Stability of a Liquid Jet Impinging on Confined Saturated Sand

Crater formation by a jet impacting a granular material.

Crater formation by impacting a granular material is ubiquitous in nature, from raindrops falling on sandy deserts to meteorites impacting moons and planets. Although previous works have considered the erosion process and surface morphology, less attention has been given to the jet hydrodynamics. Based on laboratory experiments, we show that when the liquid jet forms a crater, two situations arise. For weak or no erosion and for open craters, the jet is stationary. For vertical or overhanging crater walls, the jet displays a wide range of behaviors, from quasiperiodic oscillations to symmetry breaking and exploration of different states in time. An analysis of the different system states leads to the emergence of a bifurcation diagram depending on a dimensionless parameter, J, comparing the jet impact force to the force necessary to eject a grain. The frequency of the jet oscillations depends on the inertial velocity, the jet dispersion and the ratio between the injector cross section and the confinement length.

Reference :

Inertial drag-out problem : sheets and films on a rotating disc

Researchers from the LMFA (Univ. Lyon, Ecole Centrale de Lyon, INSA, CNRS) and LPENSL (Univ. Lyon, ENS de Lyon, CNRS) laboratories identify the physical mechanisms at work in the splashing behavior of a liquid driven by a rotating wheel.

References :

Probing fluid torque with a hydrodynamical drap: Rotation of chiral particles levitating in a turbulent jet

T. Barois, P. D. Huck, Ch. Paleo, M. Bourgoin & R. Volk, Probing fluid torque with a hydrodynamical trap : Rotation of chiral particles levitating in a turbulent jet, Physics of Fluids 31, 125116 (2019). Editors Selection and cover of Physics of Fluids.

A vertical turbulent jet is used to trap chiral particles. The particles are maintained in levitation and a stationary rotation regime is observed. The model particles used are composed of a sphere and a helical tail. The rotating performance of the particles is investigated as a function of the length and the twisting of their tails. In addition, the flow field around a spherical particle trapped in the jet is characterized by a particle tracking velocimetry technique (3D-PTV). This flow characterization is used to compute the near- field velocity around a captured particle and to predict the rotation reported for the different geometries tested.

Dispersion of Air Bubbles in Isotropic Turbulence

V. Mathai, S. G. Huisman, C. Sun, D. Lohse & M. Bourgoin, Dispersion of Air Bubbles in Isotropic Turbulence, Phys. Rev. Lett. 121, 054501 (2018).

Bubbles play an important role in the transport of chemicals and nutrients in many natural and industrial flows. Their dispersion is crucial to understanding the mixing processes in these flows. Here we report on the dispersion of millimetric air bubbles in a homogeneous and isotropic turbulent flow with a Taylor Reynolds number from 110 to 310. We find that the mean squared displacement (MSD) of the bubbles far exceeds that of fluid tracers in turbulence. The MSD shows two regimes. At short times, it grows ballistically (∝ τ^2), while at larger times, it approaches the diffusive regime where the MSD ∝ τ. Strikingly, for the bubbles, the ballistic-to-diffusive transition occurs one decade earlier than for the fluid. We reveal that both the enhanced dispersion and the early transition to the diffusive regime can be traced back to the unsteady wake-induced motion of the bubbles. Further, the diffusion transition for bubbles is not set by the integral timescale of the turbulence (as it is for fluid tracers and microbubbles), but instead, by a timescale of eddy crossing of the rising bubbles. The present findings provide a Lagrangian perspective towards understanding mixing in turbulent bubbly flows.