Airfoils.- Transitory Control of Dynamic Stall on a Pitching Airfoil.- Unsteady Lift Suppression with a Robust Closed Loop Controller.- Active Flow Control on a S10 Glider Configuration.- Numerical Investigation of Active Flow Control Applied to an Airfoil with a Camber Flap.- On Amplitude Scaling of Active Separation Control.- Lock-On to a High-Lift State with Oscillatory Forcing in a Three-Dimensional Wake Flow.- Active Flow Control on an Industry-Relevant Civil Aircraft Half Model.- Numerical Investigation of Spatially Distributed Actuation on a Three-Element High-Lift Configuration.- Robust Closed-Loop Lift Control on an Industry-Relevant Civil Aircraft Half Model.- Turbomachines.- Closed Loop Blade Tone Control in Axial Turbomachines by Flow Induced Secondary Sources in the Blade Tip Regime.- Turbofan Tone Noise Reduction by Flow-Induced Unsteady Blade Forces.- Experimental AFC Approaches on a Highly Loaded Compressor Cascade.- Robust Control in Turbomachinery Configurations.- URANS Simulations of Active Flow Control on Highly Loaded Turbomachinery Blades.- Bluff Bodies.- Application of Active Flow Control on Generic 3D Car Models.- Simulation of Active Drag Reduction for a Square-Back Vehicle.- Model Predictive Control for a 2D Bluff Body Under Disturbed Flow Conditions.- Burner and Cavities.- Closed-Loop Control of an Unstable Open Cavity.- A Zero-Mach Solver and Reduced Order Acoustic Representations for Modeling and Control of Combustion Instabilities.- Modeling the Fuel/Air Mixing to Control the Pressure Pulsations and NOx Emissions in a Lean Premixed Combustor.- Model Reduction and Feature Extraction.- Reduced Order Modeling Using Proper Orthogonal Decomposition (POD) and Wavenet System Identification of a Free Shear Layer.- Turbulence Control Based onReduced-Order Models and Nonlinear Control Design.- A New Discretization Framework for Input/Output Maps and Its Application to Flow Control.- Extraction of Coherent Structures from Natural and Actuated Flows.- Optimal Flow Control.- Optimized Waveforms for Feedback Control of Vortex Shedding.- Optimal Boundary Control Problems Related to High-Lift Configurations.
"OptimizedWaveforms for Feedback Control of Vortex Shedding (p. 391-392)
Won Tae Joe, Tim Colonius, and Douglas G. MacMynowski
Abstract. Optimal control theory is combined with the numerical simulation of an incompressible viscous flow to control vortex shedding in order to maximize lift. A two-dimensional flat plate model is considered at a high angle of attack and a Reynolds number of 300. Actuation is provided by unsteady mass injection near the trailing edge and is modeled by a compact body force.
The adjoint of the linearized perturbed equations is solved backwards in time to obtain the gradient of the lift to changes in actuation (the jet velocity), and this information is used to iteratively improve the controls. The optimized control waveform is nearly periodic and locked to vortex shedding. We compare the results with sinusoidal open- and closed-loop control and observe that the optimized control is able to achieve higher lift than the sinusoidal forcing with more than 50% lower momentum coefficients.
The optimized waveform is also implemented in a simple closed-loop controller where the control signal is shifted or deformed periodically to adjust to the (instantaneous) frequency of the lift fluctuations. The feedback utilizes a narrowband filter and an Extended Kalman Filter to robustly estimate the phase of vortex shedding and achieve phase-locked, high lift flow states.
1 Introduction
Previous work on flow control over an airfoil has used periodic excitation, such as unsteady mass injection or synthetic jets, to show that the oscillatory addition of momentum can delay boundary layer separation and reattach the separated flow [7, 8], or delay dynamic stall on a rapidly pitching airfoil [10]. Unsteady actuation was also shown to change the global dynamics of vortex shedding of post-stall flow, leading to higher unsteady lift than the natural shedding [13, 17].
In this paper, we investigate a simple model of a purely translating flat plate at high angle of attack at a Reynolds number of 300, where strong, periodic vortex shedding occurs. A small amplitude body force intended to mimic oscillatory mass injection is applied near the trailing edge in order to modulate the vortex shedding. We first consider open-loop control utilizing sinusoidal waveforms. It is observed that open-loop forcing can significantly amplify the lift, but feedback is required to tune the phase of actuation to a particular phase of the measured lift in order to lock the forcing with the phase shift associated with the highest period-averaged lift.
Rather than optimizing the phase of the control relative to the lift using only sinusoidal waveform, we investigate the possibility of optimizing the lift using more general (non-sinusoidal) actuation waveforms.We utilize a gradient-based approach that has been used previously in simulations to reduce the turbulent kinetic energy and drag of a turbulent flow in a plane channel [4], or to reduce free-shear flow noise[16]. Given the DNS for a particular actuator waveform, we solve the adjoint of the perturbed linearized equations backward in time to determine the sensitivity of the lift to the actuator input, and subsequently use this information to iteratively improve control."
