An experimental study was conducted to investigate effects of axial forcing on the flow structures near the nozzle exit in coaxial isothermal jets and jet diffusion flames. The jet was excited by adding a periodic velocity fluctuation ranging from 0 to 400% of the mean jet velocity at a tube resonating frequency. The phase-averaged axial velocity fluctuation at the jet center was measured with one-component LDV and phase-locked visualization using a light chopper and phase conditioning circuit was performed. The changes of large-scale structures in the near field of the jet are described from the visualization of horizontal and vertical cross-cut Mie scattering images. The flow structures of the forced isothermal jet are classified into four regions on the basis of the emergence of azimuthal structures and the periodic behavior of vortex structures. The jittering of azimuthal structures was characterized by the velocity fluctuation ratio and the velocity difference between the jet and the coflowing fluid. The jittering of vortices is found to be related with the vortex pairing and the axial velocity is greatly decreased at this velocity fluctuation ratio. To enhance the mixing by forcing, forcing amplitude should be adjusted to fall in the vortex jittering or break-up region. In case of the forced reacting jet, flame heights were measured from video tape recordings of the sooting images of the flame. The dependence of flame height on the velocity fluctuation ratio shows the existence of a flame length elongation and reduction region. The flame elongation is found to be related to the suppression of the flame flickering by forcing. From the Mie scattering images and flame length measurements, it is suggested that the intense mixing observed in the fully forced laminar jet and the reduction of the flame length is closely related to the development of azimuthal structures.
