This PhD investigates the effect of bed roughness on shallow mixing layers (SML) and the evolution of coherent structures that populate them, such as Kelvin-Helmholtz Coherent Structures (KHCSs), Secondary Currents (SCs), and Large and Very-Large Scale Motions (LSMs and VLSMs respectively). Two bed roughness scenarios are considered: a uniformly rough bed and a lateral transition from a rough bed to a smooth bed. The rough bed is made of artificial dense grass with a height of 5mm, while the smooth bed is made of PVC plate.
The experiments are conducted in an 18m × 1.98m laboratory flume. The flow configurations explored include both initially spanwise sheared and uniform flows. First, we explore the flows over the uniformly rough bed, comparing the acquired data to previous measurements conducted in the same flume with a hydraulically smooth bed. The objectives are to validate driving parameters for SML and KHCSs observed in the literature, assess the effect of a rough bed on the turbulent field of SML, and explore the interactions between KHCSs, LSMs, and VLSMs. Second, we investigate the longitudinal evolution of SML, KHCSs, SCs, LSMs and VLSMs over a bed with a lateral roughness transition. We evaluate the mechanisms of lateral momentum exchange and their relative importance, ultimately exploring the interaction between all four types of coherent structures.

Cite the thesis

Bastien Cerino. Large-scale flow structures in shallow mixing layers over uniform and spanwise-heterogeneous bed surfaces. Fluid mechanics [physics.class-ph]. Université Claude Bernard - Lyon I, 2024. English. ⟨NNT : 2024LYO10269⟩. ⟨tel-05180681⟩

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