74 research outputs found
Three-dimensional flow in cavity with elevated helicity driven by parallel walls
The proposed flow in a 3-D cubic cavity is driven by its parallel walls
moving in perpendicular directions to create a genuinely three-dimensional
highly separated vortical flow yet having simple single-block cubical geometry
of computational domain. The elevated level of helicity is caused by motion of
a wall in the direction of axis of primary vortex created by a parallel wall.
The velocity vector field is obtained numerically by using second-order upwind
scheme and 200^3 grid. Helicity, magnitude of normalized helicity and kinematic
vorticity number are evaluated for Reynolds numbers ranging from 100 to 1000.
Formation of two primary vortices with their axis oriented perpendicularly and
patterns of secondary vortices are discussed. Computational results are
compared to the well-known 3-D recirculating cavity flow case where the lid
moves in the direction parallel to the cavity side walls. Also results are
compared to the diagonally top-driven cavity and to cavity flow driven by
moving top and side walls. The streamlines for the proposed flow show that the
particles emerging from top and bottom of cavity do mix well. Quantitative
evaluation of mixing of two fluids in the proposed cavity flow confirms that
the mixing occurs faster than in the benchmark case.Comment: 38 pages, 13 figures The revised includes quantification of mixing
rate; numerical modeling of the transient version.The revised version has
four substantially improved figures and three new figures; number of
literature references increased from 26 to 4
Mixing in 3-D Cavity by Moving Cavity Walls
The mixing in three-dimensional enclosures is investigated numerically using
flow in cubical cavity as a geometrically simple model of various natural and
engineering flows. The mixing rate is evaluated for up to the value of Reynolds
number Re=2000 for several representative scenarios of moving cavity walls:
perpendicular motion of the parallel cavity walls (Case A), motion of a wall in
its plane along its diagonal (Case B1), motion of two perpendicular walls
outward the common edge (Case B2), and the parallel cavity walls in motion
either in parallel directions (Case B3) or in opposite directions (Case B4).
The mixing rates are compared to the well-known benchmark case in which one
cavity wall moves along its edge (Case C). The intensity of mixing for the
considered cases was evaluated for (i) mixing in developing cavity flow
initially at rest, which is started by the impulsive motion of cavity wall(s),
and (ii) mixing in the developed cavity flow. For both cases, the initial
interface of the two mixing fluids is a horizontal plane located at the middle
of the cavity. The mixing rates are ranked from fastest to slowest for twenty
time units of flow mixing. The pure convection mixing is modeled as a limit
case to reveal convective mechanism of mixing. Mixing of fluids with different
densities is modeled to show the advantage in terms of mixing rate of genuinely
3-D cases A and B1. Grid convergence study and comparison with published
numerical solutions for 3-D and 2-D cavity flows are presented. The effects of
three-dimensionality of cavity flow on the mixing rate are discussed.Comment: 52 pages, 17 figures, 4 Tables, 36 Ref
Parallelization of the Pipelined Thomas Algorithm
In this study the following questions are addressed. Is it possible to improve the parallelization efficiency of the Thomas algorithm? How should the Thomas algorithm be formulated in order to get solved lines that are used as data for other computational tasks while processors are idle? To answer these questions, two-step pipelined algorithms (PAs) are introduced formally. It is shown that the idle processor time is invariant with respect to the order of backward and forward steps in PAs starting from one outermost processor. The advantage of PAs starting from two outermost processors is small. Versions of the pipelined Thomas algorithms considered here fall into the category of PAs. These results show that the parallelization efficiency of the Thomas algorithm cannot be improved directly. However, the processor idle time can be used if some data has been computed by the time processors become idle. To achieve this goal the Immediate Backward pipelined Thomas Algorithm (IB-PTA) is developed in this article. The backward step is computed immediately after the forward step has been completed for the first portion of lines. This enables the completion of the Thomas algorithm for some of these lines before processors become idle. An algorithm for generating a static processor schedule recursively is developed. This schedule is used to switch between forward and backward computations and to control communications between processors. The advantage of the IB-PTA over the basic PTA is the presence of solved lines, which are available for other computations, by the time processors become idle
Vorticity Confinement and TVD Applied to Wing Tip Vortices for Accurate Drag Prediction
The vorticity confinement (VC) method was used with total variation
