Fig. 1: 65° delta wing at cryogenic tests in the National Transonic Facility (NTF), NASA Langley Research Centre [3.1] (By courtesy of J. M. Luckring).
The first International Vortex Flow Experiment (VFE-1) has been carried out in 1984 to 1986 on a 65° swept cropped delta wing, and the results have been used to check the Euler codes, which were available at that time. In order to validate modern Navier-Stokes solvers more detailed experimental data were necessary, and therefore a new International Vortex Flow Experiment (VFE-2) has been proposed in 2001. The corresponding investigations have been carried out in 2003 to 2008 within the framework of the RTO Task Group AVT-113 "Understanding and Modeling Vortical Flows to Improve the Technology Readiness Level for Military Aircraft". The results of this international cooperation have been published in a large number of papers and they have been collected in the Final Report of the Task Group. These documents are the basis for the continuation of the scientific work in a new open phase of VFE-2, which is carried out within the present project.
The configuration for VFE-2 has been chosen in such a way that all flow regimes (i.e attached flow, separated vortical flow without and with vortex breakdown and separated deadwater-type flow) are covered properly, and this lead again to a delta wing with a leading edge sweep of 65°. Concerning the thickness distribution a flat plate inner portion in combination with interchangeable leading edges was desired, and these requirements were fulfilled by the NASA configuration [3.1], which is shown in Fig. 1. Sets of one sharp and three rounded leading edges were available. The geometry of the wing as well as the shape of the sting is given by analytical expressions. New wind tunnel models could be built quite easily and overall aerodynamic coefficients and pressure distributions in certain cross sections were already available for a large variety of Reynolds numbers and Mach numbers [3.1].
The experimental investigations within VFE-2 have been carried out using the following wind tunnel models:
- Model 0 was the original NASA model, tested in the NTF at NASA Langley RC, USA, (Lift and pitching moment coefficients, PSI pressure distributions), see [3.1]
- Model 1 (smaller size) was on loan from NASA, USA. It has been tested at NASA, Langley RC, USA, (Aerodynamic coefficients and PSI), DLR Goettingen, Germany, (PSP and PIV) and at ONERA Lille, France, (Forces and moments, PIV).
- Model 2 has been built at TU Munich, Germany, (PIV and Hot-wire investigations of the flow field). It has been tested also at DLR Cologne, Germany, under cryogenic conditions (PIV and IR/TSP for laminar/turbulent transition).
- Model 3 has been built at Glasgow University, UK, (Unsteady force and pressure measurements at high angles of attack).
- Models 4 and 5 (Sharp and rounded LE) have been built at ONERA Lille, France, (Forces and moments, pressure distributions, flow visualization, PIV). They were also tested at TUBITAK-SAGE Ankara, Turkey, (Forces and moments, transition laminar/turbulent).
The experimental investigations were accompanied by numerical calculations of the flow on structured and unstructured grids by the partners.
Program of work. A huge matrix of possible investigations is given by the large variety of angles of attack, which lead to various flow regimes. The new measurements mainly concentrated on one (medium radius) rounded leading edge shape in comparison with the well known sharp leading edge configuration. The effects of angle of attack, Reynolds number and Mach number have been studied, but according to a decision of the Task Group detailed flow studies have been carried out for three angles of attack only, corresponding to three different flow regimes. These are
Onset of vortical flow, AoA = 13°
- Sharp leading edges: Separated flow
- Medium radius rounded leading edges: Partly attached, partly separated flow
Separated flow without vortex breakdown, AoA = 18°
- Sharp leading edges
- Medium radius rounded leading edges
Separated flow with vortex breakdown, AoA = 23°
- Sharp leading edges
- Medium radius rounded leading edges.
The scientific results obtained in the first phase of VFE-2 have been published in [3.1] – [3.41], and they are collected in the RTO AVT-113 Final Report.