Abstract
Unsteady airfoil experiments were conducted in a high-pressure wind tunnel at chord Reynolds numbers of Rec = 3.0 × 106. A moderately thick NACA0021 airfoil was pitched from rest beyond the static stall angle in six individual ramp tests with increasing and decreasing angles of attack. The variant types of motion of the pitching maneuvers were characterized by constant angular velocity, angular acceleration and angular jerk, respectively. The ramp-up experiments revealed a substantial and time-dependent excess of the aerodynamic forces from static values in all three test cases and exhibited a distinct time delay as a consequence of the variant motion types. Similarly, the ramp-down experiments were largely impacted by the progression of the pitching motion, resulting in pronounced differences in the temporal development of lift and drag. Results are shown as time series of integrated forces and surface pressure distributions.
Highlights
During operation, wind turbines are perpetually exposed to unsteady conditions of varying characteristics
The variant types of motion of the pitching maneuvers were characterized by constant angular velocity, angular acceleration and angular jerk, respectively
Phase averaged integrated forces and pressure distributions of a pitching airfoil were experimentally obtained at a chord Reynolds number of three million
Summary
Wind turbines are perpetually exposed to unsteady conditions of varying characteristics. The dynamic and unsteady inflow can be characterized as a sudden change in inflow velocity and in angle of attack, often in excess of the static stall angle, which results in a large lift and drag overshoot. This is referred to as dynamic stall, which is often described as an open research question, especially at high Reynolds numbers [1]
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