The instability of a round jet surrounded by an annular shear layer

The instability and large-scale structure of the near-nozzle region of a round jet is studied at high Reynolds numbers, using a simple stability model to identify some of the global properties of large-scale eddies. The radial dependence of the amplitudes of growing disturbances is examined in order to illustrate their ability to penetrate into the interior of the jet and into its surroundings. The results of the stability analysis are used to examine how disturbances scale as a function of shear layer thickness. It is found that the wavelength of the most amplified disturbances scales with the shear layer thickness when the latter is small compared with the jet radius. This scaling quickly breaks down as the shear layer thickness approaches the jet radius. Short-wavelength modes which are amplified near the jet nozzle do not disturb the center of the jet, but long wavelengths amplified further downstream do.