Experimental study of path instability of rising bubbles

Experimental study of path instability of rising bubbles Abstract: The main objective is to study the path instability of a freely ascending air bubble in water and silicon oil at varied Bond numbers (ratio of body forces to surface tension) and Galileo numbers (ratio of gravitational force to viscous force). Below a certain threshold of the Galileo number, the path of the bubble remains straight. But above a critical Galileo number (related to the size of the bubble) a new regime sets in, and the bubble changes its shape and follows a zig-zagging path. Experimental investigation is needed because earlier existing numerical work suffers from simplifications likely to bias predictions, such as the assumption of fixed shape [1,2] and recent simulations accounting fully for shape deformation are either still too sparse to provide a reliably established marginal stability curve [3,4] or bring controversial results [5]. Our work aims to contribute reliable observations of the onset of instability using various silicon oils. In our experimental set up, bubbles of variable size are released in a rectangular tank filled with silicon oil known to present no problem of free surface impurities. Images obtained by a fast camera and are post-processed to determine the size and shapes of the bubbles and their trajectory in the fluid. Bond and Galileo numbers are calculated based on the volume of bubble and on the physical properties of the used working fluid.  Experiments will be conducted with silicon oils of varying viscosity to obtain as many points in the Galileo ? Bond number parameter plane as possible. Bibilography : [1] Tchoufag J., Magnaudet J. and Fabre D., Linear stability and sensitivity of the flow past a fixed oblate spheroidal bubble, Physics of Fluids, 25 (2013) 052108 [2] Cano-Lozano J.C., Bohorquez P. and Martinez-Bazán C., Wake instability of a fixed axisymmetric bubble of realistic shape, Int. J. Multiphase Flow, 51 (2013) 11. [3] Tripathi M.K., Saku K.C. and Govindarajan R., Nature Comm., 6:6268, (2015). [4] Cano-Lozano J.C. , Martinez-Bazán, Magnaudet J. and Tchoufag J., Paths and wakes of deformable nearly spherical rising bubbles close to the transition to path instability, Phys. Rev. Fluids 1, 053604 (2016). [5] Zhou W. and Dusek J., Marginal stability curve of a deformable bubble, Int. J. Multiphase Flow, 89 (2017) 218

Experimental evidence for the boundary zonal flow in rotating Rayleigh-Benard convection

We report on the presence of the boundary zonal flow in rotating Rayleigh-Benard convection evidenced by two-dimensional particle image velocimetry. Experiments were conducted in a cylindrical cell of aspect ratio between its diameter (D) and height (H) of G=D/H=1. As the working fluid we used various mixtures of water and glycerol, leading to Prandtl numbers in the range 6.6<Pr<75. The horizontal velocity components were measured at a horizontal cross-section at half height. The Rayleigh numbers were in the range 10e8 < Ra < 6.3e9. The effect of rotation is quantified by the Ekman number which was between 1.5e-5<Ek< 1.2e-3 in our experiment. With our results we show the first direct measurements of the boundary zonal flow - BZF that develops near the sidewall and was recently discovered in numerical simulations as well as in sparse and localized temperature measurements. We analyse the thickness of the BZF as well as its maximal velocity as function of Pr, Ra, and Ek, and compare these results with previous DNS results

Experimental study of instabilities in turbulent Rayleigh Benard Convection and Path instability of rising bubbles

