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Simulation and feedback control of the Ahmed body flow exhibiting symmetry breaking behaviour

机译：ahmed体流的模拟和反馈控制表现出对称破坏行为

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The present work considers the low Reynolds number wake flow behind a squareback Ahmed body, in close proximity to a ground. At low Reynolds numbers such wakes are known to undergo a series of bifurcations to a state that breaks reflectional symmetry. The symmetry breaking of the wake also persists at turbulent high Reynolds numbers, where it manifests as bi-modal behaviour with random switching between the asymmetric states. Thus far, it has only been possible to study the low Reynolds number sequence of bifurcations experimentally and mathematically. The present work presents the first numerical simulations capturing the sequence of symmetry breaking bifurcations that occur. A study of how the wake topology changes throughout suggests that interaction between the closer top/bottom pair of parallel shear layers can only dominate once there is sufficient underbody flow. When this occurs, the two main vortex structures in the wake switch from being horizontally to vertically aligned. A linear feedback control strategy, designed to attenuate base pressure force fluctuations, is then implemented. This causes an accompanying reduction in drag and re-symmetrisation of the wake. Analysis using the Dynamic Mode Decomposition confirms that the wake shedding mode is re-symmetrised. This work motivates future attempts to capture wake symmetry breaking and bi-modality in numerical simulations, and application of a promising feedback control strategy at higher, turbulent Reynolds numbers.
机译：目前的工作考虑了在接近地面的方卫艾哈迈德身体后面的低雷诺数尾流。在低雷诺数下，已知此类尾波会发生一系列分叉，从而破坏反射对称性。尾波的对称破坏在雷诺数较高时也持续存在，表现为双峰行为，在非对称状态之间随机切换。到目前为止，只能通过实验和数学方法研究低分叉的低雷诺数序列。本工作提出了第一个数值模拟，捕获出现的对称破坏分叉的序列。对整个尾流拓扑结构如何变化的研究表明，只有在有足够的底部流动的情况下，平行剪切层的最接近的顶部/底部之间的相互作用才占主导地位。发生这种情况时，唤醒中的两个主要涡旋结构会从水平对齐到垂直对齐。然后实施了旨在减少基本压力波动的线性反馈控制策略。这导致阻力的减少和尾波的重新对称化。使用“动态模式分解”进行的分析确认了唤醒脱落模式已重新对称。这项工作激发了未来在数值模拟中捕获尾波对称性破坏和双峰的尝试，并在更高的湍动雷诺数下应用了有前途的反馈控制策略。

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