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].

Icing modelisation, unsteady turbulence, shock-boundary interaction, particles in blood vessels and airways, fluid-structure interaction

We are giving a significant contribution to the development of the NSMB code in the consortium of CFD Engineering and RUAG Aerospace (Switzerland) as well as a number of university laboratories (IMFT and ETH-Lausanne laboratories, ETH-Zurich, TU München, UniBw, München). This code is also used at KTH, by Airbus-France and EADS-ST. Thanks to the consortium, our contributions are made available to several European research teams. To increase flight safety and reduce the number of accidents related to the effects of icing, and to improve the certifications of aircraft to fly in all weather conditions, it is necessary to develop an effective icing simulation tool. Prediction of the accumulation of ice resulting in a degradation of aircraft performance is one of the major challenges for the scientific community in aeronautics. We developed an original approach based on an Eulerian water droplet transport formulation, a model of partial differential equations for the thermodynamics of icing, and for the first time a level-set methodology for water- Accumulation of ice. The level-set method has proven to be very effective in both 2D and 3D and greatly simplifies the multi-layer icing with ice [2-PLH16, 2-PDLH15, 4-HPHL15]. The modeling of unsteady turbulence where persistent coherent structures are found at very high Reynolds numbers and chaotic structures due to the dynamics of unorganized turbulent movements remains a very active field of research. We will be interested in hybrid models RANS-LES (DES, DDES, IDDES, SAS, PANS) or advanced models of type OES, AOES which are implemented in the NSMB solver and tested on numerous configurations in external or internal aerodynamics. Of particular interest is the prediction of shock wave interactions - turbulent boundary layers and fluid-structure interactions (fluid-elastic instabilities in tube bundles, aeroelasticity). We were able to demonstrate the relevance of 2D simulations for fluid-structure coupling compared to 3D or LES [2-EHCC16, 2-SLBB16, 4-EHDS16, 2-SMHD14, 4-HDPD14] models. We also benefit from the high level of activities in the field of bio-medical in Strasbourg. We developed tools to extract realistic geometries from medical imagery that we applied to small animal imaging [4-CG4F16, 4-XHCG12]. We have thus studied the transport and deposition of particles in airways and a collaboration with the University of Monash for the lungs and the IR4M for the blood networks is in progress.