Digital Image- and Digital Volume Correlation and advanced point tracking provides the surface or volume distribution of the displacement and strains of a structure under loading and/or in motion, respectively. The so-called inverse elasticity problem then consists in obtaining the stress distribution as well as identifying the material properties based on the knowledge of the structure's kinematics. For simple systems like a plate or a membrane of known mechanical properties, this information will be sufficient to infer not only the inertia forces but also the elastic forces. For more complex structures, the experimental observations need to be corroborated by structural mechanics modelling and elastic simulations to fill the gap between the surface measurement data and a reliable calculation of inertia- and elastic-forces by building appropriate dynamic structural models based on FEM, such as the FE updating, virtual field method, admissible displacements, or reciprocity gap formulation, can be used to solve the inverse problem. Within HOMER, the connection between the flow and surface kinematic data and the structure dynamical behavior will be pursued by both solving the inverse elasticity problem and by applying data assimilation (DA) procedures in the fluid domain. When needed, the more complex material properties will be identified offline by solving the full inverse problem using pre-defined loadings. The structural designs will therefore be modeled according to their respective constitutive material. DA procedures have already been applied to novel pressure and loads determination schemes based on 2D- /3D-PIV or dense Lagrangian particle tracking (LPT) (by e.g. Shake-The-Box or time-resolved tomo PTV/PIV data) and Navier-Stokes-regularized interpolation by synthetic experiments within the project NIOPLEX and already show very promising results. Innovative DA approaches like VIC+, FlowFit, adjoint methods and 4Dvar are under development in several groups of the consortium and have shown a significant increase of measurement spatial resolution and accuracy compared to the state-of-the-art represented by spatial cross-correlation algorithms.
DIC measurement uncertainties have been assessed in a variety of manners, and taking account of multiple experimental and numerical parameters, including the experimental design, pattern deformation, interpolation schemes, or structural discontinuities. Such approaches will be combined to those that are in development for fluid measurement to provide a comprehensive error assessment methodology for the fluid-structure problems adressed in HOMER.
The severity of flow-induced vibration issues not uncommonly arises only in a late stage of the aircraft design process, or even during final flight tests. The counter-measures that have to be applied subsequently can be expected to introduce substantial time and cost burden. The phenomena range from local small-amplitude vibrations that interfere with comfort or fatigue to large-scale LCOs or full-grown flutter that pose the airframe as a whole at serious risk.
In practice, especially in the case of local flow-induced vibrations it is often difficult to differentiate between a genuine flutter instability and a dynamic response problem in which the flow acts as a driver for the structure. In the first case the flow would immediately return to steady state if the involved structure was notionally brought to rest. In the second case the flow would keep oscillating with a certain natural frequency. The latter phenomenon can be masked by the fact that the frequency of the nonlinear fluid-dynamic oscillator can jump to the structural natural frequency once a certain structural response amplitude is exceeded (“lock-in”). An inherent flow unsteadiness can be traced back quite often to the occurrence of shock wave structures interacting with separated boundary layers for instance in the vicinity of wing-body junctions, wing-pylon-nacelle regions or flap track fairings. Other sources of flow unsteadiness are vortical structures shed from upstream or free shear layers for instance originating from engine jets.
Today’s numerical simulation methods are potentially able to cover all the mentioned types of flow unsteadiness, be it motion-induced (relevant for flutter/LCO) or due to large-scale flow instabilities (relevant for dynamic response). The current state-of-the-art is represented by the solution of the (unsteady) RANS equations that can be obtained routinely even for complex flow situation but come at the cost of usually inadequate turbulence modeling. This can be partially overcome by the concept of scale-resolving flow simulation like LES and derived so-called hybrid LES-RANS methods. All methods share a strong need for calibration and validation data for which, as an essential aim of the HOMER project, novel suited optical metrology has to be developed, tested, assessed and demonstrated up to industrial wind tunnel speeds and scales.