Free jets and other shear flows often occur in nature and various technologies and are widely studied. Turbulent jets and their breakdown have been thoroughly studied over several decades in the context of many industrial applications, including mixing, combustion, noise generation and others.

Laminar jets are studied much less because of their immediate breakdown in normal conditions due to extremely small critical Reynolds number (~20). In published experimental studies of free jets with Reynolds number of ~4000 or greater, transition to turbulence occurs near the orifice. However, in our previous work (Zayko et al., Fluid Dyn., 2018, accepted) we demonstrate a new method for the formation of free jets of 0.12 m diameter, with the Reynolds number of 10,000 and laminar region length of 5 jet diameters.

In this study, we investigate the perturbation growth in the long laminar jet. Perturbations are introduced either acoustically, or through a metal oscillating wire.

Hot-wire and PIV experimental measurements are compared with theoretical predictions of linear stability analysis.