159 research outputs found
Homogeneity and isotropy in a laboratory turbulent flow
We present a new design for a stirred tank that is forced by two parallel
planar arrays of randomly actuated synthetic jets. This arrangement creates
turbulence at high Reynolds number with low mean flow. Most importantly, it
exhibits a region of 3D homogeneous isotropic turbulence that is significantly
larger than the integral lengthscale. These features are essential for enabling
laboratory measurements of turbulent suspensions. We use quantitative imaging
to confirm isotropy at large, small, and intermediate scales by examining one--
and two--point statistics at the tank center. We then repeat these same
measurements to confirm that the values measured at the tank center are
constant over a large homogeneous region. In the direction normal to the
symmetry plane, our measurements demonstrate that the homogeneous region
extends for at least twice the integral length scale cm. In the
directions parallel to the symmetry plane, the region is at least four times
the integral lengthscale, and the extent in this direction is limited only by
the size of the tank. Within the homogeneous isotropic region, we measure a
turbulent kinetic energy of ms, a dissipation
rate of ms, and a Taylor--scale Reynolds
number of . The tank's large homogeneous region, combined with
its high Reynolds number and its very low mean flow, provides the best
approximation of homogeneous isotropic turbulence realized in a laboratory flow
to date. These characteristics make the stirred tank an optimal facility for
studying the fundamental dynamics of turbulence and turbulent suspensions.Comment: 18 pages, 9 figure
Effects of non-universal large scales on conditional structure functions in turbulence
We report measurements of conditional Eulerian and Lagrangian structure
functions in order to assess the effects of non-universal properties of the
large scales on the small scales in turbulence. We study a 1m 1m
1.5m flow between oscillating grids which produces
while containing regions of nearly homogeneous and highly inhomogeneous
turbulence. Large data sets of three-dimensional tracer particle velocities
have been collected using stereoscopic high speed cameras with real-time image
compression technology. Eulerian and Lagrangian structure functions are
measured in both homogeneous and inhomogeneous regions of the flow. We
condition the structure functions on the instantaneous large scale velocity or
on the grid phase. At all scales, the structure functions depend strongly on
the large scale velocity, but are independent of the grid phase. We see clear
signatures of inhomogeneity near the oscillating grids, but even in the
homogeneous region in the center we see a surprisingly strong dependence on the
large scale velocity that remains at all scales. Previous work has shown that
similar correlations extend to very high Reynolds numbers. Comprehensive
measurements of these effects in a laboratory flow provide a powerful tool for
assessing the effects of shear, inhomogeneity and intermittency of the large
scales on the small scales in turbulence
Kinship Care Programs: Effective Marketing and Outreach Build With Care and They Will Come!
A workshop entitled âMarketing Your RAPP to Effectively Reach Relative Caregiversâ was developed and offered at the 2019 Brookdale Foundation Training Conference for professionals who work with kinship caregivers, or âRelatives as Parents Programsâ (RAPP). The intention of this interactive brainstorming session was to give individual RAPP programs the opportunity to share and adopt proven effective methods to reach, attract, and enhance their own local or statewide caregiver programs. The attending group was comprised of over 50 individuals who operated RAPP programs that spanned the US. The three workshop facilitators (authors of this brief with 70 collective years of experience) were chosen because their programs represented kinship professionals and caregivers from differing localities (urban, suburban, and rural), races, ethnicities, and financial demographics. Since the inception of these programs in the 1990s, and despite their demographic differences, the Grandparent Resource Center (GRC) of the New York City Department for the Aging, the Grandparent Connection of the Jewish Association Serving the Aged (JASA), and the Relatives as Parents Program (RAPP) of Cornell Cooperative ExtensionâOrange County (CCE-OC), utilized similar strategies that helped grow their programs from serving single digits to hundreds of families per year. The consensus of the authors, supported and added to by workshop participants, is that the shared strategies and methods proposed in this brief could be considered âbest practicesâ and useful for any intergenerational program in the United States with a similarly defined audience
Slip-velocity of large neutrally-buoyant particles in turbulent flows
We discuss possible definitions for a stochastic slip velocity that describes
the relative motion between large particles and a turbulent flow. This
definition is necessary because the slip velocity used in the standard drag
model fails when particle size falls within the inertial subrange of ambient
turbulence. We propose two definitions, selected in part due to their
simplicity: they do not require filtration of the fluid phase velocity field,
nor do they require the construction of conditional averages on particle
locations. A key benefit of this simplicity is that the stochastic slip
velocity proposed here can be calculated equally well for laboratory, field,
and numerical experiments. The stochastic slip velocity allows the definition
of a Reynolds number that should indicate whether large particles in turbulent
flow behave (a) as passive tracers; (b) as a linear filter of the velocity
field; or (c) as a nonlinear filter to the velocity field. We calculate the
value of stochastic slip for ellipsoidal and spherical particles (the size of
the Taylor microscale) measured in laboratory homogeneous isotropic turbulence.
The resulting Reynolds number is significantly higher than 1 for both particle
shapes, and velocity statistics show that particle motion is a complex
non-linear function of the fluid velocity. We further investigate the nonlinear
relationship by comparing the probability distribution of fluctuating
velocities for particle and fluid phases
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