Results 1 - 10 of 1521
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[en] A theoretical and experimental study of the dynamic behavior of a boiling channel is presented. In particular, the existence of different basins of attraction during instabilities was established. A fully analytical treatment of boiling channel dynamics were performed using a algebraic delay model. Subcritical and supercritical Hopf bifurcations could be identified and analyzed using perturbation methods. The derivation of a fully analytical criterion for Hopf bifurcation transcription was applied to determine the amplitude of the limit cycles and the maximum allowed perturbations necessary to break the system stability. A lumped parameters model which allows the representation of flow reversal is presented. The dynamic of very large amplitude oscillations, out of the Hopf bifurcation domain, was studied. The analysis revealed the existence of new dynamical basins of attraction, where the system may evolve to and return from with hysteresis. Finally, an experimental study was conducted, in a water loop at atmospheric pressure, designed to reproduce the operating conditions analyzed in the theory. Different dynamic phase previously predicted in the theory were found and their nonlinear characteristics were studied. In particular, subcritical and supercritical Hopf bifurcations and very large amplitude oscillations with flow reversal were identified. (author). 53 refs., figs
[en] Highlights: • Direct side-by-side assessment of the Single Component Pseudopotential Lattice Boltzmann and Volume of Fluid methods. • LB roughly 10 times faster and produces 1-3 orders of magnitude lower spurious velocities than all VOF methods tested. • VOF more versatile, while LB requires further modifications to be widely applicable. • LB and VOF agree well for predicting the Reynolds number of falling droplets, while LB underpredicts droplet deformation. • Different techniques can disagree when simulating real physical problems, even upon agreeing for purely numerical benchmarks. - Abstract: While various multiphase flow simulation techniques have found acceptance as predictive tools for processes involving immiscible fluids, none of them can be considered universally applicable. Focusing on accurate simulation of liquid-liquid emulsions at the scale of droplets, we present a comparative assessment of the single-component multiphase pseudopotential lattice Boltzmann method (PP-LB, classical and modified) and the Volume of Fluid method (VOF, classical and modified), highlighting particular strengths and weaknesses of these techniques. We show that a modified LB model produces spurious velocities 1–3 orders of magnitude lower than all VOF models tested, and find that LB is roughly 10 times faster in computation time, while VOF is more versatile. Simulating falling liquid droplets, a realistic problem, we find that despite identical setups, results can vary with the technique in certain flow regimes. At lower Reynolds numbers, all methods agree reasonably well with experimental values. At higher Reynolds numbers, all methods underpredict the droplet Reynolds number, while being in good agreement with each other. Particular issues regarding LB simulations at low density ratio are emphasized. Finally, we conclude with the applicability of VOF vis-à-vis PP-LB for a general range of multiphase flow problems relevant to myriad applications.
[en] I use a representation theorem of continuum mechanics, along with a systematic approximation, to establish an exact correspondence between the momentum interaction on the solid constituent in a multiphase flow, and the Stokes drag, the Faxen force, the Saffman lift, and the Ho and Leal lift on a particle in a viscous fluid
[en] This paper addresses the explicit time integration for solving multi-model structural dynamics by the Arlequin method. Our study focuses on the stability of the central difference scheme in the Arlequin framework. Although the Arlequin coupling matrices can introduce a weak instability, the time integrator remains stable as long as the initial kinematic conditions of both models agree on the coupling zone. After showing that the Arlequin weights have an adverse impact on the critical time step, we present two approaches to circumvent this issue. Computational tests confirm that the two approaches effectively preserve a feasible critical time step and show the efficiency of the Arlequin method for structural explicit dynamic simulations. (authors)
[en] It is shown that when a point source of solute is inserted into a time-periodic, unbounded linear planar flow, the large-time, time-average transport of the solute can be described by classical anisotropic diffusion with constant effective diffusion tensors. For a given vorticity and forcing period, elongational flow is shown to be the most dispersive followed by simple shear and rotational flow. Large-time diffusivity along the major axis of the time-average concentration ellipse, whose alignment is predicted from the theory, is shown to increase with vorticity for all flows and decrease with increasing forcing frequency for elongational flow and simple shear. For the interesting case of rotational flow, there exist discrete resonant frequencies where the time-average major diffusivity reaches local maxima equal to the time-average steady flow case with zero forcing frequency
