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The effect of inter-particle contact time in granular flows -- A network theory

Description: In a kinetic theory, it is usually assumed that the time duration of particle collision is vanishingly small and only binary collisions are considered. The validity of these assumptions depends on the ratio of collision time to mean free flight time. If this ratio is small, the kinetic theory description is appropriate. In a dense system, however, this ratio is usually large, and the dynamics of the multi-particle interactions have to be considered. For instance, during a collision, the contacting pair usually has a relative tangential velocity that causes a change in the direction of rebound. This implies a dependence of the granular stress on the vorticity of the mean flows field. Due to the inherent energy dissipation in a particle collision, and the consistent rearrangement of particles, there are relaxation times associated with them. In a binary collision, this energy dissipation is represented by coefficient of restitution. In a dense granular system, multi-particle interactions occur frequently. The energy dissipation and system relaxation have to be studied by the consideration of the dynamics in the duration of particle interaction and cannot be represented by a single coefficient of restitution. In this case, the relaxation times must be introduced explicitly. By modification of the network theory for rubber material, a constitutive model for dense granular material is developed based on the dynamics of multi-particle interaction. The finite particle interaction time and system relaxation times are considered.
Date: December 31, 1996
Creator: Zhang, D.Z. & Rauenzahn, R.M.
Partner: UNT Libraries Government Documents Department


Description: Mesoscale structures such as particle clusters have been observed both in experiments and in numerical simulations of circulating fluidized beds. In a numerical simulation, in order to account for the effects of such mesoscale structures, the computational grids have to be fine enough. The use of such fine grids is impractical in engineering applications due to excessive computational costs. To predict the macroscopic behavior of a fluidized bed with reasonable computation cost, they perform a second average over the averaged equations for two-phase flows. A mesoscale inter-phase exchange force is found to be the correlation of the particle volume fraction and the pressure gradient. This force is related to the mesoscale added mass of the two-phase flow. Typically, added mass for particle scale interactions is negligible in gas-solid flows since the gas density is small compared to density of solid particles. However, for a mesoscale structure, such as a bubble, the surrounding media is the mixture of gas and particles. The surrounding fluid density experienced by the mesoscale structure is the density of the surrounding mixture. Therefore, the added mass of a mesoscale structure, such as bubbles, cannot be neglected. The property of this new force is studied based on the numerical simulation of a fluidized bed using high grid resolution. It is shown that this force is important in the region where the particle volume fraction is high. The effects of the inhomogeneity to the interphase drag are also studied.
Date: May 1, 2001
Partner: UNT Libraries Government Documents Department

Stochastic analysis of well capture zones in heterogeneous porous media

Description: In this study we present a moment-equation-based approach to derive the time-dependent mean capture zones and their associated uncertainties. The flow statistics are obtained by solving the first two moments of flow, and the mean capture zones are determined by reversely tracking the non-reactive particles released at a small circle around each pumping well. The uncertainty associated with the mean capture zones is calculated based on the particle displacement covariance for nonstationary flow fields. For comparison purpose, we also conducted Monte Carlo simulations. It has been found that our model results are in good agreement with Monte Carlo results.
Date: January 1, 2002
Creator: Zhang, D. (Dongxiao) & Lu, Z. (Zhiming)
Partner: UNT Libraries Government Documents Department

Effect of finite particle interaction time in granular systems

Description: Almost all previously published theoretical papers that propose constitutive relations for granular flows use some form of kinetic theory, which neglects effects of finite particle interaction time and multiparticle interactions. In dense systems, these effects contain essential physics and determine the evolution of the stress system in granular flows. In this paper, the authors shall demonstrate the importance of these effects and study the behavior of the granular stress in a dense system. The particle interaction time is a random variable in a granular system, and they show that its probability distribution obeys an exponential law. The temporal decay of this probability represents the destruction of contacts between particles and is related to the relaxation of the collisional stress in a granular system. By considering the balance between creation and destruction of contacts, they derive a constitutive relation for collisional stress. Depending on the form of the model chosen to approximate forces developed during particle interactions, the constitutive relation can predict either viscoelastic or viscoplastic behavior of the collisional stress. Numerical simulations are performed to verify the theoretical findings and to study further the properties of dense granular systems.
Date: July 1, 1998
Creator: Zhang, D.Z. & Rauenzahn, R.M.
Partner: UNT Libraries Government Documents Department

