Sliding mode control applied to a square-back Ahmed body

We consider the first closed-loop separation control experiment on an Ahmed body using a robust, model-based strategy called “sliding mode control” (SMC). The main objective of the control is to reduce and further maintain the aerodynamic drag of the square-back Ahmed body flow to an arbitrary value. The Reynolds number considered, based on the body height and free-stream velocity, is Reh=9×104. The wake flow is manipulated by a slotted jet placed on the top trailing edge, combined with a predefined angle direction. The flow modifications are sensed by a drag balance. Base pressure and lift measurements are also performed in real-time. The interactions between the air jet actuator and the mean near-wake flow are depicted by means of Particle Image Velocimetry. In order to evaluate the efficiency of the closed-loop strategy, periodic forcing is first investigated. Continuous blowing is initially used to directly influence the recirculation area and hence achieve a reduction in the drag. A maximum drag reduction of approximately 8% is accomplished with steady blowing. Because steady blowing leads to the highest energy consumption scheme, this strategy is only considered as a reference result. The second type of control involves periodic forcing at different frequencies. The influence of these frequencies on the near-wake and the overall drag is examined in details. Based on the open-loop results, a simplified single-input/single-output (SISO) model of the flow response is proposed and sliding mode control is applied to maintain the drag level at an arbitrarily fixed value. Finally, an experiment is conducted to show the ability of the controller to reject a disturbance, corroborating the robustness and efficiency of this control approach; the limitations of this control approach are also discussed. This kind of controller is found to be able to reduce and maintain remarkably the drag to a desired set-point regardless significant external flow perturbations, suggesting that it may be applicable in multiple experimental and industrial contexts.