On the interaction between rotation-triggered instabilities and the shear-layer instability over a laminar separation bubble on a rotating airfoil and its relation to transition*

Rotating airfoils may experience the appearance of a cross-flow velocity due to centrifugal and Coriolis forces, which can be particularly important in regions with separated flow. In non-rotating airfoils with a laminar separation bubble and low to moderate free-stream turbulence levels, laminar-turbulent transition is often triggered by the break-up of Kelvin-Helmholtz (KH) rolls formed over the separated shear layer. This work shows through direct numerical simulations that the cross-flow velocity generated by rotation is inflectional and generates inviscid instabilities that interact with the normal growth and break-up to turbulence of these KH rolls. Furthermore, this interaction may result in a transition location moving upstream if the pressure gradient is low or downstream if it is high, such as in high angles-of-attack cases. Parabolized stability equations are applied to the problem and show that the most amplified frequencies match those observed in the DNS. This indicates a convective nature of the instabilities observed. Global stability analysis is performed, confirming this character of the flow. However, it is noticed that the flow may become absolutely unstable under certain rotation conditions due to the enhancement of the reverse flow in the separation region.