Effects of long and short relaxation times of particle interactions in dense and slow granular flows

Description: In this work, dense granular flows are numerically simulated using a discrete element method. The interaction of a pair of colliding particles is modeled as a parallel connection of a linear spring and a linear dashpot. Although the force model for particle interactions is simplistic for many practical problems, a significant amount of meaningful new physics can be extracted from the numerical simulations by studying the behavior of particle interaction time and its probability distribution. For instance, it is found that the probability distribution of particle contact ages is exponential for long-term contacts. The time scale of the exponential decay of the contact age probability is related to the rheological properties of the dense granular medium.
Date: January 1, 2002
Creator: Zhang, D. Z. (Duan Z.) & Rauenzahn, Rick M.
Partner: UNT Libraries Government Documents Department

On the formation of {mu}- and {zeta}-phases in Ag-Al system by mechanical alloying

Description: Pure silver and aluminum powders have been mechanically alloyed in a Spex mill. Four selected nominal compositions Ag{sub 1{minus}x}Al{sub x} (x = 0.20, 0.23, 0.26 and 0.37) were investigated by X-ray diffraction and differential scanning calorimetry, after being subjected to different times of ball milling. In all four selected compositions, the close packed hexagonal intermediate {lambda}-phase and the fcc {alpha}-phase (Ag-based solid solution) formed together during the first hour of milling, but subsequently the {lambda}-phase disappeared during continued milling except in the two alloys corresponding to 26 at.%Al and 37 at.%Al where the {lambda}-phase is the equilibrium phase. With continued milling, the complex cubic {mu}-phase formed in the alloys of nominal composition 20, 23 and 26 at.%Al where it is expected in equilibrium alloys. No amorphous phases have been observed in the mechanically alloyed powders in this system.
Date: September 1, 1993
Creator: Paruchuri, M. R.; Zhang, D. L. & Massalski, T. B.
Partner: UNT Libraries Government Documents Department

Results from recent hydrogen pellet acceleration studies with a 2-m railgun

Description: A new 3.2-mm-diameter, two-stage, fuseless, plasma-arc-driven electromagnetic railgun has been designed, constructed, and successfully operated to achieve a record velocity of 2.67 km/s({sup b}) for 3.2 mmD {times} 4 mmL solid hydrogen pellet. The first stage of this hydrogen pellet injector is a combination of a hydrogen pellet generator and a gas fun. The second stage is a 2-m-long railgun which serves as a booster accelerator. The gas fun accelerates a frozen hydrogen pellet to a medium velocity and injects it into the railgun through a perforated coupling piece, which also serves a pressure-relieving mechanism. An electrical breakdown of the propellant gas, which has followed the pellet from the gas fun into the railgun, forms a conducting plasma-arc armature immediately behind the pellet allowing for fuseless operation of the railgun. Study of the pressure profile and the behavior of the plasma-arc armature inside the railgun bore led to elimination of spurious arcing, which prevents operation of the railgun at high voltages (and, therefore, at high currents). A timing circuit that can automatically measure the pellet input velocity and allows for accurate control of arc initiation behind the pellet helps prevent pellet disintegration and mistriggering of the arc initiation circuit. Results from the recent cryogenic operation of the two-stage pellet acceleration system are reported. 11 refs., 2 figs., 1 tab.
Date: December 1, 1989
Creator: Kim, K.; Zhang, D.J.; King, T.; Haywood, R.; Manns, W. & Venneri, F.
Partner: UNT Libraries Government Documents Department

