Statistical properties and structural analysis of three-dimensional twin round jets due to variation in Reynolds number

The aim of the current study is to delineate the effects of Reynolds number (Re = 5000-20,000) on the mean and turbulent properties as well as the turbulent structures of the flow issuing from twin round jets with nozzle spacing of 2.8 nozzle diameter (d). The measurements were performed using a high-resolution particle image velocimetry from the jet exit up to x/d = 40 to examine the merging, converging and combined regions of the twin jets. The results show that the velocity decay and jet spread rates, and the combined point location become Re-independent at Re > 10,000 while the merging points shift downstream as Re increases. Upstream of the merging point, the levels of Reynolds stresses and turbulent kinetic energy production are higher and vortical structures are more intense in the inner shear layers compared to the outer shear layers. At the merging point they equal each other and beyond this point the trends are opposite of those at the upstream. The flow analysis at turbulent/non-turbulent interface of the outer shear layer reveals that conditional streamwise velocity and spanwise vorticity are Re-independent while significantly larger transverse velocity is observed at the lowest Re. At the inner shear layer interface, on the other hand, the conditional properties do not show any dependence on Re. Two-point correlation functions exhibit the presence of larger structures in the outer shear layers due to more exposure to the ambient fluid compared to the inner shear layers. The size of the structures also decreased as Re increased indicating the effect of Re on large-scale engulfment. Joint and weighted joint probability density functions are presented to examine the flow behaviors in more details.