Thomas Séon

Buoyant mixing of miscible fluids in tilted tubes

Buoyant mixing of two fluids in tubes is studied experimentally as a function of the tilt angle θ from vertical, the density contrast and the common viscosity μ. At high contrasts and low θ, longitudinal mixing is macroscopically diffusive, with a diffusivity D increasing strongly with θ and μ. At lower contrasts and higher θ, a counterflow of the two fluids with little transverse mixing sets in. The transition occurs at an angle increasing with density contrast and decreasing with μ. These results are discussed in terms of the dependence of transerse mixing on θ and an analogy with the Boycott effect.

Buoyancy driven miscible front dynamics in tilted tubes

The velocity Vf of the fronts of light and heavy fluids in a tilted tube, interpenetrating many diameters, is studied as a function of the fluid viscosity μ, Atwood number At⪡1 and tilt angle θ from vertical. Three flow regimes are observed: starting from vertical, Vf first increases with θ, reaches a plateau and then decreases again. In the first regime, Vf is controlled by segregation and mixing effects, respectively, increasing and decreasing with θ. On the plateau, Vf is independent of the fluid viscosity and proportional to (Atgd)1∕2, indicating a balance between inertia and buoyancy. In the third regime close to horizontal, the fluids separate into two parallel countercurrents controlled by viscosity. The variations of Vf with θ, At, and μ in the second and third regimes and the crossover from one to the other are described by scaling laws based on characteristic viscous and inertial velocities.

Transient buoyancy-driven front dynamics in nearly horizontal tubes

The interpenetration of light and heavy liquids has been studied in a long tube inclined at small angles α to the horizontal. For angles greater than a critical angle αc (whose value decreases when the density contrast measured by the Atwood number At increases), the velocity of the interpenetration front is controlled by inertia and takes the steady value Vf=ki(Atgd)1∕2, with ki≃0.7. At lower angles, the front is initially controlled by inertia, but later limited by viscous effects. The transition occurs at a distance Xfc, which increases indefinitely as α increases to αc. Once the viscous effects act, the velocity of the front decreases in time to a steady value Vf∞ which is proportional to sinα. For a horizontal tube in the viscous regime, the velocity of the front decreases to zero as t−1∕2. At the same time, the profile of the interface h(x,t) only depends on the reduced variable x∕t1∕2. A quasi-unidirectional model reproduces well the variation of the velocity of the front and the profiles of the in...

Front dynamics and macroscopic diffusion in buoyant mixing in a tilted tube

The buoyancy driven interpenetration of two fluids of different densities has been studied in a long tilted tube in the strong mixing regime for which the mean concentration profile along the tube length satisfies a macroscopic diffusion equation. Variations of the corresponding macroscopic diffusion coefficient D and of the front velocity Vf are studied as a function of the Atwood number At, the viscosity ν, the tube diameter d, and the tilt angle θ. Introducing the characteristic inertial velocity Vt and the Reynolds number Ret, the normalized front velocity Vf∕Vt and dispersion coefficient D∕(Vtd) are observed to scale, respectively, as Ret−3∕4 and Ret−3∕2 for Ret≲1000. Also, Vf increases linearly with tanθ and the ratio (D∕Vf2) remains of the order of (35±10)d∕Vt in a wide range of values of the tilt angle and of the other control parameters. This close relation observed between the variations of D and Vf2 is discussed in terms of the characteristic time for transverse mixing across the flow channel.

