Characterizing a multi delta wing for aeroelastic wind tunnel experiments

Kurzfassung

The vortical flow over swept wings with several leading edge separations bears a number of challenges for the proper design of the planform and the efficient control during the flight. Leading edge vortices function as the major lifting mechanism for combat aircraft and depict the aircraft’s moment coefficients as well. The flow is either unsteady or a least quasi-steady as soon as the vortices are formed, but never steady.

Even though the mechanisms of the vortical flow and its effects on the aircraft’s forces and moments are well understood, their prediction is still not precise enough. Several failures of jet prototypes occurred in the past, which were accounted to unexpected vortex shifts and thus load changes, or to fatal vortex structure interaction. Resolving the flow topology over combat aircraft configurations numerically is not feasible, yet. Due to the unsteadiness time resolved calculations are needed in order to reveal all important aspects of the flow, which demands more CPU-capacities than are available at the moment. Thus only selected flow cases can be resolved. RANS-calculations dampen many effects significantly, which distorts the observed planform characteristics. Additionally the different positions of the vortices throughout the flight regime compromise efficient grid development. As a consequence wind tunnel experiments with time resolved measurements are needed and must be developed carefully, since they are very costly.

The trend shows that modern combat aircraft planforms provide multiple leading edge vortices, which take over specific functions such as stabilization, distribution or manoeuvring. Such a next generation planform has been developed by the DLR Institute of Aeroelasticity as well. Two wind tunnel test campaigns are planned with a semi-span model. A preliminarily selected sensor placement is crucial for the success of the measurements and additionally the structural layout must be constructed very carefully in order to ensure the structural integrity of the model throughout the entire flight regime. The flow topology of the aircraft model features a two-stage vortex systems, which leads to severe load changes between subsonic and supersonic velocities. Furthermore vortex-vortex interaction and vortex-structure interaction shall be characterized during steady positions, during pitching motions and during manoeuvres.

The aforementioned tasks have been and are still prepared by numerical simulations with the DLR TAU-code. An angle of incidence and Mach-number matrix gave a rough characterization of the new model planform. Strong gradients in the coefficient slopes show points of interest and convergence problems show possible critical points. In order to improve the resolution of the flow topology the TAU grid adaptation module was used and up to three adaptation stages were implemented. This resulted in resolving secondary separations, tertiary separations and feeding sheets effectively. The newly developed model with its unique vortex topology poses very interesting characteristics and could provide a new base for modern combat aircraft developments. The structural layout and sensor placement is in progress and accompanied by time resolved simulations. These efforts should improve the quality and the efficiency of the currently planned wind tunnel experiments at the DLR and furthermore the derived routines for the numerical preparation of experiments can be adopted by succeeding projects as well.