I found quite detailed explanation about Drag Reduction System (DRS) in F1 cars (not in FSAE cars), and it helped me to understand @dschroeder problem, as I was a patient reader of that long thread, but not grasping the subtleties. It could help other thread readers to understand how important correct meshing and simulation is in that case.
Here is driver ajustable rear wing:
Adjustable wing in closed, non-active position, with air streams ideally connected with wing contours and without any turbulences:
Adjustable rear wing in active - deployed position. Stall condition appeared. Airflow around the wing separates from the surface of the wing decreasing its effectiveness, reducing downforce and drag in the same time:
Article itself is quite long ~10 pages, with all rationale of F1 race tactic with DRS gear. Here is the link: F1 Movable Rear Wing