Experimental investigation of mean and turbulent characteristics of a free air rectangular jet with/without lip length

The influence of geometric modifications to a rectangular nozzle on the flow structure of a free jet was investigated experimentally. In this work, an adaptable nozzle was employed, featuring exchangeable upper and lower plates that could be replaced to achieve the desired lip length, while the remaining geometry was unchanged. This study aims to assess the effect of lip length on flow structure by comparing a nozzle with equal lip length, serving as a reference, to configurations with 10 and 20 mm lower lip extensions. Both the mean and turbulent properties of the jet were characterized along the streamwise and spanwise directions using a 2-dimensional hot-wire system. In addition, particle image velocimetry (PIV) was utilized for flow visualization, assessment of air entrainment, and proper orthogonal decomposition (POD) analysis. The findings indicate that increasing the lip length broadens the radial spread of the mean axial and radial velocity distributions and amplifies velocity fluctuations, resulting in enhanced mixing. Furthermore, a longer lip increases the centerline velocity decay rate and enhances velocity fluctuations along the centerline, producing a reduced potential core length relative to the equal-length configuration. It also produces a wider shear stress distribution and increases the turbulent transport of normal and shear stresses via radial velocity fluctuations. PIV imaging and POD analysis further demonstrated asymmetric vortex roll-up and mixing with ambient fluid. The newly explored lip length effect has potential applications in flow control and could be relevant to a wide range of industrial free turbulent jet technologies.