Mechanical Engineering - Theses
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ItemEvolution of canonical turbulent boundary layers( 2017)An experimental investigation of evolving turbulent boundary layers in a tow tank facility is performed in this study. The main aim here is to investigate the development of a canonical turbulent boundary layer from the trip to a high Reynolds number state. A review of the literature reveals that while the dynamic characteristics of the near-wall turbulent structures are reasonably well understood, the origin, evolution and dynamics of large-scale coherent structures in the logarithmic and outer regions of turbulent boundary layers remain largely unresolved. Unveiling the dynamics of large-scale coherent motions is essential to understanding the complicated physics behind wall-bounded turbulent flows. Therefore, an experimental set-up with a long flat plate mounted on a traversing carriage has been carefully designed and constructed. The spatially developing turbulent boundary layers formed on the bottom surface of the towed plate are investigated using a stationary PIV system, affording a unique frame of reference. Large-field-of-view high-resolution PIV and high-speed PIV techniques are employed to measure the instantaneous streamwise-wall-normal plane of the evolving turbulent boundary layer. The measurement system including the traversing carriage is fully automated such that a large number of passes can be performed to obtain converged statistics. We start our study by validating the flow statistics and canonical-state of the turbulent boundary layers. The statistics are compared with results from DNS datasets at matched Reynolds numbers. In the process of validating the PIV obtained flow statistics, we propose a robust technique to estimate small-scale missing energy due to the spatial resolution issues. The estimation tool is based on arguments that (i) the inner-scaled small-scale turbulence energy is invariant with Reynolds number and that (ii) the spatially under-resolved measurement is sufficient to capture the large-scale information, which is Reynolds number dependent. This tool can be used to diagnose whether PIV statistics are beset by some wider issue. Having validated the measurements in this way, the evolution of mean flow parameters of the developing boundary layers is compared with the predicted evolution of mean flow parameters for canonical turbulent boundary layers proposed in the literature. This assessment determines whether the turbulent boundary layers considered in this study can be classified as being in the canonical state. Good agreement has been observed between the experimental results and the predicted evolution. The unique aspect of the current experiment is that we can observe a time-resolved view of an evolving structure within turbulent boundary layers. Any large-scale coherent motions that have a convection velocity close to the freestream will remain nominally stationary within the field of view, whereas for the conventional frame of reference (i.e. wind tunnels or water channels), coherent structures convect away from the field of view with local flow velocities. Therefore, by obtaining a temporally-resolved view of developing boundary layers in this unique frame of reference, the evolution of large-scale coherent motions has been analysed. It is shown that there is a mismatch in the convection velocities between low- and high-speed regions at a matched wall-height. This convection velocity difference allows the low- and high-speed regions to interact, causing the formation of a strong internal shear layer. Both instantaneous and conditionally averaged velocity fluctuations are examined to support these findings. It is also shown that a local shear layer instability develops as the shear layer evolves. The development of this local shear layer instability leads to a shear layer roll-up process which perturbs low- and high-speed regions, causing the regions to migrate to different wall heights (ejections and sweep like events). Finally, all the dynamic properties and interactions of large-scale coherent motions discovered in this study are incorporated to construct a conceptual model. This model describes the role of shear layers and the effect of dynamic interactions of large-scale coherent motions in the outer layer of turbulent boundary layers. The model is derived with the intention of providing a better understanding of large-scale coherent motions and potentially the dynamics described in the model can be incorporated into existing kinematic models to advance future efforts for a complete model for wall-bounded turbulent flows.