29 research outputs found
Experimental study of bubble-drag interaction in a Taylor-Couette flow
No full textThis study is an experimental investigation of the interactions between the bubbles, the coherent motion and the viscous drag in a Taylor Couette flow, for the outer cylinder at rest. The cylinder radius ratio is 0.9. Bubbles are injected through a needle at the bottom of the apparatus inside the gap. Different bubble sizes are investigated (ratio between the bubble size and the gap width 0.05 and 0.12) for very small void fraction (≤0.012). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and the velocity of the inner cylinder ranging from 400 to 20000. For these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. PIV measurements of the liquid flow features, bubble tracking in a meridian plane and viscous torque of the inner cylinder measurements are performed. This study provides a first evidence of the link between the bubble localisation, the Taylor vortices and viscous torque modifications. Bubbles are attracted towards the inner cylinder, due to the rotation of the cylinder. For small buoyancy effect, bubbles are trapped and induce a decrease in the outflow intensity, thus leading to an increase of the viscous torque. When buoyancy induced bubble motion, by comparison to the coherent motion of the liquid is increased, a decrease in the viscous torque is suspected
Mitigation of underwater explosion effects by bubble curtains : experiments and modelling
No full textMine fields and UneXploded Ordnances (UXO) become a danger regarding maritime activities. Since UXOs are strongly affected by marine corrosion after decades, they cannot be handled safely. A safe solution to get rid of them would be to explode them in their locations. However, this method generates noise pollution and damaging shock waves. Mitigation of shocks and noises is made possible by the use of a bubble curtain set around the explosive charge. Physical aspects of shock propagation in bubbly flows have been the subject of numerous investigations in the past decades and theoretical models of aerated liquids now reproduce main shock features with acceptable accuracy in the case of a uniform distribution of bubbles of the same size. However, the bubble distribution obtained by air blown in a porous pipe is far to be monodisperse. So the modeling of the interaction of a shock wave with a polydisperse medium still remains a challenge. In the present study, the transmission of a shock wave propagating through a bubble curtain is investigated experimentally on a water filled tank. A microporous pipe, connected to a compressed air supply system and a flowmeter, is placed on the floor in the tank. A dual-tip fiber optical probe is used to measure the gas fraction distribution, bubble rising velocity and bubble size distribution in the curtain. A calibrated shock wave is generated by plate impact, upstream of the bubble curtain, and recorded downstream with a hydrophone. The mitigation of the pressure peak by the bubbly medium is evidenced by recorded pressure signals with and without bubble curtain. Experimental gas fraction profiles and bubble size distributions, measured in the bubble curtains, are finally used as input parameters in the numerical model developed by Grandjean et al. (2011). This numerical model enables prediction of shock wave mitigation and allows calibrating a suitable bubble curtain
Numerical simulation of bubble dispersion in turbulent Taylor-Couette flow
No full textWe investigate bubble dispersion in turbulent Taylor-Couette flow. The aim of this study is to describe the main mechanisms yielding preferential bubble accumulation in near-wall structures of the flow. We first proceed to direct numerical simulation of Taylor-Couette flows for three different geometrical configurations (three radius ratios η = R 1/R 2: η = 0.5, η = 0.72, and η = 0.91 with the outer cylinder at rest) and Reynolds numbers corresponding to turbulent regime ranging from 3000 to 8000. The statistics of the flow are discussed using two different averaging procedures that permit to characterize the mean azimuthal velocity, the Taylor vortices contribution and the small-scale turbulent fluctuations. The simulations are compared and validated with experimental and numerical data from literature. The second part of this study is devoted to bubble dispersion. Bubble accumulation is analyzed by comparing the dispersion obtained with the full turbulent flow field to bubble dispersion occurring at lower Reynolds numbers in previous works. Several patterns of preferential accumulation of bubbles have been observed depending on bubble size and the effect of gravity. For the smaller size considered, bubbles disperse homogeneously throughout the gap, while for the larger size they accumulate along the inner wall for the large gap width (η = 0.5). Varying the intensity of buoyancy yields complex evolution of the bubble spatial distribution. For low gravity effect, bubble entrapment is strong leading to accumulation along the inner wall in outflow regions (streaks of low wall shear stress). When buoyancy effect dominates on vortex trapping, bubbles rise through the vortices, while spiral patterns stretched along the inner cylinder are clearly identified. Force balance is analyzed to identify dominating forces leading to this accumulation and accumulation patterns are compared with previous experiments
