Velocity fields of round and plane isothermal micro-jets of air flowing into a submerged air space are experimentally investigated. The PIV-method and a thermal anemometer were used for the measurements. A 3D-picture of the formation and break-down of the laminar part of the jet and of the transition to turbulence is obtained. The length of the laminar zone of the round jet is shown to reach the values of L / d 0 = 80-120, with Reynolds numbers of Re = 600-3000. Such an abnormal «long range» is explained by the change in the velocity profile along the laminar part of the round jet: at the mouth of the nozzle it is Poiseuille's velocity profile, and further down the stream it first corresponds to Bay-Shi-I theory and then noticeably deviates from it. For the plane jet, the distance to the transition zone is considerably smaller than for the round one. The measured characteristics in the turbulent part of the jets agree well with the data published by other authors.
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