A sensor device for measuring flow-separation on an aerodynamic element, including a number of compliant elements with aerodynamic and/or structural properties designed to allow flow-induced vibrational motion in an air current and a sensor-element designed to measure vibrations of the number of compliant elements is provided. Further provided is an aerodynamic element, e.g. a wind turbine blade or an airfoil, with such sensor device, a method for controlling the angle of attack of an aerodynamic element, a controlling device and a wind turbine.