Near field vortex dynamics in axially forced, laminar, co-flowing jets: a descriptive study of the flow configurations

An experimental study is presented of the vortex structures that appear in the shear layer of laminar, co-flowing air jets subjected to strong axial f orcing. A set of flow visualisation experiments has been performed to eluci date the nature of the different structures and the mechanisms leading to t heir appearance and further interactions. The axial forcing sets the axisym metric instability to prescribed values of amplitude and frequency (and thu s wavelength) and produces a strong effect in the lateral spreading of the inner jet. It is shown that the near field development of the flow can be e xplained via inviscid vortex dynamics arguments, involving three vortex str uctures. Due to the strong axial forcing, all these vortices already appear as developed concentrations of vorticity in the surroundings of the nozzle exit. An azimuthal perturbation is added to the flow in the form of a lobe d nozzle exit, in order to lock the azimuthal organisation of the vortices. The results are discussed and some representative configurations are exami ned. Each configuration appears for a given range of the forcing parameters . A tentative model of the near-field vortex dynamics is developed, but qua ntitative measurements are still necessary. (C) 2001 Editions scientifiques et medicales Elsevier SAS.