Laser-induced fluorescence measurements of buoyancy driven mixing in tilted tubes

The front of a light fluid rising into a miscible heavier fluid inside a long tilted tube has been analyzed by laser-induced fluorescence. Both the local concentration field and the front velocity Vf have been studied in the inertial flow regime as a function of the tilt angle θ for a constant density contrast (At=4×10−3). We demonstrate experimentally that the velocity Vf is directly related to the local density contrast Cf by the relation Vf∝(Cf)0.5. This relation reflects a local instantaneous equilibrium between inertia and buoyancy; it is valid in the transient relaxation phase as well as in the quasistationary regime reached thereafter.The front of a light fluid rising into a miscible heavier fluid inside a long tilted tube has been analyzed by laser-induced fluorescence. Both the local concentration field and the front velocity Vf have been studied in the inertial flow regime as a function of the tilt angle θ for a constant density contrast (At=4×10−3). We demonstrate experimentally that the velocity Vf is directly related to the local density contrast Cf by the relation Vf∝(Cf)0.5. This relation reflects a local instantaneous equilibrium between inertia and buoyancy; it is valid in the transient relaxation phase as well as in the quasistationary regime reached thereafter.

On the physics of fizziness: How bubble bursting controls droplets ejection

Bubbles at a free surface usually burst in ejecting myriads of droplets. Focusing on the bubble bursting jet, prelude for these aerosols, we propose a simple scaling for the jet velocity and we unravel experimentally the intricate roles of bubble shape, capillary waves, gravity, and liquid properties. We demonstrate that droplets ejection unexpectedly changes with liquid properties. In particular, using damping action of viscosity, self-similar collapse can be sheltered from capillary ripples and continue closer to the singular limit, therefore producing faster and smaller droplets. These results pave the road to the control of the bursting bubble aerosols.

Collection of collapsing bubble driven phenomena found in champagne glasses

A simple glass of champagne or sparkling wine may seem like the acme of frivolity to most of people, but in fact it may rather be considered as a fantastic playground for any fluid physicist. In this tutorial review, the collapse of gaseous CO2 bubbles found at the free surface of a glass poured with champagne is depicted, through high speed photography and high speed video cameras. A collection of collapsing bubble driven phenomena are gathered, which illustrate the fine interplay between bubbles and the fluid around.

From turbulent mixing to gravity currents in tilted tubes

The buoyancy driven motion of a lighter fluid rising spontaneously into a heavier miscible fluid in vertical or tilted tubes give rise to flows as different as diffusive turbulent mixing and counterflow without mixing Fig. 1 . Two miscible fluids of different densities are initially in an unstable configuration, each of them occupying one halflength of a long transparent tube d=20 mm, L=4 m . The FIG. 1. The distribution of fluorescent dye in a vertical diametral plane of the tube for different tilt angles of the tube from vertical see Fig. 3 . The field of view is located between 200 and 500 mm above the gate valve Fig. 2 . Red and purple-blue shades correspond, respectively, to the pure heavy and light fluids and intermediate colors to mixtures of various compositions.

Evaporation of droplets in a Champagne wine aerosol

In a single glass of champagne about a million bubbles nucleate on the wall and rise towards the surface. When these bubbles reach the surface and rupture, they project a multitude of tiny droplets in the form of a particular aerosol holding a concentrate of wine aromas. Based on the model experiment of a single bubble bursting in idealized champagnes, the key features of the champagne aerosol are identified. In particular, we show that film drops, critical in sea spray for example, are here nonexistent. We then demonstrate that compared to a still wine, champagne fizz drastically enhances the transfer of liquid into the atmosphere. There, conditions on bubble radius and wine viscosity that optimize aerosol evaporation are provided. These results pave the way towards the fine tuning of flavor release during sparkling wine tasting, a major issue for the sparkling wine industry.

Frozen Impacted Drop: From Fragmentation to Hierarchical Crack Patterns.

We investigate experimentally the quenching of a liquid pancake, obtained through the impact of a water drop on a cold solid substrate (0 °C to -60 °C). We show that, below a certain substrate temperature, fractures appear on the frozen pancake and the crack patterns change from a 2D fragmentation regime to a hierarchical fracture regime as the thermal shock increases. The different regimes are discussed and the transition temperatures are estimated through classical fracture scaling arguments. Finally, a phase diagram presents how these regimes can be controlled by the drop impact parameters.