[en] Motivated by multiphase flow in reservoirs, we propose and study a two-species sandpile model in two dimensions. A pile of particles becomes unstable and topples if, at least one of the following two conditions is fulfilled: (1) the number of particles of one species in the pile exceeds a given threshold or (2) the total number of particles in the pile exceeds a second threshold. The latter mechanism leads to the invasion of one species through regions dominated by the other species. We studied numerically the statistics of the avalanches and identified two different regimes. For large avalanches the statistics is consistent with ordinary Bak–Tang–Weisenfeld model. Whereas, for small avalanches, we find a regime with different exponents. In particular, the fractal dimension of the external perimeter of avalanches is D f = 1.47 ± 0.02 and the exponent of their size distribution exponent is τ s = 0.95 ± 0.03, which are significantly different from D f = 1.25 ± 0.01 and τ s = 1.26 ± 0.04, observed for large avalanches. (paper)
[en] In view of the internal corrosion rate of submarine multiphase flow pipelines, this paper analyzes the related factors affecting the corrosion rate of this type of pipeline, and introduces PSO algorithm and SVM algorithm respectively. Based on the PSO-SVM combination model, the 44 groups of data was used to study the influencing factors and corrosion rate, meanwhile the 10 groups of data was used to predict. The predictions are compared with the GA-SVM model, the LS-SVM model and the CV-SVM model to verify the advancement and feasibility of the proposed method. The results show that the temperature has a relatively large influence on the corrosion rate of the multiphase flow pipeline in the seabed. The influence of pressure on the corrosion rate of the multiphase flow pipeline in the seabed is relatively small. The PSO-SVM combined model is used in the submarine multiphase flow pipeline. The e error of corrosion rate prediction is only 1.848% on average, and the model training time is only 3.17s, both of which are smaller than other models. The research proves that the PSO-SVM combination model has strong advancement and feasibility for the prediction of the internal corrosion rate of submarine multiphase flow pipelines. (paper)
[en] The primary goal of this research program is to examine the effects of two-way multiphase coupling on the development of organized vortex structures in free shear flows and the resultant multiphase dispersion. Previous research studies have determined that one-way coupled particle dispersion in free shear flows is strongly dependent on the vortex structures present in these flows and their interactions as well as the ratio of the particle aerodynamic response time to the time scale of the dominant vortex structures. Current research efforts are directed towards exploring the effects that two-way momentum, mass and energy coupling have on the multiphase dispersion processes previously uncovered. These efforts involve analytical, numerical and experimental investigations. Recent analytical and numerical results indicate that momentum coupling effects can significantly alter the global stability and potentially the large scale features of the multiphase flow field. These multiphase coupling effects may have significant importance with regard to predicting the performance of many energy conversion systems
[en] Highlights: • Good products can be achieved by accurately controlling the blend formulation and mixing. • The choice of the proper foaming agent has a great influence on the part quality. • The use of DSC allows the evaluation of the material properties after processing. - Abstract: Foam injection molding is one of the most important processes for producing lightweight plastic parts. The main feature of this process is the use of a foam blowing agent (physical or chemical) that decomposes during heating, generating gasses that are dispersed into the melt. Predicting the microstructure of the final part requires the investigation of the foam process which is a complex problem because it is necessary to combine transport phenomena of the multi-phase flow in non-isothermal conditions with foam kinetics and gas formation. The aim of this work is to evaluate the material-processing relationship in foam injection molding. The experimental measurements of PE/EVA blends, properly formulated, coupled to two types of ADC foaming agents were carried out using differential scanning calorimetry and rotational rheometry. The injection molding and direct measurements of foamed parts were then performed, based on previous measurements, to evaluate the process performance.
[en] In this contribution two novel techniques for particle characterisation in dispersed, multiphase flows will be discussed: the time-shift technique and the rainbow technique using femtosecond laser pulses. Both methods are rather new and each offers some very distinct advantages for specific applications. The purpose of this contribution is to briefly outline the physical working principles of each technique and to outline their specific features. This information is meant to stimulate use of these techniques for hitherto inaccessible applications and measurement quantities