The spray formed when a fast gas stream blows over a liquid volume presents a wide distribution of fragment sizes. The process involves a succession of changes of the liquid topology, the last being the elongation and capillary breakup of ligaments torn off from the liquid surface. The coalescence of the liquid volumes constitutive of a ligament at the very moment it detaches from the liquid bulk produces larger drops. This aggregation process has its counterpart on the shape of the size distribution associated with the ligament breakup, found to be very well represented by gamma distributions. The exponential shape of the overall distribution in the spray coincides with the large excursion wing of these elementary distributions, underlying the crucial role played by the ligament dynamics in building up the broad statistics of sprays.
International audienceWe study the relaxation of initially segregated scalar mixtures in randomly stirred media, aiming to describe the overall concentration distribution of the mixture, its shape and rate of deformation as it evolves towards uniformity. A stirred scalar mixture can be viewed as a collection of stretched sheets, possibly interacting with each other. We consider a situation in which the interaction between the sheets is enforced by confinement and is the key factor ruling its evolution. It consists of following a mixture relaxing towards uniformity around a fixed average concentration while flowing along a constant cross-section channel. The interaction between the sheets is found to be of a random addition nature in concentration space, leading to concentration distributions that are stable by self-convolution. The resulting scalar field is naturally coarsened at a scale much larger than the dissipation scale. Consequences on the mixture entropy and scalar rate of dissipation are also examined
Experiments show how a stirred scalar mixture relaxes towards uniformity through an aggregation process. The elementary bricks are stretched sheets whose rates of diffusive smoothing and coalescence build up the overall mixture concentration distribution. The cases studied, in particular, include mixtures in two and three dimensions, with different stirring protocols and Reynolds numbers which all lead to a unique family of concentration distributions stable by self-convolution, the signature of the aggregation mechanism from which they originate.
We study the relaxation of an initially segregated scalar mixture in a randomly stirred medium, aiming at describing the overall concentration distribution of the mixture, its shape, and its rate of deformation as it evolves toward uniformity. An ever dispersing mixture is realized by releasing a plume of scalar in a large scale, sustained turbulent medium on the axis and in the far field of a turbulent jet. The fluid particles constitutive of the plume are soon resolved into a set of stretched sheets whose rates of diffusive smoothing build up the overall mixture concentration distribution. The randomness of the particle's net elongation at a given instant of time induces a distribution of the mixing time from which molecular diffusion becomes effective in erasing the concentration differences. This ingredient is shown to rule the composition of this dispersing mixture, providing a detailed analytic description of the overall concentration distribution. It compares favorably with experiments using three different passive scalars, suggesting that the mixture composition results from a nonsequential, one-step lengthening process distributed among the sheets.
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