85 research outputs found
Highly turbulent Taylor-Couette flow
The research issues addressed in this mostly experimental thesis concern highly\ud
turbulent Taylor-Couette (TC) flow (Re>105, implying Ta>1011). We study it on\ud
a fundamental level to aid our understanding of (TC) turbulence and to make predictions towards astrophysical disks, and at a practical level as applications can be found in bubble-induced skin-friction drag reduction on ships. In PART I we introduce the new TC facility of our Physics of Fluids group, called the Twente turbulent Taylor-Couette (T3C) facility. It features two independently rotating cylinders of variable radius ratio with accurate rotation rate and temperature control, torque sensing, bubble injection and it is equipped with several local sensors. It is able to reach Reynolds numbers up to 3:4�106. In PART II we focus on highly turbulent single-phase TC flow. We measure the global torque as a function of the driving parameters and we provide local angular velocity measurements. The results are interpreted as the transport of angular velocity, based on the model proposed by Eckhardt, Grossmann & Lohse (2007). Furthermore, we study the turbulence transport in quasi-Keplerian profiles, mimicking astrophysical disks. In PART III we study the effect of bubbles on highly turbulent TC flow, focusing not only on\ud
global drag reduction, but also on the local bubble distribution and angular velocity profiles. We find that drag reduction is a boundary layer effect and that the deformability of bubbles is crucial for strong drag reduction in bubbly turbulent TC flow
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
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
Applying Laser Doppler Anemometry inside a Taylor-Couette geometry - Using a ray-tracer to correct for curvature effects
In the present work it will be shown how the curvature of the outer cylinder
affects Laser Doppler anemometry measurements inside a Taylor-Couette
apparatus. The measurement position and the measured velocity are altered by
curved surfaces. Conventional methods for curvature correction are not
applicable to our setup, and it will be shown how a ray-tracer can be used to
solve this complication.
By using a ray-tracer the focal position can be calculated, and the velocity
can be corrected. The results of the ray-tracer are verified by measuring an a
priori known velocity field, and after applying refractive corrections good
agreement with theoretical predictions are found. The methods described in this
paper are applied to measure the azimuthal velocity profiles in high Reynolds
number Taylor-Couette flow for the case of outer cylinder rotation
On bubble clustering and energy spectra in pseudo-turbulence
3D-Particle Tracking (3D-PTV) and Phase Sensitive Constant Temperature
Anemometry in pseudo-turbulence--i.e., flow solely driven by rising bubbles--
were performed to investigate bubble clustering and to obtain the mean bubble
rise velocity, distributions of bubble velocities, and energy spectra at dilute
gas concentrations (%). To characterize the clustering the pair
correlation function was calculated. The deformable bubbles with
equivalent bubble diameter mm were found to cluster within a radial
distance of a few bubble radii with a preferred vertical orientation. This
vertical alignment was present at both small and large scales. For small
distances also some horizontal clustering was found. The large number of
data-points and the non intrusiveness of PTV allowed to obtain well-converged
Probability Density Functions (PDFs) of the bubble velocity. The PDFs had a
non-Gaussian form for all velocity components and intermittency effects could
be observed. The energy spectrum of the liquid velocity fluctuations decayed
with a power law of -3.2, different from the found for
homogeneous isotropic turbulence, but close to the prediction -3 by
\cite{lance} for pseudo-turbulence
Optimal Taylor-Couette flow: Radius ratio dependence
Taylor-Couette flow with independently rotating inner (i) and outer (o)
cylinders is explored numerically and experimentally to determine the effects
of the radius ratio {\eta} on the system response. Numerical simulations reach
Reynolds numbers of up to Re_i=9.5 x 10^3 and Re_o=5x10^3, corresponding to
Taylor numbers of up to Ta=10^8 for four different radius ratios {\eta}=r_i/r_o
between 0.5 and 0.909. The experiments, performed in the Twente Turbulent
Taylor-Couette (T^3C) setup, reach Reynolds numbers of up to Re_i=2x10^6$ and
Re_o=1.5x10^6, corresponding to Ta=5x10^{12} for {\eta}=0.714-0.909. Effective
scaling laws for the torque J^{\omega}(Ta) are found, which for sufficiently
large driving Ta are independent of the radius ratio {\eta}. As previously
reported for {\eta}=0.714, optimum transport at a non-zero Rossby number
Ro=r_i|{\omega}_i-{\omega}_o|/[2(r_o-r_i){\omega}_o] is found in both
experiments and numerics. Ro_opt is found to depend on the radius ratio and the
driving of the system. At a driving in the range between {Ta\sim3\cdot10^8} and
{Ta\sim10^{10}}, Ro_opt saturates to an asymptotic {\eta}-dependent value.
