Skin-friction drag reduction via robust reduced-order linear feedback control

A successful application of a linear controller to a two-dimensional channel flow is presented. An optimal and robust reduced-order linear feedback controller is derived by using multi-variable linear-quadratic-Gaussian synthesis, or, in modern term, H-2 synthesis, combined with model reduction techniques. This controller based on a reduced-model of the linearized Navier - Stokes equations is applied to suppress finite-amplitude near-wall disturbances in a channel flow at Re = 1500. The controller efficiently reduced near-wall disturbances obtaining a substantial drag reduction and eventually the flow is relaminarized.