On the vortex structures and dynamics of twin jets in cross-flow
Date of Issue2016
School of Mechanical and Aerospace Engineering
The present research project focuses on the experimental study of twin jets in cross-flow (JICF) by both laser-induced fluorescence flow visualization and digital particle image velocimetry techniques. The research is divided into three parts – the first part examined the flow developments and interactions between two free-jet flows and the second and third parts study the flow behaviour and dynamics of twin jets in cross-flow arranged in tandem and parallel configurations, respectively. The separation-distances (s) used between the two jet exit centres are 1.5D, 2.0D and 3.0D, where D is the circular jet diameter and the jet Reynolds numbers range from ReD=1100 to 4400. For the twin free-jets at ReD=3300, it is observed that the merging of the jet shear layers and the development of the turbulent kinetic energy along the streamwise direction are dependent on the separation-distances. More importantly, two distinct frequency peaks are observed for the vortex formation along the outer and inner jet shear layers. Through proper orthogonal decomposition analysis, twin free-jet flow fields show staggered arrangement of the inner shear layer vortices at s/D≤2.0, reminiscent of the wake vortex shedding phenomenon behind bluff-bodies. The results suggest that when two free-jets are sufficiently close, significant interactions between the inner shear layers are able to modify the flow behaviour and developments of the jets within the near-field region. For twin jets in cross-flow experiments, the velocity-ratios used are r=2.0, 4.0, 6.0 and 8.0. When the two jets are arranged in tandem configuration and regardless of the separation-distance used here, the front jet is observed to provide ‘shielding’ for the rear jet from the cross-flow such that the rear jet penetrates deeper into the cross-flow with a higher jet trajectory. As a result, the front jet collapse upon the rear jet within the near-field region and significant jet interaction and merging occur. At smaller separation-distance of s/D≤2.0, a single pair of leading-edge and lee-side vortices is formed after the merging of the front and rear jets. On the other hand, the two jets at s/D=3.0 appear to interact more through their counter-rotating vortex pairs (CVPs). Particle-image velocimetry measurements further confirm that the near-field interaction of the two jets in tandem JICF are sensitive towards the separation-distance. While the CVPs of the front jet and the rear jet quickly merge into a single CVP shortly downstream of the rear jet exit for s/D≤2.0, the rear jet CVP structure is able to develop over a notable distance before interacting significantly with that of the front jet at the largest separation-distance. In addition, a ‘merged’ jet trajectory can be found for the tandem JICF in close proximity and the ‘rD’-scaling law remains applicable to this ‘merged’ trajectory buy introducing the concept of effective velocity ratio into the ‘rD’-scaling law proposed by Pratte and Baines (1967). When the two jets are arranged in parallel configuration, they interact most significantly along the streamwise plane of symmetry (i.e. centre between the two jet exits). Shear layers can be observed along this symmetry plane as a direct result of the two jets meet and entrain with each other regardless of the velocity-ratio used, resembling those of a single JICF. Moreover, coherent shear layer vortices are produced along this ‘symmetry-plane’ shear layer for separation-distances of s/D≤2.0. This also suggests that the two jets interact via the shear layer vortices at smaller separation-distances, whereas are more likely to interact through their CVPs when the separation-distance increases to s/D=3.0. Last but not least, cross-stream results have illustrate the merging process of the two CVPs for parallel JICF, where they undergo vorticity annihilation as well as being subjected to mutually induced downward motion. As a result, parallel JICF are likely to develop gradually into single-like CVP at far-field for separation-distances of s/D=1.5, 2.0 and 3.0 and velocity-ratios of r=2.0 to 8.0 under the present experimental conditions.