Numerical simulations of drag modulation by microbubbles in a turbulent Taylor-Couette flow
No full textThe aim of our study is to investigate numerically the interaction between a dispersed phase composed of microbubbles and a turbulent Taylor-Couette flow (flow within the gap between two cylinders). We use the Euler-Lagrange approach based on Direct Numerical Simulation of the continuous phase flow equations and a Lagrangian tracking for the dispersed phase. Each bubble trajectory is calculated by integrating the force balance equation accounting for buoyancy, drag, added-mass, pressure gradient, and the lift forces. The numerical method has been adapted in order to take into account the feed-back effect of the dispersed bubbles on the carrying flow. Our approach is based on local volume average of the two-phase Navier-Stokes equations. Local and temporal variations of the bubble concentration and momentum source terms are accounted for in mass and momentum balance equations. A number of reference cases have been tested to validate the modelling approach and its numerical implementation. Then, our previous study of bubble dispersion has been extended to two-way coupling simulations of turbulent Taylor-Couette flows (only inner cylinder is rotating). Modulation of the drag will be discussed for different geometries, Reynolds numbers and bubble sizes. The results show that near-wall turbulent structures are modified by the presence of bubbles.The aim of our study is to investigate numerically the interaction between a dispersed phase composed of microbubbles and a turbulent Taylor-Couette flow (flow within the gap between two cylinders). We use the Euler-Lagrange approach based on Direct Numerical Simulation of the continuous phase flow equations and a Lagrangian tracking for the dispersed phase. Each bubble trajectory is calculated by integrating the force balance equation accounting for buoyancy, drag, added-mass, pressure gradient, and the lift forces. The numerical method has been adapted in order to take into account the feed-back effect of the dispersed bubbles on the carrying flow. Our approach is based on local volume average of the two-phase Navier-Stokes equations. Local and temporal variations of the bubble concentration and momentum source terms are accounted for in mass and momentum balance equations. A number of reference cases have been tested to validate the modelling approach and its numerical implementation. Then, our previous study of bubble dispersion has been extended to two-way coupling simulations of turbulent Taylor-Couette flows (only inner cylinder is rotating). Modulation of the drag will be discussed for different geometries, Reynolds numbers and bubble sizes. The results show that near-wall turbulent structures are modified by the presence of bubbles
Time Resolved Two Dimensional X-Ray Densitometry of a Two Phase Flow Downstream of a Ventilated Cavity
No full textTo measure the void fraction distribution in gas-liquid flows, a two-dimensional x-ray densitometry system was developed. This system is capable of acquiring a two-dimensional projection with a 225 cm2 area of measurement through 21 cm of water. The images can be acquired at rates on the order of 1 kHz. Common sources of error in x-ray imaging, such as x-ray scatter, image distortion, veiling glare, and beam hardening were considered, and mitigated. The measured average void fraction was compared success fully to that of a phantom target and found to be within 1%. To evaluate the performance of the new system, the flow in and downstream of a ventilated nominally two-dimensional partial cavity was investigated and compared to measurements from dual tip fiber optical probes and high speed video. The measurements were found to have satisfactory agreement for void fractions above 5% of the selected void fraction measurement range.Office of Naval Research under grant N00014-10-1-097
Effect of bubble’s arrangement on the viscous torque in bubbly Taylor- Couette flow
No full textInternational audienceAn experimental investigation of the interactions between bubbles, coherent motion and viscous drag in a Taylor-Couette flow with the outer cylinder at rest is presented. The cylinder radii ratio η is 0.91. Bubbles are injected inside the gap through a needle at the bottom of the apparatus. Different bubbles sizes are investigated (ratio between the bubble diameter and the gap width ranges from 0.05 to 0.125) for very small void fraction (α<=0.23%). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and velocity of the inner cylinder, ranging from 600 to 20000. Regarding these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. A detailed characterization of the vortices is performed for the single-phase flow. The experiment also develops bubbles tracking in a meridian plane and viscous torque of the inner cylinder measurements. The findings of this study show evidence of the link between bubbles localisation, Taylor vortices and viscous torque modifications. We also highlight two regimes of viscous torque modification and various types of bubbles arrangements, depending on their size and on the Reynolds number. Bubbles can have a sliding and wavering motion near the inner cylinder and be either captured by the Taylor vortices or by the outflow areas near the inner cylinder. For small buoyancy effect, bubbles are trapped, leading to an increase of the viscous torque. When buoyancy induced bubbles motion is increased by comparison to the coherent motion of the liquid, a decrease in the viscous torque is rather observed. The type of bubble arrangement is parameterized by the two dimensionless parameters C and H introduced by Climent et al. [E. Climent, M. Simonnet and J. Magnaudet, Phys. Fluids 19, 083301(2007)]. Phase diagrams summarizing the various types of bubbles arrangements, viscous torque modifications and axial wavelength evolution are built
Urban energy transition
No full textThis issue of the Network Industries Quarterly is dedicated to the governance of energy transition in urban energy infrastructures, by providing insights from different theoretical approaches as well as analyzing multiple case studies. In the first article, Castan Broto takes a complexity approach on sustainable transitions and analyzes urban energy landscapes as the assemblage of socio-technical interactions in the case of urban energy transition in Hong Kong. Florentin, Gabillet, Gomez in the second article investigate the role of local utilities as the understudied actors in urban energy transitions, by focusing on three case studies in Grenoble, Magdeburg and Medellin. In the third article, Euston-Brown and Ndlovu consider the dynamics of urban energy transition in Sub Saharan African cities, and highlight the importance of other factors rather than the technology itself, such as learning capacity and knowledge development. Finally, Ichonose in the last article explains the climate change problems in Tokyo, and addresses different measures and requirements for climate change mitigation and adaptation programs. We hope these contributions can draw your attention towards the importance of cities in the field of sustainable energy transitions, as the urban level is getting more interest in the transition research as an important level of analysis.-- Larger, diversified and courted: the new triad of local firms of infrastructure in transition?, Florentin, D. ; Gabillet, P. and Gomez, C. D.
-- ‘Theatres of technology innovation’: supporting local government in sustainable energy transition in South Africa, Ghana and Uganda, Euston-Brown, M. and Ndlovu, M.
-- Urban Energy Landscapes and Transitions to Sustainability: Notes from Hong Kong, Castán Broto, V
-- Governing climate change in Tokyo, Ichinose, T
Analysis of the flow pattern modifications in a bubbly Couette-Taylor flow
No full textThe aim of this Brief Communication is to discuss the bubble effect on the Couette-Taylor flow patterns in the transition from laminar to turbulent flow, especially in the weakly turbulent regime. It is shown that bubble location and local void fractions both in the vortices cores and in the near wall regions directly influence the axial wavelength. Bubbles trapped in the vortices tend to increase the vorticity and reduce the axial diffusivity. Bubbles near the wall contribute to “shear induced” turbulence depending on the void fraction gradient near the wall and the bubble size
Two-phase Couette–Taylor flow: Arrangement of the dispersed phase and effects on the flow structures
No full textThis study investigates the mutual interactions between a continuous and a dispersed phase~noncondensable or condensable! in the well-known Couette–Taylor flow between two concentric cylinders at low Reynolds numbers, where the outer cylinder is immobilized. In this experiment, the turbulent structures take place progressively. The noncondensable dispersed phase ~air! is introduced either by ventilation, generated by agitation of a free surface situated at the top of the gap between the two cylinders. The condensable dispersed phase is generated by cavitation due to a drop in pressure. Comparisons are made between the single phase flow patterns and those observed in ventilated or cavitating flow. Two particular arrangements of the dispersed phase are experimentally evident, according to the Reynolds number of the flow. For low Reynolds numbers, bubbles are trapped in the core of the Taylor cells, whereas they migrate to the outflow regions near the inner cylinder for higher Reynolds numbers. Assessment of the forces applied to the bubbles and computation of their equilibrium position can act as a base in describing the bubble capture. When bubbles are located near the wall in the outflow region, it is found that the three first instabilities are strongly influenced by the dispersed phase. The cavitating flow is also characterized by an earlier appearance of the third instability
