Unification theory of instabilities of visco-diffusive swirling flows

A universal theoretical model of instabilities in swirling flows occurring in nature or industrial processes is developed. This model encompasses various open and confined flows including spiral Couette flow, spiral Poiseuille flow, and baroclinic Couette flow, which results from the combination of differential rotation and baroclinic convection due to radial heating and natural gravity. Utilizing short-wavelength local analysis, the model generalizes previous results from numerical simulations and linear stability analysis of specific flows. A general criterion for instability is derived, incorporating viscous dissipation and thermal diffusion, thereby extending and uniting existing criteria (Rayleigh for centrifugally-driven flows, L\"udwieg-Eckhoff-Leibovich-Stewartson for isothermal spiraling flows, and Goldreich-Schubert-Fricke for non-isothermal azimuthal flows).