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 – ESDU 02008 (Reference 31)

Part II: Unsteady Flow and Aero-acoustics – ESDU 04023
(Reference 32)

Part III: Alleviation of Unsteady Flow Effects,

IIIA Introduction – ESDU 08011 (This Data Item)

IIIB Acoustic suppression using passive devices – ESDU 08012
(Reference 33)

IIIC Acoustic suppression using active devices – ESDU 09001
(Reference 34 to be issued)

IIID Store deployment – ESDU 09002 (Reference 35, to be
issued)

Part IV: Overview of CFD methods – Reference 37, (Data Item to
be issued),

Part V: Bibliography and Tabular Survey – 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.