INTRODUCTION Background The effects of flow instabilities on the liquid or gas within an enclosed volume with an aperture open to a static or moving external flow@ or related matters@ have been studied for at least the last 150 years. For example@ Sondhaus in 1854 (Reference 1)@ studied the effects of a jet impinging upon an edge@ producing an acoustic effect known as an edge tone@ associated with the production of sound in organ pipes and other musical instruments. Tyndall@ in 1867 (Reference 2)@ studied the effect that sound had on the stability of jets@ while in 1868 Helmholtz published (Reference 3) the results of@ among other things@ his analysis of the natural frequency of an enclosed volume with a small aperture@ subsequently called a Helmholtz resonator. Later@ in 1877@ Lord Rayleigh (John William Strutt) published (Reference 4) a widely used book on the theory of sound@ including the effects of sound on the stability of vortex sheets and the development of a theory for the resonant conditions of an open-ended pipe. Much of the work in References 1 to 4 has provided a foundation for the subsequent development of the various theories associated with cavity aerodynamics and aero-acoustics. The study of cavity acoustics in general@ and jet impingement in relation to edge tones in particular@ continued at a steady pace for the next 70 years@ with various applications. However@ in the late 1940s and early 1950s a particular problem in the aircraft industry spurred the accelerating growth of research into cavity aerodynamics@ and especially acoustics@ that has occurred over the last 50 years. At that time there was an increasing awareness of the effects that the oscillation of the airflow in and around open cavities such as wheel wells and bomb bays were having on the aerodynamics@ and even the structural integrity* of such cavities and their contents. Early work in this area was carried out in relation to the bomb bays of the English Electric Canberra (References 6@ 8@ 9@ 13@ 19@ 21@ 29 and 39) and Boeing B47 (Reference 10). At the same time as this work on specific aircraft was being carried out in the 1950s and 1960s@ a number of investigations@ mainly wind tunnel and flight tests@ involving more general research into the unsteady aerodynamics and noise due to cavities were also under way (for example@ References 7@ 11@ 14 to 18@ 20@ 22 to 28 and 30 to 38). All these areas of research provided a firm basis for the rapidly expanding work@ both experimental and theoretical@ including computational fluid dynamics (CFD)@ from the 1970s onwards. Although nearly all the research on the unsteady aerodynamics and acoustics of cavities has been in relation to weapons bays@ much of the resulting information could also be applied to undercarriage bays or wheel wells. However@ in that application there is usually less of a problem due to the unsteadiness and noise arising from the flow within such cavities (although drag is obviously a concern)@ and more of a problem in relation to the far-field noise generated by the wheel bay and the extended undercarriage unit (for example@ References 44@ 48@ 49@ 51 and 68). Such considerations are outside the scope of this Data Item and are more relevant to the work of the ESDU Noise Committee@ see ESDU 90023 (Reference 95) for example. The main problems involving the use of rectangular planform cavities for weapons carriage are twofold. (a) Flow unsteadiness and the possibility of self-sustained oscillations and the likelihood of acoustic phenomena for deep to moderately deep cavities with open or transitional flow (see ESDU 00007 ?C Reference 97). (b) Weapon release difficulties from shallow cavities with closed flow (see ESDU 00006 ?C Reference 96) arising from the strong upwash at the forward end of the cavity coupled with the equally strong downwash at the rear. The flow is@ however@ comparatively steady compared to that in transitional or open flow. Of the two@ problem area (a) has attracted by far the most research effort@ not least because it poses the greater challenge due to the complexity of the unsteady flow involved. The adverse cavity flow effects affecting weapons carriage and release can be alleviated by various means@ both active and passive@ see Parts IIIA to IIID (References 99 to 102)*. However@ in order for the alleviation mechanism to be effective@ a knowledge of the nature of the unsteady flow is essential@ and the primary purpose of the present Data Item is to provide wide-ranging information on that aspect. * Acoustic peaks as high as 180 dB are possible@ implying pressures around 2 ?? 104 N/m2 (418 lbf/ft2); even levels of 160 dB can cause damage (Reference 71). * This Data Item forms Part II of a series on cavity flows. All the Parts are listed in Section 2.2 of Part I (Reference 98)