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31.
Fátima C.M. Portugal Moisés L. Pinto João Pires J.M.F. Nogueira 《Journal of chromatography. A》2010,1217(23):3707-3710
Polyurethane (PU) foams were applied for stir bar sorptive extraction of five triazinic metabolites (desethyl-2-hydroxyatrazine, desisopropylatrazine, desethylatrazine, 2-hydroxyatrazine and desethylterbuthylazine) in water matrices, followed by liquid desorption and high performance liquid chromatography with diode array detection (SBSE(PU)-LD/HPLC-DAD). The optimum conditions for SBSE(PU)-LD were 5 h of extraction (1000 rpm) and 5% (v/v) of methanol for the analysis of desethyl-2-hydroxyatrazine and 2-hydroxyatrazine, 15% (w/v) of sodium chloride for the remaining compounds and acetonitrile as back-extraction solvent (5 mL) under ultrasonic treatment (60 min). The methodology provided recoveries up to 26.3%, remarkable precision (RSD < 2.4%), excellent linear dynamic ranges between 5.0 and 122.1 μg/L (r2 > 0.9993) and convenient detection limits (0.4-1.3 μg/L). The proposed method was applied in the analysis of triazinic metabolites in tap, river and ground waters, with remarkable performance and negligible matrix effects. The comparison of the recoveries obtained by PU and commercial stir bars was also performed, where the yields achieved with the former were up to ten times higher proving that PU is appropriate for analysis at trace level of this type of polar compounds in water matrices. 相似文献
32.
Resolution of computational aeroacoustics problems on unstructured grids with a higher-order finite volume scheme 总被引:1,自引:0,他引:1
X. Nogueira I. Colominas L. Cueto-Felgueroso F. Navarrina 《Journal of Computational and Applied Mathematics》2010,234(7):2089-2097
Computational fluid dynamics (CFD) has become increasingly used in the industry for the simulation of flows. Nevertheless, the complex configurations of real engineering problems make the application of very accurate methods that only work on structured grids difficult. From this point of view, the development of higher-order methods for unstructured grids is desirable. The finite volume method can be used with unstructured grids, but unfortunately it is difficult to achieve an order of accuracy higher than two, and the common approach is a simple extension of the one-dimensional case. The increase of the order of accuracy in finite volume methods on general unstructured grids has been limited due to the difficulty in the evaluation of field derivatives. This problem is overcome with the application of the Moving Least Squares (MLS) technique on a finite volume framework. In this work we present the application of this method (FV-MLS) to the solution of aeroacoustic problems. 相似文献
33.
Hector Gomez Xesús Nogueira 《Communications in Nonlinear Science & Numerical Simulation》2012,17(12):4930-4946
The Swift–Hohenberg equation is a central nonlinear model in modern physics. Originally derived to describe the onset and evolution of roll patterns in Rayleigh–Bénard convection, it has also been applied to study a variety of complex fluids and biological materials, including neural tissues. The Swift–Hohenberg equation may be derived from a Lyapunov functional using a variational argument. Here, we introduce a new fully-discrete algorithm for the Swift–Hohenberg equation which inherits the nonlinear stability property of the continuum equation irrespectively of the time step. We present several numerical examples that support our theoretical results and illustrate the efficiency, accuracy and stability of our new algorithm. We also compare our method to other existing schemes, showing that is feasible alternative to the available methods. 相似文献
34.
Peixoto L Min B Martins G Brito AG Kroff P Parpot P Angelidaki I Nogueira R 《Bioelectrochemistry (Amsterdam, Netherlands)》2011,81(2):99-103
The biological oxygen demand (BOD) may be the most used test to assess the amount of pollutant organic matter in water; however, it is time and labor consuming, and is done ex-situ. A BOD biosensor based on the microbial fuel cell principle was tested for online and in situ monitoring of biodegradable organic content of domestic wastewater. A stable current density of 282±23mA/m(2) was obtained with domestic wastewater containing a BOD(5) of 317±15mg O(2)/L at 22±2°C, 1.53±0.04mS/cm and pH 6.9±0.1. The current density showed a linear relationship with BOD(5) concentration ranging from 17±0.5mg O(2)/L to 78±7.6mg O(2)/L. The current generation from the BOD biosensor was dependent on the measurement conditions such as temperature, conductivity, and pH. Thus, a correction factor should be applied to measurements done under different environmental conditions from the ones used in the calibration. These results provide useful information for the development of a biosensor for real-time in situ monitoring of wastewater quality. 相似文献
35.
The aim of this paper is to investigate the influence of uncertainties on the torsional vibration of drill-strings, in order to find out which uncertainty affects most significantly the torsional stability. The unstable torsional behavior is commonly associated to polycrystalline diamond compact bits, and manifests itself in the form of stick-slip oscillations. The stick-slip is a severe type of self-excited vibration characterized by large fluctuations in the rotation of the bit. It not only increases the bit wear, but also can cause drill-string failures. The analysis were done using a mathematical model of the drill-string based on classical torsion theory discretized by means of the finite element method. The bit-rock torque was included in the model as a nonlinear boundary condition at the bottom end of the drill-string. The values of the model parameters are typical values of a real drilling situation, which are subject to a high degree of uncertainty, what justifies a stochastic analysis. We have built probability distributions for the uncertain parameters and used Monte Carlo method to obtain the stochastic stability maps. 相似文献
36.
