89 research outputs found
Statistics of turbulent fluctuations in counter-rotating Taylor-Couette flows
The statistics of velocity fluctuations of turbulent Taylor-Couette flow are
examined. The rotation rate of the inner and outer cylinder are varied while
keeping the Taylor number fixed to
(). The azimuthal velocity component of the flow
is measured using laser Doppler anemometry (LDA). For each experiment
datapoints are acquired and carefully analysed. Using extended
self-similarity (ESS) \cite{ben93b} the longitudinal structure function
exponents are extracted, and are found to weakly depend on the ratio of the
rotation rates. For the case where only the inner cylinder rotates the results
are in good agreement with results measured by Lewis and Swinney \cite{lew99}
using hot-film anemometry. The power spectra shows clear -5/3 scaling for the
intermediate angular velocity ratios , roughly -5/3 scaling for , and no clear scaling law can be found for
(inner cylinder rotation only); the local scaling exponent of the spectra has a
strong frequency dependence. We relate these observations to the shape of the
probability density function of the azimuthal velocity and the presence of a
neutral line
Turbulence strength in ultimate Taylor-Couette turbulence
We provide experimental measurements for the effective scaling of the
Taylor-Reynolds number within the bulk , based
on local flow quantities as a function of the driving strength (expressed as
the Taylor number Ta), in the ultimate regime of Taylor-Couette flow. The data
are obtained through flow velocity field measurements using Particle Image
Velocimetry (PIV). We estimate the value of the local dissipation rate
using the scaling of the second order velocity structure
functions in the longitudinal and transverse direction within the inertial
range---without invoking Taylor's hypothesis. We find an effective scaling of
, (corresponding
to for the dimensionless
local angular velocity transfer), which is nearly the same as for the global
energy dissipation rate obtained from both torque measurements
() and Direct Numerical Simulations
(). The resulting Kolmogorov length
scale is then found to scale as
and the turbulence intensity as . With both the local dissipation rate and the local
fluctuations available we finally find that the Taylor-Reynolds number
effectively scales as Re in the
present parameter regime of .Comment: 15 pages, 8 figures, J. Fluid Mech. (In press
Heat transfer enhancement in Rayleigh-B\'enard convection using a single passive barrier
In this numerical study on Rayleigh-B\'enard convection we seek to improve
the heat transfer by passive means. To this end we introduce a single tilted
conductive barrier centered in an aspect ratio one cell, breaking the symmetry
of the geometry and to channel the ascending hot and descending cold plumes. We
study the global and local heat transfer and the flow organization for Rayleigh
numbers for a fixed Prandtl number of . We
find that the global heat transfer can be enhanced up to , and locally
around . The averaged Reynolds number is always decreased when a barrier
is introduced, even for those cases where the global heat transfer is
increased. We map the entire parameter space spanned by the orientation and the
size of a single barrier for .Comment: 19 pages, 15 figure
High humidity enhances the evaporation of non-aqueous volatile sprays
We experimentally investigate the evaporation of very volatile liquid
droplets (Novec 7000 Engineered Fluid) in a turbulent spray. Droplets with
diameters of the order of a few micrometers are produced by a spray nozzle and
then injected into a purpose-built enclosed dodecahedral chamber, where the
ambient temperature and relative humidity in the chamber are carefully
controlled. We observe water condensation on the rapidly evaporating droplet,
both for the spray and for a single acoustically levitated millimetric Novec
7000 droplet. We further examine the effect of humidity, and reveal that a more
humid environment leads to faster evaporation of the volatile liquid, as well
as more water condensation. This is explained by the much larger latent heat of
water. We extend an analytical model based on Fick's law to quantitatively
account for the data
Periodically driven Taylor-Couette turbulence
We study periodically driven Taylor-Couette turbulence, i.e. the flow
confined between two concentric, independently rotating cylinders. Here, the
inner cylinder is driven sinusoidally while the outer cylinder is kept at rest
(time-averaged Reynolds number is ). Using particle image
velocimetry (PIV), we measure the velocity over a wide range of modulation
periods, corresponding to a change in Womersley number in the range . To understand how the flow responds to a given modulation, we
calculate the phase delay and amplitude response of the azimuthal velocity.
In agreement with earlier theoretical and numerical work, we find that for
large modulation periods the system follows the given modulation of the
driving, i.e. the system behaves quasi-stationary. For smaller modulation
periods, the flow cannot follow the modulation, and the flow velocity responds
with a phase delay and a smaller amplitude response to the given modulation. If
we compare our results with numerical and theoretical results for the laminar
case, we find that the scalings of the phase delay and the amplitude response
are similar. However, the local response in the bulk of the flow is independent
of the distance to the modulated boundary. Apparently, the turbulent mixing is
strong enough to prevent the flow from having radius-dependent responses to the
given modulation.Comment: 12 pages, 6 figure
Scalar transport and nucleation in quasi-two-dimensional starting jets and puffs
We experimentally investigate the early-stage scalar mixing and transport
with solvent exchange in quasi-2D jets. We inject an ethanol/oil mixture upward
into quiescent water, forming quasi-2D turbulent buoyant jets and triggering
the ouzo effect with initial Reynolds numbers, Re_0=420, 840, and 1680. We
study starting jets with continuous injection and puffs with finite volume
injection. While both modes start with the jet stage, the puff exhibits
different characteristics in transport, entrainment, mixing, and nucleation.
For the starting jets, the total nucleated mass from the ouzo mixture seems
very similar to that of the passive scalar total mass, indicating a primary
nucleation site slightly above the virtual origin above the injection needle,
supplying the mass flux like the passive scalar injection. With continuous
mixing above the primary nucleation site, the mildly increasing nucleation rate
suggests the occurrence of secondary nucleation throughout the entire ouzo jet.
For the puffs, although the entrainment and nucleation reduce drastically when
the injection stops, the mild mixing still leads to non-zero nucleation rates
and the reduced decay of the mean puff concentrations for the ouzo mixture.
Adapting the theoretical framework established in \citet{Landel2012b} for
quasi-2D turbulent jets and puffs, we successfully model the transport of the
horizontally-integrated concentrations for the passive scalar. The fitted
advection and dispersion coefficients are then used to model the transport of
the ouzo mixture, from which the spatial-temporal evolution of the nucleation
rate can be extracted. The spatial distribution of the nucleation rate sheds
new light on the solvent exchange process in transient turbulent jet flows
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