Dynamic simulation of gas-liquid two-phase flow: effect of column aspect ratio on the flow structure.

In this paper an Eulerian/Lagrangian model, describing the hydrodynamics of a gas-liquid bubble column, is presented. The model resolves the time dependent, two-dimensional motion of small, spherical gas bubbles in a liquid using the equation of motion. The model incorporates all relevant forces acting on a bubble as it rises through the liquid, and additionally accounts for direct bubble-bubble interactions. The liquid-phase hydrodynamics are described using the volume-averaged Navier-Stokes equations. This model is used to study the hydrodynamic behaviour of bubble columns with aspect ratios ranging from 1.0 to 11.4. In addition to these theoretical results, experimental observations are presented of the flow structure in a pseudo-two-dimensional bubble column with different aspect ratios. A clear transition in the gas-liquid flow pattern could be observed, both experimentally and theoretically, from the well-known `cooling tower¿ mode of circulation (L/D = 1.0) to the staggered vortices mode of circulation (L/D 2.0). The computational results clearly showed the presence of vortical structures in the liquid phase at aspect ratios exceeding 2.0. These vortical structures in the liquid phase were studied experimentally using neutrally buoyant tracer particles and streak photography. The experimentally observed vortical structures are shown to resemble the computed structures

Computational fluid dynamics applied to gas-liquid contactors.

In this paper a `hierarchy of models¿ is discussed to study the fluid dynamic behaviour of gas-liquid bubble columns. This `hierarchy of models¿ consists of a Eulerian-Eulerian two fluid model, a Eulerian-Lagrangian discrete bubble model and a Volume Tracking or Marker Particle model. These models will be briefly reviewed and their advantages and disadvantages will be highlighted. In addition, a mixed Eulerian-Lagrangian model and a volume tracking model, both developed at Twente University, will be discussed. Some selected results obtained with these models will be presented with emphasis on the results obtained with the volume tracking model. Finally, a brief discussion on advanced experimental techniques, which reflect the recent progress in experimental fluid dynamics, will be presente

Numerical Simulation of Bubble Coalescence using a Volume of Fluid (VOF) Model

This paper presents a Volume Tracking model developed with the specific objective of studying the time – dependent behavior of multiple, “large” gas bubbles rising in an initially quiescent liquid. The model, based on the Volume – Of – Fluid concept, employs an advanced interface tracking scheme known as Youngs’ VOF to advance the gas – liquid interface through the Eulerian mesh. Additionally, the model solves the incompressible Navier – Stokes equations to obtain the flow field. Results obtained for four different cases will be discussed: the formation and rise of a skirted bubble and of a spherical cap bubble, the coalescence of two identical gas bubbles and the behavior of two gas bubbles emanating from two adjacent orifices. It could be concluded that the Volume Tracking model is able to track the motion of a gas – liquid interface, subject to appreciable changes in its topology, embedded in a flow field with significant vorticity

CESAM : The CCSO annual model of the Dutch economy

This paper presents CESAM, a macroeconometric model of the Dutch economy based on annual data. CESAM can be characterized as a Keynesian expenditure model including a neoclassical production model and a post-Keynesian financial model. This characterization holds for most of the Dutch macroeconometric models including, for instance, FREIA-KOMPAS of the Dutch Central Planning Bureau. There are, however, some interesting features that distinguish CESAM from other Dutch models: the production structure is based on a putty-clay vintage approach; the financial model is based on a system of financial accounts and is modelled using the portfolio approach; and the institutional structure of Dutch public finance is described in detail. The main objectives in using the model are to generate medium-term forecasts of the Dutch economy and to analyse economic policy

Dynamic simulation of dispersed gas-liquid two-phase flow using a discrete bubble model.

In this paper a detailed hydrodynamic model for gas-liquid two-phase flow will be presented. The model is based on a mixed Eulerian-Lagrangian approach and describes the time-dependent two-dimensional motion of small, spherical gas bubbles in a bubble column operating in the homogeneous regime. The motion of these bubbles is calculated from a force balance for each individual bubble, accounting for all relevant forces acting on them. Contributions from liquid-phase pressure gradient, drag, virtual mass, liquid-phase vorticity and gravity are considered, whereas direct bubble-bubble interactions are accounted for via an interaction model resembling the collision model developed by Hoomans et al. (1996) to model gas-fluidized beds. The liquid-phase hydrodynamics are described using the volume-averaged, unsteady, Navier-Stokes equations. A preliminary model validation has been performed by comparing the computational results with experimental observations published previously in literature by various authors. The model is shown to predict correctly the motion of a bubble plume in a pseudo-two-dimensional bubble column operated at different superficial gas velocities, provided that a detailed description of the bubble dynamics is incorporated in the model. The effect of bubble column aspect ratio on the hydrodynamic behaviour of the column has also been investigated. Our model predicts the effect of aspect ratio on the flow structure in the bubble column. The importance of the various forces acting on the bubbles will also be discussed and it will be shown that the added mass force and the lift force cannot be neglected in bubble column simulation. Finally, the model has been used to study the start-up behaviour of a two-dimensional bubble column. It will be shown that the history of the gas-liquid two-phase flow significantly affects the flow structure ultimately obtained in a bubble column. This finding has, to our knowledge, not been reported before in literature

