GALCIT Special Seminar

 Refreshments prior to the lecture at 3:45pm in the Guggenheim foyer.

The droplet breakup of a round liquid jet was one of the first linear stability problems to be solved, over a century ago. The breakup is initiated by instability waves growing on the jet surface, which were predicted analytically. Yet, tools to determine the instability of general two-phase flows are not available today. A journey towards a tool to predict, understand and control waves in one- and two-phase flows is presented in this seminar. Applications of such a tool could range from medicine to rockets, micromixers to combustors.

In the first part of this seminar, instability waves are predicted in two-fluid flows and simple geometries. It is shown that for a liquid sheet in air, both phases need to be solved for and treated as viscous, to get agreement with experiments. Next, similar techniques are applied to flows with self-sustained oscillations, which appear without external energy input, and dominate the flow behaviour. The vortex street behind a cylinder is an example of such an oscillation. It is shown how surface tension completely alters the oscillations of two-fluid wakes and jets, compared to one-phase wakes and jets.

In the second part of this seminar, adjoint-based tools are used to understand the mechanisms that create waves, and develop control strategies. We can ask questions such as: Where is the "core" of the instability? What is the optimal position for a control device? Can a steady flow be obtained by suction at the nozzle walls or a corner? The answers to these questions are demonstrated for two complex nozzles: a cross-junction with relation to microfluidics, and a combustion fuel injector. Both are treated initially as one-phase flows. The final goal is to optimize nozzle designs for two-phase flows, to achieve full control of mixing and droplet breakup.