The self-excited axisymmetric jet

The characteristics of a self-excited axisymmetric air jet driven by a whistler (i.e. pipe-collar) nozzle have been explored experimentally for various choices of the controlling parameters: namely, pipe length, jet diameter, collar length, step height, and jet speed. By appropriate choices of these parameters as well as the stage and mode (half- and full-wave), the self-sustained excitation has been induced at specific values of the excitation amplitude and the Reynolds number RD. The jet characteristics up to Rd 3.1 x 105 and x/D 60 have been documented for both laminar and turbulent flows at the pipe exit. Comparison with corresponding unexcited-jet data reveals that self-excitation produces a large increase in the fluctuation intensity in the near field of the jet, while it increases the jet spread and decay rate for the entire x-range of measurement. The dependence of the jet structure on the initial condition is stronger when self-excited than when unexcited. The first stage of excitation always produces the highest turbulence augmentation and the spectral evolution is significantly modified by self-excitation up to x/D 6. The excitation produces a significant increase in the broad-band turbulence level over that of the unexcited jet. The broad-band amplification is maximized at x/D 4 and is the highest at the largest Rd studied.These data suggest interesting possibilities for the self-excited jet in the augmentation or control of entrainment, mixing and aerodynamic noise production.