Jennifer S. Curtis

Effect of particle characteristics on particle pickup velocity

Abstract Particle entrainment is investigated by measuring the velocity required to pick up particles from rest, also known as pickup velocity. Pickup velocity is a function of individual particle characteristics and interparticle forces. Although 5–200 μm particles are investigated, the work presented here focuses on the pickup of particles in a pile in the size range of 5–35 μm. These smaller particle sizes are more typical for pharmaceutical and biomedical applications, such as dry powder inhalers (DPIs). Pickup velocities varied from 3.9 to 16.9 m/s for the range of particle sizes investigated. There is a strong correlation between particle size and the dominating forces that determine the magnitude of the pickup velocity. Preliminary data investigating pickup velocity as a function of particle size indicate the existence of a minimum pickup velocity. For larger particle sizes, the mass of the particle demands a greater fluid velocity for entrainment, and for smaller particle sizes, greater fluid velocities are required to overcome particle–particle interactions. Pickup velocity remains relatively constant at very small particle diameters, specifically, less than 10 μm for glass spheres and 20 μm for nonspherical alumina powder. This can be attributed to the negligible changes in London–van der Waals forces due to a hypothesized decrease in interparticle spacing. At intermediate particle diameters, electrostatic forces are dominant.

Granular shear flows of flat disks and elongated rods without and with friction

Granular shear flows of flat disks and elongated rods are simulated using the Discrete Element Method. The effects of particle shape, interparticle friction, coefficient of restitution, and Youngs modulus on the flow behavior and solid phase stresses have been investigated. Without friction, the stresses decrease as the particles become flatter or more elongated due to the effect of particle shape on the motion and interaction of particles. In dense flows, the particles tend to have their largest dimension aligned in the flow direction and their smallest dimension aligned in the velocity gradient direction, such that the contacts between the particles are reduced. The particle alignment is more significant for flatter disks and more elongated rods. The interparticle friction has a crucial impact on the flow pattern, particle alignment, and stress. Unlike in the smooth layer flows with frictionless particles, frictional particles are entangled into large masses which rotate like solid bodies under shear. ...

Effect of system size on particle-phase stress and microstructure formation

In this paper, we investigate the effect of particle number, or system size, on three-dimensional (3D) particle dynamic simulation results. Specifically, we simulate conditions with varying e (coefficient of restitution) and φ (solids volume fraction), containing particle numbers ranging from 250 to 300 000. Various algorithmic improvements are implemented in the simulation to efficiently handle these large numbers of particles. We observe, for the first time for 3D simulations, particle-phase microstructure formation at high coefficients of restitution. Furthermore, we show the onset of the particle-phase microstructure formation at various threshold system sizes, depending on the value of e and φ, and relate it to an increase in particle-phase stress. Visual observations are used to conduct a preliminary investigation of the nature of the microstructure formation. Multiple well-defined bands are observed for simulations of at least 100 000 particles.

Granular shear flows of flexible rod-like particles

Flexible particles are widely encountered in nature, e.g., stalks of plants, fiberglass particles, and ceramic nanofibers. Early studies indicated that the deformability of particles has a significant impact on the properties of granular materials and fiber suspensions. In this study, shear flows of flexible particles are simulated using the Discrete Element Method (DEM) to explore the effect of particle flexibility on the flow behavior and constitutive laws. A flexible particle is formed by connecting a number of constituent spheres in a straight line using elastic bonds. The forces/moments due to the normal, tangential, bending, and torsional deformation of a bond resist the relative movement between two bonded constituent spheres. The bond stiffness determines how difficult it is to make a particle deform, and the bond damping accounts for the energy dissipation in the particle vibration process. The simulation results show that elastically bonded particles have smaller coefficients of restitution comp...

Enhancing the Teaching of Fluid Mechanics and Transport Phenomena via FlowLab: A Computational Fluid Dynamics Tool

Introducing Computational Fluid Dynamics (CFD) to engineering students at the undergraduate level has become more common in recent years, although there are significant barriers for doing so using a generalized CFD solver. A common constraint is the quantity of material to be covered in a fixed amount of time in a given course, which leaves little time left for learning the use of a generalized CFD package. With this consideration in mind, FlowLab (www.flowlab.fluent.com ) was introduced by Fluent Inc. FlowLab may be described as a virtual fluids laboratory—a computer based analysis and visualization package. Using FlowLab, students solve predefined CFD exercises. These predefined exercises facilitate teaching and provide students with hands-on CFD experience. Through the design of each FlowLab exercise, students are introduced to engineering problems and concepts as well as CFD via a structured learning process. In the fall 2003 semester at Purdue University, FlowLab was used in CHE 540, a transport phenomena course offered within the School of Chemical Engineering. This course is open to advanced undergraduate engineering students and graduate students. Students were exposed to eight separate FlowLab exercises in this course. This paper gives a detailed summary of one of these specific exercises, developing flow in a pipe with and without heat transfer. The paper emphasizes how the use of CFD via FlowLab enhanced the teaching of specific concepts in transport phenomena as well as concepts in CFD such as creating a parametric geometry, discretizing the geometry, specifying boundary conditions, material properties and operating conditions, numerical solution techniques and post-processing. Experiences from this course are that FlowLab is a positive force for creating student interest and excitement in the area of fluid mechanics and transport phenomena. Using FlowLab’s post-processing capabilities, students were able to visualize complex flow fields and make direct comparison to analytical theory and experimental correlation. In addition, FlowLab provided a structured learning experience which reinforced proper pedagogy for applying CFD to engineering problems. Upon completion of the course, a student survey was performed in CHE 540 focusing on FlowLab integration and usage, and survey responses are summarized in this paper.© 2004 ASME

