297 research outputs found
Bubbly Turbulent Drag Reduction Is a Boundary Layer Effect
In turbulent Taylor-Couette flow, the injection of bubbles reduces the overall drag. On the other hand, rough walls enhance the overall drag. In this work, we inject bubbles into turbulent Taylor-Couette flow with rough walls (with a Reynolds number up to 4×105), finding an enhancement of the dimensionless drag as compared to the case without bubbles. The dimensional drag is unchanged. As in the rough-wall case no smooth boundary layers can develop, the results demonstrate that bubbly drag reduction is a pure boundary layer effec
On the excitation of inertial modes in an experimental spherical Couette flow
Spherical Couette flow (flow between concentric rotating spheres) is one of
flows under consideration for the laboratory magnetic dynamos. Recent
experiments have shown that such flows may excite Coriolis restored inertial
modes. The present work aims to better understand the properties of the
observed modes and the nature of their excitation. Using numerical solutions
describing forced inertial modes of a uniformly rotating fluid inside a
spherical shell, we first identify the observed oscillations of the Couette
flow with non-axisymmetric, retrograde, equatorially anti-symmetric inertial
modes, confirming first attempts using a full sphere model. Although the model
has no differential rotation, identification is possible because a large
fraction of the fluid in a spherical Couette flow rotates rigidly. From the
observed sequence of the excited modes appearing when the inner sphere is
slowed down by step, we identify a critical Rossby number associated with a
given mode and below which it is excited. The matching between this critical
number and the one derived from the phase velocity of the numerically computed
modes shows that these modes are excited by an instability likely driven by the
critical layer that develops in the shear layer staying along the tangent
cylinder of the inner sphere.Comment: 11 pages, 17 figure
The Twente turbulent Taylor-Couette (T3C) facility: Strongly turbulent (multiphase) flow between two independently rotating cylinders
A new turbulent Taylor-Couette system consisting of two independently
rotating cylinders has been constructed. The gap between the cylinders has a
height of 0.927 m, an inner radius of 0.200 m, and a variable outer radius
(from 0.279 to 0.220 m). The maximum angular rotation rates of the inner and
outer cylinder are 20 and 10 Hz, respectively, resulting in Reynolds numbers up
to 3.4 x 10^6 with water as working fluid. With this Taylor-Couette system, the
parameter space (Re_i, Re_o, {\eta}) extends to (2.0 x 10^6, {\pm}1.4 x 10^6,
0.716-0.909). The system is equipped with bubble injectors, temperature
control, skin-friction drag sensors, and several local sensors for studying
turbulent single-phase and two-phase flows. Inner cylinder load cells detect
skin-friction drag via torque measurements. The clear acrylic outer cylinder
allows the dynamics of the liquid flow and the dispersed phase (bubbles,
particles, fibers, etc.) inside the gap to be investigated with specialized
local sensors and nonintrusive optical imaging techniques. The system allows
study of both Taylor-Couette flow in a high-Reynolds-number regime, and the
mechanisms behind skin-friction drag alterations due to bubble injection,
polymer injection, and surface hydrophobicity and roughness.Comment: 13 pages, 14 figure
Acquired Immune Deficiency Syndrome Policies in Central Illinois: A Survey, Analysis, and Model
One purpose of this study was to develop model policies for school districts to use in dealing with Acquired Immune Deficiency Syndrome (AIDS). A second purpose was to identify which school districts in the Educational Service Region (ESR) comprised of Clark, Coles, Cumberland, Edgar, Moultrie, and Shelby counties had addressed the issue of AIDS policies. The population surveyed in the initial phase consisted of 22 superintendents within the ESR. A survey was developed utilizing many items from a national study (Keough and Seaton, 1987). The survey was administered during a monthly meeting of superintendents hosted by Regional Superintendent Rosemary Shepherd. A Likert scale was used where possible and means and positive percentages were calculated. Comparisons with the national results were made. The results clearly showed that local superintendents were representative of their national counterparts. 86% of the districts surveyed had developed or were developing AIDS policies. In developing model policies, the author gathered 14 national, state, and local policies dealing with AIDS. These policies were rated excellent, average, or poor on 13 different criteria suggested by the Illinois State Board of Education and the Illinois Department of Public Health. The results showed that most districts addressed AIDS within the context of a Chronic Contagious Disease Policy, that distinct student and employee policies were preferred, and that the policies were generally weak concerning student and employee rights. Model policies were then developed relying heavily on information gathered in the policy evaluation process
Dynamics of a piecewise smooth map with singularity
Experiments observing the liquid surface in a vertically oscillating
container have indicated that modeling the dynamics of such systems require
maps that admit states at infinity. In this paper we investigate the
bifurcations in such a map. We show that though such maps in general fall in
the category of piecewise smooth maps, the mechanisms of bifurcations are quite
different from those in other piecewise smooth maps. We obtain the conditions
of occurrence of infinite states, and show that periodic orbits containing such
states are superstable. We observe period-adding cascade in this system, and
obtain the scaling law of the successive periodic windows.Comment: 10 pages, 6 figures, composed in Latex2
Boolean Chaos
We observe deterministic chaos in a simple network of electronic logic gates
that are not regulated by a clocking signal. The resulting power spectrum is
ultra-wide-band, extending from dc to beyond 2 GHz. The observed behavior is
reproduced qualitatively using an autonomously updating Boolean model with
signal propagation times that depend on the recent history of the gates and
filtering of pulses of short duration, whose presence is confirmed
experimentally. Electronic Boolean chaos may find application as an
ultra-wide-band source of radio wavesComment: 10 pages and 4 figur
- …