diminishing (TVD) schemes to reduce possible over-confinement and applied to
tip vortices shed by edges of wings in order to predict induced drag using
far-field integration. The optimal VC parameter was determined first by
application to 2-D vortices and then to tip vortices shed by a 3-D wing. The
3-D inviscid simulations were post-processed using the wake-integral technique
to determine lift-induced drag force. Dependence of the VC parameter on the
flight Mach number and the angle of attack was evaluated. Grid convergence
studies were conducted for 2-D vortices and for induced drag generated by 3-D
wing. VC was used with TVD minmod and differentiable flux limiters to evaluate
their effect on the VC method. Finally, the VC approach was combined with the
Reynolds stress equation turbulence model, and the results were compared to
experimental data of tip vortex evolution.Comment: 40 pages, 12 Figure
Coupled Continuum and Molecular Model of Flow through Fibrous Filter
A coupled approach combining the continuum boundary singularity method (BSM) and the molecular direct simulation Monte Carlo (DSMC) is developed and validated using Taylor-Couette flow and the flow about a single fiber confined between two parallel walls. In the proposed approach, the DSMC is applied to an annular region enclosing the fiber and the BSM is employed in the entire flow domain. The parameters used in the DSMC and the coupling procedure, such as the number of simulated particles, the cell size, and the size of the coupling zone are determined by inspecting the accuracy of pressure drop obtained for the range of Knudsen numbers between zero and unity. The developed approach is used to study flowfield of fibrous filtration flows. It is observed that in the partial-slip flow regime, Kn ⩽ 0.25, the results obtained by the proposed coupled BSM-DSMC method match the solution by BSM combined with the heuristic partial-slip boundary conditions. For transition molecular-to-continuum Knudsen numbers, 0.25 \u3c Kn ⩽ 1, the difference in pressure drop and velocity between these two approaches is significant. This difference increases with the Knudsen number that confirms the usefulness of coupled continuum and molecular methods in numerical modeling of transition low Reynolds number flows in fibrous filters
Modeling of Sedimentation of Particles near Corrugated Surface by Boundary Singularity Method
The velocity and trajectory of particle moving along the corrugated surface
under action of gravity is obtained by meshless Boundary Singularity Method
(BSM). This physical situation is found often in biological systems and
microfluidic devices. The Stokes equations with no-slip boundary conditions are
solved using the Green function for Stokeslets. In the present study, the
velocity of a moving particle is not known and becomes a part of the BSM
solution. This requires an adjustment of the matrix of BSM linear system to
include the unknown particle velocity and incorporate in the BSM the balance of
hydrodynamic and gravity forces acting on the particle. Comparison has been
made to prior published analytical and experimental results to verify the
effectiveness of this methodology to predict the trajectory of particle
including its deviation from vertical trajectory and select the optimal set of
computational parameters. The developed BSM methodology is applied to
sedimentation of two spherical particles in proximity for which the analytical
solution is not feasible.Comment: 14 pages, 6 figure
Modeling of Chemical Vapor Infiltration Using Boundary Singularity Method
Boundary Singularity Method (BSM) was used to model Chemical Vapor
Infiltration (CVI) in a fibrous preform. Straight, long fibers of varying
cross-sectional geometry, representing fibers of a preform, were placed within
a domain of a pre-determined size. The preparation of dense fiber-reinforced
Silicon-Carbon (SiC) composites was considered as a representative of CVI
methodology, where methyl-trichlorosilane (MTS) was used as both the silicon
and carbon donor for the silicon carbide matrix. Concentrations of MTS were
then set at the domain boundaries, and the domain was gradually infiltrated
with MTS as time progressed. The concentration of MTS at the surface of the
preform fibers was calculated using the adopted BSM. For quasi-equilibrium
considered, the reaction rate at solid surface is equal to the diffusion rate
towards the surface. The Robin or third type boundary condition, which is a
linear combination of the values of a function and the values of its derivative
on the boundary of the domain, are developed and implemented to BSM. From the
fibers surface concentrations obtained by BSM, deposition rates were
calculated, and the geometry was updated to reflect the fiber growth during the
time step, therefore, the fiber size growth and pore filling was modeled over
time. The BSM analysis was verified by comparisons to a known analytical
solution of concentric cylinders with a concentration set at the outer cylinder
and a reaction at the inner. BSM solutions were also compared to experimental
data as well as computational results obtained by a Level-Set Method (LSM).