Theoretical predictions for the asymptotic value of Ro_{opt} are compared to
the experimental results, and found to differ notably. Furthermore, the local
angular velocity profiles from experiments and numerics are compared, and a
link between a flat bulk profile and optimum transport for all radius ratios is
reported.Comment: Submitted to JFM, 28 pages, 17 figure
Boundary Zonal Flow in Rotating Turbulent Rayleigh-Bénard Convection
For rapidly rotating turbulent Rayleigh–Bénard convection in a slender cylindrical cell, experiments and direct numerical simulations reveal a boundary zonal flow (BZF) that replaces the classical large-scale circulation. The BZF is located near the vertical side wall and enables enhanced heat transport there. Although the azimuthal velocity of the BZF is cyclonic (in the rotating frame), the temperature is an anticyclonic traveling wave of mode one, whose signature is a bimodal temperature distribution near the radial boundary. The BZF width is found to scale like Ra1/4Ek2/3 where the Ekman number Ek decreases with increasing rotation rate
The importance of bubble deformability for strong drag reduction in bubbly turbulent Taylor-Couette flow
Bubbly turbulent Taylor-Couette (TC) flow is globally and locally studied at
Reynolds numbers of Re = 5 x 10^5 to 2 x 10^6 with a stationary outer cylinder
and a mean bubble diameter around 1 mm. We measure the drag reduction (DR)
based on the global dimensional torque as a function of the global gas volume
fraction a_global over the range 0% to 4%. We observe a moderate DR of up to 7%
for Re = 5.1 x 10^5. Significantly stronger DR is achieved for Re = 1.0 x 10^6
and 2.0 x 10^6 with, remarkably, more than 40% of DR at Re = 2.0 x 10^6 and
a_global = 4%.
To shed light on the two apparently different regimes of moderate DR and
strong DR, we investigate the local liquid flow velocity and the local bubble
statistics, in particular the radial gas concentration profiles and the bubble
size distribution, for the two different cases; Re = 5.1 x 10^5 in the moderate
DR regime and Re = 1.0 x 10^6 in the strong DR regime, both at a_global = 3 +/-
0.5%.
By defining and measuring a local bubble Weber number (We) in the TC gap
close to the IC wall, we observe that the crossover from the moderate to the
strong DR regime occurs roughly at the crossover of We ~ 1. In the strong DR
regime at Re = 1.0 x 10^6 we find We > 1, reaching a value of 9 (+7, -2) when
approaching the inner wall, indicating that the bubbles increasingly deform as
they draw near the inner wall. In the moderate DR regime at Re = 5.1 x 10^5 we
find We ~ 1, indicating more rigid bubbles, even though the mean bubble
diameter is larger, namely 1.2 (+0.7, -0.1) mm, as compared to the Re = 1.0 x
10^6 case, where it is 0.9 (+0.6, -0.1) mm. We conclude that bubble
deformability is a relevant mechanism behind the observed strong DR. These
local results match and extend the conclusions from the global flow experiments
as found by van den Berg et al. (2005) and from the numerical simulations by
Lu, Fernandez & Tryggvason (2005).Comment: 31 pages, 17 figure
Ultimate Turbulent Taylor-Couette Flow
The flow structure of strongly turbulent Taylor-Couette flow with Reynolds
numbers up to Re_i = 2*10^6 of the inner cylinder is experimentally examined
with high-speed particle image velocimetry (PIV). The wind Reynolds numbers
Re_w of the turbulent Taylor-vortex flow is found to scale as Re_w ~ Ta^(1/2),
exactly as predicted for the ultimate turbulence regime, in which the boundary
layers are turbulent. The dimensionless angular velocity flux has an effective
scaling of Nu_{\omega} ~ Ta^0.38, also in correspondence with turbulence in the
ultimate regime. The scaling of Nu_{\omega} is confirmed by local angular
velocity flux measurements extracted from high-speed PIV measurements: though
the flux shows huge fluctuations, its spatial and temporal average nicely
agrees with the result from the global torque measurements
Wall roughness induces asymptotic ultimate turbulence
Turbulence is omnipresent in Nature and technology, governing the transport
of heat, mass, and momentum on multiple scales. For real-world applications of
wall-bounded turbulence, the underlying surfaces are virtually always rough;
yet characterizing and understanding the effects of wall roughness for
turbulence remains a challenge, especially for rotating and thermally driven
turbulence. By combining extensive experiments and numerical simulations, here,
taking as example the paradigmatic Taylor-Couette system (the closed flow
between two independently rotating coaxial cylinders), we show how wall
roughness greatly enhances the overall transport properties and the
corresponding scaling exponents. If only one of the walls is rough, we reveal
that the bulk velocity is slaved to the rough side, due to the much stronger
coupling to that wall by the detaching flow structures. If both walls are
rough, the viscosity dependence is thoroughly eliminated in the boundary layers
and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of
transport, whose existence had been predicted by Robert Kraichnan in 1962
(Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be
extrapolated to arbitrarily large Reynolds numbers
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