Visualization of Turbulent Eddies in a Free Jet by a Laser Sheet Method.

Abstract

The structure of the mixing layer in an axisymmetric round free jet was investigated on the basis of the two-dimensional instantaneous photographs of turbulent eddies, which were taken using a laser-light sheet method with a YAG pulse laser and a 35-mm still camera. Eddies were successfully visualized by titanium dioxide particles seeded in the flow for nitrogen, helium and carbon-dioxide jets having different kinematic viscosities. The observation of the coherent structure of air entrainment eddies shows that the turbulent eddies have unexpectedly coarse spatial distribution and their scale in the horizontal plane is almost identical with that in the vertical one. This suggests that three-dimensional structure is established at the upstream position. It is also shown that in the helium jet having a larger kinematic viscosity and a lower density, eddies of a larger scale are formed and retard mixing with the surrounding air. In addition, we present a detailed discussion of the structure of mixing eddies, based on the formation of the elliptic and hyperbolic points in the shear flow.