Drift-flux modeling of hyper-concentrated solid-liquid flows in dredging applications

Transporting large amounts of sand is mostly done hydraulically in dredging and mining. This method of sand transport is efficient and is used in land reclamation projects or extraction of oil from tar sands. Large pieces of equipment, such as pumps and pipe line systems, dredging vessels etc., are used enabling the sand water mixtures to be transported hydraulically. Therefore, a good understanding of the hydrodynamical behavior of sand water mixtures is eminent in order to further improve these kind of systems. In this thesis a numerical model has been developed which describe the hydraulic behavior of sediment fluid mixtures. In the model the volume concentration of solids varies from 0.0 to 0.6. Moreover, the model is able to describe mixtures consisting of multiple sized sand particles.Offshore and Dredging Engineerin

Advection-diffusion sediment models in a two-phase flow perspective

Sediment profiles in open channels are usually predicted by advection-diffusion models. Most basic forms consider the terminal settling velocity of a single particle in still clear water. Alternative forms account for hindered settling at higher concentrations. It is not known, however, how these modifications relate to mass and momentum conservation of each phase. For dilute flow, it is known that the original form can be derived from a two-phase analysis, assuming a dilute suspension, neglect of inertial effects in the momentum balance and using a linear drag force formulation. Here we study how and if it is possible to understand the hindered-settling modifications for the non-dilute case, and formulate a relation between advection-diffusion models and parameters involved in the turbulent drag force. This note verifies that the transient two-phase flow solutions converge to steady state, and compares the results to experimental data.Offshore and Dredging Engineerin

An extension of the drift-flux model for submarine granular flows

To model submarine flows of granular materials we propose an extension of the drift-flux approach. The extended model is able to represent dilute suspensions as well as dense granular flows. The dense granwular flow is modelled as a Herschel–Bulkley fluid, with a yield stress that depends on the dispersed phase pressure. Qualitative numerical experiments show that the model is able to correctly reproduce the stability of submerged sand heaps with different internal angles of friction and initial slopes. When initially starting with heaps with an angle smaller than the internal angle of friction, the heaps are stable. When starting with heaps with angles larger than the internal angle of friction, a flow of solid material is initiated. The flow later stops when the bed is at an angle smaller than the internal angle of friction.Offshore and Dredging Engineerin

Systematic review of economic aspects of alternative modes of delivery

Objective To carry out a systematic review of the literature relating to economic aspects of alternative modes of delivery. Methods A comprehensive literature search of the years 1990–1999 was conducted of electronic and non-electronic sources using a tested search strategy. Papers considered to contain useful cost or resource use data were read in full and classified according to their relevance to the review and their methodological quality. Relevant cost and resource use data were converted to £ sterling and inflated to 1998-1999 price levels. Results The literature search resulted in 975 papers, 49 of which met criteria for the review. Thirty-two papers were from the USA where the organisation, structure and costs of health care are significantly different from that of other industrialised countries. The aggregate costs of different modes of delivery reported in these American studies were between four and five times higher than costs reported in other studies. The majority of included studies were of poor quality. Data from the better quality studies demonstrated that caesarean section costs a health service substantially more than other modes of delivery. The range of costs of an uncomplicated vaginal delivery were £629-£1298 compared with £1238-£3551 for a caesarean section. However, papers have so far only considered short term health service costs. Conclusions Research is required to estimate the cost and resource use attributable to alternative modes of delivery. Future research should investigate the long term health service costs and the costs that arise outside the health service which are likely to vary according to mode of delivery