A wind turbine blade includes a porous window defined in the suction side of the blade. The porous window is permeable to airflow from within an internal cavity of the blade through the suction side. A deployable cover member is configured with the porous window and is variably positionable from a fully closed position wherein airflow through the porous window is blocked, to a fully open position wherein airflow is established through an entirety of the porous window. An actuating mechanism is disposed within the internal cavity of the blade and is configured with the cover member to move the cover member between the fully closed and fully open positions in response to a control signal.