The Flow on the Apex of a Sharp-Edged Delta Wing

Abstract

This paper describes a study of the topology of the skin-friction line pattern on a sharp-edged 65-deg swept delta wing. Special attention is paid to the flow pattern on the apex. Because the pattern on this part of the wing is poorly resolved in experiments, a large-scale model of the apex was constructed to obtain more insight into the flow topology. The angle of attack is shown to have a strong effect on the apex flow topology. The topology shows evidence of primary and secondary separation. ELTA-WING planforms are used for many current fighter air- craft and in designs of future fighter and UCAV configurations. Such aircraft rely on lift generated by vortex flow to enhance maneu- verability. Strong leading-edge vortices generated along the leading edge of the highly swept delta wings have a dominant effect on the flow and air loads. At high angle of attack, a sudden disorganiza- tion of the structure of the leading-edge vortex core flow occurs, known as vortex breakdown. The disorganized core flow induces strong fluctuations of the wing surface pressures downstream of the vortex breakdown point. Interaction with tail surfaces results in so- called tail buffeting. The consequences of these phenomena are a deterioration of aircraft performance, stability, and control. Mitchell et al. 1 have recently given an overview of experimental databases on vortex core breakdown and of state-of-the-art numerical solutions. The present study is part of a research program that was con- ducted at the Delft University of Technology (TUD) to investigate the causes of the strong nonlinearities (critical states) occurring in the forces and moments of a 65-deg delta wing at sideslip. 2 The au- thors used flow visualization data to determine the effects of sideslip on the topology of the surface flow in an effort to explain the behavior of the surface pressures with sideslip. Difficulties were encountered in inferring the topology of the flow at the apex due to poor resolu- tion of the streakline pattern. The apex region is of interest because the vortices on the delta wing can be expected to originate there. The present study pays attention to the flow at the apex. At this stage only the flow at zero sideslip is considered. For this condition, several theoreticians 3−6 have postulated the topology of the flow at the apex of a sharp-edged delta wing wherein the apex is consid- ered on a microscopic scale. Because few experimental data exist to verify their postulates, a scale model of the apex was constructed to improve the resolution of the apex flow. Preliminary tests conducted on this so-called apex model showed a complex skin-friction line pattern to exist at α = 30 deg. 7 To better understand the formation of this pattern, in the present study the flow on the apex model has been visualized at α between 0 and 30 deg.