13 research outputs found
Towards boiling Taylor-Couette turbulence
Taylor-Couette (TC) flow is the flow in-between two concentric cylinders which rotate independently. It is together with Rayleigh-Bénard (RB) convection, the flow in-between two plates, heated from below and cooled from above, one of the canonical flows where turbulence can be probed and put to the test. As compared to RB flow, where the flow is thermally-driven, the driving in TC flow is purely mechanical which is far more efficient when designing experiments. In this thesis, we focus our investigation towards a better understanding on the effect of vapor bubbles in high-Reynolds TC flow. These bubbles are the product of boiling a low-boiling point liquid which we use as the working fluid. We have designed an experiment, where the boiling can be well-controlled and monitored in order to quantify the physical response of the flow due to the bubbles, in particular its drag. We find that vapor bubbles are just as efficient in reducing the drag in the flow as air bubbles. Almost 50% drag reduction can be achieved when the volume fraction of the bubbles is ~8% , the Taylor number is ~10^12 and the Weber number is We>1
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
Controlling secondary flow in Taylor-Couette turbulence through spanwise-varying roughness
Highly turbulent Taylor-Couette flow with spanwise-varying roughness is
investigated experimentally and numerically (direct numerical simulations (DNS)
with an immersed boundary method (IBM)) to determine the effects of the spacing
and axial width of the spanwise varying roughness on the total drag and
{on} the flow structures. We apply sandgrain roughness, in the form of
alternating {rough and smooth} bands to the inner cylinder. Numerically, the
Taylor number is and the roughness width is varied between
, where is the gap width. Experimentally,
we explore and .
For both approaches the radius ratio is fixed at , with
and the radius of the inner and outer cylinder respectively. We
present how the global transport properties and the local flow structures
depend on the boundary conditions set by the roughness spacing .
Both numerically and experimentally, we find a maximum in the angular momentum
transport as function of . This can be atributed to the
re-arrangement of the large-scale structures triggered by the presence of the
rough stripes, leading to correspondingly large-scale turbulent vortices.Comment: 20 pages, 7 figures, draft for JF
Catastrophic phase inversion in high-Reynolds number turbulent Taylor--Couette flow
Emulsions are omnipresent in the food industry, health care, and chemical
synthesis. In this Letter the dynamics of meta-stable oil-water emulsions in
highly turbulent () Taylor--Couette
flow, far from equilibrium, is investigated. By varying the oil-in-water void
fraction, catastrophic phase inversion between oil-in-water and water-in-oil
emulsions can be triggered, changing the morphology, including droplet sizes,
and rheological properties of the mixture, dramatically. The manifestation of
these different states is exemplified by combining global torque measurements
and local in-situ laser induced fluorescence (LIF) microscopy imaging. Despite
the turbulent state of the flow and the dynamic equilibrium of the oil-water
mixture, the global torque response of the system is found to be as if the
fluid were Newtonian, and the effective viscosity of the mixture was found to
be several times bigger or smaller than either of its constituents.Comment: 5 pages, 4 figure
Effect of axially varying sandpaper roughness on bubbly drag reduction in Taylor-Couette turbulence
We experimentally investigate the influence of alternating rough and smooth
walls on bubbly drag reduction (DR). We apply rough sandpaper bands of width
between and , and roughness height ,
around the smooth inner cylinder (IC) of the Twente Turbulent Taylor-Couette
facility. Between sandpaper bands, the IC is left uncovered over similar width
, resulting in alternating rough and smooth bands, a constant pattern in
axial direction. We measure the DR in water that originates from introducing
air bubbles to the fluid at (shear) Reynolds numbers ranging
from to . Results are compared to bubbly DR
measurements with a completely smooth IC and an IC that is completely covered
with sandpaper of the same roughness . The outer cylinder is left smooth for
all variations. Results are also compared to bubbly DR measurements where a
smooth outer cylinder is rotating in opposite direction to the smooth IC. This
counter rotation induces secondary flow structures that are very similar to
those observed when the IC is composed of alternating rough and smooth bands.
