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Nonlinearity of density stratification modulates buoyancy effects. We report results from a body-inclusive large eddy simulation (LES) of a wake in nonlinear stratification, specifically for a circular disk at diameter-based Reynolds number ($Re$) of $5000$. Five density profiles are considered; the benchmark has linear stratification and the other four have hyperbolic tangent profiles to model a pycnocline. The disk moves inside the central core of the pycnocline in two of those four cases and, in the other two cases with {a} shifted density profile, the disk moves partially/completely outside the pycnocline. The maximum buoyancy frequency ($N_{max}$) for all the profiles is the same. The first part of the study investigates the centered cases. Nonuniform stratification results in increasing wake turbulence relative to the benchmark owing to reduced suppression of turbulence production as well as the wave trapping in the pycnocline. Steady lee waves are also quantified to understand limitations of linear theory. The second part pays attention to the effect of a relative shift between the pycnocline and the disk. The wake defect velocity decays faster in the cases with a shift. The effect of disk location on the Kelvin wake waves (a family of steady waves within the pycnocline) and its modal form is obtained and explained by solving the Taylor-Goldstein equation. The family of unsteady internal gravity waves that are generated by the wake is also studied and the effect of disk shift is quantified., 23 pages, 15 plots, submitted to Journal of Fluid Mechanics

The analog signals generated in the read-out electronics of particle detectors are shaped prior to the digitization in order to improve the signal to noise ratio (SNR). The real amplitude of the analog signal is then obtained using digital filters, which provides information about the energy deposited in the detector. The classical digital filters have a good performance in ideal situations with Gaussian electronic noise and no pulse shape distortion. However, high-energy particle colliders, such as the Large Hadron Collider (LHC) at CERN, can produce multiple simultaneous events, which produce signal pileup. The performance of classical digital filters deteriorates in these conditions since the signal pulse shape gets distorted. In addition, this type of experiments produces a high rate of collisions, which requires high throughput data acquisitions systems. In order to cope with these harsh requirements, new read-out electronics systems are based on high-performance FPGAs, which permit the utilization of more advanced real-time signal reconstruction algorithms. In this paper, a deep learning method is proposed for real-time signal reconstruction in high pileup particle detectors. The performance of the new method has been studied using simulated data and the results are compared with a classical FIR filter method. In particular, the signals and FIR filter used in the ATLAS Tile Calorimeter are used as benchmark. The implementation, resources usage and performance of the proposed Neural Network algorithm in FPGA are also presented.

Alumina/SiC ‘nanocomposites’ show significant property improvements compared with pure alumina. The improvements are thought to stem at least in part from the microstresses caused by the thermal expansion mismatch between alumina and SiC. These microstresses have been measured previously by neutron and X-Ray diffraction. This paper reports stress measurements using Cr 3+ fluorescence microscopy of the alumina matrix. The results show that although fluorescence microscopy is less powerful than the diffraction techniques in terms of the range of information provided, it does provide an alternative method of measuring subsurface microstresses in these materials which is quicker, cheaper and higher in spatial resolution.

Porous titanium specimens have been produced by means of the space holder method. Titanium grade 3 (TiCP3) has been used as material and ammonium bicarbonate as spacer. Process parameters (compaction pressure, metal grain size range and spacer volume) influence on porosity morphology and distribution and bending strength has been analysed. The results denote an important loss of strength when comparing samples sintered without and with spacer. On the other hand, a higher bending strength was observed in the porous samples with a smaller size of ammonium bicarbonate particles. Finally, the evolution of bending strength with the compaction pressure depends on the spacer volume, having a direct dependency for reduced amounts and inverse for higher contents.

Metal matrix composite (MMC) rods made of an aluminum alloy matrix with a chemical composition close to AA6061, reinforced with 5, 10, and 15 vol pct of titanium nitride particles, were produced by hot extrusion of cold compacted cylindrical forms to obtain better mechanical properties than the aluminum alloy itself. Artificial aging by heat treatment makes composites have higher increments in yield and tensile strength. Aging kinetics was analyzed by hardness and differential scanning calorimetry (DSC) measurements, showing acceleration with a direct relationship to the reinforcement percentage.

6061/X (X = AlN, Si3N4, TiN) aluminum-matrix composites have been prepared by a powder metallurgy process: by mixing aluminum alloy 6061 powder with nitride particles and extruding the mixture at high temperatures. After sintering and aging, the Vickers hardness of the composites has been measured. The results indicate that the Vickers hardness of the composites increases significantly with nitride particle content both before and after aging. The aging behavior of the composites can be interpreted in terms of a simple kinetic model, which was used earlier to predict changes in the yield strength of age-hardened unreinforced aluminum alloys. The model is shown to adequately describe the changes in the hardness of the composites. It is concluded that reinforcing nitride particles increase the precipitation rate, without influencing the nature of the precipitation process in the supersaturated solid solution.

