Algebraic stability modes in rotational shear flow

We investigate the two-dimensional (2D) stability of rotational shear flows in an unbounded domain. The eigenvalue problem is formulated by using a novel algebraic mode decomposition distinct from the normal modes with temporal evolution exp(ωt) . Based on the work of Nold and Oberlack (2013 Phys. Fluids 25 104101), we show how these new modes can be constructed from the symmetries of the linearized stability equation. For the azimuthal base flow velocity V(r)=r−1 an additional symmetry exists, such that a mode with algebraic temporal evolution t s is found. s refers to an eigenvalue for the algebraic growth or decay of the kinetic energy of the perturbations. An eigenvalue problem for the viscous and inviscid stability using algebraic modes is formulated on an infinite domain with r→∞ . An asymptotic analysis of the eigenfunctions shows that the flow is linearly stable under 2D perturbations. We find stable modes with the algebraic mode ansatz, which can not be obtained by a normal mode analysis. The stability results are in line with Rayleigh’s inflection point theorem.