Piroz Zamankhan

Complex Flow Dynamics in Dense Granular Flows—Part I: Experimentation

By applying a methodology useful for analysis of complex fluids based on a synergistic combination of experiments, computer simulations, and theoretical investigation, a model was built to investigate the fluid dynamics of granular flows in an intermediate regime where both collisional and frictional interactions may affect the flow behavior In Part I, the viscoelastic behavior of nearly identical sized glass balls during a collision have been studied experimentally using a modified Newtons cradle device. Analyzing the results of the measurements, by employing a numerical model based on finite element methods, the viscous damping coefficient was determined for the glass balls. Power law dependence was found for the restitution coefficient on the impact velocity. In order to obtain detailed information about the interparticle interactions in dense granular flows, a simplified model for collisions between particles of a granular material was proposed to be of use in molecular dynamic simulations, discussed in Part II.

Complex Flow Dynamics in Dense Granular Flows—Part II: Simulations

By applying a methodology useful for analysis of complex fluids based on a synergistic combination of experiments, computer simulations, and theoretical investigation, a model was built to investigate the fluid dynamics of granular flows in an intermediate regime, where both collisional and frictional interactions may affect the flow behavior In Part I, experiments were described using a modified Newtons Cradle device to obtain values for the viscous damping coefficient, which were scarce in the literature. This paper discusses detailed simulations of frictional interactions between the grains during a binary collision by employing a numerical model based on finite element methods. Numerical results are presented of slipping, and sticking motions of a first grain over the second one. The key was to utilize the results of the aforementioned comprehensive model in order to provide a simplified model for accurate and efficient granular-flow simulations with which the qualitative trends observed in the experiments can be captured. To validate the model, large scale simulations were performed for the specific case of granular flow in a rapidly spinning bucket. The model was able to reproduce experimentally observed flow phenomena, such as the formation of a depression in the center of the bucket spinning at high frequency of 100 rad/s. This agreement suggests that the model may be a useful tool for the prediction of dense granular flows in industrial applications, but highlights the need for further experimental investigation of granular flows in order to refine the model.

Particle interactions in a dense monosized granular flow

Abstract Simulations of bounded dense sheared granular flows were performed to investigate multi-phase flow phenomena, in particular the behavior of the ordered phase. In the absence of gravity, collapse of the granular structures was found to occur for a wide range of shear rate, as evidenced by the disappearance of the signature of the ordered structure in the wall normal stress signal. However, normal stress signals matching those detected in recent experiments were obtained for a system whose dynamics were collision-dominated rather than friction-dominated. Moreover, the system was found to exhibit a similar flow behavior in the presence of gravity. The stress signals were analyzed using wavelet transforms, which indicated the existence of stick–slip dynamics, characterized by harmonic frequencies. It is shown that these observations might elucidate the origin of stick–slip dynamics in the system, which experienced instability due to gravitational compactification at shear rates below a certain critical value.

Flowing grains in an inclined duct

Large scale, three-dimensional computer simulations were performed to investigate flow dynamics of monosized, viscoelastic, spherical solid particles past a stationary wedge located in the middle of an inclined duct. At low flow rates of solid particles, a continuous flow was observed similar to that excited by steadily and rapidly adding particles to the top of a heap. However, at high flow rates, a totally different situation arises, where a flow with a different nature was established in the duct. In this case, the granular flow within the upper part of the duct accelerates adjacent to the pointed tip of the wedge, and develops into vast masses of solid particles thrust and folded over each other. This is similar to the supercritical nappes in an open-channel flow of a liquid. In addition, some experimental evidences have been presented that suggest the existence of supercritical nappes in flowing grains over a stationary wedge within an inclined duct at high flow rates.

Some qualitative features of the Couette flow of monodisperse, smooth, inelastic spherical particles

Computer simulations have been performed to examine the occurrence of power-law correlations for the stresses exerted on the confining walls by the particles in the three dimensional Couette flow of hard, smooth, dissipative spherical particles of uniform size. At high particle concentrations, the wavelet analysis of the wall shear stress has revealed the existence of anomalous, long-ranged temporal correlations. Based on the results obtained, there are indications that the dense Couette flow of monodisperse, smooth, inelastic, spherical particles is a system which may be characterized by continuous distributions of the physical measures of its particles, such as size.

