on remaining challenges. The article examines the interplay between hierarchical and network organizations in Viet Nam, as stakeholders seek to better define the respective roles and authority of the party, and other public, quasi-public and private organizations at different territorial levels

Industrial location and public policy: the case of Kenya's textile industry

The purpose of this interim research report is to develop models of the relationship between industrial location and public policy in Kenya, to test the validity of these models using empirical data, and to use those parts of the models which appear to be valid to help improve policy outcomes. The approach will be contextual, in that government interventions will be considered in relation to the wider context of overall industrial, spatial and population policies, and in relation to the political economy of which these policies are a part. Data have been collected in interviews with government administrators, and with officials from 17 textile, knitwear and, clothing manufacturing firms. Data analysis will be based on procedures taken from the disciplines of political science and regional economics

Some considerations on coastal processes relevant to sea level rise

The effects of potential sea level rise on the shoreline and shore environment have been briefly examined by considering the interactions between sea level rise and relevant coastal processes. These interactions have been reviewed beginning with a discussion of the need to reanalyze previous estimates of eustatic sea level rise and compaction effects in water level measurement. This is followed by considerations on sea level effects on coastal and estuarine tidal ranges, storm surge and water level response, and interaction with natural and constructed shoreline features. The desirability to reevaluate the well known Bruun Rule for estimating shoreline recession has been noted. The mechanics of ground and surface water intrusion with reference to sea level rise are then reviewed. This is followed by sedimentary processes in the estuaries including wetland response. Finally comments are included on some probable effects of sea level rise on coastal ecosystems. These interactions are complex and lead to shoreline evolution (under a sea level rise) which is highly site-specific. Models which determine shoreline change on the basis of inundation of terrestrial topography without considering relevant coastal processes are likely to lead to erroneous shoreline scenarios, particularly where the shoreline is composed of erodible sedimentary material. With some exceptions, present day knowledge of shoreline response to hydrodynamic forcing is inadequate for long-term quantitative predictions. A series of interrelated basic and applied research issues must be addressed in the coming decades to determine shoreline response to sea level change with an acceptable degree of confidence. (PDF contains 189 pages.