This thesis presents an experimental investigation of the effects of offset height ratio and boundary condition (free surface and solid wall) on the turbulence characteristics of submerged twin jets issuing from two parallel round nozzles. A particle image velocimetry (PIV) technique was used to conduct two sets of experiments. The first set of experiments focused on twin surface jets at Reynolds number, Re = 4400, separation ratio (G/d = 2.3) and offset heights, 1d, 2d, 3d and 4d from the free surface. The second set of experiments, which was designed based on the main findings of the previous experiment, examined the effects of free surface and wall on the turbulence structure of the twin jets. These experiments were performed at Re = 5000, G/d = 2 and offset heights, 1d, 2d and 4d from the free surface or wall.

For the surface jets, the mean velocity and turbulence intensities at the free surface rapidly increased beyond the attachment point by amounts that increases with decreasing offset ratio. Further downstream, the surface transverse turbulence intensities decayed to negligible values while the streamwise turbulence intensities were persistent because of the energy redistribution from the transverse Reynolds normal stress to the tangential (streamwise and spanwise) Reynolds stresses.

For both boundary conditions, the attachment length, Reynolds stresses and the spatial extents of the large-scale structures increased with increasing offset ratio. The transition of the twin jets to a single jet was more rapid as offset ratio increases due to the enhanced entrainment and mixing for larger offset ratios. The evolution of the jet farther away from the free surface or wall behaved like a free jet, but the decay rate of the jet interacting with each boundary was substantially reduced in the far field due to confinement effects. The difference between the effects of free surface and wall on the mean velocity, Reynolds stresses and triple velocity correlations were confined to the immediate vicinity of each boundary.

The interaction between the large-scale structures and the boundaries were also investigated using various multi-point statistical techniques such as discrete wavelet transform, linear stochastic estimation, swirling strength, joint probability density function and two-point velocity correlations. The results showed that, the wall considerably reduced the spatial coherence of the structures embodied in the transverse auto-correlations compared to those near the free surface.