INTRODUCTION 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: Part I: Time-averaged Flow ?C ESDU 02008 (Reference 31) Part II: Unsteady Flow and Aero-acoustics ?C ESDU 04023 (Reference 32) Part III: Alleviation of Unsteady Flow Effects@ IIIA Introduction ?C ESDU 08011 (This Data Item) IIIB Acoustic suppression using passive devices ?C ESDU 08012 (Reference 33) IIIC Acoustic suppression using active devices ?C ESDU 09001 (Reference 34 to be issued) IIID Store deployment ?C ESDU 09002 (Reference 35@ to be issued) Part IV: Overview of CFD methods ?C Reference 37@ (Data Item to be issued)@ Part V: Bibliography and Tabular Survey ?C Reference 38@ (Data Item to be issued). 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 (ESDU 08012) deals with acoustic suppression using passive devices. The use of active devices is considered in Part IIIC (ESDU 09001)@ while the improvement of store deployment conditions is dealt with in Part IIID (ESDU 09002). Section 2.1@ below@ gives general background information behind the need for the alleviation of weapons bay noise and the improvement of store deployment conditions@ two important problem areas concerning the internal carriage of weapons. 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 in Section 3. Sections 3.1 and 3.2 consider open or transitional flow and closed flow@ respectively@ with particular attention being paid to the possible alleviation strategies. Section 4 deals with the classification of devices used to control cavity unsteady flow@ whether related to noise or store deployment conditions. Of the two main categories@ passive devices are usually not categorised further. On the other hand@ active devices are commonly categorised as having either open-loop or closed-loop operation. Many of the wind-tunnel tests or CFD simulations involving acoustic suppression devices concern simple rectangular box cavities@ often with no doors fitted and usually with no stores installed. Therefore@ Section 5 presents data giving some idea of the effects of cavity shaping (Section 5.1)@ doors (Section 5.2) and stores (Section 5.3) 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. Section 6 deals with drag considerations in the choice of an acoustic suppression device. The few instances which feature the effect of a suppression device on the drag@ in addition to the acoustics@ of a cavity are discussed in Section 6.1. In order to understand the effect on drag in the assessment of various passive devices@ the concept of cavity baseline drag (gross drag) and its relationship with the drag (net drag) predicted using ESDU 00006 (Reference 29) and ESDU 00007 (Reference 30) is outlined in Section 6.2. Finally@ the assessment of the installed drag of each of the passive devices dealt with in Sections 3 to 9 of ESDU 08012 is considered in Section 6.3.