Dynamics of the jet flow issued from a lobed Nozzle: Tomographic particle image velocimetry measurements

Abstract

This study focuses on the dynamics of the three-dimensional jet flow issued from a lobed nozzle. Flow field measurements were performed using a split-screen dual-camera tomographic particle image velocimetry (tomo-PIV) system. The lobed nozzle was constructed using three-circle configuration at the nozzle exit, where the ratio of the circular centre offset to the circle radius was a/b=0.8. Two high-speed cameras were used with an appropriate combination of prismatic and planar mirrors to split each camera into two views. The Reynolds number was fixed at Re=2400, and the circular jet at the same Reynolds number was also measured for comparison. Fourier mode decomposition was used to identify the large-scale structures superposed in the unsteady flow field. The lobe trough in the jet decreased the jet’s width due to the smooth connection between the pipe section and the lobed exit. Turbulence was intensified in the jet shear layer, and the passing fluid puffs in the circular jet instantaneous flow field were also broken in the lobed jet. The successively passing fluid puffs in the circular jet were observed to be the axisymmetric large-scale structure in Fourier modes dominating at St=0.39. However, breakdown of the axisymmetric structures was induced by the lobed nozzle at St=0.51 and 0.65, and the double helical modes arose at St=0.28 and 0.38 (also 0.4) in the shear layer of the jet potential core and interacted with the entire jet column.