ESDU 00006:2011

ESDU 00006 develops a method for predicting the drag where the
cavity length is large compared to the depth and the shear flow
enters the cavity and attaches to the floor before separating to
pass over the rear wall with a stagnation point near the top of the
wall (i.e. closed flow). A family of curves suggests an upper limit
of cavity depth to length ratio for closed flow in terms of
free-stream Mach number and cavity width to length or width to
height ratio. In supersonic flow it is possible for vortices to
form as the shear flow spills over the side edges of the cavity and
their impingement on the rear wall gives rise to an increase in
drag for which an estimation method is also provided. Tables give
the ranges of parameters covered in the construction of 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 1 for low-speed flow (Mach number less than 0.1) and to
within 2 for high-speed flow (Mach number between 0.5 and 3).
Worked examples illustrate the use of the method. A companion ESDU
document, ESDU 00007, deals with other types of cavity flow, known
as transitional and open. 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.