Large Eddy Simulations of a Brine-Mixing Tank

Traditionally, solid-liquid mixing has always been regarded as an empirical technology with many aspects of mixing, dispersing, and contacting related to power draw. One important application of solid-liquid mixing is the preparation of brine from sodium formate. This material has been widely used as a drilling and completion fluid in challenging environments such as in the Barents Sea. In this paper large-eddy simulations, of a turbulent flow in a solid-liquid, baffled, cylindrical mixing vessel with a large number of solid particles, are performed to obtain insight into the fundamental aspects of a mixing tank. The impeller-induced flow at the blade tip radius is modeled by using the sliding mesh. The simulations are four-way coupled, which implies that both solid-liquid and solid-solid interactions are taken into account. By employing a soft particle approach the normal and tangential forces are calculated acting on a particle due to viscoelastic contacts with other neighboring particles. The results show that the granulated form of sodium formate may provide a mixture that allows faster and easier preparation of formate brine in a mixing tank. In addition it is found that exceeding a critical size for grains phenomena, such as caking, can be prevented. The obtained numerical results suggest that by choosing appropriate parameters a mixture can be produced that remains free-flowing no matter how long it is stored before use.

Localized Structures in Vertically Vibrated Granular Materials

Granular materials exhibit unusual kinds of behavior, including pattern formations during the shaking of the granular materials; the characteristics of these various patterns are not well understand. Vertically shaken granular materials undergo a transition to convective motion that can result in the formation of bubbles. A detailed overview is presented of collective processes in gas-particle flows that are useful for developing a simplified model for molecular dynamic type simulations of dense gas-particle flows. The governing equations of the gas phase are solved using large eddy simulation technique. The particle motion is predicted by a Lagrangian method. Particles are assumed to behave as viscoelastic solids during interactions with their neighboring particles. Inter-particle normal and tangential contact forces are calculated using a generalized Hertzian model. The other forces that are taken into account are gravitational and drag force resulting from velocity difference with the surrounding gas. A simulation of gas-particle flow is performed for predicting the flow dynamics of dense mixtures of gas and particles in a vertical, pentagonal, prism shaped, cylindrical container The base wall of the container is subjected to sinusoidal oscillation in the vertical direction that spans to the bottom of the container The model predicts the formation of oscillon type structures on the free surface. In addition, the incomplete structures are observed. Interpretations are proposed for the formation of the structures, which highlights the role played by the surrounding gas in dynamics of the shaken particles.

Shear induced diffusive mixing in simulations of dense Couette flow of rough, inelastic hard spheres

Large-scale numerical simulations of a system of inelastic, rough, hard spheres of volume fraction φs=0.565, which are initially distributed randomly in a Couette geometry, show clear evidence of the movement of the particles in directions transverse to the bulk motion. This behavior of the aforementioned system, which has been considered as a model for a granular fluid, is consistent with recent experimental observations [Hsiau and Hunt, J. Fluid Mech. 251, 299 (1993)]. Based on the results obtained, there are indications that a bounded rapid granular flow could be a diffusive system at volume fractions even higher than 0.56. This finding contradicts earlier computer experiments [Campbell, J. Fluid Mech. 348, 85 (1997)] which found a rapidly flowing granular material is a diffusive system except at large solids concentrations (i.e., φs>0.56).

Analysis of Submarine Pipeline Scour Using Large-Eddy Simulation of Dense Particle-Liquid Flows

Using large-eddy simulation technique for dense particle-fluid flows, the current-induced scour is predicted for both the mono- and bidispersed systems below a horizontal submarine pipeline exposed to unidirectional flow. The simulations are four-way coupled, which implies that both solid-liquid and solid-solid interactions are taken into account. Particles are assumed to behave as viscoelastic solids during interactions with their neighboring particles, and their motion are predicted by a Lagrangian method. The interparticle normal and tangential contact forces between particles are calculated using a generalized Hertzian model. The other forces on a particle that are taken into account include gravitational pressure gradient force accounting for the acceleration of the displaced liquid, the drag force resulting from velocity difference with the surrounding liquid, and the Magnus and Saffman lift forces. The predicted scour profiles for monodispersed system are found to compare favorably with the laboratory observations. For the bidispersed system, a seepage flow underneath the pipe (which is a major factor to cause the onset of scour below the pipeline) is found to be weakened using an appropriate size for the sand bed. This fiffnding highlights the importance of the bed particle size distribution on the onset of scour below the pipelines.

Multiscale modeling of fluid turbulence and flocculation in fiber suspensions

A mathematically rigorous, multiscale modeling methodology capable of coupling behaviors from the Kolmogorov turbulence scale through the full scale system in which a fiber suspension is flowing is presented. Here the key aspect is adaptive hierarchical modeling. Numerical results are presented focus of which are on fiber floc formation and destruction by hydrodynamic forces in turbulent flows. Specific consideration was given to molecular-dynamics simulations of viscoelastic fibers in which the fluid flow is predicted by a method which is a hybrid between direct numerical simulations and large eddy simulation techniques, and fluid fibrous structure interactions were taken into account. The present results may elucidate the physics behind the breakup of a fiber floc, opening the possibility for developing a meaningful numerical model of the fiber flow at the continuum level where an Eulerian multiphase flow model can be developed for industrial use.