Cratering of a Lunar Soil Simulant, JSC-1A, by a Turbulent Subsonic Jet

Impinging rocket plumes from spacecraft interact with the lunar regolith surface and release a high velocity particle-spray that is potentially hazardous to surrounding surface structures and surface architecture. Experiments performed at NASA-KSC and the University of Florida provide data on the cratering of a particle bed composed of lunar soil simulant JSC-1A by a turbulent subsonic jet of gas. Cratering experiments were also performed on beach sand, which has a narrower particle size distribution than JSC-1A. JSC-1A contains a wide particle size distribution with a high number of fine particles (20% by volume less than 35 μm). JSC-1A is highly compressible and contains much rougher and more angular particles than beach sand. The differences in the particle properties between JSC-1A and beach sand leads to stark differences in the cratering of the two materials. The cratering of beach sand results in a dual crater shape consisting of a conical outer crater and a paraboloid inner crater. While the jet is impinging on the sand, the crater grows logarithmically with time and the outer crater cycles between the angle of repose and angle of failure as particles are ejected from the inner crater onto the outer crater. The JSC-1A crater is similar to a scour hole in shape, which grows deeper over time, with large chunks of material being eroded by the jet. The growth rate of the crater of JSC-1A is highly dependent on the bulk bed density of the JSC-1A. For the same jet velocity the cratering rate of a bed of JSC-1A with a higher bulk density (more compact) was slower than a bed with a lower bulk density (less compact). Even for a lower jet velocity, the less compact bed can crater faster than the more compact bed. The cratering of a sieved fraction of JSC-1A in the absence of fine particles is studied by sieving to a particle size range of 150 to 425 μm. The cratering of sieved JSC-1A is more similar to the cratering of beach sand than to unsieved JSC-1A indicating that the presence of fine particles in unsieved JSC-1A is the cause for its atypical cratering behavior. 36 Earth and Space 2012

The Effect of Spanwise Width on Rectangular Jets With Sidewalls

A CFD study on the effect of spanwise width on a rectangular jet with sidewalls is conducted using a standard k-ɛ model with wall functions. An order of magnitude analysis reveals the role played by spanwise turbulent shear terms, arising from the wall bounded flow, as the aspect ratio is decreased at high streamwise distances. A comparative study involving experimental data and other turbulence models is also presented to validate the k-ɛ model for this confined jet flow. It is found that the effect of bounding walls is negligible up to a streamwise distance of at least 105 jet diameters for an aspect ratio of 40, however this distance, within which the flow can be approximated as two-dimensional, decreases with decrease in aspect ratio.

Collisional dissipation rate in shearing flows of granular liquid crystals

We make use of discrete-element-method numerical simulations of inelastic frictionless cylinders in simple shearing at different length-to-diameter ratios and solid volume fractions to analyze the rate of collisional dissipation of the fluctuation kinetic energy. We show that the nonspherical geometry of the particles is responsible, by inducing rotation, for increasing the dissipation rate of the fluctuation kinetic energy with respect to that for frictionless spheres. We also suggest that the partial alignment of the cylinders induced by shearing concurs with the particle inelasticity in generating correlation in the velocity fluctuations and thus affecting the collisional dissipation rate as the solid volume fraction increases. Finally, we propose simple phenomenological modifications to the expression of the collisional dissipation rate of kinetic theory of granular gases to take into account our findings.

Stresses and orientational order in shearing flows of granular liquid crystals

We perform discrete element simulations of homogeneous shearing of frictionless cylinders and show that the particles are characterized by orientational order and form a granular liquid crystal. For elongated and flat cylinders, the alignment is in the plane of shearing, while cylinders having an aspect ratio equal to 1 and 0.8 show no orientational order. We show that the particle pressure is insensitive to the cylinder aspect ratio and well predicted by the kinetic theory of granular gases, with a singularity in the radial distribution function at contact different from that for frictionless spheres. The numerical results quantitatively agree with physical experiments on different geometries. The particle shear stress is affected by orientational anisotropy. We postulate that, for frictionless cylinders, the viscosity is roughly due to the motion of the orientationally disordered fraction of the particles, and show that it is proportional, through the order parameter, to the expression of kinetic theory. Finally, we suggest that the orientational order is the result of the competing effects of the shear rate, which induces alignment, and the granular temperature, which ramdomizes.

Deposition of non-spherical particles in bifurcating airways

Abstract Particle morphology plays an important role in pulmonary drug delivery. Not only does particle shape affect how particles flow and deposit, the shape also influences the drug release rate from the particles. In this work, a semi-theoretical relationship is developed to describe deposition efficiency as a function of fluid and particle properties, incorporating the effect of particle shape. For the 10 different particle types studied (with aerodynamic diameters between 1 and 10 µm), three key deposition mechanisms are identified. All particles deposit through inertial impaction, and additionally deposit via sedimentation or diffusion, depending on the particle specific momentum.