Vortex Shedding Lock-In due to Pitching Oscillation of a Wind Turbine Blade Section at High Angles of Attack

Abstract EN

The unsteady flow around a pitching two-dimensional airfoil section (NREL S809) has been simulated using unsteady RANS with the transition SST turbulence model.

This geometry is chosen to represent a wind turbine blade in a standstill configuration.

The Reynolds number is Re=106 based on a chord length of 1 m.

A prescribed sinusoidal pitching motion has been applied at a fixed amplitude of 7° for a range of high angles of attack 30°<α<150°.

At these incidences, the airfoil will behave more like a bluff body and may experience periodic vortex shedding.

It is well known that, in bluff body flows, oscillations can lead to a lock-in (lock-in) of the vortex shedding frequency, fv, with the body’s motion frequency, fp.

In order to investigate the susceptibility of airfoil to lock-in, the frequency ratio r (r=fp/fv0) has been varied around r=1.

The lock-in region boundaries have been proposed, and an analysis of the effect of the oscillation amplitude has been conducted.

The lock-in map obtained suggests that, for the vibration amplitude considered, the risk of vortex-induced vibration is more significant in the regions of α≈40° and α≈140°, i.e., for shallower characteristic lengths.

Finally, a lumped parameter wake oscillator model has been proposed for pitching airfoils.

This simple model is in qualitative agreement with the CFD results.