For the measurements with roughness, the bubbly DR is found to initially
increase more strongly with , before levelling off to reach a
value that no longer depends on . This is attributed to a more
even axial distribution of the air bubbles, resulting from the increased
turbulence intensity of the flow compared to flow over a completely smooth wall
at the same . The air bubbles are seen to accumulate at the
rough wall sections in the flow. Here, locally, the drag is largest and so the
drag reducing effect of the bubbles is felt strongest. Therefore, a larger
maximum value of bubbly DR is found for the alternating rough and smooth walls
compared to the completely rough wall
Factores asociados con la asfixia perinatal en el Hospital Nacional Alberto Sabogal Sologuren de enero – diciembre 2014
INTRODUCCION: La asfixia perinatal se define como la agresión producida al recién nacido o feto alrededor del momento del parto o nacimiento por la falta de oxÃgeno y/o de una perfusión tisular adecuada, conduciendo una hipoxemia e hipercapnia con acidosis metabólica significativa. Un tercio de toda la mortalidad global infantil en los menores de cinco años corresponde a muertes durante el perÃodo neonatal. OBJETIVOS: Determinar los factores asociados con la asfixia perinatal en el servicio de NeonatologÃa del HNASS de Enero – Diciembre 2014. PACIENTES Y MÉTODOS: Estudio de casos y controles. Los casos fueron los recién nacidos con diagnóstico de asfixia perinatal, menores de 8 dÃas de nacido, atendidos en el servicio de NeonatologÃa. Se revisaron las historias clÃnicas, de donde se obtuvo los datos epidemiológicos y clÃnicos, los que fueron registrados en un instrumento de recolección de datos. RESULTADOS: Un total de 80 casos y 160 controles completaron la muestra. Las variables que resultaron asociadas fueron: Grado de instrucción superior OR=0.14 (IC 95% 0.03 – 0.57), control prenatal adecuado OR=0.29 (IC 0.10 – 0.83), preeclampsia OR=4.31 (IC 95% 1.52 – 11.48), trabajo de parto prolongado OR=9.89 (IC 95% 2.56 – 38.24), desprendimiento prematuro de placenta OR=16.45 (IC 95% 2.41 – 112.46), corioamnionitis OR=8.14 (IC 95% 1.19 – 55.67), recién nacido pretérmino OR=4.07 (IC 95% 21.64 – 10.11), oligohidramnios OR=6.65 (IC 95% 1.33 – 33.27) y restricción de crecimiento intrauterino OR=15.86 (IC 95% 2.52 – 99.9).
CONCLUSIONES: En el Hospital Nacional Alberto Sabogal Sologuren, los principales factores de riesgo prenatales obstétricos para el desarrollo de asfixia perinatal son: Desprendimiento prematuro de placenta, trabajo de parto prolongado, corioamnionitis y preeclampsia. Asà mismo se evidenció que los principales factores de riesgo fetales son: Restricción de crecimiento intrauterino, oligohidramnios y recién nacido pretérmino. Se evidenciaron factores protectores para asfixia perinatal y son: Grado de instrucción superior y control prenatal adecuado
Statistics, plumes and azimuthally travelling waves in ultimate Taylor-Couette turbulent vortices
In this paper, we experimentally study the influence of large-scale Taylor rolls on the small-scale statistics and the flow organization in fully turbulent Taylor-Couette flow for Reynolds numbers up to ReS = 3×105. The velocity field in the gap confined by coaxial and independently rotating cylinders at a radius ratio of η=0.714 is measured using planar particle image velocimetry in horizontal planes at different cylinder heights. Flow regions with and without prominent Taylor vortices are compared. We show that the local angular momentum transport (expressed in terms of a Nusselt number) mainly takes place in the regions of the vortex in- and outflow, where the radial and azimuthal velocity components are highly correlated. The efficient momentum transfer is reflected in intermittent bursts, which becomes visible in the exponential tails of the probability density functions of the local Nusselt number. In addition, by calculating azimuthal energy co-spectra, small-scale plumes are revealed to be the underlying structure of these bursts. These flow features are very similar to the one observed in Rayleigh-Bénard convection, which emphasizes the analogies of these systems. By performing a complex proper orthogonal decomposition, we remarkably detect azimuthally travelling waves superimposed on the turbulent Taylor vortices, not only in the classical but also in the ultimate regime. This very large-scale flow pattern, which is most pronounced at the axial location of the vortex centre, is similar to the well-known wavy Taylor vortex flow, which has comparable wave speeds, but much larger azimuthal wavenumbers
Double maxima of angular momentum transport in small gap η=0.91 Taylor-Couette turbulence
We use experiments and direct numerical simulations to probe the phase space of low-curvature Taylor-Couette flow in the vicinity of the ultimate regime. The cylinder radius ratio is fixed at, where is the inner (outer) cylinder radius. Non-dimensional shear drivings (Taylor numbers) in the range are explored for both co- and counter-rotating configurations. In the range, we observe two local maxima of the angular momentum transport as a function of the cylinder rotation ratio, which can be described as either 'co-' or 'counter-rotating' due to their location or as 'broad' or 'narrow' due to their shape. We confirm that the broad peak is accompanied by the strengthening of the large-scale structures, and that the narrow peak appears once the driving (Ta) is strong enough. As first evidenced in numerical simulations by Brauckmann et al. (J. Fluid Mech., vol. 790, 2016, pp. 419-452), the broad peak is produced by centrifugal instabilities and that the narrow peak is a consequence of shear instabilities. We describe how the peaks change with as the flow becomes more turbulent. Close to the transition to the ultimate regime when the boundary layers (BLs) become turbulent, the usual structure of counter-rotating Taylor vortex pairs breaks down and stable unpaired rolls appear locally. We attribute this state to changes in the underlying roll characteristics during the transition to the ultimate regime. Further changes in the flow structure around cause the broad peak to disappear completely and the narrow peak to move. This second transition is caused when the regions inside the BLs which are locally smooth regions disappear and the whole boundary layer becomes active