This paper describes numerical studies conducted for one promising active flow control (AFC) technology for high-lift configurations, namely the pulsed blowing through slots. The selected configuration is a 2D high-lift airfoil, known as DLR-F15, and represents the state of the art for transonic turbulent airfoil for modern civil transport aircraft. The slot-actuators are applied on the suction side of the trailing edge flap to prevent local flow separation. The studies are addressed to evaluate the feasibility of unsteady RANS simulations to reproduce the trends discovered in experiments. Within the European framework of JTI-Clean Sky, numerical and experimental studies for low speed AFC are addressed. In this project, a computational study was performed at ONERA and DLR for verifying their standard approaches for AFC with available time-averaged experimental data and performing a direct comparison of computed flow unsteadiness and its implications. The results for moderate angles of attack with and without control show that the numerical predictions are in good agreement with the experiment and in between each other provided that the laminar to turbulent boundary layer transition on the flap is considered.