An experimental investigation of the near-field region of a free turbulent coaxial jet using LDA

The near field-region of the flow field of a coaxial jet configuration is investigated experimentally using Laser Doppler Anemometer. In order to obtain a better definition and, possibly, a deeper physical understanding of the Bow field under consideration, the Bow was simplified so that the jet is made to issue from concentric round profiled nozzles and to discharge freely into still ambient air. The velocity ratio between the inner and the outer streams of the jet, lambda, is varied for a fixed upstream conditions. In addition a relatively wide interface between the two streams was introduced. This interface affects the structure of the shear layers between the two streams, thus affecting the velocity decay characteristics along the centerline. The obtained results show that the inner potential core length of the of coaxial jet strongly depends on the velocity ratio lambda while the outer potential core for jets having velocity ratios greater than unity seems to be insensitive to the velocity ratio. The investigated coaxial jet Bow fields did not show self-similarity up to x/D = 25. Jets with velocity ratio less than unity were resulted to develop faster than those with lambda greater than unity. In addition increasing the velocity ratio was seen to accelerate the jet decay along the centerline. (C) 1999 Elsevier Science Ltd. All rights reserved.