INTRODUCTION

This Data Item is the second in the series of four Items (Parts IIIA to IIID) comprising Part III of the wider five-part series (Parts I to V) dealing with the aerodynamics and aero-acoustics of rectangular planform cavities, see Section 2.2 in Part I (Reference 108). The general subject area of Part III is the alleviation of unsteady flow effects and is introduced in Part IIIA (Reference 110). The present Data Item (Part IIIB) deals with acoustic suppression using passive devices. The use of active devices is considered in Part IIIC (Reference 111), while the alleviation of store deployment conditions is dealt with in Part IIID (Reference 112).

The very wide range of passive devices employed in both wind-tunnel and flight tests can be separated into seven main types. In the discussion of the various types tested, even the less successful ones are mentioned, since ineffectiveness in one situation should not be taken to imply ineffectiveness in all situations - although that may be true of some. In this Item the particular types of passive device (or actuator*) covered are fences or spoilers (Section 3), vortex generators (Section 4), front and/or rear wall geometry changes (Section 5), the rod-in-crossflow (Section 6), passive resonance tubes (Section 7), Helmholtz resonators (Section 8), and baffles and absorptive materials (Section 9). The work is prefaced, in Section 2.1, by a consideration of the various means whereby passive devices affect the cavity shear flow. The first four device types, on which a considerable amount of research has been carried out, are tackled in Sections 3 to 6 in a common fashion involving firstly a historical overview of the research, secondly an assessment of the effectiveness of the device in terms of acoustic suppression, and thirdly whatever can be said concerning the drag of the combined cavity-plus-device compared to the cavity baseline drag, i.e. the drag of the cavity without the device. Cavity baseline drag (gross drag), and its relationship with the drag (net drag) predicted using ESDU 00006 (Reference 106) for closed flow and ESDU 00007 (Reference 107) for open and transitional flows, is outlined in Section 6.2 of Part IIIA (Reference 110). Each of Sections 3 to 6 contains a list of overall conclusions concerning the device under consideration. The final three device types, considered in Sections 7 to 9, have had relatively little research attention compared to the first four, and so are dealt with in a more general way, with conclusions being given where possible.

Section 10 contains the overriding conclusions concerning the use of each passive device and may be consulted prior to a consideration of the reviews of device performance and the fuller conclusions set out in Sections 3 to 9. In summary, carefully designed spoilers and rear wall ramps have been found to be successful cavity noise suppression devices at subsonic speeds, and satisfactory at low supersonic speeds, as evidenced by their widespread past and present use on military aircraft. The newer rod-in-crossflow concept shows considerable promise in modal and background noise alleviation over a wide Mach number range. Of the other devices, the small amount of available data suggests that rearward-facing steps ahead of the front wall, vortex generators, Helmholtz resonators and acoustic liners could be successful in appropriate situations, or perhaps in combination with another device. The particular question of the intrinsic drag of the various devices and the overall drag of a cavity before and, especially, after suppression requires more research.

Finally, the inclusion of the rod-in-crossflow in the device types considered, being a high frequency (HF) suppression device, required more information on the physical processes involved, and Appendices A and B outline those processes, while Appendix C considers the characteristic signature of effective (and ineffective) HF devices.

* It might reasonably be thought that an actuator would be capable of some form of actuation other than deployment. However, in the literature the term "actuator" is commonly used for any device, whether active or passive.