Loads are too low or there are unexplained spikes when the dynamic stall models are active but the results are reasonable when the dynamic stall models are deactivated
Bladed versions affected: all Bladed versions adopting two-point approximation method.
Date of last article update: 24 November 2022
One of the reasons this problem occurs is that the gradient calculations needed for the dynamic stall models are not accurate (see this link). This occurs most of the time due to gradient changes in the assumed linear region for Bladed to compute the data. As an illustration, see figure below where the gradient changes in the important area between points A and B. Some airfoils indeed have these characteristics, e.g., to introduce an aerodynamic breaking.
For the time being, two solutions can be proposed:
(1) If the load analysis is ensured not to deal with these small angles of attack, i.e. calculations never reach these angles, then the polar data in this area can be modified. A simple edit can be done by fitting a linear curve to match the correct polar gradient, then extrapolate the lift data below point B.
(2) If the assumption in point (1) does not hold true, then dynamic stall calculations may be deactivated in this blade region.
This issue is fixed from Bladed 4.13 using a newly implemented "linear fit gradient method" described here: Linear fit gradient method
