Modal Instability Analysis of Light Jets

Linear global modes are computed for round helium jets in air at low Reynolds number. The governing equations are formulated in the limit of zero Mach number, and all parameters are chosen such as to model the experiments of Hallberg & Strykowski1. A single axisymmetric mode is found to become unstable beyond a critical Reynolds number value that depends on the initial shear layer thickness. The neutral curve for linear global instability is traced as a function of these two parameters, and this result compares favorably with the experimental reference measurements.