Transition in synthetic jets

This paper is a sequel to several recent papers that analysed experimental data on axisymmetric synthetic jets using similarity transformation of governing equations involving a one-equation model of turbulence. Unusual feature there is variation of parameters (“constants”) in transformed equations along the synthetic jet axis. Interestingly, this variation is non-monotonous: there is an abrupt change at a certain distance from the nozzle. Velocity spectra of one synthetic jet have earlier shown this change to be associated with disappearance of sharp peaks in the spectra, characteristic for presence of organised vortices. The change thus indicates transition into fully chaotic turbulence. In the present paper, the study of the transition is extended to a whole range of synthetic jets, from Stokes number Sk = 11.5–108.8 (corresponding to frequencies from ∼10 Hz to ∼100 Hz). In a search for a rule governing the transition, the vortices in the organised part of the synthetic jets were modelled as spherical objects. It is found that the transition occurs at the axial distance from the nozzle equal to ∼10 diameters of these spherical model objects.