Understanding the vortex patterns on the leeside of a body of revolution is a major problem in missile aerodynamics Above incidences of about 5 ° the boundary layer separates from the leeside of the body along a separation line and hence introduces circulation into the flow Up to about 25 ° to 35 ° incidence the circulation will concentrate itself around two symmetric vortices in the cross-flow plane The interaction of these vortices with downstream surfaces such as controls can induce large variations in local angle of attack causing control problems.
In an effort to improve aerodynamic prediction codes British Aerospace (BAe) has had a continuing interest in vortex-dominated flowfields. As an aid to understanding the qualitative aspects of such flowfields a water tunnel has been found to be extremely useful in flow visualization. The high density and low diffusivity of the working fluid enable dense flow tracers with high reflectivities, such as dyes or particles to be used successfully BAe have amassed over the years data in the form of flow visualization photographs concerned primarily with the characteristics of vortices shed from bodies of revolution and subsequent interactions with downstream surfaces Studies have included the effects of curved flowfields on body vortices, the interaction of body vortices with intakes (Beaman, 1986) and of course, the effects of lifting panels on body vortex development (Deane, 1978) Some of the more qualitative results of the studies have assisted in the development of aerodynamic prediction methods (Beaman, 1986) However, concern has been expressed as to the validity of water tunnel techniques in representing flow at higher Reynolds numbers Reynolds numbers are orders of magnitude lower in the water tunnel than in wind tunnels or free flight, although at the high angles of incidence normally considered when investigating vortex effects and with the supersonic wing sections usually used in missile technology, it is expected that separation on a wing is fixed at the sharp leading edge and hence the wing flow will be well represented For the body, however, no such prominent feature fixes the separation position which, as is well known, can change significantly with Reynolds number
As a step towards validating the use of water tunnels in simulating the flow around missile configurations at moderate angles of attack BAe have undertaken a comparative study in which reliable wind tunnel data on vortex locations have been reproduced in a water tunnel.
The derivation of vortex locations from wind tunnel data can be a difficult problem to solve In many cases separation points are derived from surface pressure distributions, which can give rise to large uncertainties due to the interpretation required Vortex locations are generally derived from flow visualization in a wind tunnel, for instance vapour screen techniques which may require varying degrees of interpretation and tend not to be the primary objective of the tests with a consequent decrease in accuracy and scope BAe were therefore fortunate in performing some work for the Royal Aircraft Establishment, Bedford, that included the analysis of flowfield data around a body of revolution collected for the specific purpose of investigating vortex locations and strengths