223,532 research outputs found
DNS of vertical plane channel flow with finite-size particles: Voronoi analysis, acceleration statistics and particle-conditioned averaging
We have performed a direct numerical simulation of dilute turbulent
particulate flow in a vertical plane channel, fully resolving the phase
interfaces. The flow conditions are the same as those in the main case of
"Uhlmann, M., Phys. Fluids, vol. 20, 2008, 053305", with the exception of the
computational domain length which has been doubled in the present study. The
statistics of flow and particle motion are not significantly altered by the
elongation of the domain. The large-scale columnar-like structures which had
previously been identified do persist and they are still only marginally
decorrelated in the prolonged domain. Voronoi analysis of the spatial particle
distribution shows that the state of the dispersed phase can be characterized
as slightly more ordered than random tending towards a homogeneous spatial
distribution. It is also found that the p.d.f.'s of Lagrangian particle
accelerations for wall-normal and spanwise directions follow a lognormal
distribution as observed in previous experiments of homogeneous flows. The
streamwise component deviates from this law presenting significant skewness.
Finally, a statistical analysis of the flow in the near field around the
particles reveals that particle wakes present two regions, a near wake where
the velocity deficit decays as 1/x and a far wake with a decay of approximately
1/(x*x).Comment: accepted for publication in Int. J. Multiphase Flo
Development and Evaluation of the Nebraska Assessment of Computing Knowledge
One way to increase the quality of computing education research is to increase the quality of the measurement tools that are available to researchers, especially measures of students’ knowledge and skills. This paper represents a step toward increasing the number of available thoroughly-evaluated tests that can be used in computing education research by evaluating the psychometric properties of a multiple-choice test designed to differentiate undergraduate students in terms of their mastery of foundational computing concepts. Classical test theory and item response theory analyses are reported and indicate that the test is a reliable, psychometrically-sound instrument suitable for research with undergraduate students. Limitations and the importance of using standardized measures of learning in education research are discussed
Divergence and convergence of inertial particles in high Reynolds number turbulence
Inertial particle data from three-dimensional direct numerical simulations of
particle-laden homogeneous isotropic turbulence at high Reynolds number are
analyzed using Voronoi tessellation of the particle positions, considering
different Stokes numbers. A finite-time measure to quantify the divergence of
the particle velocity by determining the volume change rate of the Voronoi
cells is proposed. For inertial particles the probability distribution function
(PDF) of the divergence deviates from that for fluid particles. Joint PDFs of
the divergence and the Voronoi volume illustrate that the divergence is most
prominent in cluster regions and less pronounced in void regions. For larger
volumes the results show negative divergence values which represent cluster
formation (i.e. particle convergence) and for small volumes the results show
positive divergence values which represents cluster destruction/void formation
(i.e. particle divergence). Moreover, when the Stokes number increases the
divergence takes larger values, which gives some evidence why fine clusters are
less observed for large Stokes numbers. Theoretical analyses further show that
the divergence for random particles in random flow satisfies a PDF
corresponding to the ratio of two independent variables following normal and
gamma distributions in one dimension. Extending this model to three dimensions,
the predicted PDF agrees reasonably well with Monte-Carlo simulations and DNS
data of fluid particles.Comment: 23 pages, 9 figure
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