Fluid flows coupled by a deformable solid layer (DS) may be found in both natural and industrial settings. To analyse the stability of such systems, we consider plane Couette flows coupled by a DS. The shear stress exerted by the flowing fluids on the DS leads to its deformation, coupling the motion in each fluid. We demonstrate that, as a consequence of the coupling and irrespective of the thickness of the DS, a viscous instability exists, which can lead to absolute instability at sufficiently high dimensionless speed of the lower plate. The predicted instability apparently arises due to the energy exchange between the fluids and DS via the tangential velocities at the interface. A linear elasticity model for the DS is employed, removing the non-physical growth rate predicted by previous models. A spatio-temporal analysis further reveals the existence of an absolute instability. Insight on this instability could be potentially utilised in manipulating mixing in microfluidic systems and membrane separation as well as for understanding morphologies evolving during membrane formation via interfacial polymerisation.

To investigate the coherent structures in turbulent wall-bounded bubbly flows, a local flow topology analysis has been performed in this work. Using the invariants of the velocity-gradient tensor, all possible small-scale flow structures can be categorized into two nodal and two focal topologies for incompressible flows. The analysed direct numerical simulation database of freely moving and deforming bubble swarms is based on a multi-marker geometrical volume-of-fluid method. It appears that bubbles are acting as mixing elements and fragmenting the large topology structures. Since the behaviour in both phases is quite different, key quantities were evaluated in a conditional manner, i.e. separately for the gaseous dispersed phase and the liquid carrier phase. Several similarities with an analytical solution for laminar creeping flows have been observed although the investigated channel flow is turbulent. It has been concluded that the inclusion of bubbles results in a partial shift from focal to nodal flow character whereas the share of stable and unstable flow types remains fairly constant at approximately 50 % in both phases. No clear analogy can be discerned in terms of the global effects of bubble inclusion and wall confinement on flow topologies. At least with respect to flow topologies, regions in the vicinity of the interface show a clearly different behaviour depending on the phase. The size distribution of coherent structures in the bubble-laden channel flow has also been analysed. The measured flow topology size spectrum in the liquid phase can be approximated by a simple model with reasonable accuracy.