Intial characterization fo a commerical electron gun for profiling high intensity proton beams in Project X

Description: Measuring the profile of a high-intensity proton beam is problematic in that traditional invasive techniques such as flying wires don't survive the encounter with the beam. One alternative is the use of an electron beam as a probe of the charge distribution in the proton beam as was done at the Spallation Neutron Source at ORNL. Here we present an initial characterization of the beam from a commercial electron gun from Kimball Physics, intended for use in the Fermilab Main Injector for Project X. Despite the fact that the horizontal spot size is abnormally large in the high current measurement, the spot size at the downstream cross X2 is reasonable in the context of measuring the deflection. A thin foil OTR would help with the beam heating and should be tried. The next phase of this experiment is to simulate the proton beam with a pair of current carrying wires and to design and construct a fast deflector. Some of the remaining issues to be considered include determining the minimum beam current needed to observe the deflected beam for a given sweep time and the impact of longitudinal variations in the charge density of the bunch.
Date: March 1, 2011
Creator: Thurman-Keup, R.; Johnson, A.S.; Lumpkin, A.H.; Thangaraj, J.C.T.; Zhang, D.; /Fermilab et al.
Partner: UNT Libraries Government Documents Department

Quantum cascade light emitting diodes based on type-II quantum wells

Description: The authors have demonstrated room-temperature CW operation of type-II quantum cascade (QC) light emitting diodes at 4.2 {micro}m using InAs/InGaSb/InAlSb type-II quantum wells. The type-II QC configuration utilizes sequential multiple photon emissions in a staircase of coupled type-II quantum wells. The device was grown by molecular beam epitaxy on a p-type GaSb substrate and was compared of 20 periods of active regions separated by digitally graded quantum well injection regions. The maximum average output power is about 250 {micro}W at 80 K, and 140 {micro}W at 300 K at a repetition rate of 1 kHz with a duty cycle of 50%.
Date: January 21, 1997
Creator: Lin, C. H.; Yang, R. Q.; Zhang, D.; Murry, S. J.; Pei, S. S.; Allerman, A. A. et al.
Partner: UNT Libraries Government Documents Department

High Resolution BPM Upgrade for the ATF Damping Ring at KEK

Description: A beam position monitor (BPM) upgrade at the KEK Accelerator Test Facility (ATF) damping ring has been accomplished, carried out by a KEK/FNAL/SLAC collaboration under the umbrella of the global ILC R&D effort. The upgrade consists of a high resolution, high reproducibility read-out system, based on analog and digital down-conversion techniques, digital signal processing, and also implements a new automatic gain error correction schema. The technical concept and realization as well as results of beam studies are presented. The next generation of linear colliders require ultra-low vertical emittance of <2 pm-rad. The damping ring at the KEK Accelerator Test Facility (ATF) is designed to demonstrate this mission critical goal. A high resolution beam position monitor (BPM) system for the damping ring is one of the key tools for realizing this goal. The BPM system needs to provide two distnict measurements. First, a very high resolution ({approx}100-200nm) closed-orbit measurement which is averaged over many turns and realized with narrowband filter techniques - 'narrowband mode'. This is needed to monitor and steer the beam along an optimum orbit and to facilitate beam-based alignment to minimize non-linear field effects. Second, is the ability to make turn by turn (TBT) measurements to support optics studies and corrections necessary to achieve the design performance. As the TBT measurement necessitates a wider bandwidth, it is often referred to as 'wideband mode'. The BPM upgrade was initiated as a KEK/SLAC/FNAL collaboration in the frame of the Global Design Initiative of the International Linear Collider. The project was realized and completed using Japan-US funds with Fermilab as the core partner.
Date: August 17, 2011
Creator: Eddy, N.; Briegel, C.; Fellenz, B.; Gianfelice-Wendt, E.; Prieto, P.; Rechenmacher, R. et al.
Partner: UNT Libraries Government Documents Department