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The objective of this research is to investigate the effects of nozzle spacing on the mean velocity and higher order turbulent statistics of free round twin jets produced from sharp contraction nozzles. The experiments were performed in an air chamber for four nozzle spacing ratios, S/d = 2.8, 4.1, 5.5 and 7.1 and at a fixed Reynolds number of 10,000. A planar particle image velocimetry system was used to conduct the velocity measurements. The results show that downstream of the potential core, a reduction in spacing ratio leads to an earlier and more intense interaction between the jets, indicated by enhanced half-velocity width spread rate in the inner shear layers and a significant rise of turbulent intensities and vorticity thickness along the symmetry plane. A reduction in spacing ratio, however, diminishes the ambient fluid entrainment along the inner shear layers leading to reduced core jet velocity decay rate. A closer proximity of the jets also leads to the suppression of Reynolds stresses in the inner shear layers but not in the outer shear layers, suggesting that there exists no significant communication between the inner and outer shear layers. The Reynolds stress ratios along the jet centerline reveals the highest anisotropy in the potential core region. Skewness and flatness factors are used to examine the asymmetry and intermittency of the velocity fluctuations, while two-point correlation analysis is employed to investigate the effects of nozzle spacing ratio on the spatial coherence of large-scale vortical structures.ii ACKNOWLEDGMENT

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The effects of nozzle orientation on the mixing and turbulent characteristics of elliptical free twin jets were studied experimentally. The experiments were conducted using modified contoured nozzles with a sharp linear contraction. The centers of the nozzle pair had a separation ratio of 5.5. Two nozzle configurations were tested, twin nozzles oriented along the minor plane (Twin_Minor) and twin nozzles oriented along the major plane (Twin_Major) and the results were compared with single jet. In each case, the Reynolds number based on the maximum jet velocity and the equivalent diameter was 10,000. A planar particle image velocimetry system was used to measure the velocity field in the jet symmetry plane. It was observed that the velocity decay rate is not sensitive to nozzle orientation. However, close to the jet exit the spread rate was highest in the minor plane. In addition, contour plots of swirling strength, Reynolds shear stress and turbulent intensities revealed significant differences between the minor and major plane. Velocity profiles showed little variation close to the jet exit, while further downstream the variations between the velocity profiles were more pronounced between the major and minor planes.