Nonaxisymmetric instability in slowly swirling jet flows

Very small amounts of swirl are shown to strongly destabilize a round jet flow to large n inertial modes, where (k,n) are, respectively, the axial and azimuthal wave numbers of the disturbance. This mechanism is operative if the scale of the small swirl, ε, is O(n−1) for k=O(1). Further, viscosity is shown to exert a stabilizing influence on these modes if the Reynolds number is no larger than O(n3), hence, neutral curves have been obtained for a particular example—the slowly-swirling Squire jet. For this case, there is a very broad range of unstable wave numbers—all k between εn/2 and n. It is further determined that in the context of an initial-value problem for such a weak vortex on which is imposed a concentrated initial disturbance, the entire vortex is temporally unstable downstream of the initial disturbance.