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Bi2Ca2Co1.7Ox bulk polycrystalline ceramics were prepared by the solid state method and by directional growth. Moreover, the effect of annealing on the textured materials has been studied. Microstructure has shown randomly oriented grains in the classical sintered materials while in the textured samples those were well oriented with their c-axis nearly perpendicular to the growth direction. Furthermore, the textured-annealed samples showed much lower amount of secondary phases than the as-grown ones. These microstructural changes are reflected on the thermoelectric properties which increase with the grain orientation and with the decrease on the secondary phase content mainly due to the electrical resistivity reduction. As a consequence, a raise on the power factor of about 6 and 9 times, compared with the classically sintered samples, was obtained for the as-grown and textured-annealed ones, respectively. The maximum power factor obtained at 650 ºC in the textured-annealed samples (~ 0.31 mW/K2m) is about 50 % higher, at the same temperature, than the obtained in one of the most used methods for texturing this kind of materials, the sinter-forging process.Se han preparado cerámicas policristalinas volumétricas de Bi2Ca2Co1.7Ox por medio del método clásico de estado sólido y por crecimiento direccional. Además, se ha estudiado el efecto del recocido en los materiales texturados. La microestructura ha mostrado granos orientados al azar en las muestras preparadas por sinterizado en estado sólido, mientras que en las muestras texturadas se orientaban con el eje c perpendicular a la dirección de crecimiento. Por otro lado, las muestras texturadas-recocidas mostraban una cantidad mucho menor de fases secundarias que las crecidas. Estos cambios en la microestructura se reflejan en las propiedades termoeléctricas, que aumentan con la orientación de los granos y con la disminución del contenido en fases secundarias, principalmente por la reducción en los valores de la resistividad eléctrica. Como consecuencia, se han producido aumentos del factor de potencia entre 6 y 9 veces los valores obtenidos en las muestras sinterizadas, en las crecidas y texturadas-recocidas, respectivamente. El máximo valor del factor de potencia se ha obtenido a 650 ºC en las muestras texturadas-recocidas (~ 0.31 mW/K2m), que es alrededor del 50 % mayor, a la misma temperatura, del que se ha obtenido en muestras procesadas por forjado en caliente, que es uno de los procesos más utilizados para texturar este tipo de materiales.

Ca3-xMgxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Mg have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Mg has been incorporated into the Ca3Co4O9 structure and no Mg-based secondary phases have been identified. Apparent density measurements have shown that samples do not modify their density until 0.05 Mg content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Mg content increases until 0.05 Mg addition. The improvement in both parameters leads to higher power factor values than the usually obtained in samples prepared by the conventional solid state routes and close to those obtained in textured materials.Se han preparado cerámicas termoeléctricas policristalinas de Ca3-xMgxCo4O9, con pequeñas cantidades de Mg, utilizando el método clásico de estado sólido. La caracterización microestructural ha mostrado que el Mg se ha incorporado en la estructura del Ca3Co4O9 y que no se han formado fases secundarias con el Mg. Las medidas de densidad aparente han mostrado que las muestras prácticamente poseen la misma densidad hasta un contenido de 0.05 Mg, disminuyendo para contenidos mayores. La resistividad eléctrica disminuye y el coeficiente Seebeck aumenta cuando el contenido de Mg se incrementa hasta valores de 0.05. La mejora de ambos parámetros conduce a valores del factor de potencia mayores que los obtenidos en muestras preparadas por la vía convencional de estado sólido, alcanzando valores próximos a los que se encuentran en materiales texturados.

Bi1.6Pb0.4Sr2Co1.8Ox thermoelectric ceramics with small Ag additions (0, 1, and 3 wt.%) have been successfully produced by a sol-gel method via nitrates. Microstructure has shown a reduction on the amount of secondary phases and an increase on the bulk density with increasing Ag contents. The microstructural evolution, as a function of Ag content, is confirmed with the electrical resistivity values which show an important decrease for the 3 wt.% Ag samples, leading to maximum power factor values of about 0.025 mW/K2.m at room temperature, which is about two times higher than the obtained for the Ag-free sintered samples.Se han preparado cerámicas termoeléctricas de composición Bi1.6Pb0.4Sr2Co1.8Ox con pequeñas adiciones de Ag (0, 1, y 3 % en peso) por medio de un método sol-gel a partir de los nitratos metálicos. La microestructura ha mostrado una reducción de la cantidad de fases secundarias y un aumento de la densidad del material al aumentar la cantidad de Ag. La evolución microestructural, en función del contenido de Ag, se ha confirmado con los valores de la resistividad eléctrica que muestra una reducción importante para las muestras con 3 % en peso de Ag, lo que lleva a alcanzar valores máximos del factor de potencia de alrededor de 0.025 mW/K2.m a temperatura ambiente, que es dos veces mayor que el obtenido para muestras sinterizadas sin Ag.

Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Bi2Sr2Co1.8Ox ceramics with improved thermoelectric properties, compared to the classical solid state reaction. The products obtained by these different methods have been compared using DTA-TGA, powder X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show higher homogeneity and lower content of secondary phases. The main differences in thermoelectrical properties are due to the decrease of electrical resistivity in samples obtained by solution methods, compared with the solid state obtained samples. Between them, the decrease is especially high for those samples prepared by the polymer solution method. Therefore, the polymeric solution synthesis route is shown to yield a power factor four times higher than the obtained for the solid state and sol-gel methods at room temperature.Se han obtenido cerámicas termoeléctricas Bi2Sr2Co1.8Ox utilizando varios métodos de síntesis, el clásico por estado sólido, sol-gel y de matriz polimérica. Se han comparado los productos obtenidos por las diferentes vías a través de técnicas de DTA-TGA, difracción de rayos X en polvo, microscopía electrónica de barrido y de caracterización termoeléctrica. Todas las muestras obtenidas por los métodos en disolución poseen una mayor homogeneidad y menor contenido de fases secundarias. La principal diferencia en las propiedades termoeléctricas es debida a la disminución de la resistividad eléctrica en las muestras obtenidas por medio de métodos en disolución, comparado con las que se obtienen por estado sólido. Entre ellas, la reducción de la resistividad es mucho mayor para las que se obtienen por el método de matriz polimérica. Además, los materiales obtenidos por este último método poseen un factor de potencia, a temperatura ambiente, cuatro veces mayor que el obtenido en las sintetizadas por los otros dos métodos estudiados.

The present study uses results of eddy‐resolving numerical simulations to investigate the open channel flow over large clusters of freshwater mussels (Unio elongatulus) partially buried in a rough, gravel bed. The density of the mussels forming the array varies from 26 to 500 mussels/m2. The flow structure is analyzed at large distances from the leading edge of the mussel bed, where the flow can be considered fully developed. The effects of changing the mussel bed density, the filtering discharge, the burial level and the roughness of the bed surface in which mussels are burrowed, are investigated in terms of flow field, turbulent structures, drag forces, and bed shear stresses. It is found that strong interactions occur between energetic eddies generated by the larger gravels on the exposed bed surface and by the mussel shells. Simulations results show that for a burial depth close to 50% and a ratio between the average gravel size and the mussel protruding height of 0.13, the shell induced turbulence becomes dominant for mussel bed densities around 50 mussels/m2. The influence of the bed roughness becomes less relevant with increasing mussel density, as the generation of energetic eddies is mostly controlled by mussel‐to‐mussel interactions. For fixed bed roughness, burial level and filtering velocity, the mean streamwise drag force and the associated drag coefficient for the exposed part of each mussel decrease with increasing mussel density, even if strong variations are observed for individual mussels. For constant mussel bed density and burial level, the mean streamwise drag force and the mean drag coefficient decrease slightly with increasing bed roughness. Increasing the burial level decreases not only the drag forces but also the drag coefficients because of the more streamlined shape of the top of the mussels. Strong active filtering acts toward decreasing the mean streamwise force and the mean drag coefficient. The spanwise drag forces contribute significantly to the total drag force, especially for high mussel bed densities. Based on smooth bed calculations, bed‐averaged shear stresses are reduced in highly dense clusters. Key Points: Mussel‐to‐mussel interactions are important for dense arrays and influence flow structure and turbulenceEddy resolving simulations showed that the effect of bed roughness become less significant with increasing mussel bed densityIn dense clusters of mussels, forces on the shells and bed shear stresses are reduced thus favoring mussel stability [ABSTRACT FROM AUTHOR]

