Vortex interaction in patches of randomly placed emergent cylinders
The flow field of multiple-cylinder configurations exhibits complex interactions between shear layers, vortexes and wakes. For high stem-Reynolds numbers, the flow is turbulent and, low and intermediate areal number-densities of cylinders, and turbulence is produced mostly by the work of Reynolds shear stresses in the horizontal plane (uv component) against the time-averaged shear rate characteristic of vertical-axis vortex shedding in the wake of cylinders. The spatial pattern of turbulent production and of other terms of the equation of conservation of Turbulent Kinetic Energy (TKE) is thus mostly determined by the interaction of vortexes shed by individual cylinders and by the distance between cylinders. The main objective of this paper is to advance on the understanding of vortex interaction in patches of randomly placed emergent and rigid cylinders. In particular, the relation between cylinder Strouhal numbers, vortex decay and vortex path statistics is investigated for isolated cylinder and for a cylinder within an array of randomly placed cylinders with a areal-number density of 980 cylinders/m2. Results are compared to shed light on the influence of neighbouring cylinders. An experimental database acquired with 2D Particle Image Velocimetry (PIV) was explored. A methodology to detect vortexes in 2D flow fields is proposed. It features a point-based criterion and a global search to detect all the possible vortex core locations, combined with a curve-based criterion, to decide whether the detected point corresponds to a vortex, depending on the geometry of streamlines. The results show a decrease on the amount of vortexes and a shorter vortex life for the cylinder within the array when compared with the isolated cylinder. The averaged vortex path is also affected by the presence of neighbouring cylinders. Concerning the Strouhal number, the normalized shedding frequency is approximately the same for both studied cases.