Asymmetries in Nominally Symmetric Flows

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.