Combined vertex-based - cell-centred finite volume method for flows in complex geometries

CFD modelling of 'real-life' processes often requires solutions in complex three dimensional geometries, which can often result in meshes where aspects of it are badly distorted. Cell-centred finite volume methods, typical of most commercial CFD tools, are computationally efficient, but can lead to convergence problems on meshes which feature cells with high non-orthogonal shapes. The vertex-based finite volume method handles distorted meshes with relative ease, but is computationally expensive. A combined vertex-based - cell-centred (VB-CC) technique, detailed in this paper, allows solutions on distorted meshes that defeat purely cell-centred physical models to be employed in the solution of other transported quantities. The VB-CC method is validated with benchmark solutions for thermally driven flow and turbulent flow. An early application of this hybrid technique is to three-dimensional flow over an aircraft wing, although it is planned to use it in a wide variety of processing applications in the future.

The 2001 World Trade Centre evacuation

The WTC evacuation of 11 September 2001 provides an unrepeatable opportunity to probe into and understand the very nature of evacuation dynamics and with this improved understanding, contribute to the design of safer, more evacuation efficient, yet highly functional, high rise buildings. Following 9/11 the Fire Safety Engineering Group (FSEG) of the University of Greenwich embarked on a study of survivor experiences from the WTC Twin Towers evacuation. The experiences were collected from published accounts appearing in the print and electronic mass media and are stored in a relational data base specifically developed for this purpose. Using these accounts and other available sources of information FSEG also undertook a series of numerical simulations of the WTC North Tower. This paper represents an overview of the results from both studies.

Computational modeling of mold filling and related free-surface flows in shape casting: an overview of the challenges involved

Accurate representation of the coupled effects between turbulent fluid flow with a free surface, heat transfer, solidification, and mold deformation has been shown to be necessary for the realistic prediction of several defects in castings and also for determining the final crystalline structure. A core component of the computational modeling of casting processes involves mold filling, which is the most computationally intensive aspect of casting simulation at the continuum level. Considering the complex geometries involved in shape casting, the evolution of the free surface, gas entrapment, and the entrainment of oxide layers into the casting make this a very challenging task in every respect. Despite well over 30 years of effort in developing algorithms, this is by no means a closed subject. In this article, we will review the full range of computational methods used, from unstructured finite-element (FE) and finite-volume (FV) methods through fully structured and block-structured approaches utilizing the cut-cell family of techniques to capture the geometric complexity inherent in shape casting. This discussion will include the challenges of generating rapid solutions on high-performance parallel cluster technology and how mold filling links in with the full spectrum of physics involved in shape casting. Finally, some indications as to novel techniques emerging now that can address genuinely arbitrarily complex geometries are briefly outlined and their advantages and disadvantages are discussed.

On Reynolds number and scaling effects in microchannel flows

This paper presents a numerical study of the Reynolds number and scaling effects in microchannel flows. The configuration includes a rectangular, high-aspect ratio microchannel with heat sinks, similar to an experimental setup. Water at ambient temperature is used as a coolant fluid and the source of heating is introduced via electronic cartridges in the solids. Two channel heights, measuring 0.3 mm and 1 mm are considered at first. The Reynolds number varies in a range of 500-2200, based on the hydraulic diameter. Simulations are focused on the Reynolds number and channel height effects on the Nusselt number. It is found that the Reynolds number has noticeable influences on the local Nusselt number distributions, which are in agreement with other studies. The numerical predictions of the dimensionless temperature of the fluid agree fairly well with experimental measurements; however the dimensionless temperature of the solid does exhibit a significant discrepancy near the channel exit, similar to those reported by other researchers. The present study demonstrates that there is a significant scaling effect at small channel height, typically 0.3 mm, in agreement with experimental observations. This scaling effect has been confirmed by three additional simulations being carried out at channel heights of 0.24 mm, 0.14 mm and 0.1 mm, respectively. A correlation between the channel height and the normalized Nusselt number is thus proposed, which agrees well with results presented.

Approximating the evacuation of the World Trade Center North Tower using computer simulation

This article concerns an investigation of the full scale evacuation of a building with a configuration similar to that of the World Trade Center (WTC) North Tower using computer simulation. A range of evacuation scenarios is explored in order to better understand the evacuation of the WTC on 11 September 2001. The analysis makes use of response time data derived from a study of published WTC survivor accounts. Geometric details of the building are obtained from architects' plans while the total building population used in the scenarios is based on estimates produced by the National Institute of Standards and Technology formal investigation into the evacuation. This paper attempts to approximate the events of 11 September 2001 and pursue several `what if' questions concerning the evacuation. In particular, the study explores the likely outcome had a single staircase survived intact from top to bottom. More generally, this paper explores issues associated with the practical limits of building size that can be expected to be efficiently evacuated using stairs alone.

