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.