Shock Effects on Delta Wing Vortex Breakdown

Abstract

It has been observed that delta wings placed in a transonic freestream can experience a sudden movement of the vortex breakdown location as the angle of incidence is increased. The chapter reports on the use Computational Fluid Dynamics (CFD) to examine this behaviour in detail. The study shows that a shock-vortex interaction is responsible. The balance of the vortex strength and axial flow, and the shock strength, is examined to provide an explanation of the sensitivity of the breakdown location. Limited experimental data is available to supplement the CFD results in certain key respects, and the ideal synergy between CFD and experiments for this problem is considered. 29.1 INTRODUCTION The occurrence of shocks on delta wings introduces complex shock/vortex interactions, particularly at moderate to high angles of incidence. These interactions can make a significant difference to the vortex breakdown behaviour. For subsonic flows the motion of the location of onset of breakdown towards the apex is relatively gradual with increasing incidence [29-1]. The strengthening of the shock which stands off the sting as the incidence is increased can lead to a shock/vortex interaction triggering breakdown. The location of breakdown can shift upstream by as much as 30% of the chord in a single one degree incidence interval [[29-2], [29-3]] due to this interaction. From the study of the interaction between longitudinal vortices and normal shocks in supersonic flow [29-4] it has been found that it is possible for a vortex to pass through a normal shock without being weakened sufficiently to cause breakdown. The flow over slender delta wings is potentially more complex as the shock is not necessarily normal to the freestream in the vortex core region [29-5]. Investigation is needed to consider the behaviour and onset of vortex breakdown, particularly with respect to shock/vortex interactions. To consider this behaviour, the flow over a sharp leading edged, slender delta wing was considered under subsonic and transonic conditions. This investigation was undertaken as part of the 2nd International Vortex Flow Experiment (VFE-2), a facet of the NATO RTO AVT-113 Task Group, which was set up to consider the flow behaviour both experimentally and computationally over a specified 65° delta wing geometry. The work of VFE-2 built on the first International Vortex Flow Experiment (VFE-1) [29-6] carried out in the late eighties, which was used to validate the inviscid CFD codes of the time. Progress has been made in both experimental and computational aerodynamics, particularly in turbulence models since the conclusion of the VFE-1. Therefore, it was proposed by Hummel and Redecker [29-7] that a second experiment should be undertaken to provide a new, comprehensive database of results for various test conditions and flow regimes, to further the understanding of vortical flows. The test conditions considered under the VFE-2 framework include both subsonic and transonic Mach numbers for low, medium and high angles of incidence at a range o...