The stability of swirling jets

The stability of a swirling cylindrical jet of compressible fluid is examined by performing a normal mode analysis and numerically solving the eigenvalue problem. Perturbations are considered and instabilities which are characteristic of both a non-swirling (top-hat) jet and a Rankine vortex are investigated for a particular axial wavenumber. The vortex instabilities are weak, and are found to remain weak when axial flow is present. The jet instabilities are much stronger, but axial flow is a stabilizing influence. The positive helicity (km greater than 0) nonaxisymmetric modes (m not equal to 0) are stabilized by a small component of azimuthal flow. The axisymmetric mode (m = 0) and the negative helicity nonaxisymmetric modes persists in rapidly swirling jets, but with a greatly reduced growth rate.