Abstract This study summarises some new characteristics of the fluid flow over a confined circular cylinder at low Reynolds numbers. Results from both two- and three-dimensional direct numerical simulations are presented at blockage ratio between 0.1 and 0.9 and Reynolds number between 120 and 500. Floquet stability analysis of selected cases will also be presented. From the two-dimensional simulations, it is found that the fluctuating lift forces decreases with blockage ratio and becomes zero (where the flow is steady) at blockage ratio of approximately 0.7–0.8. Upon further increasing the blockage ratio to 0.9, the simulations show a dramatic increase in the fluctuating lift forces, nearly an order of magnitude greater than previously reported for an unconfined cylinder flow. It is also found that for blockage ratio of 0.5, there is a long term two-dimensional instability that becomes more prominent with increasing Reynolds number. This instability has a time scale of approximately 105 time units ( D / U max ) at Reynolds number of 500. In addition, the transition between two- and three-dimensional flow at blockage ratios up to 0.5 is investigated. It is shown that the transition Reynolds number decreases with increasing blockage ratio. At high blockage ratio of 0.5, as we increase the Reynolds number, the transition to three-dimensional flow is shown to go from unsteady two-dimensional to steady three-dimensional before transitioning to unsteady three-dimensional flow.

中文翻译：

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低雷诺数下圆柱体上密闭流动的一些新特性

摘要 本研究总结了低雷诺数下流体在受限圆柱体上流动的一些新特性。二维和三维直接数值模拟的结果在阻塞比介于 0.1 和 0.9 之间，雷诺数介于 120 和 500 之间时呈现。还将呈现所选案例的 Floquet 稳定性分析。从二维模拟中发现，波动升力随堵塞比减小，并在堵塞比约为 0.7-0.8 时变为零（其中流动稳定）。在进一步将阻塞比增加到 0.9 后，模拟显示波动的升力显着增加，几乎比之前报告的无限制气缸流大一个数量级。还发现对于 0.5 的堵塞比，随着雷诺数的增加，长期二维不稳定性变得更加突出。这种不稳定性在雷诺数为 500 时具有大约 105 个时间单位 (D / U max ) 的时间尺度。此外，研究了堵塞比高达 0.5 时二维和三维流动之间的过渡。结果表明，过渡雷诺数随着堵塞比的增加而降低。在 0.5 的高阻塞比下，随着我们增加雷诺数，向三维流动的过渡显示为从不稳定的二维到稳定的三维，然后再过渡到不稳定的三维流动。此外，还研究了堵塞比高达 0.5 时二维和三维流动之间的过渡。结果表明，过渡雷诺数随着堵塞比的增加而降低。在 0.5 的高阻塞比下，随着我们增加雷诺数，向三维流动的过渡显示为从不稳定的二维到稳定的三维，然后再过渡到不稳定的三维流动。此外，还研究了堵塞比高达 0.5 时二维和三维流动之间的过渡。结果表明，过渡雷诺数随着堵塞比的增加而降低。在 0.5 的高阻塞比下，随着我们增加雷诺数，向三维流动的过渡显示为从不稳定的二维到稳定的三维，然后再过渡到不稳定的三维流动。