Closed-loop control of laminar separation was investigated numerically for a low-pressure turbine blade and an airfoil at relatively low operating Reynolds-numbers. Open-loop investigations show that within a certain frequency range disturbances are amplified. This can be exploited for making the control more effective. The amplification is attributed to a hydrodynamic instability of the base flow. A simple closed-loop controller, where a downstream pressure signal is fed back to an upstream actuator, is shown to be as effective as an optimized open-loop controller. The simple closed-loop controller can be improved by continuously adjusting the controller parameters during the run time of the simulation using a minimization algorithm that minimizes a desired objective such as drag over lift and control effort. This allows for an optimization of the gain and phase of the feedback resulting in a better "synchronization" or coupling of the actuation and the flow dynamics.