Three-dimensional wake transition for CO₂flow at supercritical pressure over single heated cylinder

The three-dimensional wake transition for supercritical CO2 (SCO2) with Reynolds number (Re) ranging from 100 to 300 is systematically investigated by large eddy simulation. Numerical results of the constant-property fluid are validated with the classical conclusions. The secondary instabilities, i.e., mode A (spanwise scale of ∼4.5 times cylinder diameter) and mode B (approximately equal to cylinder diameter), are well captured in the wake. The results at Re = 120 show that the evolution of vortex dislocations is mainly manifested in the alternating occurrence of three stages, i.e., the two-dimensional vortex shedding stage, pure mode A stage, and dislocation stage. The stable state of pure mode A is found at Re = 140. As Re continues to increase, the wake shows a gradual transition from mode A to mode B, rather than the transition from mode A* (mode A + dislocation) to mode B. The frequency spectrum in this process is the gradual shift of a single peak from the low frequency to the high frequency. The wake cycle sequence changes from the alternation of four structures at Re = 155 to the alternation of pure and "contaminated"mode B structures at Re = 170. The critical condition of wake "resonance"is determined at Re = 175, where the final state of the wake is relatively ordered mode B structure, representing the weakest three-dimensionality in the transition regime. After that, the three-dimensionality of the wake enhances with Re, and the probability of the occurrence of disordered structures increases accordingly. The vortex shedding frequency of the pure mode follows the order: 2D > B > A > A*. The width of the frequency spectrum is mainly related to whether the wake flow state is a continuous single mode or the alternation of multiple modes. Based on the Strouhal-Reynolds number relationship, flow pattern in the wake transition process is finally discussed.