Obtained dynamics of pore size and location will help to evaluate quality of
material manufactured by CVI. Porosity transients were obtained to show the
relation between initial and current porosities as time progresses.Comment: 23 pages, 13 Figure
Predictors of In-School Weapon-Carrying
Delinquent and violent behaviors have become a major concern of parents, teachers, and school administrations across the country as media images of school violence permeate perceptions of school safety. Although national surveys show a slight decline in school delinquency, schools continue to search for ways to improve safety. This investigation seeks to understand the predictors of weapon-use in schools. Literature related to school shootings, disorder, and weapon-carrying as well as various theories, including control, social disorganization, and subculture, guide hypotheses that explore the school characteristics related to in-school weapon-carrying as well as the interaction effects between school and student characteristics. Using a large, national sample, this unprecedented investigation explores whether school characteristics predict weapon-carrying net of individuals' propensity to carry weapons. The study also investigates whether school characteristics condition the relationships between student characteristics and weapon-carrying. Findings indicate that school characteristics, specifically those related to school location and violent environments, are important in explaining recent in-school weapon-carrying even when controlling for past weapon-carrying. Further, results suggest that school-level predictors are more important in explaining student weapon-carrying in urban schools than in non-urban schools. Implications and directions for future research will be discussed
Teen Court: Does it Reduce Recidivism?
Teen Court is a diversion program designed to intercept the process of sending first-time misdemeanor juvenile offenders though the traditional juvenile justice system. Despite its widespread popularity throughout the United States, very little research has been conducted on Teen Court's effectiveness at reducing recidivism. Those studies that do exist lack rigorous methodologies. The present evaluation uses data from a Teen Court in Maryland and a comparable group of juvenile offenders who went through the Department of Juvenile Services. A logistic regression indicates that Teen Court is positively related to recidivism. Conclusions and recommendations are discussed
Link-wise Artificial Compressibility Method
The Artificial Compressibility Method (ACM) for the incompressible
Navier-Stokes equations is (link-wise) reformulated (referred to as LW-ACM) by
a finite set of discrete directions (links) on a regular Cartesian mesh, in
analogy with the Lattice Boltzmann Method (LBM). The main advantage is the
possibility of exploiting well established technologies originally developed
for LBM and classical computational fluid dynamics, with special emphasis on
finite differences (at least in the present paper), at the cost of minor
changes. For instance, wall boundaries not aligned with the background
Cartesian mesh can be taken into account by tracing the intersections of each
link with the wall (analogously to LBM technology). LW-ACM requires no
high-order moments beyond hydrodynamics (often referred to as ghost moments)
and no kinetic expansion. Like finite difference schemes, only standard Taylor
expansion is needed for analyzing consistency. Preliminary efforts towards
optimal implementations have shown that LW-ACM is capable of similar
computational speed as optimized (BGK-) LBM. In addition, the memory demand is
significantly smaller than (BGK-) LBM. Importantly, with an efficient
implementation, this algorithm may be one of the few which is compute-bound and
not memory-bound. Two- and three-dimensional benchmarks are investigated, and
an extensive comparative study between the present approach and state of the
art methods from the literature is carried out. Numerical evidences suggest
that LW-ACM represents an excellent alternative in terms of simplicity,
stability and accuracy.Comment: 62 pages, 20 figure
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