Les présents travaux portent sur deux problèmes classiques de mécanique des fluides: la convection turbulente de Rayleigh-Bénard en rotation et l’instabilité de la trajectoire de bulles en ascension dans un liquide. Pour ce qui concerne la convection turbulente de Rayleigh-Bénard en rotation, les champs de vitesse et de température ont été mesurés respectivement par PIV (Particle Image Velocimetry) et par LIF (Laser Induced Fluorescence) dans le plan de symétrie vertical de notre cellule cylindrique de rapport d’aspect 1. Les résultats de PIV confirment l’affaiblissement de la circulation à grande échelle en fonction des nombres Rossby décroissants – jusqu’à sa disparition totale – ainsi que la formation de colonnes de vortex en régime dominé par la rotation. Par des corrélations croisées sur le champ de vitesse, il a été possible de montrer expérimentalement que la vorticité dans les colonnes changeait de signe au centre de la cellule. Les fluctuations de vitesses dans la cellule sont fortement anisotropiques et suivent une loi d’échelle en Ro0.2 en régime affecté par la rotation. La température des colonnes de vortex tout comme celle des panaches a été estimée par des mesures LIF. Pour ce qui concerne l’instabilité de la trajectoire de bulles en ascension, les bulles petites montent verticalement, alors qu’au-dessus d’une certaine taille, elles montent en zigzag ou en suivant une trajectoire hélicoïdale. De nouveaux points expérimentaux sur la courbe de stabilité marginale ont été obtenus en travaillant dans des huiles de silicone de 5 et 10 cst et dans l’eau. L’accord avec les résultats des simulations numériques les plus récentes n’est que partiel. La vitesse ascensionnelle, la fréquence et l’amplitude des oscillations ont été mesurées et suggèrent une bifurcation de Hopf supercritiqueThe present work focuses on two common fluid flow problems namely, Turbulent Rayleigh-Bénard Convection and Path instability of rising bubbles immersed in a liquid. Concerning Rotating Turbulent Rayleigh-Bénard Convection, the flow field and temperature field were measured respectively by Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) in a vertical plan of symmetry of our cylindrical cell of aspect ratio 1. The weakening of the Large Scale Circulation with decreasing Rossby number - leading to its complete disappearance - was confirmed as well as the formation of vortex columns in the rotation dominated regime. By doing velocity cross correlations, it has been possible to prove experimentally that the vorticity of the columns change direction in the cell’s center. The velocity fluctuations in the cell are highly anisotropic and follow a scaling of Ro0.2 in the rotation affected regime. The temperature of the vortex columns as well as of individual plumes has been estimated by LIF measurements. Concerning the Path instability of rising bubbles, small bubbles rise in straight path, whereas beyond a critical size, bubbles rise in zigzag or helical path. Some new experimental points on the marginal stability curve have been obtained by working in silicon oils of 5 and 10 cst and in water. The agreement with the most recent numerical simulations is only partial. The rise velocity, frequency and amplitude of oscillation have also been measured and suggest a supercritical Hopf bifurcation

Etude expérimentale d'instabilités à travers la convection turbulente de Rayleigh-Bénard et les instabilités de trajectoires de bulles en ascension

The present work focuses on two common fluid flow problems namely, Turbulent Rayleigh-Bénard Convection and Path instability of rising bubbles immersed in a liquid. Concerning Rotating Turbulent Rayleigh-Bénard Convection, the flow field and temperature field were measured respectively by Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) in a vertical plan of symmetry of our cylindrical cell of aspect ratio 1. The weakening of the Large Scale Circulation with decreasing Rossby number - leading to its complete disappearance - was confirmed as well as the formation of vortex columns in the rotation dominated regime. By doing velocity cross correlations, it has been possible to prove experimentally that the vorticity of the columns change direction in the cell’s center. The velocity fluctuations in the cell are highly anisotropic and follow a scaling of Ro0.2 in the rotation affected regime. The temperature of the vortex columns as well as of individual plumes has been estimated by LIF measurements. Concerning the Path instability of rising bubbles, small bubbles rise in straight path, whereas beyond a critical size, bubbles rise in zigzag or helical path. Some new experimental points on the marginal stability curve have been obtained by working in silicon oils of 5 and 10 cst and in water. The agreement with the most recent numerical simulations is only partial. The rise velocity, frequency and amplitude of oscillation have also been measured and suggest a supercritical Hopf bifurcation.Les présents travaux portent sur deux problèmes classiques de mécanique des fluides: la convection turbulente de Rayleigh-Bénard en rotation et l’instabilité de la trajectoire de bulles en ascension dans un liquide. Pour ce qui concerne la convection turbulente de Rayleigh-Bénard en rotation, les champs de vitesse et de température ont été mesurés respectivement par PIV (Particle Image Velocimetry) et par LIF (Laser Induced Fluorescence) dans le plan de symétrie vertical de notre cellule cylindrique de rapport d’aspect 1. Les résultats de PIV confirment l’affaiblissement de la circulation à grande échelle en fonction des nombres Rossby décroissants – jusqu’à sa disparition totale – ainsi que la formation de colonnes de vortex en régime dominé par la rotation. Par des corrélations croisées sur le champ de vitesse, il a été possible de montrer expérimentalement que la vorticité dans les colonnes changeait de signe au centre de la cellule. Les fluctuations de vitesses dans la cellule sont fortement anisotropiques et suivent une loi d’échelle en Ro0.2 en régime affecté par la rotation. La température des colonnes de vortex tout comme celle des panaches a été estimée par des mesures LIF. Pour ce qui concerne l’instabilité de la trajectoire de bulles en ascension, les bulles petites montent verticalement, alors qu’au-dessus d’une certaine taille, elles montent en zigzag ou en suivant une trajectoire hélicoïdale. De nouveaux points expérimentaux sur la courbe de stabilité marginale ont été obtenus en travaillant dans des huiles de silicone de 5 et 10 cst et dans l’eau. L’accord avec les résultats des simulations numériques les plus récentes n’est que partiel. La vitesse ascensionnelle, la fréquence et l’amplitude des oscillations ont été mesurées et suggèrent une bifurcation de Hopf supercritiqu