Turbulent multiphase Taylor-Couette flows

In this thesis we investigate turbulent multiphase Taylor–Couette flows. In chapter 1 and 2 we investigate bubbly Taylor–Couette flows in light of drag reduction for the shipping industry. Using a global air volume fraction of 4% we achieve 40% drag reduction in fresh water. We investigate the effects of different salts (NaCl, MgCl2, Na2SO4, substitute sea water, and NaCH3COO) on the bubbles and their drag reducing effects. These salts, bar sodium acetate, in the water inhibit bubble coalescence leading to a smaller equilibrium average bubble size. These bubbles are less effective for drag reduction. For sodium acetate solutions we observe an increasing drag reduction with the salt concentration. We connect the bubble Weber number, characterizing the bubble deformability, with the observed drag reduction and find a good correlation.
In chapter 3 we use swelling hydrogel particles to study the effect of volume fraction on turbulent Taylor–Couette flow up to volume fraction of 75%. Increasing the volume fraction leads to an increase in effective viscosity, increasing the torque on the inner cylinder. At higher volume fractions the particles weaken the large-scale turbulent structures, leading to decrease in torque. At high volume fractions the system switches from a volume imposed flow to a pressure imposed flow, further increasing the torque. We observe the particle angular velocity profile in the gap and at Φ=75% the system shows solid body rotation.
Chapter 4 focusses on the transition from the classical turbulent regime to the ultimate turbulent regime single-phase Taylor–Couette flow, where the boundary layer transitions from laminar to turbulence. We observe this transition to be hysteretic. At the transition, we observe a decrease in Taylor vortices and an increase in torque. We explain this by that the vortices have become of a more turbulent nature.