Effect of the contraction ratio upon viscoelastic fluid flow in three-dimensional square–square contractions

In this work we investigate the laminar flow through square–square sudden contractions with various contraction ratios (CR=2.4, 4, 8 and 12), using a Newtonian fluid and a shear-thinning viscoelastic fluid. Visualizations of the flow patterns were carried out using streak line photography and detailed velocity field measurements were performed using particle image velocimetry. The experimental results are compared with numerical predictions obtained using a finite-volume method. For the Newtonian fluid, a corner vortex is found upstream of the contraction and increasing flow inertia leads to a reduction of the vortex size. Good agreement is observed between experiments and numerical simulations. For the shear-thinning fluid flow a corner vortex is also observed upstream of the contraction independently of the contraction ratio. Increasing the elasticity of the flow, while still maintaining low inertia flow conditions, leads to a strong increase of the vortex size, until an elastic instability sets in and the flow becomes time-dependent at De≈200, 300, 70 and 450 for CR=2.4, 4, 8 and 12, respectively. At low contraction ratios, viscoelasticity brings out an anomalous divergent flow upstream of the contraction. For both fluids studied the flow presents a complex three-dimensional helical vortex structure which is well predicted by numerical simulations. However, for the viscoelastic fluid flow the maximum Deborah number achieved in the numerical simulations is about one order of magnitude lower than the critical Deborah number for the onset of the elastic instability found in the experiments.     

A surrogate indicator of criticality for stochastic project networks

The study of the delay that can be caused by any activity of a stochastic project network is a key topic because of the increasing importance of risk and time control in project management. The main concept adopted for this purpose has been the notion of critical activity developed for deterministic project networks but, in this paper, the inadequacy of the concept critical activity for stochastic project networks is shown and a new surrogate indicator of criticality (SIC) is built, using a regression model applied to a large set of generated project networks. This new indicator explains more than 90% of the initial variance estimated for more than 80,000 activities belonging to a wide range of project networks (580 nets), with very different morphological types.     

Using Constraint Satisfaction Problem approach to solve human resource allocation problems in cooperative health services

In developing countries, the increasing utilization of health services, due to a great life expectancy, is followed by a reduction in incomes from the public health system and from private insurance companies, to the payment of medical procedures. Beyond this scenery, it is mandatory an effective hospital cost control though the utilization of planning tools.This work is intended to contribute to the reduction of hospital costs, proposing a new tool for planning human resources utilization in hospital plants. Specifically, it is proposed a new tool for human resources allocation in health units. The solution to the allocation problem uses the CSP technique (Constraint Satisfaction Problem) associated with the backtracking search algorithm. With the objective of enhancing the backtracking search algorithm performance a new heuristics is proposed. Through some simulations the performance of the proposed heuristics is compared to the other heuristics previously published in literature: remaining minimum values, forward checking and grade heuristics.Another important contribution of this work is the mathematical modeling of the constraints, that could be unary, multiple, numeric and implicit constraints. In the results it is presented a case study of a human resource allocation in a cooperative health service.Based on the results, it is proposed that for a real allocation problems solution, the best approach is to combine the remaining minimum values heuristics with the grade heuristics, to select the best unit allocation to be filled, and then use the proposed heuristic to select the best physician to the chosen unit allocation. This association shows a satisfactory result for the human resource allocation problem of the case study, with an algorithm convergence time of 46.7 min with no backtracks. The same problem when manually resolved took about more than 50 h.     

Effects of chlorophyll fluorescence on the estimation of microphytobenthos biomass using spectral reflectance indices

The communities of benthic microalgae that form dense biofilms at the surface of aquatic sediments, or microphytobenthos, are important primary producers in estuarine intertidal flats and shallow coastal waters. The microalgal biomass present in the photic zone of the sediment is a key parameter for ecological and photophysiological studies on microphytobenthos, and has been routinely estimated using hyperspectral reflectance indices based on the chlorophyll (Chl) a red absorption peak at 675 nm, usually the Normalised Difference Vegetation Index (NDVI). This study reports that red region-based biomass indices measured on microphytobenthos biofilms can be significantly affected by the enrichment of reflected light with solar-induced Chl fluorescence emitted by the microalgae. Chl fluorescence emission peaks at 683 nm, counterbalancing the decrease in reflectance centered at 675 nm, thus causing the underestimation of NDVI. The interference of Chl fluorescence was found to be easily identified by a conspicuous double-peak feature in the 670-700 nm region of the second-derivative reflectance spectra. The fluorescence-induced NDVI underestimation was shown to be most pronounced for high surface biomass levels and low incident solar irradiance. Particular aspects of microphytobenthos biofilms, such as the increase in surface Chl fluorescence due the contribution of emission by subsurface layers, and vertical migratory responses by motile microalgae to changes in ambient light, further complicate the effects on biomass estimation using NDVI-like indices. By comparing NDVI with a fluorescence-independent biomass index for a wide range of natural light conditions, it was found that Chl fluorescence interference may cause the underestimation of microalgal biomass to reach over 25%, with errors above 10% being expected for more than half of the measuring occasions. These results indicate that the use of NDVI may compromise the correct assessment of important aspects of microphytobenthos ecology, such as the characterisation of migratory behaviour or the determination of biomass-specific productivity rates, and call for the use of alternative biomass indices, not based on the Chl a red absorption peak.     

