  Laminar-turbulent transition on a rotating wind turbine blade at a chord
Reynolds number of $1 \times 10^5$ and varying angles of attack ($AoA$) is
studied with direct numerical simulations and linear stability theory. The
rotation effects depend on the streamwise pressure gradient and direct/reverse
flow state. In the $AoA=12.8^\circ$ case, rotation retards the flow in the
laminar separation bubble (LSB) and renders the mixed
Tollmien-Schlichting/Kelvin-Helmholtz (TS/KH) instability more unstable.
Rotation fosters an oblique secondary instability mechanism, rapidly breaking
the KH rolls into small-scale turbulence. A sub-harmonic mechanism is dominant
in the non-rotating case, retarding transition. However, rotation accelerates
the boundary layer upstream of separation, subject to a strong adverse pressure
gradient (APG), stabilizing TS waves and delaying transition in $3\%$ and
reattachment in $4\%$. In the $AoA=4.2^\circ$ and $AoA=1.2^\circ$ cases,
rotation decelerates the flow upstream of separation, subject to a favorable
pressure gradient (FPG), which makes TS waves more unstable. Nonetheless,
rotation accelerates the separated flow, partially stabilizing the KH
mechanism. Rotation further promotes the appearance of inflectional crossflow
velocity profiles that triggers stationary and traveling crossflow modes. These
modes are less unstable than the TS/KH mechanism of the separated shear layer
but lead to the formation of coherent spanwise-velocity structures. For
$AoA=4.2^\circ$, rotation strengthens the sub-harmonic secondary instability
mechanism in the rotating case over the oblique one under no rotation. For
$AoA=1.2^\circ$, the sub-harmonic mechanism is dominant regardless of rotation.
Notice that the transition location is not changed by rotation in the
$AoA=4.2^\circ$ and $AoA=1.2^\circ$ cases. Finally, rotation stabilizes the
absolute instability found in these cases.
