Local and global instability of buoyant jets and plumes

Abstract eng:
The local and global linear stability of buoyant jets and plumes has been studied as a function of the Richardson number Ri and density ratio S in the low Mach number approximation. Only the m = 0 axisymmetric mode is shown to become globally unstable, provided that the local absolute instability is strong enough. The helical mode of azimuthal wavenumber m = 1 is always globally stable. A sensitivity analysis indicates that in buoyant jets (low Ri), shear is the dominant contributor to the growth rate, while, for plumes (large Ri), it is the buoyancy. A theoretical prediction of the Strouhal number of the self-sustained oscillations in helium jets is obtained that is in good agreement with experimental observations over seven decades of Richardson numbers.

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