Wake Dynamics of Circular Cylinders at Moderate Reynolds Numbers (300 < Re < 10000)

Understanding the evolution of flow structures around bluff bodies across different Reynolds number regimes is crucial both for practical engineering applications and for advancing the fundamental knowledge of flow stability and wake dynamics. One of the canonical cases in this context is the flow past a circular cylinder. Despite its geometric simplicity, this configuration exhibits complex unsteady behavior, particularly in the [endif]-->variation with Reynolds number. In this study, we focus on three Reynolds numbers, Re = 300, Re = 1000 and Re = 5000, within the subcritical regime (300 < Re < 1.4 × 10⁵), where a well-documented sharp decline in [endif]--> occurs, known as the first lift crisis. Although both cases feature vortex shedding, they display fundamentally different wake dynamics and force characteristics. To investigate the mechanisms underlying this transition, we performed DNS using the solver NEKTAR++ in a 2.5D setup. Flow structures were identified using instantaneous and time-averaged fields of vorticity and the Q-criterion. Furthermore, to extract coherent spatiotemporal patterns and dominant dynamical modes, we applied Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) to the fields.