Structural and dynamic differences in the turbulent wake of cantilevered square and circular cylinders protruding a thin laminar boundary layer
Date
2019-08-16
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
This thesis documents a comparative experimental study of the vortical structures and dynamics in the turbulent wake of aspect ratio 4 cantilevered square and circular cylinders at a Reynolds number of 10500 protruding a thin laminar boundary layer. Antisymmetric Kármán-like vortex shedding of half-loop structures is observed in the phase-averaged field of both cylinders. The signature of these shed vortices are dipole structures imprinted on the mean wakes. The half-loop structures, associated spectral signatures, and dipole vortices are concentrated at lower elevations in the circular cylinder wake but span the square cylinder’s entire height and are significantly stronger. A low-frequency instability is observed at high elevations of both cylinder wakes but at different frequencies and is more broadband for the square cylinder. The low frequency signature spans the entire height of the square cylinder and therefore interacts with the vortex shedding. Both low-frequency signatures are shown to be unique to laminar boundary layers. However, these cannot be directly related to instabilities of the laminar horseshoe vortex system. The square cylinder exhibits a significantly more complex mean wake structure with vortices descending from the dipole pair and growing from the ground plate into the far wake. Increased bluffness of the square cylinder geometry leads to stronger rotation of merging structures in the phase-averaged field and formation of these additional mean wake structures. Furthermore, a vortex along the ground plate forms in the mean field of both cylinders which is the signature of a complex interaction between the primary horseshoe vortex legs and the shedding structures. Above the cylinder free-ends, both cylinders exhibit tip vortices planted to their free-end surface that extend into the near wake region. These tip vortices are mostly steady for the circular cylinder but oscillate vertically at the vortex shedding frequency for the square cylinder. Thus, the square cylinder’s free-end flow directly interacts with the vortex shedding in the wake. This work establishes that square and circular cylinder geometries give rise to topologically and dynamically distinct wakes and that the boundary layer state influences the dynamics.
Description
Keywords
Vortex Shedding, Cantilevered Bluff Bodies, Particle Image Velocimetry, Turbulent Wake Flows, Horseshoe Vortex System, Tip Flows, Laminar Boundary Layers, Low Frequency Wake Dynamics
Citation
Kindree, M. G. (2019). Structural and dynamic differences in the turbulent wake of cantilevered square and circular cylinders protruding a thin laminar boundary layer (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.