From meadows to milk to mucosa – adaptation of Streptococcus and Lactococcus species to their nutritional environments

Lactic acid bacteria (LAB) are indigenous to food-related habitats as well as associated with the mucosal surfaces of animals. The LAB family Streptococcaceae consists of the genera Lactococcus and Streptococcus. Members of the family include the industrially important species Lactococcus lactis, which has a long history safe use in the fermentative food industry, and the disease-causing streptococci Streptococcus pneumoniae and Streptococcus pyogenes. The central metabolic pathways of the Streptococcaceae family have been extensively studied because of their relevance in the industrial use of some species, as well as their influence on virulence of others. Recent developments in high-throughput proteomic and DNA-microarray techniques, in in vivo NMR studies, and importantly in whole-genome sequencing have resulted in new insights into the metabolism of the Streptococcaceae family. The development of cost-effective high-throughput sequencing has resulted in the publication of numerous whole-genome sequences of lactococcal and streptococcal species. Comparative genomic analysis of these closely related but environmentally diverse species provides insight into the evolution of this family of LAB and shows that the relatively small genomes of members of the Streptococcaceae family have been largely shaped by the nutritionally rich environments they inhabit.

Nanoscale coherent imaging of photonic structures by PSTM

We present an alternative instrument to map local optical field distributions: a photon scanning tunneling microscope (PSTM). In a PSTM a near-field optical fiber probe is used to frustrate the evanescent field above an integrated optical device. The evanescent wave is converted into a propagating wave that is coupled into the fiber, guided through it and subsequently detected by a photomultiplier tub

The path from schizotypy to depression and aggression and the role of family stress

Background. Schizotypy is a multidimensional construct that is linked to the vulnerability for psychosis. Positive schizotypy includes having paranormal beliefs. Negative schizotypy includes social anhedonia. Disorganized schizotypy includes social anxiety and communication disorder. Schizotypy relates to depression and aggression. Family stress from high expressed emotion (EE; a rating of criticism, hostility, and emotional overinvolvement in a close relative toward a person showing signs of mental disorder) may mediate the link between schizotypy, depression and aggression. This study tested, using path analyses, the hypotheses that schizotypy predicts depression and aggression through high perceived EE as criticism and irritability (hypothesis 1) and praise and intrusiveness in a close relative (hypothesis 2). Methods. One hundred and four healthy participants listened to and rated the self-relevance of standard criticism and standard praise that denote EE. Participants rated their level of schizotypy, depression, aggression, and perceived EE in self-report questionnaires. Two path models tested the hypotheses. Results. Disorganized schizotypy, more than positive schizotypy, predicted the path to depression and aggression when perceived criticism and perceived EE-irritability were mediators. Disorganised schizotypy, more than negative schizotypy, predicted the path to depression and aggression when perceived praise and perceived EE-intrusiveness were mediators. Conclusions. Greater perceived criticism and less perceived praise in family communication explain the path from disorganized schizotypy (more so than positive or negative schizotypy) to depression and aggression. These findings indicate a need to consider the thought disorder-EE link as a potential contributor to depression and aggression in people with schizophrenia

Local phase measurements of light in a one-dimensional photonic crystal

For the first time the local optical phase evolution in and around a small, o­ne-dimensional photonic crystal has been visualized with a heterodyne interferometric photon scanning tunnelling microscope. The measurements show an exponential decay of the optical intensity inside the crystal, which consists of a periodic array of subwavelength air rods fabricated in a conventional ridge waveguide. In addition it is found that the introduction of the air rods has a counter- intuitive effect o­n the phase development inside the structure. The heterodyne detection scheme allows the detection of low- intensity scattered wanes. In the vicinity of the scattering air rods phase singularities are found with a topological charge of plus or minus o­n