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Tracking the trajectories of individual droplets in Lagrangian numerical simulations of electrosprays involves a large number of computations, due to the calculation of the electrostatic repulsion force between droplets (space cha...
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Tracking the trajectories of individual droplets in Lagrangian numerical simulations of electrosprays involves a large number of computations, due to the calculation of the electrostatic repulsion force between droplets (space charge force). Two strategies are proposed in this study to reduce such number while preserving accuracy. In one strategy, the force contribution from distant droplets is simplified by replacing the droplet charges belonging to small volumes or cells within the spray with a single charge per cell located at the cell's center-of-charge. In the other strategy, the integration of the droplets' motion in different axial regions of the spray is carried out using two very different time steps, using higher time resolution where the electrical force variation sensed by the droplets is larger. With these methods the CPU time was shortened by a factor of 39 (from 1658 to 42 h), in a simulation of an electrospray characterized by a count mean diameter (CMD) of 8.84 μm and around 26,000 droplets in the steady state. In another spray, comprising about 3400 droplets (CMD=32 μm), the CPU time was reduced by a factor of 4.4. In this case, the CPU reduction is smaller because the proposed methodologies become less efficient with a smaller number of droplets. This study is also concerned with the selection of a proper integration time step. We show that an acceptable upper bound to the time step is based on the proper description of numerical collisions between electrospray droplets. Interestingly, in both systems simulated, a similar maximum acceptable time step is found (2 |xs).
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A series of direct numerical simulations, both in two- and three-dimensions, of the flow past a circular cylinder for Reynolds numbers Re 600 has been conducted. From these simulations, the time-mean (and, for the three-dimensiona...
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A series of direct numerical simulations, both in two- and three-dimensions, of the flow past a circular cylinder for Reynolds numbers Re 600 has been conducted. From these simulations, the time-mean (and, for the three-dimensional simulations, the spanwise spatial-mean) flow has been calculated. A global linear stability analysis has been conducted on these mean flows, showing that the mean cylinder wake for Re 600 is marginally stable and the eigenfrequency of the leading global mode closely predicts the eventual saturated vortex shedding frequency. A local stability analysis has also been conducted. For this, a series of streamwise velocity profiles has been extracted from the mean wake and the stability of these profiles has been analysed using the Rayleigh stability equation. The real and imaginary instability frequencies gained from these profiles have then been used to find the global frequency selected by the flow using a saddle-point criterion. The results confirm the success of the saddle-point criterion when the mean flow is quasi-parallel in the vicinity of the saddle point; however, the limitations of the method when the mean flow exhibits higher curvature are also elucidated.
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In this work we discuss some recent contributions to the random search problem.Our analysis includes superdiffusive Levy processes and correlated randomwalks in several regimes of target site density, mobility and revisitability.W...
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In this work we discuss some recent contributions to the random search problem.Our analysis includes superdiffusive Levy processes and correlated randomwalks in several regimes of target site density, mobility and revisitability.We present results in the context of mean-field-like and closed-form averagecalculations, as well as numerical simulations. We then consider randomsearches performed in regular lattices and lattices with defects, and we discussa necessary criterion for distinguishing true superdiffusion from correlatedrandom walk processes. We invoke energy considerations in relation to criticalsurvival states on the edge of extinction, and we analyze the emergence of Levybehavior in deterministic search walks. Finally, we comment on the randomsearch problem in the context of biological foraging.
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The paper reports on the results of some recently performed numerical and physical experiments with steel structures. The experiments consisted of a bolt pull-out from a steel plate and the loading of different geometric arrangeme...
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The paper reports on the results of some recently performed numerical and physical experiments with steel structures. The experiments consisted of a bolt pull-out from a steel plate and the loading of different geometric arrangements of a trapezoidal stiffener of a steel orthotropic plate. Both numerical and physical experiments have been performed until full material failure. The simulations have been performed using LS-DYNA, a commercial finite element code. The paper discusses several modelling and numerical problems that have to be solved for the successful simulation. The simulation results are compared with the results of the physical experiments.
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A tsunami-like sea disturbance of nonseismic origin was observed on 7 May 2007 on the Bulgarian Black Sea coast. The maximum wave amplitude was three meters, and the sea oscillations lasted for several hours. The anomalous behavio...
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A tsunami-like sea disturbance of nonseismic origin was observed on 7 May 2007 on the Bulgarian Black Sea coast. The maximum wave amplitude was three meters, and the sea oscillations lasted for several hours. The anomalous behaviour was seen in many places of the coast of Bulgaria and was stronger in the north. Several accounts of eyewitnesses as well as reports of local port authorities and three tide-gauge recordings are available and have been collected and analysed. The tsunami could have been produced by a submarine landslide or by atmospheric pressure pulses. Here, we explore the former hypothesis, i.e., the possibility that the tsunami was induced by a landslide that may have occurred on the shelf margin off Bulgaria. By making use of numerical modelling we find that submarine mass movements taking place within a certain delimited source area off Bulgaria may have generated tsunamis compatible with the observations.
