Coherent flow structures upstream a circular cylinder near a plane wall

This study investigates the flow behavior in front of a circular cylinder placed near a plane wall using large eddy simulation. It identifies two distinct coherent vortical flow structures occurring during the transition from laminar to turbulent flow upstream of the cylinder. These coherent flows can be triggered by either reducing the gap between the cylinder and the wall or increasing the Reynolds number (Re_D, based on the cylinder diameter). The first coherent flow emerges at relatively low Re_D and features periodic vortical tubes aligned parallel to the longitudinal axis of the cylinder. These vortical tubes move downstream with the flow, transforming into Λ-shaped structures before jumping over the cylinder. The second type, observed at higher Re_D and farther upstream, initially appears as elongated, streaky, streamwise vortices, which evolve into larger hairpin-type vortices as they approach the cylinder. These hairpin vortices also form periodically but at a lower frequency than the vortical tubes and are arranged in a staggered formation. Both types of coherent flows are found to affect the amplitude and frequency of forces acting on the cylinder, which is important in many engineering applications. A similar transition process from laminar to turbulent flow has been observed in the canonical boundary layer; however, the two types of coherent flow structures emerge, develop, and break down within a short distance in front of the cylinder, setting them apart from previous studies.