Influence of freestream turbulence and porosity on porous disc-generated wakes

Abstract

This study aims to evaluate the effect of freestream turbulence (FST) on wakes produced by discs with different porosity. The wakes are exposed to various freestream turbulence "flavours", where turbulence intensity and integral length scale are independently varied. The turbulent wakes are interrogated through hot-wire anemometry from 3 to 15 diameters downstream of the discs. It is found that discs with low porosity behave similarly to a solid body, both in terms of entrainment behaviour and scaling laws for the centreline mean velocity evolution. Far from the discs, the presence of FST reduces both the wake growth rate and entrainment rate, with a clear effect of both turbulence intensity and integral length scale. As porosity increases, these "solid body" FST effects gradually diminish and are reversed above a critical porosity. The entrainment behaviour in disc-generated wakes is significantly influenced by the presence of large scale coherent-structures, which act as a shield between the wake and the surrounding flow, thus impeding mixing. We found that higher porosity, turbulence intensity, or integral length scale weaken the energy content of these structures, thereby limiting their influence on wake development to a shorter distance downstream of the disc. This, in turn, potentially reduces the influence of large-scale engulfment to the overall entrainment mechanism to a shorter distance downstream of the discs as well. For low porosity discs, freestream turbulence intensity initially promotes near wake growth through the suppression of large-scale structures; however, further downstream, the wakes grow faster when the background is non turbulent.