The flow, sediment transport and bathymetry at equilibrium conditions in loose-bed open channels are predicted using a fully three-dimensional (3D) Reynolds-Averaged Navier-Stokes model that was validated for cases in which the bed-load transport is dominant. The paper focuses on two test cases corresponding to laboratory experiments performed in an S-shaped channel in which the suspended sediment load is comparable to the bed load. A modified version of the original model that accounts for gravitational force effects due to the total sediment load is proposed. It is shown that the way bed-slope effects are accounted for plays a major role in improving the predictive capabilities of the model for a case in which the equilibrium bathymetry is characterized by relatively large values and sharp variations of the transverse slope. The model is then used to qualitatively and quantitatively analyze the distributions of several flow variables that are difficult to measure in the laboratory. They include the distributions of the bed-shear stress and of a Chezy-type friction coefficient assumed to be constant by most lower-order (e.g., two-dimensional depth-averaged) models. DOI: 10.1061/(ASCE)HY.1943-7900.0000213 CE Database subject headings: River beds; Morphology; Open channels; Sediment transport. Author keywords: Bed morphology; CFD model; Open channel; Sediment transport.

This paper uses results from detached eddy simulation to reveal the dynamics of large-scale coherent eddies in the flow around a circular pier with an equilibrium scour hole. This is important for the sediment transport because the local scour process is controlled to a large extent by the large-scale coherent structures present in the near-bed region. The present paper investigates the dynamics of these coherent structures, their interactions and their role in entraining sediment in the later stages of the scour process when the horseshoe vortex system is stabilized by the presence of a large scour hole. The pier Reynolds number was 2.06 x [10.sup.5], outside the range of well-resolved large-eddy simulation (LES). Additionally, scale effects are investigated based on comparison with LES results obtained at a much lower Reynolds number of 16,000 in a previous investigation. The paper provides a detailed study of the dynamics of the main necklace vortices of the horseshoe vortex system, including an investigation of the bimodal oscillations, their effect on the amplification of the turbulence within the scour hole and the interactions of the necklace vortices with the downflow. Several mechanisms for the growth of the downstream part of the scour hole in the later stages of the scour process are discussed. Similar to the low- Reynolds-number simulation, and consistent with experimental observations, the presence of strong upwelling motions near the symmetry plane resulted in the suppression of the large-scale vortex shedding in the wake. The fact that the nondimensional values of the turbulent kinetic energy and pressure RMS fluctuations in the higher Reynolds number simulation were consistently lower inside the regions of high turbulence amplification associated with the main necklace vortex, the separated shear layers and the near-wake shows that changes in the flow and turbulence due to the Reynolds number and scour hole geometry can be quantitatively significant over Reynolds numbers between [10.sup.4] and [10.sup.5]. DOI: 10.1061/(ASCE)HY.1943-7900.0000101 CE Database subject headings: Scour; Bridges; Piers; Vortices; Eddies; Simulation; Turbulence.

Measurements of water discharge and flow velocities in riverine and tidal environments are commonly made with acoustic Doppler current profilers (ADCPs) mounted on a moving boat. This paper presents results of high-resolution Large Eddy simulations (LES) conducted to investigate two elemental error sources in ADCP measurements from a moving boat. One of these errors is due to the flow disturbance induced by the boat-mounted ADCR The other error is due to the lack of flow homogeneity in horizontal layers assumed by the ADCP algorithm to compute orthogonal velocities from the measured radial velocities along the acoustic beams. The first error is investigated by comparing LES results for an undisturbed flow field with LES results for a flow field disturbed by a boat-mounted ADCR The second error is investigated by comparing the velocities beneath the ADCP simulated by LES with virtual ADCP velocities, which are obtained by applying the ADCP algorithm to LES velocities data mined along the path of the acoustic beams of the virtual profiler. The distribution of the Reynolds stresses beneath the ADCP estimated with the ADCP algorithm from the virtual ADCP velocity data are also compared with those obtained from the LES solutions for both the undisturbed and ADCP-disturbed flows. Results show that the boat significantly disturbs the flow field and that the disturbed flow field is qualitatively different from the flow fields observed around an isolated ADCP (no boat). DOI: 10.1061/(ASCE)HY.1943-7900.0000083 CE Database subject headings: Simulation; Computational fluid dynamics technique; Acoustic techniques; Flow measurement; Turbulence; Errors; Uncertainty principles.