Wake merging and turbulence transition downstream of side-by-side porous discs

The wake merging of two side-by-side porous discs with varying disc spacing is investigated experimentally in a wind tunnel. Two disc designs used in the literature are employed: a non-uniform disc and a mesh disc. Hot-wire anemometry is utilised to acquire two spanwise profiles at 8 and 30 disc diameters downstream and along the centreline between the dual-disc configuration up to 40 diameters downstream. The spanwise Castaing parameter profiles confirm the appearance of rings of internal intermittency at the outermost parts of the wakes. These rings are the first feature to interact between the discs. After this point, the turbulence develops to a state whereby an inertial range is observable in the spectra. Farther downstream, the internal intermittency shows the classical features of homogeneous, isotropic turbulence. These events are repeatable and occur in the same order for both types of porous discs. This robustness allows us to develop a general map of the merging of the two wakes.