Affecting drag in turbulent Talor-Couette flow

Abstract

Turbulent flows are omnipresent in nature and technology. The majority of flows encountered in daily life and in industrial applications deal with rough walls and transient effects. Furthermore, many flows can be regarded as multiphase flows, i.e. the flow consisting of multiple phases of liquids, gasses and solids. Surprisingly maybe, the understanding of these flows is still limited, and many studies focus on idealised situations, which do not take the aforementioned phenomena into account. To study these types of flow, we used a Taylor-Couette system, i.e. the flow between 2 concentric independently rotating cylinders. This system is one of the canonical flow setups in which the physics of fluids is studied, and it has been used to study a.o. pattern formation, instabilities, viscosity measurements, turbulence and multiphase flows. Taylor-Couette flow is known to be mathematically similar to Rayleigh-Benard convection. That is, written in the correct dimensionless form, the relevant scaling laws are identical for both systems. In that sense, one can learn about Rayleigh-Benard convection by studying Taylor-Couette flow, and vice versa. In this thesis, we chose to specifically study transient effects, rough walls and air lubrication in turbulent flows, not only to increase our fundamental understanding of these of ubiquitous flows, but also to address highly relevant questions in collaboration with industrial partners. In maritime industry, the use of air lubrication is seen as a promising method to reduce the overall friction between a ship and the surrounding water, and thus the fuel consumption. However, the relevant parameters optimizing air lubrication are not yet well understood. Wall roughness is known to increase the drag, but given the enormous variety of roughness types, many open questions remain to be unanswered. The thesis is divided in three parts, i.e. Part 1: Transient turbulence, Part 2: Roughness in turbulence, and Part 3: Air lubrication in turbulent flows.