Active Control For Separated Flow By Lorentz Force

Active flow control (AFC) is a new, interdisciplinary study area that crosses control sci-ence and fluid mechanics. By injecting energy into the flow, AFC can control the separation and transition, and reduce the drags of vehicles by improving the flow stability, or reduce the pressure fluctuations, or reduce the radiation noises and other effects. Compared with the passive control, active flow control is more efficient and robust, and thus has broad ap-plication prospects. However, AFC also faces with more complex theocratical and practical problems than passive flow control.Applying electromagnetic force to to change the local flow field of weak electrical conductivity media, is a new active flow control approach. In this dissertation, two separated flow pattern, cylinder wake and high-attack-angle hydrofoil, controlled by the Lorentz force are studied. To fill the gap between theory and engineering, the model reduction, controller design and analysis issues are studied.Active flow control can be classified into two types, model-based and model-independent. Both types of AFC are studied in this dissertation. First, the model reduction study, obtained using the finite volume method of data flow analysis, based on the snapshot-style proper orthogonal decomposition and Galerkin projection method, the electromagnetic force was established under the reduced-order model for flow around a cylinder. We apply the improved particle swarm optimization algorithm for model parameter calibration, to improve the accuracy of the model reduction. Based on the reduced order model, and by applying nonlinear system theory, a dissipative controller is designed.Second, in the study of the model-independent control method, we propose a novel swarm-intelligent based control algorithm-particle swarm optimisation based extremum seeking control algorithm (PSOESC). The proposed PSOESC algorithm can steer the sys-tem state to the vicinity of the best point through online measurement and optimization. Furthermore, to enhance the practicality of the algorithm, we propose a state sequence re-arrangements algorithm to handle the random, swarm characteristics of the PSO. In the proposed algorithm, the control process is smoothed by reshuffle-then-insertion procedure to reduce the oscillation and to avoid large control gains. We also propose an accelerat-ing algorithm for PSOESC to accelerate the convergence rate of PSOESC for limit cycle control, by employing the periodical nature of the limit cycle and the characteristics of PSO.Finally, we conduct a theoretical analysis of the stability of PSOESC in a noisy environ- ment by stochastic differential equations theory. Sufficient condition is given to guarantee the convergence of PSO in noisy environment. We also analyse the influence of the objective function and the noise intensity on the optimisation accuracy.Numerical analysis and model experiments show that the proposed control strategies of the two studied active flow control method can both achieve better results than literatures. Closed-loop control can improve the efficiency and reduce the electrochemical corrosion. Using the proposed model reduction method, the flow separations are controlled by the electromagnetic force. The obtained reduced order model can accurately describe the main flow characteristics, and has high robustness. The control law designed based on the reduced order model can effectively reduce the flow separation, thereby reducing the resulting flow around a cylinder oscillating transverse force and resistance, or increase the airfoil lift-drag ratio. The proposed PSOESC can obtain similar results, but more robust.