Abstract:  Confining flows is often thought as stabilizing. In this seminar, I will discuss two situations where it happens to have a strong destabilizing effect on jets and wakes.

First we experimentally study two phase jets in co-axial microfluidic injection devices, known to exhibit two distinct regimes of microdroplet production: dripping, in which droplets pinch off near the capillary tip, and jetting, in which droplets pinch off from an extended thread downstream of the capillary tip. Recently, good agreement has been found between the absolute to convective instability transition predicted by the linear stability analysis of co-axial Poiseuille flow and the dripping to jetting transition observed in experiments.

We conduct an experimental analysis of the dripping and jetting regimes, in order to validate the theoretical predictions issuing from the absolute/convective theory in terms of frequency and wavenumber selection.
We find indeed that a single frequency dominates the system in the dripping regime, while a broad range of frequencies are present in the jetting regime. We also explore the behavior of the jetting regime as a noise amplifier by means of a pulsating laser beam to locally destabilize the flow. While no reaction is observed in the dripping regime, a strong synchronization occurs in the jetting regime.

In a second study, the influence of confinement on spatially developing wakes at moderate Reynolds numbers (100<Re<500)  is analyzed by means of Direct Numerical Simulations.  The destabilizing influence of moderate
confinement is confirmed. The governing frequencies extracted from the nonlinear simulations are compared to the predictions of weakly non parallel linear absolute/convective theory.  For Re=500, another destabilization mechanism is active, that yields to low frequency modulations of the wave front.

Host:  Prof. Eckart Meiburg