Influence of upstream generated turbulence on an axisymmetric jet

Various jet-based applications rely on an adequate understanding of the upstream turbulence conditions. While active grids are commonly used to alter the turbulent properties of the flow in a wind tunnel, a unique concept of such a grid, as applied to an axisymmetric jet, is proposed in this study. The aim is to control the inlet flow conditions and investigate the subsequent influence on the shear layer roll-up and the self-similar behaviour of the jet. The active grid fabricated is circular in shape and consists of 4 independently controlled paddles. A parametric study is carried out with 3 different jet contraction ratios, C_R, of 2.4, 7.5 and 12.25, while paddles have a mean rotational speed, Ω, of 0.1, 1 and 10 Hz. Hot-wire anemometry is performed at multiple radial and axial locations in order to characterize the flow. Additionally low speed planar PIV is performed for selected cases to quantify self-similarity. Initial results highlight that the rotational speed of the paddles has limited influence on the turbulent intensity but alters the integral length scale in the flow, while suppression of the shear layer roll-up is also noted for specific configurations.