Date of Award

2025

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Circular cylinders; Flow past cylinders; Gap ratio effects; Near-bed bluff bodies; Wake characteristics

Supervisor

Ghassan Nasif

Supervisor

Ram Balachandar

Rights

info:eu-repo/semantics/openAccess

Abstract

This thesis investigates the wake characteristics of a circular cylinder located near a bed within a thick, turbulent boundary layer. Two distinct models with identical diameters (D) but varying gap (G) sizes between the cylinder and the bed were analyzed, resulting in gap-to-diameter ratios (G⁄D) of 0.25 and 0.5. This computational study uses large eddy simulation (LES) with the wall-adaptive local eddy-viscosity model to capture the detailed features and turbulence characteristics. The mean characteristics showed the presence of a bed vortex in the case of the larger gap ratio (G⁄D=0.5), which diminishes as the gap size decreases. Meanwhile, turbulence quantities revealed the suppression of the vortex shedding and shear layer instabilities for the lower gap ratio (G⁄D=0.25). The suppression of the alternating vortex shedding pattern was further investigated through the instantaneous spanwise vorticity analysis, providing a qualitative understanding of the vortex dynamics for these two gap ratio cases. Additionally, λ_2 criterion was employed to explore the three-dimensional turbulent flow structures in the cylinder wake, identifying the formation of horseshoe vortices (HSVs) along the span of the cylinder and quasi-2D structures in the wake. The analysis showed that with decreasing gap size, the frequency of HSV formation increased significantly, while the occurrence of quasi-2D flow structures decreased. This transition in the flow behavior emphasizes the role of bed proximity in shaping the wake dynamics. The findings from this study provide a fundamental understanding of bed proximity effects in the wake characteristics of a cylinder in a turbulent boundary layer. These insights have significant implications for structural engineering applications, subsea pipeline design, and thermal management systems. By identifying coherent flow structures and their evolution, this study contributes to a broader understanding of turbulence in confined geometries, especially in systems involving cylindrical structures near surfaces or boundaries.

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