An experimental study of coherent structures, secondary currents and surface boils and their interrelation in open-channel flow

River and open-channel flows are free surface boundary layer flows with complex 3D, large-scale, turbulent structures. The study of 2D and 3D large-scale turbulent flow structures is a great challenge for physicists, mathematicians and engineers from such different domains as civil, environmental and mechanic engineering. Different processes can generate 3D, large-scale, turbulent structures which occur at the same time. On the one hand, large scale vortical structures such as secondary currents of Prandtl's second kind play an important role in the understanding of 3D turbulent structures in straight channels and rivers. Secondary currents affect bottom shear stress and longitudinal mean velocity, and contribute to sediment transport and air-water gas exchange by creating upwelling and downwelling motion in the water column. At the free surface, such upwelling and downwelling motion is an important mechanism for the air-water gas exchange and is considered to be responsible for surface boils. On the other hand, experimental work in turbulent boundary layers revealed the existence of bursting resulting in hairpin shaped structures which are responsible for the link between the inner and the outer layer. The interaction between these two layers in turbulent boundary layers is considered in terms of the dynamics of momentum, energy, and Reynolds shear stress transport. In order to advance in the understanding of this fundamental problem in turbulent open-channel flow, recently developed measurement and observation techniques are used in this Ph.D study. A non-intrusive Acoustic Doppler Velocity Profiler (ADVP), Surface Large Scale Particle Image Velocimetry (LSPIV) and a hot-film probe were combined in the investigation of coherent structures, secondary currents, surface boils and their interaction in turbulent rough-bed open-channel flow. The ADVP permits to measure 3D quasi-instantaneous velocity profiles in the entire water depth and to investigate the mean field and the fluctuating field of all three velocity components. The LSPIV system, developed at the LHE, allows visualizing the water surface and obtaining the surface velocity information in relation to instantaneous surface vortical structures. Bottom shear stress was measured with a sensor based on the hot film principle. The instruments provided the mean and instantaneous velocity field in the entire water depth and at the free surface. Six sets of experiments were carried out in turbulent rough bed open-channel for three different width-to-depth ratios (12.25, 15 and 20) at high, moderate and low Reynolds numbers. The results of the ADVP measurements show mean longitudinal velocity patterns undulating across the channel which indicate patterns of secondary currents in the mean flow structure. Upwelling regions can be identified by lower relative mean longitudinal velocities close to the free surface, and downwelling regions can be identified by higher relative mean longitudinal velociti