Some comments on experimental results of three lift controllers for a wind turbine blade section using an active flow control

This paper considers three control schemes to extract maximum power by maintaining the highest aerodynamic lift value using local blade section control. Robust PID, model-free, and adaptive super-twisting controllers are proposed. It is concluded that model-free control is a good compromise in case of saturation constraints.

I value the experimental validations that have been performed, showing the performance of the three controllers. Furthermore, I appreciate that you do not draw hard conclusions that one controller type is superior to the others. The introduction should be improved, discussing what has been done in the field of local blade section control and referencing other work. Furthermore, the motivation for choosing these three controllers should be improved upon. The presentation of results should improve, and the description of how you arrive at the controller calibrations should be better.

I advise a major revision of this paper before it can be considered for publication in WES.

MAJOR COMMENTS:

- I think the paper title is weak because of the first three words: "Some comments on." Can you just leave these words out?

- There has been done quite a lot of research on local blade section control. Please extend your introduction by discussing and referencing these works and positioning your work in relation to what has already been done.

- The introduction does not state anything about the outcome of this research, please include this.

- The introduction should clearly state the contributions. Is it that you evaluated three different controller types on an already existing test bench?

- The introduction should make clear why you chose these three controllers. Now feels a bit random. Also, these controllers are not well described in the introduction.

- 2.3: It is not clear to me whether you can individually open/close each of the holes in the blade (you say something about multiple solenoid valves?). Maybe indicate and make it clear using a figure?

- Section 3 is not introduced, you directly begin with 3.1. 

- 3.1: So you consider a SISO system and no actuation of individual holes? I.e.: Output lift force (y), that needs to be controlled (follow a reference lift, y*) by a pressure input (u)? Be more clear on this.

- The performance that you can attain with each of the controller types is very dependent on the calibration and implementation of the controller. I think it is hard to conclude that one of the controllers is superior with respect to the other controllers, using your current approach. That is, how do you know for sure that you cannot attain the performance levels of the other controllers, simply by a more optimal calibration of the gains? Please provide more discussion and insights on this point. What would your approach be to get the most optimal performance of all the controller so that you can draw these conclusions?

MINOR COMMENTS:

- Captions underneath figures need to be more extensive, so that figures are interpretable independent from the main text.

- Subscripts that are not indices should be nonitalic, e.g., K_\mathrm{p}