On turbulence and transport in shallow wake flows

Natural flows can often be characterized as shallow turbulent shear flows. Due to local disturbances large coherent vortical structures are introduced in such base flows, and participate in the transport of momentum and mass. In order to gain more insight in the genesis and evolution of large-scale flow structures, the shallow flow around a single cylindrical obstacle was examined as a basic study in a turbulent vertical shear flow. Experiments were conducted in a shallow flow test facility applying an improved combined LDA-LIF measurement technique. Thus, coincident information was obtained about the horizontal velocity and scalar tracer with high temporal and spacial resolution. For shallow turbulent wake flows behind single cylindrical obstacles different instability types are presented differing significantly from unbounded wake flows. From the analysis of the ID power spectral distributions the simultaneous occurance of large-scale 2D and small-scale 3D turbulent features in the flow is demonstrated as well as their interaction for different types of instability. The consequences of distinguishable turbulent scales are evidenced for the longitudinal evolution of turbulent kinetic energy, mass variance and longitudinal mass transport. The experimental data can be used to compute Rather characteristic measures for shallow wake flows. e.g. the energy and scalar dissipation rates. Because of the simple flow configuration and the well controlled boundary conditions, the data sets also provide a data base for the validation of numerical models, which are to be used in more complicated "real world" applications.