Direct numerical simulations of transient turbulent jets: vortex-interface interactions
Résumé
The breakup of an interface into a cascade of droplets and their subsequent coalescence
is a generic problem of central importance to a large number of industrial settings
such as mixing, separations and combustion. We study the breakup of a liquid jet
introduced through a cylindrical nozzle into a stagnant viscous phase via a hybrid
interface-tracking/level-set method to account for the surface tension forces in a
three-dimensional Cartesian domain. Numerical solutions are obtained for a range of
Reynolds (Re) and Weber (We) numbers. We find that the interplay between the azimuthal
and streamwise vorticity components leads to different interfacial features and flow
regimes in Re–We space. We show that the streamwise vorticity plays a critical role
in the development of the three-dimensional instabilities on the jet surface. In the
inertia-controlled regime at high Re and We, we expose the details of the spatio-temporal
development of the vortical structures affecting the interfacial dynamics. A mushroom-like
structure is formed at the leading edge of the jet inducing the generation of a liquid
sheet in its interior that undergoes rupture to form droplets. These droplets rotate inside
the mushroom structure due to their interaction with the prevailing vortical structures.
Additionally, Kelvin–Helmholtz vortices that form near the injection point deform in the
streamwise direction to form hairpin vortices, which, in turn, trigger the formation of
interfacial lobes in the jet core. The thinning of the lobes induces the creation of holes
which expand to form liquid threads that undergo capillary breakup to form droplets.