Wake of tandem cylinders near a wall

Abstract

This article reports an experimental investigation on the near wake of two identical circular cylinders in crossflow arranged in tandem configuration in the streamwise direction and with the additional interference of the ground. The Reynolds number based on the cylinders diameter is 4.9×103. The present study analyses the effect of longitudinal pitch-to-diameter ratios spanning the well known flow regimes of tandem cylinders in absence of ground effect, i.e. single bluff body, shear layer reattachment and vortex co-shedding of twin cylinders. Particle Image Velocimetry measurements are performed to extract first and second order statistics of the wake. The wake features have been analyzed and compared with the bluff-body wake models present in literature. Additionally, the flow fields are decomposed in Proper Orthogonal modes to characterize the main coherent structures. The time coefficients of the modes are analyzed to extract phase relations between vortical features.Far enough from the wall the cylinders wake is symmetric, with Von Kármán vortices shed symmetrically in the wake with respect to cylinders centerline. The measured average wake characteristics and vortical structures are consistent with the data reported in the literature for tandem cylinders without ground effect. For a wall-to-cylinder gap equal to 1 diameter, the ground strongly influences the flow field, introducing asymmetry in the typical Von Kármán wake. The ground boundary layer is thickened past the downstream cylinder and a wall-jet appears, enclosed between the wall boundary layer and the cylinders wake. Eventually these flow structures are not distinguishable when the cylinders wake and the wall boundary layer are grown enough to merge. From POD analysis, vorticity blobs appear on the wall, paired with vortex shedding.If the wall gap is decreased to 0.3 diameters, an extended low speed region appears close downstream of the cylinders. A POD mode similar to a vortex shedding is still present; however it cannot be associated to an alternate Von Kármán street. The shed structures show a shorter wavelength than the Von Kármán shedding in far-from-the ground cases and are accompanied by a flapping jet-like structure ejected from the wall gap between the cylinder and the wall. The jet strongly changes the features of the wake on the side opposite to that of the wall, thus suggesting a possible coexistence between jet oscillations and shedding.