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 $\text{Re}_{\lambda,\text{bulk}}$, 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 $\epsilon(r)$ 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 $\epsilon_{\text{bulk}} /(\nu^{3}d^{-4})\sim \text{Ta}^{1.40}$, (corresponding to $\text{Nu}_{\omega,\text{bulk}} \sim \text{Ta}^{0.40}$ for the dimensionless local angular velocity transfer), which is nearly the same as for the global energy dissipation rate obtained from both torque measurements ($\text{Nu}_{\omega} \sim \text{Ta}^{0.40}$) and Direct Numerical Simulations ($\text{Nu}_{\omega} \sim \text{Ta}^{0.38}$). The resulting Kolmogorov length scale is then found to scale as $\eta_{\text{bulk}}/d \sim \text{Ta}^{-0.35}$ and the turbulence intensity as $I_{\theta,\text{bulk}} \sim \text{Ta}^{-0.061}$. With both the local dissipation rate and the local fluctuations available we finally find that the Taylor-Reynolds number effectively scales as Re$_{\lambda,\text{bulk}}\sim \text{Ta}^{0.18}$ in the present parameter regime of $4.0 \times 10^8 < \text{Ta} < 9.0 \times 10^{10}$.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 $s$ 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 $\mathcal{O}(10^9)$ and the roughness width is varied between $0.47\leq \tilde{s}=s/d \leq 1.23$, where $d$ is the gap width. Experimentally, we explore $\text{Ta}=\mathcal{O}(10^{12})$ and $0.61\leq \tilde s \leq 3.74$. For both approaches the radius ratio is fixed at $\eta=r_i/r_o = 0.716$, with $r_i$ and $r_o$ 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 $\tilde{s}$. Both numerically and experimentally, we find a maximum in the angular momentum transport as function of $\tilde s$. 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 ($10^{11}\leq\text{Ta}\leq 3\times 10^{13}$) 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 $s$ between $48.4\,mm$ and $148.5\,mm$, and roughness height $k = 695\,{\mu}m$, around the smooth inner cylinder (IC) of the Twente Turbulent Taylor-Couette facility. Between sandpaper bands, the IC is left uncovered over similar width $s$, 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 $\textit{Re}_s$ ranging from $0.5 \times 10^6$ to $1.8 \times 10^6$. 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 $k$. 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 $\textit{Re}_s$, before levelling off to reach a value that no longer depends on $\textit{Re}_s$. 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 $\textit{Re}_s$. 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 oxígeno 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 período neonatal. OBJETIVOS: Determinar los factores asociados con la asfixia perinatal en el servicio de Neonatología 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 días de nacido, atendidos en el servicio de Neonatología. Se revisaron las historias clínicas, de donde se obtuvo los datos epidemiológicos y clínicos, 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. Así 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