Aerodynamics and aero-acoustics of rectangular planform cavities Part IIIC: Alleviation of Unsteady Flow Effects – Acoustic suppression using active devices
ESDU 09001 provides information on the use of active devices for acoustic suppression in rectangular planform cavities. Active devices require an input of external energy in some form@ unlike passive devices (see ESDU 08012)@ which require no external energy input. The wide range of active devices employed in wind-tunnel tests@ flight tests and CFD simulations is conveniently separated into four broad classes: flap@ jet@ plasma and laser actuators. The various actuators in each of these four classes@ their design and modes of operation@ together with the References pertaining to their use in the acoustic suppression of cavities are discussed. The two modes of operation for active actuators@ i.e. open-loop and closed-loop@ together with their advantages and disadvantages@ are also discussed in general terms. Open-loop and closed-loop control are further considered in some detail@ with a historical overview of the work followed by a general assessment of the effectiveness of the more successful devices. Open-loop control is dealt with in terms of steady operation using mass injection (jets@ micro-jets and micro-slots) and pulsed operation (flaps@ jets@ and plasma and laser actuators). Further consideration of steady versus pulsed operation is given@ and the contentious role of pulsing in acoustic suppression is dealt with. Closed-loop control using piezo-electrically driven flaps@ plasma actuators and synthetic jets is discussed@ including the important role of the control methodology@ which can have a significant influence on the suppression effectiveness. Currently@ actuators suitable for use in closed-loop control (at least for the small-scale models tested) only have sufficient authority to control cavity acoustics at low free-stream Mach numbers (less than 0.5)@ and almost all of the work was carried out using synthetic jets in the form of voice-coil drivers. Other actuator types in the form of piezo-electrically driven flaps and plasma (dielectric barrier discharge) actuators are also considered. It is concluded that in open-loop@ steady mass injection using spanwise rows of jets@ either large diameter or micro-jets or micro-slots can provide good acoustic suppression@ both tonal and broadband@ at high subsonic and low supersonic Mach numbers. Disadvantages are the relatively high mass flow rates required and the problems of adapting to off-design conditions in open-loop. Low-frequency pulsing has no benefit. Tests using high-frequency pulsing also suggest little or no benefit@ although CFD simulations suggest otherwise@ given an ideal slot resonator (yet to be designed). In closed-loop@ no current actuator is wholly satisfactory@ even for the low Mach numbers at which they have reasonable authority. There is a need for an actuator able to control cavity acoustics at high subsonic and low supersonic Mach numbers@ and of a type suitable for use in a fully active closed-loop system. Only then will the full potential of such a system@ in terms of its adaptability to changing flow conditions with a low power consumption@ be achieved. Four Appendices provide background information on various topics of relevance to active devices. Appendix A deals with the flow structures and operating modes of under-expanded supersonic jets in relation to Hartmann-Sprenger resonance tubes. Also included are the operating frequency and other properties of pulsed micro-jets. Appendix B describes the A* actuator@ capable of applying pulsed flow to a micro-jet using a piezo-electric stack in a closed hydraulic system. Appendix C deals with the relationships between the various flow parameters used for mass injection@ while Appendix D gives a brief outline of low-dimensional modelling as used in closed-loop feedback control of cavity acoustics.