Loss probability estimation and control for OFDM/TDMA wireless systems considering multifractal traffic characteristics

In this paper, we analyze the queueing performance in terms of loss rate of an OFDM (orthogonal frequency-division multiplexing)/TDMA (time division multiplexing access) based wireless system taking into account the multifractal behavior of the wireless traffic flows. To this end, first, we show evidences of multifractal characteristics on wireless traffic traces. These findings motivated us to propose a traffic policing and control scheme based on a multifractal envelope process in order to maintain the traffic flows well-behaved, i.e., in accordance to the desired QoS parameters. Furthermore, by assuming a multifractal traffic model, we derive a data loss probability equation for wireless traffic flows that was applied to the OFDM/TDMA based wireless system. Simulations and comparisons to other methods were carried out in order to verify the efficiency of the proposed traffic policing scheme as well as of the loss probability estimation approach.     

Forced convection in electro-osmotic/Poiseuille micro-channel flows of viscoelastic fluids: fully developed flow with imposed wall heat flux

The analytical solution for heat transfer in a dynamic and thermally fully developed channel flow of the simplified Phan-Thien–Tanner fluid induced by combined electro-osmosis and pressure gradient was obtained assuming that material properties are independent of temperature. The flow forcing was quantified by an appropriate dimensionless parameter and its effect and that of all other relevant dimensionless numbers is presented and discussed. Specifically, the forced convection occurs under conditions of constant wall heat flux and the solution includes the effects of Weissenberg number, electric double layer (EDL) thickness, forcing ratio parameter, viscous dissipation as well as of Joule heating due to the electric currents and was obtained under the simplifying Debye–Hückel approximation. Generally speaking, the Joule effect is stronger than the viscous dissipation except in very narrow channels, but these fall outside the validity of the Debye–Hückel conditions. For pure electro-osmosis, viscous dissipation is restricted to the near-wall region and virtually nonexistent elsewhere, so it is irrelevant for thin electric double layers and Joule heating is more relevant. As the EDL thickens and/or the pressure gradient contribution increases, the role of viscous dissipation grows and shear-thinning effects also appear more clearly on the Nusselt number. Generally speaking, an increase in internal heating results in lower Nusselt numbers and this effect is stronger than the effect of shear-thinning, which is responsible for a slight increase in the Nusselt number.     

Model reduction by CPOD and Kriging

This paper proposes a novel approach for multi-objective optimization when the criteria of interest rely on a functional output from an expensive-to-evaluate numerical simulator. More specifically, the proposed method is developed in the frame of an automotive application. The aim of this application is to design the shape of an intake port in order to maximize the mass flow (denoted by Q) and the tumble (denoted by T), which both depend on a 3D velocity field obtained by numerical flow simulation. Since the considered flow simulator is time-consuming, using regular multi-objective genetic algorithms (MOGA) directly on integral quantities depending on the simulator output is prohibitive. Three different Reduced Order Models (ROMs) are presented. The first one consists in directly Kriging the integral quantities Q and T on the basis of the outputs computed at an initial design of experiments, and basing the optimization search on the sequentially obtained couples of response surfaces. The other methods explored in the present work consist in building a parametrized representation of the whole velocity field by different variants of the Proper Orthogonal Decomposition (POD). Instead of directly Kriging Q and T at un-sampled locations, the proposed technique is hence to proceed in two steps: first approximate the functional outcome by Kriging the POD coefficients, and then compute the integral quantities Q and T associated with the approximate 3D field. However, such an approach induces new difficulties since the truncated POD does not preserve the global (integrated) quantities, and that surrogate-based MOGA with this kind of POD are therefore likely to fail locating the (Q, T)-Pareto front accurately. This is what motivates to propose an original constrained POD method (called CPOD) meant to overcome the bias created by the truncation made in regular POD. More precisely, this means modifying the way of calculating the POD coefficients by imposing the integral quantities Q and T based on the truncated POD to match with the actual Q and T values obtained by flow simulation at the design of experiments. A detailed comparison of the Pareto sets obtained from the three ROMs demonstrates the interest of the CPOD approach.     

Query expansion using an immune-inspired biclustering algorithm

Query expansion is a technique utilized to improve the performance of information retrieval systems by automatically adding related terms to the initial query. These additional terms can be obtained from documents stored in a database. Usually, this task is performed by clustering the documents and then extracting representative terms from the clusters. Afterwards, a new search is performed in the whole database using the expanded set of terms. Recently, the authors have proposed an immune-inspired algorithm, namely BIC-aiNet, to perform biclustering of texts. Biclustering differs from standard clustering algorithms in the sense that the former can detect partial similarities in the attributes. The preliminary results indicated that our proposal is able to group similar texts effectively and the generated biclusters consistently presented relevant words to represent a category of texts. Motivated by this promising scenario, this paper better formalizes the proposal and investigates the usefulness of the whole methodology on larger datasets. The BIC-aiNet was applied to a set of documents aiming at identifying the set of relevant terms associated with each bicluster, giving rise to a query expansion tool. The obtained results were compared with those produced by two alternative proposals in the literature, and they indicate that these techniques tend to generate complementary results, as a consequence of the use of distinct similarity metrics.     

A hierarchical model for concurrent material and topology optimisation of three-dimensional structures

This paper presents an extension of the hierarchical model for topology optimisation to three-dimensional structures. The problem addressed covers the simultaneous characterisation of the optimal topology of the structure and the optimal design of the cellular material used in its construction. In this study, hierarchical suggests that the optimisation model works at two interconnected levels, the global and local levels identified, respectively, with the structure and its material. The class of cellular materials, defining the material microstructure, is restricted to single scale cellular materials, with the cell geometry locally optimised for the given objective function and constraints. The model uses the asymptotic homogenisation model to obtain the equivalent material properties for the specific local microstructures designed using a SIMP based approach. The necessary optimality conditions for the hierarchical optimal design problem are discussed and approximated numerically by a proper finite element discretisation of the global and local analysis and design problems. Examples to explore and demonstrate the model developed are presented.