117 research outputs found
Fluid-particle interaction from lattice Boltzmann simulations for flow through polydisperse random arrays of spheres
No full textFluid–solid drag force correlations, such as the Ergun relation, are widely used in many areas of chemical engineering. In many practical applications, the solid phase consist of an assembly of spheres which are, more often than not, polydisperse. In this paper we report on a study of the fluid–particle interaction by fully resolved DNS-type simulations (lattice Boltzmann) of flow through polydisperse random arrays of spheres, both for log-normal and Gaussian size distributions. In a recent paper [Van der Hoef, M.A., Beetstra, R., Kuipers, J.A.M., 2005. Lattice Boltzmann simulations of low Reynolds number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force. J. Fluid Mech. 528, 233] we have shown that a correction factor should be applied to the monodisperse drag force relations, when used for bidisperse systems. On the basis of the data reported in this paper, we conclude that the correction factor also applies to general polydisperse systems
Impact of a steel ball on soft sand
No full textThe radius of the steel ball is 1.25cm,
the falling height 2.5 m.
For more information, see:
Lohse, D., Bergmann, R.P.H.M., Mikkelsen, R., Zeilstra, C., Meer, D. van der, Versluis, M. , Weele, J.P. van der, Hoef, M.A. van der, & Kuipers, J.A.M.
Impact on soft sand: Void collapse and jet formation.
Phys. Rev. Lett., 93 (19), 198003-1-198003-4. (2004).The movies shows the impact of a steel ball on soft granular matter
Clinical significance of the microvasculature in coronary syndromes
No full textThis thesis explores the role of coronary physiology in the diagnosis, management, and prognosis of chronic and acute coronary syndromes. Part A focuses on the procedural considerations of assessing coronary physiology invasively using pressure and flow measurements. The authors discuss the limitations of resting indices in evaluating coronary stenosis severity and the importance of performing physiological assessments meticulously. Moreover, they investigate the comprehensive diagnosis of chronic coronary syndromes and the insights provided by combined pressure and flow measurements. Also, the impact of percutaneous coronary intervention (PCI) on resolving myocardial ischemia in discordant stenoses based on the novel coronary-flow-capacity (CFC) concept is assessed. Part B focuses on microvascular function in acute coronary syndromes, particularly in patients with ST-segment elevation myocardial infarction (STEMI). The authors discuss the pathophysiology, diagnostic armamentarium, and treatment strategies related to microvasculature in patients with STEMI. They investigate the microvascular function in the acute event of myocardial infarction and how it restores following a successful primary percutaneous intervention. Overall, the thesis provides a comprehensive overview of the role of the coronary microcirculation in the diagnosis, management, and prognosis of chronic and acute coronary syndromes. The authors emphasize the importance of performing physiological assessments meticulously and highlight the limitations and challenges associated with current diagnostic methods. They also provide insights into the potential future directions for the field
The effects of particle and gas properties on the fluidization of Geldart A particles
No full textWe report on 3D computer simulations based on the soft-sphere discrete particle model (DPM) of Geldart A particles in a 3D gas-fluidized bed. The effects of particle and gas properties on the fluidization behavior of Geldart A particles are studied, with focus on the predictions of Umf and Umb, which are compared with the classical empirical correlations due to Abrahamsen and Geldart [1980. Powder Technology 26, 35¿46]. It is found that the predicted minimum fluidization velocities are consistent with the correlation given by Abrahamsen and Geldart for all cases that we studied. The overshoot of the pressure drop near the minimum fluidization point is shown to be influenced by both particle¿wall friction and the interparticle van der Waals forces. A qualitative agreement between the correlation and the simulation data for Umb has been found for different particle¿wall friction coefficients, interparticle van der Waals forces, particle densities, particle sizes, and gas densities. For fine particles with a diameter Click to view the MathML source, a deviation has been found between the Umb from simulation and the correlation. This may be due to the fact that the interparticle van der Waals forces are not incorporated in the simulations, where it is expected that they play an important role in this size range. The simulation results obtained for different gas viscosities, however, display a different trend when compared with the correlation. We found that with an increasing gas shear viscosity the Umb experiences a minimum point near Click to view the MathML source, while in the correlation the minimum bubbling velocity decreases monotonously for increasing ¿g
Effects of heterogeneity on the drag force in random arrays of spheres
No full textThe modelling of the gas-solid interaction is a prerequisite in order to accurately predict fluidized bed behaviour using models such as the Discrete Particle Model (DPM) or the Two Fluid Model (TFM). Currently, the drag force is usually modelled purely based on porosity and slip velocity, which are averaged with respect to the grid size used to solve the model equations. Interfaces at heterogenous structures such as bubbles or free
board are not accounted for. As recently pointed out by Xu et al. (2007), sub-grid information for the particle position is available in DPM simulations, thus the local porosity is known and can be used when calculating the drag. Direct Numerical Simulation of flow in particulate systems were done using the lattice Boltzmann method. These simulations
were carried out with random arrays of spheres which only have a slight degree of heterogeneity and the gas-solid interaction force on each particle was measured. First we compared these results, which can be considered as the true drag force, with the drag force one would predict from a correlation typically used in larger scale models (such as the relation of van der Hoef et al. (2005)). Even for the random arrays, the drag on some individual particles differed considerably (up to 40%) from the predicted drag. Then we evaluate the effectiveness of improved drag models, that use information on local
