Xavier Ruiz

Generation of a Monodisperse Microbubble Jet in Microgravity

A new method to create a jet of a virtually monodisperse microbubble suspension of prescribed bubble size into a quiescent cavity is proposed. The method is insensitive to gravity and is based on the creation of a slug flow at a T junction in a capillary tube before injection. We develop a theoretical analysis that establishes the validity and efficiencyofthe method,ascontrolledbythecrossflowWebernumber,andyields asimple explicitprediction forthe bubble size in terms of the injection parameters. The method operates efficiently for small Weber numbers, yet it generates small bubbles of very uniform size. The reduced size dispersion is also explained within the theoretical model. The method of bubble formation, injection, and spreading by the resulting turbulent jet is validated experimentally in 4.7 s of free fall in the drop tower at the University of Bremen. Experiments demonstrate the physical principle behind the method of bubble formation and allow us to explore the dynamics of the resulting bubble jet after injection of the slug flow into a quiescent cavity in microgravity conditions as an efficient method of bubblespreadingandtransport.Detailed measurements of averagelocal velocities show thatbubblesare essentially passivewithrespecttothecarriermean flow,andtheinherentturbulenceofthe flowiscrucialforoptimalspreading of the bubbledistribution andreduction of bubble coalescence. Theshape of the bubblejet isstudied asa function of the Reynolds number. Finally, the degree of coalescence is also characterized and found to be remarkably small.

Bifurcations and chaos in single-roll natural convection with low Prandtl number

Convective flows of a small Prandtl number fluid contained in a two-dimensional cavity subject to a lateral thermal gradient are numerically studied by using different techniques. The aspect ratio (length to height) is kept at around 2. This value is found optimal to make the flow most unstable while keeping the basic single-roll structure. Two cases of thermal boundary conditions on the horizontal plates are considered: perfectly conducting and adiabatic. For increasing Rayleigh numbers we find a transition from steady flow to periodic oscillations through a supercritical Hopf bifurcation that maintains the centrosymmetry of the basic circulation. For a Rayleigh number of about ten times that of the Hopf bifurcation the system initiates a complex scenario of bifurcations. In the conductive case these include a quasiperiodic route to chaos. In the adiabatic one the dynamics is dominated by the interaction of two Neimark-Sacker bifurcations of the basic periodic solutions, leading to the stable coexistence...

Numerical Studies of Fluid Flow in Microgravity Conditions for Confined Crystal Growth

We study the convective flow induced by residual accelerations in microgravity conditions for different geometric arrangements which are relevant to crystal growth experiments. We consider both constant and oscillating acceleration and focus mostly on the transient relaxation dynamics. Results are relevant to estimate impact of more realistic residual accelerations in crystal growth experiments.