Instabilities, multiphase turbulence
Presentation
The theme "Instabilities, multiphase turbulenec" deals with the phenomena of multiphase transport from the point of view of the instabilities induceded by the movement of particles, the effects of turbulence, the phase change, the interfaces and free surfaces as well as the complexe of geometries.
Instabilities in trajectories of spheres, disks, ellipsoids and bubbles ; transition to turbulence in wakes, receptivity of linearly stable flows
We focus on the instabilities in wakes and their effect on the trajectory of the particles moving in the fluids under the effect of gravity. The activity benefits from a specific spectral code developed on the basis of the theoretical analysis of the breakage of axisymmetry due to the transition to turbulence. This is a very widespread situation affecting any solid object that is indeformable or deformable but axisymmetric at rest, placed in a Newtonian fluid and exposed to gravity. The low cost and the precision of the simulations allowed us to be the first or among the firsts on subjects as fundamental as the exact prediction of the different regimes of trajectories of spherical particles [2-ZD15], disks [2-CBD13], elipsoids in fall or in free ascension. Some publications are references (article Jenny et al., JFM 2004) and our results serve as a benchmark to validate multi-particle simulation codes at the KIT IFH [2-UD14]. Our database is public and accessible by the scientific community. The published state diagrams illustrate the effects of the transition on the trajectories and allow to compare the numerical and theoretical predictions to experimental observations. Currently, work is ongoing with the study of the stability of the trajectory of a bubble (deformable). We were the first to obtain a marginal stability curve that takes into account the deformation of the bubble due to the instability. Two other key issues were also addressed during the five-year period. Direct simulations of the transition to three-dimensionality in the wake of a cylinder have made it possible to explain experimental observations dating from the 1980s [2-ABD11,2-ABD14]. A numerical and theoretical study resulted in the proposal of a new theoretical framework explaining the turbulence in flows not undergoing linear instabilities [2-SDD11].
Frost modelisation, unsteady turbulence, shock-boundary interaction, particles in blood vessels and airways, fluid-structure interaction
