On the effect of orifice thickness and divergence angle in the near and intermediate fields of axisymmetric jets

We experimentally explored the distinct effects induced by the geometry of various axisymmetric orifices in the near and intermediate regions of jets at a Reynolds number of Re≈10⁵ based on the diameter, d=10 mm, of the orifices. The orifices inspected had thicknesses of ∊/d=0.2, 1 and 2 and divergence angles of θ=0^∘,20^∘ and 45^∘. High-speed planar particle image velocimetry (PIV) was used to characterize the instantaneous flow field, high-order statistics, and dominant coherent motions at the jets symmetry plane within a streamwise distance of x/d∈(0,10]. Time-averaged flow fields reveal the effects of each orifice; velocity profiles in the near region exhibited saddle-backed, top-hat, and developing flow profiles with the short (sharp-edged), medium (contraction) and long (pipe-like) nozzles, which are modulated by ∊/d. The mixing rate was also affected by the orifices; differences between the cases are associated with the formation of distinct turbulent structures. Instantaneous vorticity fields, velocity spectra, and modal decomposition revealed relatively strong vortices and large scale meandering shed at relatively high frequency. The divergence angle strongly modulated the onset of large-scale oscillations.