Structured-light-based surface measuring for application in fluid–structure interaction

Structured-light-based profilometry techniques provide a simple and suitable tool to characterize experimentally the dynamics of the interaction between fluid and structure. The study implements a method based on the analysis of a sinusoidal fringe pattern. This method allows the retrieval of a three dimensional surface shape from a two dimensional video recording of the deformation of fringes projected on the surface. A detailed description of the method and the implementation is provided which will facilitate its adaption to and utilization in custom FSI applications (the code is published under open-source license). An application example is presented in the form of an experiment. A hyperflexible hydrofoil is set under forced rotational oscillation in a closed water tunnel to mimic the characteristics of a vertical-axis hydrokinetic cross-flow turbine. The flexible structures encounter large deformations which are linked to corresponding hydrodynamic forces. To capture this interdependence, the deformations were recorded in sync with forces registered with a six-axis load cell. A validation of the method is presented and shows good accuracy in the deformation measurements of a hyperflexible hydrofoil. A benefit compared to other common techniques like Laser-interferometer measurements is the simplicity and modest hardware requirements of the method with the possibility to acquire height fields with good spatial and temporal resolution, which allows a spectral analysis of the surface deformation. The hydrodynamic forces are presented together with the corresponding deformation of the structure, and spectral analyses are performed which demonstrates the possibilities of the proposed method. In summary, the presented method allows the analysis of fluid–structure interactions using a simple assembly consisting of a projector and a high-speed camera.