porosity
Fluid-particle interaction force for polydisperse systems from lattice boltzmann simulations
No full textGas-solid fluidized beds are almost always polydisperse in industrial application. However, to describe the fluid-particle interaction force in models for large-scale gas-solid flow, relations are used which have been derived for monodisperse system, for which ad-hoc modifications are made to account for polydispersity. Recently it was shown, on the basis of detailed lattice Boltzmann simulations, that for bidisperse systems these
modifications predict a drag force which can be factors different from the true drag force. In this work fluid-particle interaction forces for polydisperse system are studied by means of
lattice Boltzmann simulation, using a grid that is typically an order of magnitude smaller than the sphere diameter. Two different lognormal size distributions are considered for this study. The systems consist of polydisperse random arrays of spheres in the diameter range of 8-24 grid spacing and 8-40 grid spacing, a solid volume fraction of 0.5 and 0.3 and Reynolds number 0.1 to 500. The data confirms the observations made for bidisperse
systems, namely that an extra correction factor for the drag force is required to adequately capture the effect of polydispersity. It was found that the correction factor derived by van
der Hoef et al (J. Fluid Mech. 528 (2005) 233) on the basis of bidisperse simulation data, applies also to general polydisperse systems
Effects of heterogeneity on the drag force in random arrays of spheres
No full textThe modelling of the gas-solid interaction is a prerequisite in order to accurately predict fluidized bed behaviour using models such as the Discrete Particle Model (DPM) or the Two Fluid Model (TFM). Currently, the drag force is usually modelled purely based on porosity and slip velocity, which are averaged with respect to the grid size used to solve the model equations. Interfaces at heterogenous structures such as bubbles or free board are not accounted for. As recently pointed out by Xu et al. (2007), sub-grid information for the particle position is available in DPM simulations, thus the local porosity is known and can be used when calculating the drag. Direct Numerical Simulation of flow in particulate systems were done using the lattice Boltzmann method. These simulations were carried out with random arrays of spheres which only have a slight degree of heterogeneity and the gas-solid interaction force on each particle was measured. First we compared these results, which can be considered as the "true drag force, with the drag force one would predict from a correlation typically used in larger scale models (such as the relation of van der Hoef et al. (2005)). Even for the random arrays, the drag on some individual particles differed considerably (up to 40%) from the predicted drag. Then we evaluate the effectiveness of improved drag models, that use information on local porosity
A numerical study of fluidization behavior of Geldart A particles using a discrete particle model
No full textThis paper reports on a numerical study of fluidization behavior of Geldart A particles by use of a 2D soft-sphere discrete particle model (DPM). Some typical features, including the homogeneous expansion, gross particle circulation in the absence of bubbles, and fast bubbles, can be clearly displayed if the interparticle van der Waals forces are relatively weak. An anisotropy of the velocity fluctuation of particles is found in both the homogeneous fluidization regime and the bubbling regime. The homogeneous fluidization is shown to represent a transition phase resulting from the competition of three kinds of basic interactions: the fluid¨Cparticle interaction, the particle¨Cparticle collisions (and particle¨Cwall collisions) and the interparticle van der Waals forces. In the bubbling regime, however, the effect of the interparticle van der Waals forces vanishes and the fluid¨Cparticle interaction becomes the dominant factor determining the fluidization behavior of Geldart A particles. This is also evidenced by the comparisons of the particulate pressure with other theoretical and experimental results
Comparison between integral equation method and molecular dynamics simulation for three-body forces: Application to supercritical argon
No full textThe prediction of the structural and thermodynamic properties of supercritical argon has been carried out by two independent routes: semianalytical calculations and numerical simulations. The first one is based on the hybridized mean spherical approximation (HMSA) conjugated with an effective pair potential that incorporates multipole dispersion interactions. The second one uses a very recent numerical simulation technique, inspired by the Car–Parrinello method [van der Hoef et al., J. Chem. Phys. 111, 1520 (1999)], which contains an effective quantum-mechanical representation of the underlying electronic structure. The latter approach allows us to treat the contribution of the three-body effects as well, and to validate the use of an effective pair potential for them in the framework of the self-consistent integral equation method. For all the supercritical argon states studied, the results obtained with the semianalytical approach are in good agreement with the predictions of the numerical simulation. Here it is shown that HMSA remains competitive with molecular dynamics simulation when the triple-dipole and the dipole–dipole–quadrupole three-body terms are taken into accoun
Deviations from Fick's law in Lorentz gases
No full textWe have calculated the self-dynamic structure factorF(k,t) for tagged particle motion in hopping Lorentz gases. We find evidence that, even at long times, the probability distribution function for the displacement of the particles is highly non-Gaussian. At very small values of the wave vector this manifests itself as the divergence of the Burnett coefficient (the fourth moment of the distribution never approaching a value characteristic of a Gaussian). At somewhat larger wave vectors we find thatF(k,t) decays algebraically, rather than exponentially as one would expect for a Gaussian. The precise form of this power-law decay depends on the nature of the scatterers making up the Lorentz gas. We find different power-law exponents for scatterers which exclude certain sites and scatterers which do not
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