Unlike civil aviation, where aircraft fly relatively steadily and generate lift at low angles of attack, military aircraft are often required to perform extreme manoeuvres. These manoeuvres can push the wings to very high angles of attack, where the flow around the wings is close to stalling. In this regime, the airflow can separate from the wing, leading to a sudden loss of lift—and potentially the loss of the aircraft.
Certain wing shapes generate vortices, particularly along the leading edge. These vortices have a lift-enhancing effect on the upper side of the wing due to the additional speed component. The stall area can thus be shifted to higher angles of attack, resulting in significantly improved manoeuvrability.
A central question of current research concerns the problem of flutter stability of aircraft configurations under vortex-dominated flow conditions.
A flying wing model was analyzed in several wind tunnel tests. The aim was to better understand the unsteady aerodynamics of such configurations under vortex-dominated flow and to compare the findings with numerical methods (1). To avoid aeroelastic influences, the model was designed to be especially rigid.
Selected flow conditions were used for validation. Numerical parameters, such as the turbulence model, were adjusted so that the results matched the measured data as closely as possible—for both stationary and unsteady flow variables. Once the numerical aerodynamic model had been validated using the wind tunnel data and adapted so that a close match with the measurements could be achieved, it served as the basis for further analyses. In particular, complex stationary and unsteady effects in vortex-dominated flows were analyzed and their potential influence on aeroelastic phenomena was considered (2).
Combining the numerical model with experimental results provided a sound basis for realistically simulating the interaction between vortex-induced flow structures and aeroelastic effects.
To better understand the results of the investigations described below, two important aerodynamic parameters should be explained here. The first is the Mach number, which describes the ratio of the speed of an object to the speed of sound in the surrounding medium. The speed of sound in air at a temperature of 20°C is approximately 340 m/s, or 1,224 km/h. Another important parameter is the Reynolds number. It describes the ratio of inertial forces to viscous forces in the flow. Geometrically similar bodies exhibit similar flow conditions when their Reynolds numbers are comparable. This is important for comparing the results of wind tunnel experiments with scaled-down models to the flow conditions of full-scale aircraft. For full-scale aircraft, Reynolds numbers typically range from 10 to 30 million, while wind tunnel experiments usually range from 1 to 5 million.
In the course of the investigations, aerodynamic flow conditions were varied. In particular, slowly increasing the Mach number while maintaining a constant Reynolds number and an angle of attack with a pronounced vortex flow produced an interesting effect. When the Mach number—and thus the airspeed—increases, the flow conditions and the position and strength of the vortices on the upper wing surface change abruptly. A so-called flow separation occurs on the upper side and the vortices shift towards the aircraft fuselage, weakening. This, in turn, alters the forces and moments acting on the structure, significantly affecting the aircraft’s flutter stability. When the Mach number is reduced, the flow does not return to its original state until a significantly lower Mach number is reached. This is a hysteresis. In practice, this means that two very different flow states can exist within the same Mach number range under identical conditions, resulting in markedly different flutter stability behavior
The flutter curves shown illustrate this effect: although the Mach number is identical in both cases, the different flow states in the hysteresis range lead to clearly different flutter curves for the individual elastic modes.
The hysteresis behaviour described here is not an isolated case. Similar effects occur, for example, in the stall of two-dimensional profiles (3) or in the aeroelasticity of thin panels (4). Such abrupt changes highlight the complexity of nonlinear flow–structure interactions, showing that aeroelastic stability depends not only on current flow conditions but also on the history of how the parameters have varied. How decisive this influence can be in different scenarios remains an exciting question for future investigations.
Guido Voß, Department Aeroelastic Simulation, DLR Institute of Aeroelasticity
