Vortex dynamics on a generic UCAV configuration

A water tunnel investigation of the vortex dynamics on a generic UCAV configuration has been conducted using flow visualization and Particle Image Velocimetry (PIV). The planform under investigation comprised of a tandem delta wing arrangement known as a chinard configuration. This planform was shown to exhibit a significant delay in the development of vortex breakdown compared to a simple delta wing of similar sweep due to a favorable interaction between the foreplane and main wing vortices. PIV measurements confirmed the flow visualisations that merging of the foreplane and main wing vortices takes place at zero and non-zero roll angles. At a particular roll angle (when breakdown is downstream of the foreplane trailing-edge), a double-breakdown structure was observed, with simultaneous existence of breakdown on the foreplane and main wing. This phenomenon was periodic and manifest as a sudden appearance of an upstream bubble-type burst on the foreplane which propagated downstream on to the main wing. Dynamic roll experiments showed that large hysteresis effects exist even at small reduced frequencies, with vortex breakdown occurring 50% chord further forward at zero roll angle compared to the static location. The hysteresis and non-linearity of the vortex breakdown response increases with increasing reduced frequency, with the maximum differences between the static and dynamic cases reaching approximately 75% chord at the maximum roll angle. Variations of the amplitude and phase lag of the breakdown displacement with frequency are similar to those of slender delta wings, although the phase lag is larger for this configuration.