This Data Item is the first 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, as summarised below:

Part I: Time-averaged Flow – ESDU 02008,

Part II: Unsteady Flow and Aero-acoustics – ESDU 04023,

Part III: Alleviation of Unsteady Flow Effects,

IIIA: Introduction – ESDU 08011,

IIIB: Acoustic suppression using passive devices – ESDU 08012,

IIIC: Acoustic suppression using active devices – ESDU 09001,

IIID: Store deployment – ESDU 09002,

Part IV: Overview of CFD methods – (Data Item to be issued),

Part V: Bibliography (up to 2020) – ESDU 16004.

The general subject area of Part III is the alleviation, or suppression, of unsteady flow effects and is introduced by the present Item (Part IIIA). Part IIIB deals with acoustic suppression using passive devices. The use of active devices is considered in Part IIIC, while the improvement of store deployment conditions is dealt with in Part IIID.

The Data Item gives general background information behind the need for the alleviation of weapons bay noise and the improvement of store deployment conditions. It is noted that, ideally, the means to overcome those problems should not increase the drag of the aircraft; preferably, it should decrease it.

The general principles involved in the suppression of the effects of unsteady flow are discussed with reference to open or transitional flow and closed flow, with particular attention being paid to possible alleviation strategies.

The classification of devices used to control cavity unsteady flow, whether related to noise or store deployment conditions, is explained. Of the two main categories, passive devices are usually not categorised further. Active devices are commonly categorised as having either open-loop or closed-loop operation.

Data are presented which give some idea of the effects of cavity shaping, doors and stores on the noise characteristics of cavities with no suppression devices. Such material can be used in a number of ways; for example, as a guide to the relative effectiveness of a given suppression device in different baseline (i.e. no suppression device fitted) situations.

Where possible the effects of a suppression device on the drag, in addition to the acoustics, of a cavity are discussed.