Abstract
Direct numerical simulations of cross-flow over a circular cylinder are performed for , 220, 300, 800, and 1575, spanning laminar-to-turbulent transition and progressively capturing the initial two-dimensional global instability, three-dimensional mode and mode instabilities, a distinct instability, and Kelvin-Helmholtz instability, respectively. For , vortex topology and snapshot proper orthogonal decomposition (sPOD) analyses of the three-dimensional flow fields reveal significant alterations in the dynamics of secondary vortices preceding shear layer transition, denoted instability. The defining instantaneous characteristic of instability is hairpin vortex formations straddling mode structures. An iterative scheme for three-dimensional vortex tracking is employed and the probability density functions of secondary vortex statistics show a marked broadening for , associated with the increased variability in the wake vortex dynamics. The significant broadening of the modal energy spectrum of the sPOD serves as a marker for the topological change to instability, reflecting the increased complexity of the dynamics. Integral effects on the primary shedding instability due to the increase in secondary vortex formation are deduced through a decomposition of the vorticity transport in the wake. An increase in streamwise and transverse reorientation of spanwise vorticity is identified for increasing Re, leading to a decrease of up to in the advected spanwise vorticity of the primary vortices. Therefore, the reorientation of spanwise vorticity may be identified as a subsidiary mechanism for the attenuation of fluctuating structural loads in the basic cylinder wake.
6 More- Received 15 October 2018
DOI:https://doi.org/10.1103/PhysRevFluids.4.124702
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