Proceedings of the Institution of Mechanical Engineers,Part I: Journal of Systems and Control Engineering

Several fluid flow problems related to propulsion and power generation exhibit strong acoustic resonances. Produced due to interactions of the acoustics with other underlying unsteady mechanisms such as unsteady heat release or shear flow instability, these resonances manifest as large and sustained pressure oscillations. Active control of such resonances has been shown to be highly effective, resulting in a dramatic reduction in the acoustic resonances using a very small fraction of the system energy. In this paper, a specific fluid flow problem is discussed, that of supersonic impinging jets, and the control of the acoustic resonances that are produced by these jets, via detailed experimental investigations. The actuator used for active control consists of a pulsed microjet, by which it is shown that in a scaled supersonic experimental facility acoustic resonances can be reduced, utilizing a fraction of the mass flowrate needed with a steady microjet. Several parameters related to pulsed injection are varied to evaluate their impact on the resonances, and it is found that duty cycle and pulsing frequency have the most dominant effect on the jet noise as well as on the overall flow field. System identification based models of the uncontrolled and controlled jet are derived to explain the results obtained. The effect of low-frequency pulsing on resonance suppression is explained via the introduction of a controller in the closed-loop whose parameters vary non-linearly with the pulsing parameters.

Turbulent flow has a significantly higher drag than the corresponding laminar flow at the same flow conditions, and therefore incurs a significant penalty of increased fuel consumption due to the extra thrust required. One possible way of decreasing the drag is to apply surface suction to delay the transition from laminar to turbulent flow. In this paper an aerofoil with three non-overlapping panels covering up to 20 per cent of chord for boundary layer transition control is considered. The problem is complicated by the fact that panels can change both their positions and lengths. The complexity of the optimization problem is such that it is not practical to perform the investigation using a single processor. A constrained global parallel algorithm based on a combination of deformed configuration methods and a controlled random search method is developed. It is shown that for the problem considered, good solutions can be found efficiently.

To speed up gradient estimation in a slope-seeking controller two different modifications are proposed in this study. In a first approach, the gradient estimation is based on a locally identified black-box model. A further improvement is obtained by applying an extended Kalman filter to estimate the local gradient of an input—output map. Moreover, a simple method is outlined to adapt the search radius in the classical extremum- and slope-seeking approach to reduce the perturbations near the optimal state. To show the versatility of the slope-seeking controller for flow control applications two different wind tunnel experiments are considered, namely with a two-dimensional bluff body and a generic three-dimensional car model (Ahmed body).

The feedback control of laminar plane Poiseuille flow is considered. In common with many flows, the dynamics of plane Poiseuille flow is very non-normal. Consequently, small perturbations grow rapidly with a large transient that may trigger non-linearities and lead to turbulence, even though such perturbations would, in a linear flow, eventually decay. This sensitivity can be measured using the maximum transient energy growth. The linearized flow equations are discretized using spectral methods and then considered at one wave-number pair in order to obtain a model of the flow dynamics in a form suitable for advanced control design. State feedback controllers that minimize an upper bound on the maximum transient energy growth are obtained by the repeated solution of a set of linear matrix inequalities. The controllers are tested using a full Navier—Stokes solver, and the transient energy response magnitudes are significantly reduced compared with the uncontrolled case.

This paper presents actuator models for fluidic thrust vectoring and circulation control and they are used in the design of a robust controller for an unmanned air vehicle. The pitching and rolling moments for the aircraft are produced through the use of a co-flow fluidic thrust vectoring arrangement at the wing trailing edges. Experimental results for the co-flow actuators are used to derive mathematical models and their performance is compared with conventional control surfaces. For the controller design, nonlinear dynamic models are approximated by a simplified linear parameter varying (LPV) model. The polytopic nature of the controller is exploited to reformulate the LPV controller design problem into a μ-synthesis problem. The LPV controllers exhibit superior stability properties over the entire operating region, when compared to conventional gain-scheduling schemes.

The induced flow field in the vicinity of a single, symmetric, AC plasma actuator has been studied in initially static air at atmospheric pressure. Hot-wire and cold-wire anemometry, together with flow visualization, were used to observe the temporal and spatial structure of the induced flow. The plasma discharge is pulsed on a millisecond scale and the flow forms a series of pulsed wall jets, which can be maintained indefinitely, similar to a synthetic jet. It was observed that the plasma actuator initiates a pair of vortices at the instant of plasma creation, moving at 25° to the surface. After an initiation period, the plasma develops into a laminar wall jet. Thermal imagery has been used to estimate the surface temperature of the dielectric sheet during plasma operation.

This paper aims to develop understanding of the systems costs associated with the application of flow control systems to civil transport aircraft based on the use of electrically powered synthetic jet actuators (SJAs). The study is based on the development of a low-order mass model using estimated power specific masses of generation, management, distribution, and conversion subsystems; application of existing empirical rules for application of pneumatic boundary layer mixing flow control devices to determine the required fluid power for an A320 case study application; and characterization and optimization of lab-based SJA technology to establish realistic estimates for power conversion efficiency and actuator maximum authority. The peak velocity obtained from a velocity-optimized synthetic jet actuator was 130 m/s, at a corresponding power efficiency of 7 per cent. The highest power efficiency obtained was 14 per cent, corresponding to a peak velocity of 70 m/s. The power specific mass for the overall flow control system considered for the A320 application is estimated to be around 1 kg of system mass per kW of electrical power required, of which around 50 per cent is due to power generation and 30 per cent is due to power conversion (actuation).

Synthetic jets have been proposed for efficient flow control due to their unique zero-net-mass-flux feature and the conceptual simplicity of the system. However, the effectiveness of the control is heavily dependent upon the complicated flow interaction of the jets with the flow stream on which the jet is applied. To understand the flow control mechanism better, a numerical simulation method is investigated in this paper. Using a computational fluid dynamics technique, the detached eddy simulation for an isolated synthetic jet for flow control is presented. A cubic-root filter is introduced with continuous eddy viscosity variation at the switching point between the Reynolds-averaged Navier—Stokes solution region and the large eddy simulation region. The intention is to explore the effects of unstructured grids for such a filtering strategy. A dissipation-controlled Roe scheme is applied and a dynamic grid technique is employed to implement the periodically moving diaphragm. The results obtained are compared with the experimental data and a previous study using structured grids.

Flow control techniques for increasing the rate of jet mixing in axisymmetric nozzle flows have been investigated. A combination of water tunnel and high-speed airflow facilities is used to assess the near-field jet behaviour. Solid tabs, steady fluid tabs (i.e. discrete radially discharged control jets located close to the core jet exit), and pulsed fluid tabs are compared. The effect of fluid tab velocity amplitude, pulse rate, and pulse phase are studied using open-loop control. The measurements indicate that fluid tabs generate a similar streamwise vortex formation process (and hence display increased mixing) as previously observed in solid-tabbed nozzle flows. In incompressible testing the mixing effectiveness with a pair of pulsed fluid tabs 180° out-of-phase was as good as a twin solid tab nozzle for a control jet flowrate of only 0.5 per cent of the primary (core) jet flow. In preliminary high-speed testing similar benefits of fluid tabs over solid tabs were observed. Further study of pulsed fluid tabs is recommended; they have the attractive performance benefit that they can be easily switched off when not needed and offer increased flexibility as the basis of an optimized active control jet mixing device.