Origin and control of vortex breakdown of unmanned combat air vehicles

The instantaneous and averaged flow structures past a delta wing of lowsweep angle are investigated using a technique of high-image-density particle imagevelocimetry. Three different UCAV planforms have been used. The UCAVplanforms were subjected to static variations of angle-of-attack, with the aim ofunderstanding the underlying flow physics and methods for their control. It was alsosubjected to perturbations for the case of basic delta wing in order to control thevortex breakdown or stall phenomenon.At all values of angle-of-attack up to the fully stalled condition for the basicdelta wing, the averaged vorticity layer exhibits an elongated form; a large-scaleconcentration of vorticity within the leading-edge vortex of a highly swept wing isnot present. At low angle-of-attack α, this elongated, averaged layer can exhibit,however, well-defined concentrations of vorticity. These elongated vorticity layersare accompanied by narrow recirculation zones adjacent to the wing surface. Controlin the form of a small amplitude perturbation of the basic wing, at a frequencycorresponding to the subharmonic of the spectral component in the initial region ofdevelopment of the separated layer, can restabilize the time-averaged patterns ofstreamline topology and vorticity, such that they resemble those occurring at lowerangle-of-attackElongated layers of vorticity in the crossflow plane exhibit well-defined time-averaged and instantaneous concentrations of vorticity for the case of diamond andlambda planforms. Flow features are interpreted in conjunction with patterns ofstreamline topology, as well as images of root-mean-square velocity fluctuation andvelocity spectra obtained from space-time imaging.Keywords: UCAV, Delta Wing, PIV, Vorticity, Turbulent FlowI