The paper describes a new, simple method for the formation of free round jets with long laminar regions by a jet-forming device of ∼1.5 jet diameters in size. Submerged jets of 0.12 m diameter at Reynolds numbers of 2000–12 560 are experimentally studied. It is shown that for the optimal regime, the laminar region length reaches 5.5 diameters for Reynolds number ∼10 000 which is not achievable for other methods of laminar jet formation. To explain the existence of the optimal regime, a steady flow calculation in the forming unit and a stability analysis of outcoming jet velocity profiles are conducted. The shortening of the laminar regions, compared with the optimal regime, is explained by the higher incoming turbulence level for lower velocities and by the increase of perturbation growth rates for larger velocities. The initial laminar regions of free jets can be used for organising air curtains for the protection of objects in medicine and technologies by creating the air field with desired properties not mixed with ambient air. Free jets with long laminar regions can also be used for detailed studies of perturbation growth and transition to turbulence in round jets.

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Experimental and numerical investigations of a flow in a unseparated conical diffuser with a small tilt angle of the generatrix are carried out. A steady turbulent air flow was delivered at the diffuser inlet through a tube with a diameter ∼0.3 of the diffuser inlet diameter. The velocity and turbulent stresses were measured at the outlet of the diffuser, at 600 < Reout < 3000. A strong influence of the channel expansion on the flow pattern was discovered. In contrast to a tube, where for Re < ∼2000 only laminar flow is observed as steady, at the outlet of the diffuser a turbulent flow is formed for Reout > ∼1000. The formation of a turbulent flow is confirmed by both the results of Reynolds stresses measurements and calculations. While decreasing Reout below ∼1000, flow becomes laminar and turbulent perturbations disappear. For 600 < Reout < 3000 the velocity and the turbulent stresses measured in the diffuser outlet are well described by the three-parameter model of shear turbulence. The same calculations showed, that for Reout > ∼1300 the turbulent flow can be considered as steady, that means that its outlet parameters are almost independent of the conditions at the inlet of the diffuser, if these conditions are sufficient for the formation of a turbulent flow.