This experimental study is concerned with the mixing in the initial region of turbulent coaxial streams. The investigation is limited to the low-speed incompressible case with the inner stream of lower velocity than the outer stream. Velocity ratios ranged from infinity to one. The inner to outer stream density ratio was either 4 or 1. Experimental measurements were made with a hot-wire anemometer system, static pressure probes, and shadowgraphy. The data show that a backflow region is formed in this particular apparatus at outer to inner stream velocity ratio of 13 for the homogeneous case and at outer to inner stream velocity ratio of 26 for the heterogeneous case. For higher velocity ratio, a circulating toroidal vortex is established which enhances mixing between the inner and outer streams. This vortex pattern exhibits similarity of velocity with velocity ratio variations.

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