Conventionally, it is believed that as the Reynolds number increases from around 195, the flow around a circular cylinder exhibits three-dimensional vortex shedding modes corresponding to spanwise instability in the wake. The shedding modes initiate with mode A, transition to mode A* with large-scale dislocation, and eventually transition to the small-scale mode B structure as the Reynolds number increases. At higher Reynolds numbers, such as Re=300 in our case, it is generally assumed that the flow field globally maintains a chaotic mode B structure. Through high-fidelity numerical simulations of the three-dimensional flow around a cylinder at Re=300 and Ma=0.3, we discovered, in addition to the small-scale three-dimensional mode B structure, a large-scale streamwise vortical structure named transient mode A at certain local spanwise positions of the cylinder and the wake in both developing stage and fully developed stage of the simulation. This transient mode A structure differs from the three types of structures mentioned above: it combines the generalized symmetry of the mode A structure at large scales with partial small-scale symmetry features of the mode B structure, lacks the dislocation characteristics present in mode A*, and exhibits a lower drag coefficient than both mode A and mode B. Dynamic analysis reveals that mode A, mode B, and transient mode A share similar properties. Under the influence of viscous effects, the streamwise vortical structures that are not parallel to the flow direction rapidly dissipate and recede, resulting in an overall increase in the streamwise alignment of the structures.
Su et al. (Tue,) studied this question.