Dynamics of free jets submitted to upstream acoustic modulations

The response of low Reynolds number air jets to acoustic modulations is investigated with systematic experiments. Flow transfer functions, power spectral densities of the perturbations plotted in the form of space-frequency maps, instantaneous jet images recorded using laser tomography, and phase-conditioned velocity fields serve to identify the different regimes of motion. Results indicate that five regions can be distinguished in a diagram where the jet Strouhal number StD serves as a horizontal coordinate while the shear layer Strouhal number Stθ constitutes the vertical coordinate. Boundaries of the different regions are defined by the characteristic values of these two dimensionless groups at preferred mode and neutral wave conditions, corresponding, respectively, to a maximum of amplification and a vanishing rate of growth. The forcing frequencies imparted to the flow can be ordered with respect to these characteristic parameters. The flow transfer functions at the excitation frequency indicate that perturbations have a different nature depending on the value of the jet Strouhal number based on the exhaust diameter StD. Convective, mixed type, and acoustic ranges are identified. Detailed measurements of the spatial evolution of the forced components and corresponding harmonics provide information on the nonlinear power transfer between the different modes for modulation amplitudes reaching 10% of the mean flow velocity. The StD−Stθ mapping is used to categorize the various modes of interaction observed in the experiments.