Vortex dynamics for flow over a circular cylinder in proximity to a wall

Abstract

The dynamics of vortical structures in flow over a circular cylinder in the vicinity of a flat plate is investigated using particle image velocimetry (PIV). The cylinder is placed above the flat plate with its axis parallel to the wall and normal to the flow direction. The Reynolds number $Re_{D}$ based on the cylinder diameter $D$ is 1072 and the gap $G$ between the cylinder and the flat plate is varied from gap-to-diameter ratio $G/D=0$ to $G/D=3.0$. The flow statistics and vortex dynamics are strongly dependent on the gap ratio $G/D$. Statistics show that as the cylinder comes close to the wall ($G/D\leqslant 2.0$), the cylinder wake becomes more and more asymmetric and a boundary layer separation is induced on the flat plate downstream of the cylinder. The wake vortex shedding frequency increases with decreasing $G/D$ until a critical gap ratio (about $G/D=0.25$) below which the vortex shedding is irregular. The deflection of the gap flow away from the wall and its following interaction with the upper shear layer may be the cause of the higher shedding frequency. The vortex dynamics is investigated based on the phase-averaged flow field and virtual dye visualization in the instantaneous PIV velocity field. It is revealed that when the cylinder is close to the wall ($G/D=2.0$), the cylinder wake vortices can periodically induce secondary spanwise vortices near the wall. As the cylinder approaches the wall ($G/D=1.0$) the secondary vortex can directly interact with the lower wake vortex, and a further approaching of the cylinder ($G/D=0.5$) can result in more complex interactions among the secondary vortex, the lower wake vortex and the upper wake vortex. The breakdown of vortices into filamentary debris during vortex interactions is clearly revealed by the coloured virtual dye visualizations. For $G/D<0.25$, the lower shear layer is strongly inhibited and only the upper shear layer can shed vortices. Investigation of the vortex formation, evolution and interaction in the flow promotes the understanding of the flow physics for different gap ratios.

Unsteady Flow Physics of Airfoil Dynamic Stall

“…It should be noted that previous experiments in the literature on vortex shedding in the vicinity of a rigid wall have indicated that the critical shedding parameters remain unaffected for models that are installed beyond two characteristic length scales away from the wall. [37,38]…”

Wake-induced transition in the low-Reynolds-number flow over a multi-element airfoil

“…The finite-time Lyapunov exponents (FTLEs) method (Haller & Yuan 2000; Haller 2001; Shadden, Dabiri & Marsden 2006; Green, Rowley & Haller 2007; Shadden, Astorino & Gerbeau 2010) and virtual dye visualization (VDV) (He et al. 2017; Wang et al. 2017) can extract Lagrangian coherent structures (LCSs) from complex flows based on tracking fluid particle trajectories.…”

“…The VDV can clearly visualize the interactions between different flow patterns by emitting virtual dyes with different colours (He et al. 2017; Wang et al. 2017).…”

Incompressible SPH simulation of flow past horizontal cylinder between plane wall and free surface

“…The shed vortex from the upper part of the cylinder advects nearly parallel to the wall while the shed vortex from the lower part of the cylinder advects away from the wall and eventually crosses over the trajectory of the upper shed vortex. The final relative vertical positions of both vortices are opposite to those in the infinite fluid case [8,10]. He et al [10] found that a secondary vortex also forms, as a consequence of the roll-up of the separated wall boundary layer.…”

“…The final relative vertical positions of both vortices are opposite to those in the infinite fluid case [8,10]. He et al [10] found that a secondary vortex also forms, as a consequence of the roll-up of the separated wall boundary layer. At G/D = 1.0, regular vortex shedding is observed.…”