A critical step in the processing of Metal Matrix Composites (MMCs) reinforced with ceramic particles is the insertion of these particles into the metal matrix alloy. This greatly influences the strength of the composite since it is controlled by the metal-particle interfacial bond strength. Because of the difficulty in wetting ceramic particles with molten metal, the Powder Metallurgy or PM route was developed. Powder Extrusion consolidates the composite to over 98% dense, and can be carried out below the Solidus Temperature of the alloy. The most important aspect of the microstructure is the distribution of the reinforcing particles, and this depends on the processing and fabrication routes involved, as well as the relative size of the matrix and reinforcing particles. Extrusion can homogenize the structure to some extent, but minimizing reinforcement inhomogeneity during initial processing is important to achieve optimum properties.

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No. 154. 8. L . H . H I H A R A and R. M. L A T A N I S I O N , Scr. Met. Mat. 22 (1990) 231. 9. H . H . U H L I G , “Corrosion Handbook” (John Wiley & Sons, New York, 1951) p. 165. 10. D . A L T E N P O H L and W. P O S T , J. Electrochem. Soc. 108 (1961) 628. 11. H . B O H N I and H.H. U H L I G , ibid. 116 (1969) 906. 12. Z . A . F O R O U L I S and M. J . T H U B R I K A R , ibid. 122 (1975) 1296. 13. A . B R O L I and H. H O L T A N , Corr. Sci. 13 (1973) 237. 14. V . A M I G O , J . L . O R T I Z and M.D. S A L V A D O R , Scr. Mat. 42 (2000) 383. 15. A . J . S E D R I C K S , “Corrosion of Stainless Steels” (John Wiley & Sons, New York, 1979) p. 54. 16. G . R . W A L L W O R K and B. H A R R I S , “Localized Corrosion” (NACE, Houston, TX, 1981) p. 292.

It has recently been shown that small additions of SiC to alumina can significantly improve both the surface finish for a given grinding treatment, and the resistance to severe wear. This paper describes experiments designed to obtain a clearer understanding of the mechanisms involved by correlating quantitative measurements of surface fracture during a standard abrasive wear test with the wear rate for a range of microstructures. The surface fracture parameters measured were (i) the proportion of the surface in which pieces of material had been removed by brittle fracture, and (ii) the size (equivalent circular diameter and depth) of the individual pullouts. Microstructures with systematic variations in grain size, SiC volume fraction, SiC particle size, SiC position (inter- or intra-granular) and sintering aids were tested. The results indicate that the reduction in wear rate on adding SiC or reducing the grain size is caused by a reduction in the area fraction of surface pullout by brittle fracture. For low SiC contents (~2vol%), this reduction in surface pullout is a consequence of the reduction in size of the individual pullouts. For higher SiC contents (10vol%), there is evidence that the nucleation of surface cracking is also inhibited. The underlying micromechanisms responsible for these effects are discussed.

Abstract The present paper describes the basis and applications of some gas lift troubleshooting techniques being used in the production operations of Lake Maracaibo field, production operations of Lake Maracaibo field, Venezuela. By integrating simple physical models and analytical procedures, engineers are able to diagnose a broad. procedures, engineers are able to diagnose a broad. range of gas lift wells. The presented techniques have been applied successfully on a large amount of gas lift wells, making them an alternative to some methodologies that appear in the literature. Representative field cases have been included to illustrate the explanations. Introduction In Lake Maracaibo (Fig. 1), gas lift has been used as an artificial lift method since the early sixties. As a result of the depletion of the reservoirs and the addition of new production areas, the number of wells placed under production areas, the number of wells placed under artificial lift increased. Fig. 2 illustrates the evolution of the daily average number of active wells in the 1950-1988 period. By 1988 almost 80% of the total number of active wells was produced by artificial lift and more than 95% out of this produced by artificial lift and more than 95% out of this percentage was associated to gas lift. percentage was associated to gas lift. Due to the amount of wells and the geographic extension covered by the production system, Lake Maracaibo can be considered the largest offshore gas lift oil field in the world. In such a large system, early detection of gas lift malfunctions is necessary to accomplish the desired level of production. Troubleshooting is also used to improve designing techniques by thoroughly analyzing the response of gas the wells. In this manner, continuous, intermittent, and slug gas lift designing techniques have been enhanced. The reasons listed above have justified plenty of engineering and operational work leading to the development and improvement of techniques for troubleshooting gas lift equipment under the specific production operational conditions of Lake Maracaibo. production operational conditions of Lake Maracaibo. However, the philosophy behind these methodologies can be easily applied to any other gas lift field in the world. These techniques have worked successfully in more than 6000 wells over a period of 10 years. To troubleshoot gas lift wells, designing calculations are performed in which field data is used to model the performed in which field data is used to model the system's behavior. Matching the results with the field data will indicate where the gas lift gas is being injected. Deviations between the model and field data will indicate possible problems. possible problems. The present paper concentrates on presenting numerical models as well as analytical procedures for troubleshooting continuous and intermittent gas lift wells. Representative field cases are used to illustrate the applications of these techniques. THE GAS LIFT SYSTEM The typical gas lift system to be considered for troubleshooting purposes is illustrated in Fig. 3. Gas at high pressure from compressor plants is received and distributed in the gas manifold, where gas injection rate is controlled and measured with a choke (variable orifice controller) and a gas flow recorder (orifice plate meter). In the well, gas passes from annulus to tubing through the gas lift operating valve placed in a mandrel. Fluids delivered by the reservoir are then gas lifted to the surface. P. 579

