Multiphase wall-bounded turbulence

In many geophysical situations and in all industrial applications, turbulent flows are wall-bounded. Many of these flows are multi-phase, i.e. flows consisting of one or multiple inclusions. The current understanding of these flows is still limited and this makes it important to study them. In this thesis we study these wall-bounded multi-phase flows in two canonical systems: Taylor-Couette flow (TC) and Rayleigh-Bénard convection (RBC). In this work we used spherical and cylindrical particles to investigate if we have reduced skin friction similar to bubbly drag reduction. The global torque measurements showed that these particles barely alter the drag, even at very large particle volume fractions. Surprisingly, we found a preferential alignment for the cylindrical particles with respect to the inner cylinder wall. Using oil and water we are able to create deformable inclusions. Increasing the oil volume fraction over a critical point results in phase inversion with water droplets in oil. In this regime we found drag reduction due to the large water droplets in the flow. This is confirmed with in-situ microscopic imaging. In the last two chapters of this thesis we study the effect of non-homogeneous boundaries in both TC and RBC. Using bands of sandgrain roughness we were able to control the secondary flows in TC. This means that for example roughness like barnacles on the hull of a ship can induce secondary flows that push air bubbles away and thereby, reducing the drag reducing effect.