Reynolds number dependence of plane jet development in the transitional regime

Abstract

The transitional characteristics of plane turbulent jets have been investigated in the present study. Hot wire measurements have been performed for a jet issuing from a rectangular nozzle of aspect ratio 20, for Reynolds number varying in the range 250⩽Re⩽6250. In this range, the characteristics of flow development are found to be Reynolds number dependent, in contrast to the fully developed turbulent jets which show features independent of initial conditions such as inlet Re. For low Re jets, the jet spread is significantly influenced by the low frequency oscillations caused by shear layer instability. Large sized vortices are formed in the shear layers at the fundamental frequency of the instability, which lead to subharmonic low frequency oscillations due to vortex pairing and merger, at larger axial distances. Consequently, the far field flow structure of a low Re jet is dominated by large size vortices which give rise to a higher level of flow intermittency, larger entrainment of ambient fluid, and faster jet decay, as compared to high Re jets. Also, in the absence of finer scales and broader spectrum of eddies, the mean flow field achieves self-similar structure much ahead of the fluctuating components and fully developed turbulent flow characteristics are not observed, even in the far field. In high Re jets, on the other hand, the vortex break-up processes also simultaneously occur along with vortex pairing and merger. Therefore, energy transfers to a broad spectrum of scales and finer scales are observed even in the near field of the jet. Although the achievement of self-similarity for the mean field is slightly faster than that for the fluctuating components, turbulence also attains a fully developed state at about a nondimensional axial distance of 20. The associated probability density functions of the fluctuating components evolve into Gaussian profiles, implying isotropic turbulence. Due to the dominance of finer scales, the overall entrainment level is less and decay is slower for a high Re jet.