Published online by Cambridge University Press: 29 October 2019
In the energy stability theory, the critical Reynolds number is usually defined as the minimum of the first positive eigenvalue $R_{1}$ of an eigenvalue equation for all wavenumber pairs
$(\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D6FD})$, where
$\unicode[STIX]{x1D6FC}$ and
$\unicode[STIX]{x1D6FD}$ are the streamwise and spanwise wavenumbers of the normal mode. We prove that
$(\cos \unicode[STIX]{x1D703}\pm 1)R_{1}$ are decreasing functions of
$\unicode[STIX]{x1D703}=\arctan (\unicode[STIX]{x1D6FD}/\unicode[STIX]{x1D6FC})$ for the parallel flows between no-slip or slip parallel plates with or without variations in temperature. Numerical results inspire us to conjecture that
$R_{1}$ is also a decreasing function of
$\unicode[STIX]{x1D703}$ for the parallel shear flows under the no-slip boundary condition and without variations in temperature. If the conjecture is correct, the least stable normal modes for the energy stability will be streamwise vortices for these base flows.