Core Turbulence Effect on Naturally Occurring Modes in a Circular Jet

Abstract

Measurements were made of instability modes (axisymmetric and azimuthal) triggered by naturally occurring disturbances in a circular jet. A modal spectrum technique was developed using eight circum-ferentially located hot-wire probes, and applied to a high Reynolds number (400,000) jet for both untripped (transitional) and tripped (turbulent) nozzle exit boundary layers and for core turbulence levels of 0.15 and 5 percent. The energy content of the higher order modes (m > 1) was significantly lower than the axisymmetric (m = 0) and the azimuthal (m = ±1) modes. In the early evolution of the jet the axisymmetric mode was dominant with the azimuthal modes dominating only after the end of the potential core. For a fixed level of core turbulence (0.15 percent) the growth of the axisymmetric and azimuthal modes occurred closer to the nozzle exit for the untripped (transitional) boundary layer case than for the tripped (turbulent) case. For the tripped (turbulent) boundary layer case, when the core turbulence was increased from 0.15 to 5 percent, the growth of the instability wave and its subsequent “saturation” amplitude were reduced.