Why do only some riblets generate spanwise rollers?

Spanwise-coherent rollers form over certain riblet-treated surfaces, increasing skin-friction drag. Acute-angled triangular riblets are particularly susceptible to this instability. Previous linear stability analyses have struggled to correctly predict the streamwise wavelength of these rollers, nor determine which riblets generate rollers. This is rectified by imposing an empirical boundary condition which captures the streamwise-scale dependence of the riblet admittance, as measured in direct numerical simulations. The rollers are then predicted to be marginally stable at streamwise wavelengths matching those observed in direct numerical simulations (≈ 150 viscous units) and at similar wall-admittance magnitudes. The admittances required for roller growth are much lower than previously believed, explaining how changes to the riblet tip angle, and thereby the wall-admittance, can impact the likelihood of rollers being generated.