MODE SPECTRA OF NATURAL DISTURBANCES IN A CIRCULAR JET AND THE EFFECTOF ACOUSTIC FORCING

A modal spectrum technique was used to study coherent instability mode s (both axisymmetric and azimuthal) triggered by naturally occurring d isturbances in a circular jet. This technique was applied to a high Re ynolds number (400,000) jet for both untripped (transitional) and trip ped (turbulent) nozzle exit boundary layers, with both cases having a core turbulence level of 0.15%. The region up to the end of the potent ial core was dominated by the axisymmetric mode, with the azimuthal mo des dominating further downstream. The growth of the azimuthal modes w as observed closer to the nozzle exit for the jet with a transitional boundary layer. Whether for locally parallel flow or slowly diverging flow, even at low levels of acoustic forcing, the inviscid linear theo ry is seen to be inadequate for predicting the amplitude of the forced mode. In contrast, the energy integral approach reasonably predicts t he evolution of the forced mode.