Predicting the flow field where gases of different kinds are mixing is important in providing design guidance in a number of engineering systems, e.g. diffusion type chemical lasers, rocket injectors and scramjets.
By using mixtures of air and helium, density and velocity distributions in axially symmetric jet exhausting into the free atmosphere are measured with a high accuracy. It is shown that the far downstream radial distributions of velocity and density are similar and they may all be approximated by the Gaussian distribution. The velocity and density at the centerline are inversely proportional to the streamwise coordinate. The linear increase of the half radius with the axial coordinate is observed, however, the rate of linear spread is found to be strongly dependent upon the difference in density between the jet and the receiving medium. On the basis of the measurements the eddy viscosity and the turbulent Schmit number in fully developed region are calculated by numerical analysis.

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The experimental investigations were carried out to explore the wake-rotor interference effects on the however performance of the co-axial rotor.
The effect of various combinations of variable geometry parameters such as the axial spacing and the pitch difference between rotors on the hover performance were systematically tested.
Flow visualization testings were also conducted to clarify the relationships between the rotor performance and the tip vortex behaviours at the near wake of each rotor.
It was found that the performance of co-axial rotor was largely depended on these parameters and there was the optimum combination of the parameters which could offer more efficient performance than a single rotor one.
It was also shown that the optimum pitch difference and the load sharing of the rotor had a unique property to define the optimum condition of the co-axial rotor.
The possibility to improve the hover performance might be originated from the fact that the co-axial rotor arrangement could alleviate the wake-rotor interference effects by the relocation of the tip vortex in the near wake structure.

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