Many flows that are expected to be symmetric are actually observed to be asymmetric. The appearance of asymmetry in the face of no particular cause is a widespread although underappreciated occurrence. This rather puzzling and sometimes frustrating phenomenon can occur in wide-angle diffusers, over the forebody of axisymmetric bodies at high angles of attack, in the wake downstream of streamlined as well as bluff bodies, and in the flow over three-dimensional bumps and ramps. We review some notable examples and highlight the extreme sensitivity of many such flows to small disturbances in the body geometry or the incoming flow. Some flows appear to be permanently asymmetric, while others are bistable on timescales that are orders of magnitude longer than any convective timescale. Convective or global instabilities can occur, bistability is common, and mode interactions become important when multiple similar but distinct timescales and length scales are present. Our understanding of these phenomena is still very limited, and further research is urgently required; asymmetries in otherwise symmetric flows can have serious real-world consequences on vehicle control and performance.

中文翻译：

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名义对称流中的不对称性


许多预期是对称的流实际上被观察到是不对称的。在没有特定原因的情况下出现不对称是一种普遍但未被充分认识的情况。这种相当令人费解且有时令人沮丧的现象可能发生在广角扩散器中，在高攻角下轴对称体的前体上，在流线型和钝体下游的尾流中，以及在三维颠簸和坡道上的流动中。我们回顾了一些值得注意的例子，并强调了许多此类流动对身体几何形状或入流中的小干扰的极度敏感。一些流动似乎是永久不对称的，而另一些流动在比任何对流时间尺度长几个数量级的时间尺度上是双稳态的。可能会发生对流或全局不稳定性，双稳态很常见，当存在多个相似但不同的时间尺度和长度尺度时，模态相互作用变得很重要。我们对这些现象的理解仍然非常有限，迫切需要进一步的研究;否则对称流中的不对称性可能会对车辆控制和性能产生严重的现实后果。