An experimental investigation of the near-field flow development in coaxial jets

The near-field region of a coaxial jet having inner to outer diameter ratio di/do=0.39 is investigated experimentally for four ratios of annular to central jet velocities of η=0.18, 0.48, 0.8, and 1.11. Measurements were acquired nonintrusively using molecular tagging velocimetry at downstream distances up to six inner jet diameters. High spatial-resolution profiles of mean axial velocity, axial turbulent intensity, skewness, kurtosis, radial gradients of mean velocity, and velocity gradient fluctuations are presented and discussed. In the near-exit region of the inner mixing layer, evidence suggests the existence of two trains of vortices shed from the inner jet wall for velocity ratios η>0.18. The results also indicate that for the flow configuration examined the length of the annular potential core is a function of the velocity ratio. Turbulence characteristics of the flow appear to be influenced by both the velocity ratio and absolute velocity of the annular jet. Integral length and Taylor microscales in the inner shear layer are shown to grow monotonically with downstream distance, with their ratio reaching a constant value. It is concluded that the turbulence structure in the inner mixing layer is highly anisotropic and that the levels of anisotropy are a relatively strong function of the velocity ratio.