In aircraft design, safety is a crucial factor. As the dynamic interaction of structure and aerodynamics is particularly important with respect to stability, the field of aeroelastics as their combination has to be taken into account explicitly in order to ensure a reasonable optimisation of the design. The circulation of a profile section may be influenced by blowing out a thin air jet on its upper surface
Example: Flow past an aerofoil without and with circulation control
The Coanda effect prevents flow separation even at a very large flap deflection. This way, a significant lift gain can be achieved.
The aerodynamics of an aircraft using active circulation control cannot be described with the methods of potential theory and is therefore examined by numerical simulations based on the Reynolds-averaged Navier-Stokes equations (RANS). However, this approach only allows for limited analyses of the coupled system due to the computational effort.
In the framework of the Collaborative Research Center SFB 880, the finite element model of the wing structure is reduced using modal reduction. The aeroelastic coupling follows in the modal space.
Example: Flutter modes of the wing
The resulting reduced-order model allows for systematic stability investigations. They show two dynamic instabilities occurring only due to the active circulation control, which are mere bending flutter and mere torsion flutter. Both phenomena are single degree of freedom flutter occurring independent of the approach velocity, i.e. also at low velocities as in landing approach.
Aiming at active vibration damping of slender structures due to circulation the structure is extended to actuators and sensors, which are elements of a control unit. By means of the sensors deformations of the structure may be identified. They can be counteracted specifically by triggering the actuators.
The investigated structure as well as the fluid are described by the Space-Time Finite Element method. The movements of the mesh describing the circulation of the fluid incorporate large rotations. Therefore they are modelled by PS-SSMU-method, which combines the Shear-Slip-Mesh-Update-Method with a pseudo-structural approach.
Example: Rotating plate with piezoelectric layer exposed to fluid circulation
The figures show the fields of velocity and pressure as well as the movement of the fluid mesh regarding a rotating plate with piezoelectric sensor and actuator layer.
Due to deformation-dependent exposure to fluid flow structures may show instabilities in system behaviour even at small wind velocities which can result in short-time failure. In addition to extensive experiments in wind tunnels the evidence of load carrying capacity is often proven by numerical simulations, whereat the forces related to the fluid flow are formulated depending on the pressure coefficients. Here the complex fluid flow situations are considered simplifying the pressure coefficients to mean values of the spatiotemporal fluctuations. Based on measured distributions of pressure the stochastic discretisation results from spectral methods. Thereby probabilistic investigations of the response behaviour are provided in time as well as in frequency domain.
Interaction of wind flow and structural motion is to be considered in particular, when self-excitation mechanisms or resonance phenomena lead to instability of deformation behavior. To solve fluid-structure interaction problems classical concepts with measured power spectra for wind excitation are developed as well as approaches related to the numerical solution of the Navier-Stokes differential equations with direct coupling to the elastic structure, which are applied both for engineering structural analysis. Thus the stress-strain behavior and the vibration characteristics can be investigated in borderline situations.
Example: Aeroelasticity of H-shaped cross section
The fields of pressure depict the the flow separation of an eddy at the leading edge of the cross section.