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9 to occur and how they could proceed into asymmetric motions of breakdown location. Such studies are impossible to achieve experimentally. Experiments have a crucial role to play in validating the predictions in the sense of breakdown movement (from visualization), core properties and frequencies (from surface measurements or LDA). Although this kind of interaction is more of a concern for slender wings, evidence of such interactions at a relatively low sweep angle of Λ = 60 o was reported recently [24]. Wing tip accelerations occurred in an asymmetric structural mode for a slightly flexible delta wing when vortex breakdown occurred on the wing. Time-accurate CFD simulations could provide evidence of the underlying reasons for the instability and guide detailed flowfield measurements to further the understanding. 3.4 Nonslender Vortices Much of our knowledge of vortex flows is related to slender vortices. There is very little known about the structure of vortices over nonslender delta wings (Λ ≤ 55 o ) and unsteady flow phenomena. Figure 7 shows an example of flow visualisation for a Λ = 50 o delta wing, where a dual vortex structure is identified. Both PIV measurements [25] and DNS calculations [26] confirmed that both vortices have the same sign of vorticity. It has been found that nonslender wings (with sweep angles as low as 40 o ) at angles of attack as low as a few degrees can produce strong vortical flows. An example of surface flow visualization for α = 2.5 o is shown in Figure 8 for a Λ = 50 o delta wing, where the secondary separation and reattachment lines are visible. For α = 15 o , there is a change in the curvature of the secondary separation line around the midchord, which is presumably due to the vortex breakdown. Figure 9 shows root mean square values of fluctuating velocity together with the surface streamline pattern obtained from velocity measurements close to the wing surface. For α = 15 o , the signature of vortex breakdown starting around 40% of the chord length is visible. However, for α = 20 o , it is not the breakdown, but the reattachment of the shear layer which produces unsteadiness near the wing surface. Reattachment of shear layer, vortex breakdown, and stall over wings with rounded leading-edges are very complex and can benefit from numerical simulations for better understanding of the general flow topology which can then guide detailed measurements. Such numerical studies are problematic due to the difficulty in accurately predicting the (non-fixed Reynolds number dependent)
10 separation location over rounded leading edges. However, it is unknown to what extent the vortical structures are dependent on the accurate prediction of the separation location. Again experiments focussing on the leading edge region to provide detailed velocity and turbulence data for separation onset would provide valuable validating data for the predictions. Also, there is a need to understand separated and vortical flows at nonzero roll angles for nonslender wings. Recently, it was discovered that nonslender delta wings can exhibit wing rock phenomenon [27]. 3.5 Multiple Vortices Another area that has received little attention is the interaction of multiple vortices such as those found on double delta wings (see Figure 10). Interactions of multiple vortices, complex vortex patterns, coiling-up and merging, vortex breakdown, and unsteady interactions are highly challenging vortical flows. These aspects are even more complex and challenging for manoeuvring aircraft. This is a particularly interesting area in which CFD can provide much needed understanding since the entire unsteady flowfield can be visualised and studied. Experimental flow visualisation techniques can be applied for static cases though this is harder for manoeuvring cases. As such time accurate CFD simulations would be able to track core motions, examine vortex interactions, highlight interaction induced vortex breakdown, as other phenomena currently poorly understood. Location of interesting phenomena with CFD would also have the advantage of guiding experimentalists in finding measurement locations of interest. 3.6 Manoeuvring wing vortices The spectrum of unsteady flow phenomena over stationary delta wings is shown in Figure 3 as a function of dimensionless frequency [15]. Also shown is the frequency range of aerodynamic manoeuvres for current fighter aircraft. Future unmanned aircraft could be highly manoeuvrable and flexible, with the capability of performing extreme manoeuvres at high g (with a 30g vehicle envisioned). At such high reduced frequencies, there is the possibility of a coupling of aerodynamic manoeuvres with vortex instabilities. For highly manoeuvrable aircraft configurations, nonlinear unsteady aerodynamics presents major challenges for the development of flight control laws. The dynamic response of leading edge vortices and breakdown is important for flight of unmanned
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