Results are reported from a three-dimensional computational stability analysis of flow
over a backward-facing step with an expansion ratio (outlet to inlet height) of 2 at
Reynolds numbers between 450 and 1050. The analysis shows that the first absolute
linear instability of the steady two-dimensional flow is a steady three-dimensional
bifurcation at a critical Reynolds number of 748. The critical eigenmode is localized
to the primary separation bubble and has a flat roll structure with a spanwise
wavelength of 6.9 step heights. The system is further shown to be absolutely stable
to two-dimensional perturbations up to a Reynolds number of 1500. Stability spectra
and visualizations of the global modes of the system are presented for representative
Reynolds numbers.