Turbulent wake structure and dynamics for the thin flat plate normal to a uniform flow: a study of two dynamically stable solutions

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This thesis presents a comparative experimental study of the differences in the structure and dynamics of two nominally two-dimensional turbulent wakes behind a thin flat plate placed normal to a uniform flow. The flows are differentiated by their end conditions: with and without end plates. Both cases are characterized by Karman-like vortex shedding with broadband low frequency unsteadiness. Both wakes evidence a low frequency flapping motion, associated with a flow normal oscillation of the shear layers, as well as a slowly drifting baseflow that is common to cylinder wakes. However, significant differences in the mean velocity fields, back pressures, shedding frequencies, turbulence levels, and Reynolds stress magnitudes/spatial distributions indicate the existence of two dynamically stable solutions. Thus, the flat plate distinguishes itself from standard bluff body flows featuring unique solutions. Low-order representations of the flow fields are used to reconstruct the wake dynamics. The results show that the underlying dynamics differ in terms of the energy distribution and content of otherwise similar modes of coherent motion. For the open end case, a greater cycle-to-cycle variation of the shedding process is associated with a comparatively stronger slow-varying mode in the base region. In contrast, for the closed end case, a shear layer flapping mode is more strongly expressed, which may account for greater variations in the trajectories of the shed vortices. These differences are then related to the structure and intensity of the Reynolds stress fields. A better understanding of the vortex formation process is developed to account for differences in the vortex streets. A careful accounting of the vorticity transport in the wake is conducted and the contributions of different mechanisms are assessed. The work further contributes a new model for estimating the circulation associated with shed vortices which accounts for vorticity not captured in the core region. Despite the strength of the shed vortices being similar between the two cases, differences in the formation regions, such as the rates of vorticity decay, suggest that the concentration of vorticity within the separated shear layers and forming vortices are important to the wake dynamics.
low-order modelling, bluff-body wakes, vorticity, vorticity transport, fluid dynamics, fluid mechanics, turbulence
Braun, E. A. (2021). Turbulent wake structure and dynamics for the thin flat plate normal to a uniform flow: a study of two dynamically stable solutions (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.