Local field correction particle image velocimetry (LFC-PIV) has become an established alternative among high-resolution PIV techniques. Previous works by the authors introduced its implementation by means of simple algorithms. In these works the initial limitation of the method, which was related to the mean distance between particles, was removed. Comparison with other contemporary high-resolution techniques indicates that it offers advantages in robustness and accuracy. The trade-off for this better performance is a heavier computing load. Until now, the computing time that this load requires has not been characterized in detail, since this computing time could be substantially reduced by accepting a reduction in accuracy. This paper focuses on the characterization of the trade-off between time and accuracy, thus offering a new perspective to PIV. In this field, LFC-PIV offers a wide range of possibilities that are described in the paper. Several alternative schemes for LFC-PIV are tested, together with an analysis of the influence of the number of iterations. Performance figures for both accuracy and expended time are given. Metrological evaluation is carried out over synthetic images. A test of coherence between these results and the performance on real images is also presented. The paper shows that even for a limited number of iterations this technique offers advantages. 相似文献
37.
J. Nogueira A. Lecuona P. A. Rodríguez J. A. Alfaro A. Acosta 《Experiments in fluids》2005,39(2):314-321
Elsewhere in this volume (Nogueira et al. (2005) Exp Fluids, in press), the conceptual background that explains the possibility of resolving wavelengths smaller than the size of the interrogation window, with no basic restrictions but sampling, has been explained. Here, a practical implementation of the concepts is performed. To achieve this resolution in iterative PIV processing, an appropriate weighting function can be used, as commented in that reference. Here, the constraints for the design of such weighting functions are presented and analysed. This opens a line of work on possible weighting functions to develop, since the weightings used in these iterative methods, like local field correction particle image velocimetry (LFC-PIV) (Nogueira et al. (1999) Exp Fluids 27(2):107–116), have not been optimised yet. As an example, different weighting functions are commented and tested both on synthetic and real images. The results on these new weightings indicate that the current ones can be improved and the optimisation criteria are open for further advancement. 相似文献
38.
The spatial resolution of correlation particle image velocimetry (PIV) is a frequently addressed issue that still raises scientific
interest. In conventional non-iterative PIV, the spatial resolution limits are of common knowledge (Willert and Gharib (1991) Exp Fluids 10:181–193; Raffel et al. (1998) ISBN 3-540-63683-8, Springer, Berlin Heidelberg New York, among others). On the contrary, those advanced iterative multipass
methods that use image distortion techniques or multigrid techniques present a more complex scenario. One of the concepts
that raises more debate is the limiting effect of the interrogation window size. This paper focuses on the subject, trying
to clarify key points. The results indicate that iterative algorithms using an appropriate weighting function eliminate the
window size from the ensemble of spatial resolution limits. 相似文献
39.
Although alkyl carbonic acids (ACAs) and their salts are referred to as instable species in aqueous medium, we demonstrate that a monoalkyl carbonate (MAC) can in fact be easily formed from bicarbonate and an alcohol even in the presence of a high amount of water. A CE system with two capacitively coupled contactless conductivity detectors (C?Ds) was used to obtain different parameters about these species and their reactions. Based on the mobilities obtained for a series of alcohols ranging from 1 to 5 carbons, the coefficients of diffusion and the hydrodynamic radii were calculated. When compared with the equivalent carboxylates, MACs have radii systematically smaller. Although the precise pK(a) values of the ACAs could not be obtained, because of the fast decomposition in acid medium, it was possible, for the first time, to show that they are below 4.0. This result suggests that the acidity of an ACA is quite similar to the first hydrogen of H?CO?. Using a new approach to indirectly calibrate the C?D, the kinetic constants and the equilibrium constants of formation were also obtained. The results suggest that the increase in the chain length makes the MACs less stable and more inert. 相似文献
40.
Bar adsorptive micro-extraction using three powdered activated carbons (ACs) as adsorbent phases followed by liquid desorption and high performance liquid chromatography with diode array detection (BAμE(ACs)-LD/HPLC-DAD), was developed to monitor triazinic herbicides (atrazine, simazine and terbutylazine) in environmental water matrices. ACs used present apparent surface areas around 1000 m2 g−1 with an important mesoporous volume and distinct surface chemistry characteristics (pHPZC ranging from 6.5 to 10.4). The textural and surface chemistry properties of the ACs adsorbent phases were correlated with the analytical data for a better understanding of the overall enrichment process. Assays performed on 10 mL water samples spiked at the 10.0 μg L−1 levels under optimized experimental conditions yielded recoveries around 100% for the three herbicides under study. The analytical performance showed good precision (RSD < 15.0%), convenient detection limits (≈0.1 μg L−1) and suitable linearity (1.0-12.0 μg L−1) with good correlation coefficients (r2 > 0.9914). By using the standard addition method, the application of the present method on real water matrices, such as surface water and wastewater, allowed very good performances at the trace level. The proposed methodology proved to be a suitable sorptive extraction alternative for the analysis of priority pollutants with polar characteristics, showing to be easy to implement, reliable, sensitive and requiring a low sample volume to monitor triazinic compounds in water matrices. 相似文献