The scientific work of the VFE-2 facet of the RTO AVT-113 Task Group has been carried out from 2003 to 2007, and the achieved status of knowledge has been documented in a large number of papers [3.1 – 3.41]. Many aspects of vortical flow could be treated successfully, but on the other hand some problems are still unsolved. Therefore, at the end of the RTO-phase of VFE-2 in 2008 the scientific program is not terminated. As with VFE-1, the scientific work will continue in the next decade: Many problems have not been solved completely or have even been started, and others have not yet been looked at within the framework of the RTO Task Group. Therefore new experiments will be carried out worldwide in a new open phase of VFE-2. The configuration under consideration is relatively simple and described analytically, and therefore new wind tunnel models can be built quite easily. Numerical calculations will go on as well, and better results as well as new solutions will be achieved.
The results achieved so far in phase 1 of VFE-2 are collected in the RTO AVT-113 Final Report [3.42]. They will be the starting point for future investigations. In the resuming Chapter 35 of the Final Report the achievements as well as the unsolved problems have been summarized. On this basis an outlook for the next needs of further investigations is given subsequently.
Fig.1: Schematic view of the vortex formation on the VFE-2 configuration with medium radius rounded leading edge at M = 0.4, Rmac = 3x106, AoA = 13.3° according to the numerical solutions of EADS and DLR. (Region between the onset of the outer primary vortex and the separation of the two primary vortices enlarged, secondary vortices omitted)
On the experimental side the needs are
- Measurements on the boundary layer status laminar/turbulent for the VFE-2 configuration with sharp and rounded leading edges. Fully developed vortical flow without vortex breakdown at AoA = 18° should be the starting point, and later also partly separated vortical flow at AoA = 13° could be added. The already existing experimental results will be further evaluated, but there is an urgent need for new measurements.
- Experimental investigations on the shock formation in transonic fully developed vortical flow AoA = 23° for the VFE-2 configuration with sharp and medium radius rounded leading edges. Subject should be the mutual interference between the terminating and the cross flow shocks on the one hand and vortex breakdown on the other.
- New measurements on the partly separated vortical flow at AoA = 13°. Up to now the structure of the initial flow separation due to the thickness distribution near the apex of the configuration (see Fig. 1) is unknown. At DLR numerical simulations have shown vorticity distributions without a dominating concentrated vortex in this area, whereas experiments indicate some weak longitudinal vortices in this area as shown in Fig. 1.
On the numerical side the needs are
- New calculations related to the fully developed vortical flow without vortex breakdown at AoA = 18° for the VFE-2 configuration with sharp and medium radius rounded leading edges in incompressible flow and at very low Reynolds numbers. Converged solutions should be achieved for various turbulence models, and in comparisons with the available results from Hot-Wire-Anemometry (HWA) [3.42, Chapter 21], the best suitable turbulence model for calculations of vortical flows should be found.
- Attempts should be made to predict the laminar/turbulent transition in fully developed vortical flows without vortex breakdown at AoA = 18°. Starting point could be the sharp edged VFE-2 configuration. For this purpose two steps would be helpful
- Calculation of the vortical flow for prescribed laminar and turbulent boundary layer regions on the wing as a guess. A converged solution should be achieved. As input experimental results could be used.
- Point by point stability analysis of the calculated flow field. Based on the results corresponding modification of the original laminar/turbulent transition guess.
- For partly separated vortical flow at AoA = 13° the CFD codes should be further validated using the already available or new experimental data. The vortical flow field in the vicinity of the onset point of the outer primary vortex should be analysed in detail in order to check the validity of the schematic view presented in Fig. 1.