Luo, Qianyu; (2023) Large-scale turbulence structures in open channel flow over topographical heterogeneity. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This research employs the method of Large Eddy Simulation (LES) to elucidate and quantify the complex flow over topographical heterogeneity with particular emphasis on large-scale flow structures and water surface deformations. Two sets of numerical simulations are performed. First of all, free-surface flow over a backward-facing step (streamwise topographical heterogeneity) for a range of relative submergence is investigated. A relationship between the relative submergence and the extent of the recirculation zone, as well as the strength of the shear layer is revealed. The lower the relative submergence, the longer the recirculation zone and the stronger the shear layer downstream of the step, which is characterised by elevated levels of turbulent shear stress and contains significant amounts of turbulent kinetic energy. The exact nature of the water surface deformation is studied further: it is governed by three types of gravity waves as well as turbulence-induced forced waves. The analysis reveals the interplay between low frequency large-scale turbulence structures, which are the result of flow separation from the step and reattachment behind the step, and the decomposed dynamics of the water surface. The free surface deformation is mainly the result of freely propagating gravity waves, forced waves, and turbulence in the form of rollers and/or hairpin vortices. The water surface is dominated by quasi-1D fluctuations which are modulated at a low frequency following the cyclic evolution of the recirculation zone behind the step. Further research focuses on free-surface flows over longitudinal ridges (spanwise topographical heterogeneity) for a range of spanwise spacings. A unique type of large-scale structure, turbulence-driven secondary currents, is observed in these flows. The analysis reveals although time-averaged clockwise and counter-clockwise secondary current cells are symmetrical about the ridge-axis, they exhibit a periodical increase and decrease in size as well as lateral and vertical movement in space over the meandering-period.

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