No 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) INTRODUCTIONThe 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 . 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], ] due to this interaction.From the study of the interaction between longitudinal vortices and normal shocks in supersonic flow 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 . 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) 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 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 of Reynolds numbers .

The objective of the Cranked-Arrow Wing Aerodynamics Project International (CAWAPI) was to allow a comprehensive validation of Computational Fluid Dynamics methods against the CAWAP flight database. A major part of this work involved the generation of high-quality computational grids. Prior to the grid generation an IGES file containing the air-tight geometry of the F-16XL aircraft was generated by a cooperation of the CAWAPI partners. Based on this geometry description both structured and unstructured grids have been generated. The baseline structured (multi-block) grid (and a family of derived grids) has been generated by the National Aerospace Laboratory NLR. Although the algorithms used by NLR had become available just before CAWAPI and thus only a limited experience with their application to such a complex configuration had been gained, a grid of good quality was generated well within four weeks. This time compared favourably with that required to produce the unstructured grids in CAWAPI. all-tetrahedral and hybrid unstructured grids has been generated at NASA Langley Research Center and the USAFA, respectively. To provide more geometrical resolution, trimmed unstructured grids have been generated at EADS-MAS, the UTSimCenter, Boeing Phantom Works and KTH/FOI. All grids generated within the framework of CAWAPI will be discussed in the article. Both results obtained on the structured grids and the unstructured grids showed a significant improvement in agreement with flight test data in comparison with those obtained on the structured multi-block grid used during CAWAP. Nomenclature

No abstract

The behaviour of the flow over slender delta wings under transonic conditions is highly complex. With the occurrence of a number of shocks in the flow the behaviour of vortex breakdown is quite different to that for subsonic flow. This investigation considers this behaviour over the 65 o sharp leading edge delta wing used in the 2nd International Vortex Flow Experiment (VFE-2) using Computational Fluid Dynamics. Three institution involved in the VFE-2 have collaborated to consider the wing under conditions of M = 0.85 and Re = 6 × 10 6 at two incidences: α = 18.5 o and 23 o. The flow solutions are compared to existing experimental data and show good agreement for the cases considered. However, a discrepancy with the experimental data is shown where the critical incidence for the onset of vortex breakdown on the wing is under-predicted. From analysis of the solutions, it is determined that the onset of vortex breakdown is highly dependent on the vortex strength and the strength and location of the shocks in the flow. The occurrence of a critical relationship between these parameters is suggested for vortex breakdown to occur and is used to explain the discrepancies between the computational and experimental results based on the under-prediction of the vortex core axial velocity. A sensitivity study of the flow to a number of computational factors, such as turbulence model, is also undertaken. However, it is found that these parameters have little effect on the overall behaviour of the transonic flow. Nomenclature α Angle of incidence α cr Critical incidence for vortex breakdown Ω Rotation Rate M ∞ Free-stream Mach number P 1 , P 2 Pressures defined upstream and downstream of shock surface r c Radius of vortex core at point of max. swirl velocity Re Reynolds number Ro Rossby number U θ Tangential or Swirl Velocity U x Axial Velocity x/c r Non-dimensional stream-wise location y/s Non-dimensional span-wise location