This paper presents a new optimization method for reactive-power planning of a subtransmission network under normal operating conditions. The planning problem is divided into a master problem and two subproblems. The master problem is to determine yearly investment in reactive-power compensation devices for expected load growth over a planning horizon. The subproblems are to determine the optimal operation of the power system under normal conditions. The master problem is reduced to a yearly Hamiltonian minimization problem, thereby reducing the dimensionality-of-optimization algorithm. Two subproblems, P-optimization and Q-optimization, both minimize the fuel requirement using a unified approach.

A method for optimal operation of large-scale power systems is presented that it is similar to the one utilized by the Houston Lighting and Power Company. The main objective is to minimize the system fuel costs while maintaining an acceptable system performance in terms of limits on generator real and reactive power outputs, transformer tap settings, and bus voltage levels. Minimizing the fuel costs of such large-scale systems enhances the performance of optimal real power generator allocation and of optimal power flow that results in an economic dispatch. To handle large-scale systems of this nature, the problem is decomposed into a real and a reactive power optimization problem. The control variables are generator real power outputs for the real power optimization problem and generator reactive power outputs, compensating capacitors, and transformer tap settings for the reactive power optimization. The gradient projection method (GPM) is utilized to solve the optimization problems. It is an iterative procedure for finding an extremum of a function of several constraint variables without using penalty functions or Lagrange multipliers. Mathematical models are developed to represent the sensitivity relationships between dependent and control variables for both real and reactive-power optimization procedures and thus eliminate the use of B-coefficients. >

This paper presents a unified method for optimal real and reactive power dispatch for the economic operation of power systems. As in other methods, the problem is decomposed into a P-optimization module and a Q-optimization module, but in this method both modules use the same generation cost-objective function. The control variables are generator real power outputs for the real power module; and generator reactive power outputs, shunt capacitors/reactors, and transformer tap settings for the reactive power module. The constraints are the operating limits of the control variables, power line flows, and bus voltages. The optimization problem is solved using the gradient projection method (GPM) which is used for the first time in the power systems study. The GPM allows the use of functional constraints without the need of penalty functions or Lagrange multipliers among other advantages. Mathematical models are developed to represent the sensitivity tivity relationships between dependent and control variables for both, real and reactive power, optimization modules, and thus eliminate the use of B-coefficients. Results of two test systems are presented and compared with conventional methods.

This paper presents a method for optimal real and reactive power dispatch for the economic operation of power systems. As in other methods, the problem is decomposed into a P-optimization module and a Q-optimization module, but in this method both modules use the same generation cost objective function. The control variables are generator real power outputs for the real power module; and generator reactive power outputs, shunt capacitors/reactors, and transformer tap settings for the reactive power module. The constraints are the operating limits of the control variables, power line flows, and bus voltages. The optimization problem is solved using the gradient projection method (GPM) which is used for the first time in the power systems study. Among other advantages, the GPM allows the use of functional constraints without the need for penalty functions or Lagrange multipliers. Mathematical models are developed to represent the sensitivity relationships between dependent and control variables for both (real and reactive power) optimization modules, and thus eliminate the use of B-coefficients. Results of two test systems are presented and compared with conventional methods.

The fuel cost formula is developed for optimal real-and reactive-power dispatch for the economic operation of power systems. The problem is decomposed into a P-optimisation and a Q-optimisation module, where both modules use the same fuel cost objective function resulting in the optimal load flow. The control variables are generator real-power outputs for the real-power module; and generator reactive-power outputs, shunt capacitors/reactors and transformer tap settings for the reactive-power module. The constraints are the operating limits of the control variables, power-line flows and busbar voltages. The optimisation problem is solved using the gradient projection method (GPM) for the quadratic objective function and linear constraints. The GPM allows the use of functional constraints without the need of penalty functions or Lagrange multipliers among other advantages. Mathematical models are developed to represent the sensitivity relationships between dependent and control variables for both real- and reactive-power optimisation modules; and thus eliminate the use of B-coefficients. Results of two test systems are presented and compared with conventional methods.