Ronald E. Hanson

Feedback control of slowly-varying transient growth by an array of plasma actuators

Closed-loop feedback control of boundary layer streaks embedded in a laminar boundary layer and experiencing transient growth, which is inherent to bypass boundary layer transition, is experimentally investigated. Streaky disturbances are introduced by a spanwise array of cylindrical roughness elements, and a counter disturbance is provided by a spanwise array of plasma actuators, which are capable of generating spanwise-periodic counter rotating vortices in the boundary layer. Feedback is provided by a spanwise array of shear stress sensors. An input/output model of the system is obtained from measurements of the boundary layer response to steady forcing, and used to design and analyze a proportional-integral controller, which targets a specific spanwise wavenumber of the disturbance. Attention is directed towards a quasi-steady case in which the controller update is slower than the convective time scale. This choice enables addressing issues pertinent to sensing, actuation, and control strategy that are...

Effect of Plasma Actuator Excitation for Controlling Bypass Transition in Boundary Layers

The response of a zero-pressure-gradient laminar boundary layer to forcing by a spanwise array of plasma actuators is investigated experimentally. The plasma actuators used in this study are designed to produce counter-rotating vortex pairs in a spanwise periodic arrangement inside the boundary layer. It was demonstrated by Hanson et al., in related studies, that this plasma actuator array can be successfully used to control the transient growth instability occurring in a Blasius boundary layer. However, it was also demonstrated that the control effectiveness could be greatly affected by the actuator array geometry, which affects the modal content and energy distribution of the resulting disturbance. The focus of the present work is on studying the effect of excitation signal parameters, including the driving frequency and voltage, on the disturbance introduced by the plasma actuators. It is demonstrated that the disturbance energy increases with increasing frequency and voltage, consistent with previous studies of the thrust force or induced velocity from simpler actuator geometries. A crucial finding of the present study is that the individual modes produced by the actuator array are not affected in the same proportional manner by varying the voltage. The control disturbance approached a pure spanwise-harmonic disturbance for increasing voltage. In contrast, the excitation frequency did not have a discernible effect on the relative magnitude of each mode produced by the actuator array. These results highlight the complexity of the response of the boundary layer to the various actuation parameters and have critical implications for the practical integration of these actuators as part of a closed-loop control system.

Steady-state closed-loop control of bypass boundary layer transition using plasma actuators

The overarching objective motivating this work is a physical demonstration of modelbased, closed-loop control of bypass transition using plasma actuators. The present work is concerned with the closed-loop control of bypass transition using plasma actuators. This manuscripts extends the work by Hanson et al., 1,2 who demonstrated that a spanwise array of plasma actuators can produce significant attenuation of the transient growth disturbances introduced by roughness elements. In the present work, the control loop is closed based on feedback from wall-shear stress measurements. The control signal is based on empirical modelling of the input/output flow response for several flow conditions. The latter is obtained for both the main disturbance, generated by a roughness-element array, as well as the control disturbance, forced using a spanwise plasma actuator array. The output is characterized using wall-shear-stress measurements downstream of the actuation location. The controller is designed to minimize the residual disturbance energy in the output measurements at the target instability spanwise wavenumber. The control model developed in this work was applied to three steady disturbance cases, including one that is outside of the parameter range for which the input/output model was developed. The closed-loop control model is shown to effectively attenuate the boundary layer disturbance by 1,end > 95% in each case, with the initial control iteration accounting for 1,1 > 89%.

Effect of Dielectric Degradation on Dielectric Barrier Discharge Plasma Actuator Performance

The objective of this study was to investigate a method to monitor the health of plasma actuators. Common actuator dielectric materials were used in this research, namely Kapton polyimide tape, and those manufactured from printed circuit boards (PCB) having a dielectric of FR-4 composite material. Actuators utilizing these dielectric materials are known to visually degrade during operation. However, a quantitative measure of the degradation of these actuators has not yet been examined. A parametric study was used to identify the role of operational parameters, actuator excitation voltage and frequency, on the rate of the degradation process over long durations. Measurements of the actuator power consumption and capacitance were used quantify the change in actuator health over time. It was shown that both the power consumed and effective capacitance of the plasma actuator may increase considerably during the degradation process, which was related to the thinning of the dielectric layer. Specifically, only the top exposed layer of Kapton was affected, whereas the underlying adhesive layer remained intact. The variation in actuator power consumption and capacitance could be mitigated by considering actuators with an increased number of Kapton tape layers. For actuators having a dielectric of the FR-4 composite material, the presence of the plasma affected the acrylic epoxy however, the woven fibreglass remained in tact. It was shown that over the duration of the experiments the power consumption and effective capacitance decreased for these actuators. Measurements of the wall-jet induced for each actuator type was used to monitor the momentum production over time. The momentum production decreased for the FR-4 actuators over time. For the Kapton polyimide tape actuators, momentum production could increase.

Effect of DBD Plasma Actuator Geometry and Excitation for Producing Transient Growth Modes in a Laminar Boundary Layer

This work is concerned with the response of a Blasius boundary layer to plasma actuators for the purpose of using these devices in bypass transition control. The structure of actuator-induced boundary layer streaks is measured using hot-wire anemometry over a streamwise distance of a approximately 100 boundary layer thicknesses, based on a thickness of 4 mm at the point of actuation. Spanwise power spectra of the streamwise disturbance velocity show that 90% of the actuator-induced disturbance energy is contained within a mode related to the spanwise spacing of the exposed actuator electrodes, and its first 3 harmonics. The modal content and corresponding streamwise growth characteristics are discussed for four plasma actuator geometries over multiple excitation voltages in the 3:5 5:5 kVPP range. Actuator output was found to have a closer correlation to actuator power consumption than the excitation voltage. A common relationship between disturbance energy and power consumption was found among actuators of different dielectric thickness and similar electrode geometry. Degradation of the actuators was observed over the course of their running lifetime, causing test cases to diverge gradually from this relationship.

Control of transient growth induced boundary layer transition using plasma actuators

This study investigates an actuation scheme that can be readily implemented and integrated as part of a feedback control system in the laboratory for the purpose of negating the effect of the transient growth instability in order to delay boundary layer transition. The actuators investigated here consist of a spanwise array of symmetric plasma actuators, which are capable of generating spanwise periodic counter-rotating vortices. Two different actuator geometries are investigated, resulting in a reduction of the total disturbance energy by 46% and 68%. It is demonstrated that the control authority of an actuator can be significantly improved by optimizing the geometry of the array.