ESDU 00007:2011

ESDU 00007 develops an empirical method for predicting the drag
where the cavity length is small compared to its depth and the
shear flow bridges the gap between the front and rear walls, with
the dividing streamline ending in a stagnation point at or near the
top of the rear wall, trapping one or more vortices in the cavity
(i.e. open flow). A single trapped vortex is typical for
depth-to-length ratios between 0.5 and 1. For lower depth-to-length
values a tandem pair of vortices can arise, while for higher
depth-to-length values vertically stacked double or even triple
vortices may occur. Families of curves are given to suggest a lower
limit of cavity depth-to-length ratio for open flow in terms of
freestream Mach number and cavity width-to-length ratio. If the
cavity length is long compared to its depth, the shear flow enters
the cavity and attaches to the floor before separating to exit over
the rear wall with a stagnation point near the top of the wall
(i.e. closed flow); the closed flow case is treated in the
companion document, ESDU 00006. A family of curves repeated from
that document is given to suggest an upper limit of cavity
depth-to-length ratio for closed flow in terms of freestream Mach
number and cavity width-to-length ratio. For a given cavity width
and depth, at given flow conditions, as the cavity length is
progressively increased from zero there is a range of values over
which, for subsonic freestream speeds, the flow type gradually
changes from open to closed flow, with the flow entering the cavity
over the front wall but not attaching to the floor before passing
over the rear wall (i.e. transitional flow). For supersonic speeds
a similar range exists but the change from open to closed flow is
more complex and abrupt, passing through two intermediate stages
(i.e. transitional-open and transitional-closed). ESDU 00007
continues the open flow prediction method with a smooth progression
into the transitional region. The interface region between
transitional and closed flows, which is not precise and may need
the construction of a short fairing, is discussed and illustrated
by means of an example. Tables give the ranges of parameters
covered by the method. The prediction of the ratio of the drag
coefficient, based on floor area, to the local skin friction
coefficient at the cavity mid-length station (in the absence of the
cavity) is assessed to be within 2. However, that accuracy requires
certain data to be excluded from the analysis, and for freestream
Mach numbers greater than 0.5 their inclusion would lower the
agreement to within 5. The concerns with the data are discussed and
all the details of the analysis are explained. Worked examples
illustrate the use of the method. The third item in the series,
ESDU 10016, deals with the effect on cavity drag of a pair of doors
open at 90°, including the effects of three different treatments of
the door leading and trailing edges