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In this paper, a trajectory shaping guidance law, which considers constraints of field-of-view (FOV) angle, impact angle, and terminal lateral acceleration, is proposed for a constant speed missile against a stationary target. Fir...
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In this paper, a trajectory shaping guidance law, which considers constraints of field-of-view (FOV) angle, impact angle, and terminal lateral acceleration, is proposed for a constant speed missile against a stationary target. First, to decouple constraints of the FOV angle and the terminal lateral acceleration, the third-order polynomial with respect to the line-of-sight (LOS) angle is introduced. Based on an analysis of the relationship between the looking angle and the guidance coefficient, the boundary of the coefficient that satisfies the FOV constraint is obtained. The terminal guidance law coefficient is used to guarantee the convergence of the terminal conditions. Furthermore, the proposed law can be implemented under bearings-only information, as the guidance command does not involve the relative range and the LOS angle rate. Finally, numerical simulations are performed based on a kinematic vehicle model to verify the effectiveness of the guidance law. Overall, the work offers an easily implementable guidance law with closed-form guidance gains, which is suitable for engineering applications.
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Efforts towards a quantification of tsunamis, even though it started about seventy-five years ago, is still a puzzling aspect, since there are several scales proposed to measure tsunami size that are confusing regarding the intens...
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Efforts towards a quantification of tsunamis, even though it started about seventy-five years ago, is still a puzzling aspect, since there are several scales proposed to measure tsunami size that are confusing regarding the intensity or magnitude, or they are difficult to apply. In the present work, twelve different studies that relate specifically to numerical modeling of tsunami prediction and handling in Greek areas are reviewed. Even though current knowledge cannot guarantee the appearance and severity of a tsunami, warnings about tsunamis and predicting them in general can become more attuned and warnings can be more specific, with the development of numerical modeling, possibly leading in the future to wave power plants. (C) 2016 Elsevier Ltd. All rights reserved.
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High-resolution numerical simulations of cellular detonations are performed using a parallelized adaptive grid solver, in the case where the channel width is very wide. In particular, the nonlinear response of a weakly unstable Ze...
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High-resolution numerical simulations of cellular detonations are performed using a parallelized adaptive grid solver, in the case where the channel width is very wide. In particular, the nonlinear response of a weakly unstable Zeldovich-Neumann-Doering (ZND) detonation to two-dimensional perturbations is studied in the context of the idealized one-step chemistry model. For random perturbations, cells appear with a characteristic size in good agreement with that corresponding to the maximum growth rate from a linear stability analysis. However, the cells then grow and equilibrate at a larger size. It is also shown that the linear analysis predicts well the ratio of cell lengths to cell widths of the fully developed cells. The evolutionary dynamics of the growth are nonetheless quite slow, in that the detonation needs to run of the order of 1000 reaction lengths before the final size and equilibrium state is reached. For sinusoidal perturbations, it is found that there is a large band of wavelengths/cell sizes which can propagate over very long distances (1000 reaction lengths). By perturbing the fully developed cells of each wavelength, it is found that smaller cells in this range are unstable to symmetry breaking, which again results in cellular growth to a larger final size. However, a range of larger cell sizes appear to be nonlinearly stable. As a result it is found that the final cell size of the model is non-unique, even for such a weakly unstable, regular cell case. Indeed, in the case studied, the equilibrium cell size varies by 100% with different initial conditions. Numerical dependencies of the cellular dynamics are also examined.
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In this paper, we show that the droplet morphology varies significantly during the transport process in digital microfluidic (DMF) systems depending on the gap height between the top and bottom plates. The experimental results elu...
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In this paper, we show that the droplet morphology varies significantly during the transport process in digital microfluidic (DMF) systems depending on the gap height between the top and bottom plates. The experimental results elucidate the effect of changing the gap height on the initiation and progression of droplet motion. For example, extreme necking is observed during transport from one electrode to another at low gap heights. In essence, this necking is unlike the previously reported necking occurring during the splitting process over three electrodes. Pronounced elongation of droplets at certain stages during the droplet motion was examined. The effect of changing the gap height on the droplet velocity profile is illustrated. We found that prolonged actuation time is needed to transport the droplet successfully at low gap heights. This paper entails detailed experimental and numerical characterization of droplet morphology to insinuate useful design and prototyping tips for enhanced DMF devices. Such deformation, elongation, and necking corroborate that the existing analytical models may be inadequate for certain gap heights as they typically assume a circular shape for the droplet during motion. [2014-0292]
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