Rating effective pitch rates for dynamic stall prediction

The delay in the onset of dynamic stall varies with the unsteadiness of the pitching kinematics. The unsteadiness of linear ramp-up pitching motions is characterised by the non-dimensional pitch rate. With increasing pitch rate, dynamic stall onset occurs at higher angles of attack and the temporal stall delay decreases. For sinusoidal motions, an effective unsteadiness parameter is introduced as the rate of change of angle of attack when the static stall angle is exceeded. For large amplitude pitching motions around a mean angle of attack close to the static stall angle, the effective unsteadiness predicts the dynamic stall delay well as non-linear variations in the angle of attack remain small. Here, we explore the effect of highly non-linear pitching kinematics on the onset and delay of dynamic stall using time-resolved force and flow-field measurements. We prescribe non-linear pitching profiles with a constant mean pitch rate but different values of the pitch rate and the pitch acceleration around the static stall angle. The non-linearity of the kinematics significantly influences the dynamic stall development and delay. New candidate effective unsteadiness parameters are proposed to characterise the effect of pitch angle accelerations on the dynamic stall onset and delay.