Trade unions, civil society organisations and health reforms

This article provides an analysis of resistance to neoliberalism and commodification in the public healthcare sector as seen from a trade union perspective. It uses recent research on social-movement unionism and new labour internationalism to structure a series of case studies examining resistance to different dimensions of healthcare commodification in four countries. The range of alliances trade unions are making do not fit tidily into one model, but give insights into the movement elements of trade unionism. This dimension must be strengthened, but can also be in tension with collective bargaining and other institutional processes. How to constantly reconcile these different positions is the future challenge facing trade unions.

Trade credit linkages along the supply chain: evidence for the Italian textile sector

We examine the trade credit linkages among firms within a supply chain to reckon the effect of such linkages on the propagation of liquidity shocks from downstream to upstream firms. We choose a sample appropriate for this task, consisting of a large data set of Italian firms from the textile industry, a well known example of a comprehensive manufacturing cluster featuring a large number of small and specialized firms at each level of the supply chain. The results of the analysis indicate that the level of trade credit that firms provide to their suppliers is positively related to the level of trade credit granted to their clients: when the level of trade credit granted to clients divided by sales goes up by 1, the level of trade credit provided to suppliers divided by cost-of goods-sold goes up by an amount that varies between 0,22 and 0,52. Since all firms along the chain are linked by trade credit relationships, an increase in the level of trade credit granted by wholesalers generates a liquidity cascade throughout the chain. We designate the overall increase in the level of trade credit among all firms in the chain as a result of a unitary impulse in the level of trade credit granted by wholesalers as the multiplier effect of trade credit for the industry chain. We estimate such multiplier to vary between 1.28 and 2.04. We also investigate the effect of final demand on the level of trade credit sourced by firms at various levels of the chain and, in particular, whether such effect is amplified for firms further up in the chain as a result of liquidity propagation via trade credit linkages. We uncover evidence of such amplification when the links of liquidity transmission along the chain are individually modeled and estimated. An unitary increase in wholesalers’ sales is found to produce an effect on trade payables among firms at the top of the chain (i.e., Preparers and Spinners) that is more than twice as big as the corresponding effect among firms at the bottom of the chain (i.e., Wholesalers).

Developing global trade union "space"

International unions and international solidarity strategies have been changing partly as a response to changes in the global economy. Global union federations have played an important role in expanding communication and developing alliances with other social movements. One of the issues facing trade unions wanting to be effective at an international level is to what extent national concerns can inform and even be transcended by international perspectives. Proposals for studying the global labor force "horizontally", that is, according to different categories and forms of work rather than on a nation-by-nation basis, demonstrates a recognition of global- national dimensions (Harrod and O'Brien 2002:49). This paper will discuss the development of a global trade union "space", which enables national and global unions to work together effectively, using a series of case studies drawn from the experience of global and European public service unions.

Characterising noise signals in 2-phase solid gas flows

Pneumatic conveying of powder and granular material involve the mixed flow of solid particles in air. Characterisation of solid/gas flow regimes is important for the design, operation and control of plants involving such two-phase processes. This paper describes preliminary studies directed at identifying flow regimes in solid/gas flows by analysis of the process `noise' signals from a flow transmitter which has a relatively wide frequency response.

Laser Doppler velocimetry measurements of particle velocity profiles in gas-solid two-phase flows

The measurement of particle velocities in two-phase gas-solid systems has a wide application in flow monitoring in process plant, where two-phase gas-solids systems are frequently employed in the form of pneumatic conveyors and solid fuel injection systems. Such measurements have proved to be difficult to make reliably in industrial environments. This paper details particle velocity measurements made in a two phase gas-solid now utilising a laser Doppler velocimetry system. Tests were carried out using both wheat flour and pulverised coal as the solids phase, with air being used as the gaseous phase throughout. A pipeline of circular section, having a diameter of 53 mm was used for the test work, with air velocities ranging from 25 to 45 m/s and suspension densities ranging from 0.001 kg to 1 kg of solids per cubic meter of air. Details of both the test equipment used, and the results of the measurements are presented.