This study builds on an earlier study of low-Reynolds number flow about a cylinder forced to oscillate in-line with the main flow, which found vortex switches at some oscillation amplitude values. Here we extend the Reynolds number domain to Re = 60–350, utilize a computational domain characterized by R2/R1 = 360, and do computations at two frequency ratios of f/St0 = 0.8 and 0.9. Computations were carried out using a thoroughly tested finite-difference code. Some results were compared with those obtained by Ansys CFX, and good agreement was found. When plotted against oscillation amplitude, rms and time-mean values of force coefficients revealed a shift toward lower amplitude with higher Re. Findings for the effect of frequency ratio are similar. Where vortex switches occurred, a pre- and post-jump analysis is carried out. POD analysis of the cylinder wake flow field is employed to reveal the detailed wake dynamics as the forcing parameters are varied. The analysis provides further details on the transition of the dominant wake modes in response to the symmetry breaking bifurcation underlying the vortex switches observed in the simulations.
The effects on ion-acoustic wave propagation caused by the introduction of a large biased metal plate close to and parallel to the separation grid in the target chamber of a double plasma machine are experimentally examined. It is found that the biased plate can re-excite ion-acoustic waves and can act as an externally controlled phase shifter.
The periodically forced cylinder wake exhibits complex but highly symmetrical patterns. In recent work, the authors have exploited symmetry-group equivariant bifurcation theory to derive low order equations describing, approximately, the dominant nonlinear dynamics of wake mode interactions. The models have been shown to qualitatively predict the observed bifurcations suggesting that the Karman wake remains, dynamically, a fairly simple system at least when viewed in 2D. Preliminary experimental data are presented supporting the feasibility of using 2D simulation results for the derivation of the low order model parameters. A POD analysis of the wake PIV velocity field yields flow modes closely similarly to those obtained via 2D CFD computations for Re in the 1000 range. The paper presents new results of simulations for Re = 200. For this low Reynolds number, the forced Karman wake exhibits rich dynamics dominated by quasi-periodicity, mode locking, torus doubling and chaos. The low Re torus breakdown may be explained by the Afraimovich-Shilnikov theorem. Interestingly, in a previous analysis for the higher Re number, Re = 1000, transition to a period-doubled flow state was found to occur via a route akin to the Takens-Bogdanov bifurcation scenario.
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