On the wake-like vortical arrangement and behaviour associated with twin jets in close proximity

Experiments on twin circular free jets with separation-distances of 1.5, 2.0 and 3.0 jet diameters (D) have been conducted using time-resolved particle-image velocimetry technique. Mean velocity results show that the merging of inner shear layers of the twin jets occurs nearer to the jet exit surface as the separation-distance decreases. Along the mean centre location of the twin jets, turbulent kinetic energy at 3.0D separation-distance increases monotonically up to 8.0D downstream of the jet exits, whereas it decreases gradually from a maximum level for smaller separation-distances. Frequency and proper orthogonal decomposition (POD) analyses show that two distinct peak frequencies are found to correlate with vortex formations associated with the outer and inner shear layers, regardless of the separation-distances. Velocity fields from POD analysis reveal that the inner shear layers vortices from the twin jets merge in a staggered configuration at 1.5D and 2.0D separation-distances, reminiscent of the wake vortex shedding phenomenon behind bluff-bodies. On the other hand, they remain essentially independent at 3.0D separation-distance. The present study indicates that the inner shear layer interactions between the twin jet flows are sensitive towards their separation-distance and they are governed by considerably different mechanisms. Lastly, comparisons of the frequencies associated with the outer shear layer, centreline and wake-like region between the present twin jets and annular jets investigated previously demonstrate good agreements in the overall trends.