Control Optimization Systems Of Turbulent Jet And Flow Around An Airfoil

Flow control is a very important research area in fluid mechanics.Most of the flow control studies focus on passive or open-loop control.However,passive control requires permanent fixtures which are not readily modified or removed.Although open-loop con-trol can easily change the control parameters artificially.In the case of a varying operating condition,the open-loop control cannot optimize the control parameters automatically and instantaneously.Therefore,it is important to design a closed-loop control system which can optimize control parameters automatically.Flow separation and jet mixing are typical turbulence phenomena.How to suppress the flow separation and increase the jet mixing efficiency is a research hotspot.Thus,the objective of this research is to develop different closed-loop control systems for flow separation control and jet mixing enhancement.A closed-loop flow separation control system is firstly proposed to manipulate flow around the NACA 0015 airfoil,which includes an airfoil,closed-loop controller,load cell and a novel dielectric barrier discharge plasma actuator.Open-loop control experiments show that the novel plasma actuator can delay the stall angle by 5 degrees and increase the maximum lift coefficient by 9%.The closed-loop control uses slope seeking with an extended Kalman filter(EKF)to adjust the driving voltage amplitude of the plasma actuator.Compared with the traditional slope seeking algorithm,the improved control algorithm can greatly improve the convergence speed and system robustness.The slope seeking control has made great progress in flow separation control,there-fore,this method is applied to jet mixing applications.In this study,a single-input-single-output(SISO)extremum-seeking control system for jet mixing enhancement based on a radially injected unsteady minijet was proposed.The jet centerline velocity decay rate K at 5 times jet diameters D downstream is used as the feedback signal,the minijet excita-tion frequency f_eor the mass flow rate Q of the minijet is used as the control signal.The extremum-seeking method is used to optimize f_eor Q independently.The system could find the optimal f_eor Q automatically.When an extremum-seeking with EKF method is applied,the system can significantly shorten the convergence time and improve the dynamic response of the control system.The SISO control system could only optimize one parameter.However,the operat-ing environment may change frequently in some applications,which may lead to different optimal control parameters.Thus,a dual-input-single-output(DISO)extremum-seeking control system has been developed to manipulate the turbulent round jet based on the unsteady minijet.It has been demonstrated that this system may obtain rapidly and si-multaneously the optimal f_eand Q of the minijet,thus achieving the maximum K,as confirmed by the open-loop control results.This technique is robust to the parametric un-certainty with respect to Re_Dwithin the tested range and adaptable when the initial control parameters are varied.It has been further found that the control performance is essentially independent of Re_D.Compared with the former studies,this DISO control system has a better control performance and can be extended to other applications easily.The extremum-seeking method is a linear control method,a nonlinear control method:machine learning control(MLC),is then used to optimize mixing of the turbulent round jet with an aim to improve the control performance.The flow is monitored with two hotwire sensors located at 5D and 3D downstream of the nozzle exit.The mixing per-formance is monitored with mean velocity at the 5D downstream of the nozzle exit.A reduction of this velocity correlates with increased entrainment near the potential core.MLC is employed to optimize sensor feedback,a general open-loop broadband frequency actuation and combinations of both.MLC has identified the optimal periodic forcing with a small duty cycle as the best control policy eventually.In addition,MLC results indicate that neither new frequencies nor sensor feedback improves mixing further under current experiment condition.The jet mixing results demonstrate the untapped potential of MLC in quickly learning optimal general control policies,even deciding between open-and closed-loop control.The convergence time of MLC is comparable with typical optimiza-tion of periodic forcing with two free parameters(frequency and duty cycle).However,it is much longer than that of the extremum-seeking control methods.In summary,different control methods have been used to build closed-loop control systems for flow separation control on an airfoil and jet mixing enhancement.The DISO control shows better control performance than the SISO control.The MLC gives the best control results compared with the other two linear control methods.Closed-loop control shows great potential in the field of flow control and can be further extended to other flow control areas.