Linear Quadratic Regulation Control for Falling Liquid Films

SIAM Journal on Applied Mathematics, Volume 84, Issue 3, Page 940-960, June 2024.
Abstract. We propose and analyze a new methodology based on linear-quadratic regulation (LQR) for stabilizing falling liquid films via blowing and suction at the base. LQR methods enable rapidly responding feedback control by precomputing a gain matrix, but they are only suitable for systems of linear ordinary differential equations (ODEs). By contrast, the Navier–Stokes equations that describe the dynamics of a thin liquid film flowing down an inclined plane are too complex to stabilize with standard control-theoretical techniques. To bridge this gap, we use reduced-order models—the Benney equation and a weighted-residual integral boundary layer model—obtained via asymptotic analysis to derive a multilevel control framework. This framework consists of an LQR feedback control designed for a linearized and discretized system of ODEs approximating the reduced-order system, which is then applied to the full Navier–Stokes system. The control scheme is tested via direct numerical simulation (DNS) and compared to analytical predictions of linear stability thresholds and minimum required actuator numbers. Comparing the strategy between the two reduced-order models, we show that in both cases we can successfully stabilize towards a uniform flat film across their respective ranges of valid parameters, with the more accurate weighted-residual model outperforming the Benney-derived controls. The weighted-residual controls are also found to work successfully far beyond their anticipated range of applicability. The proposed methodology increases the feasibility of transferring robust control techniques towards real-world systems and is also generalizable to other forms of actuation.