Published online by Cambridge University Press: 13 July 2021
The streamwise vortex instability of boundary layers caused by wall roughness in the form of surface undulations is investigated. The instability is characterised by a roughness parameter $\varGamma$ depending on the geometry and fluid properties. At $O(1)$ values of $\varGamma$ disturbances develop on the same length scale as the basic boundary layer flow. The instability is driven by a boundary condition relating the disturbance wall shears in the streamwise and normal directions. The undulations have a wavelength comparable with the boundary layer depth and the amplitude is asymptotically small compared with the depth. If the roughness parameter is large then, apart from a narrow window of vortex wavenumbers, the instability responds in a quasi-parallel manner. Falkner–Skan boundary layers are considered in detail and the dependence on the angle of the wedge associated with the flows investigated. A particular susceptibility to roughness instabilities of flows past $90^{\circ }$ wedges is uncovered. The limits of small and large wavenumbers are considered and universal results given for the critical roughness height $h$ and wavelength $b$ needed for instability.