Null projection screen test for aerodynamic surfaces on wind turbine blades

Abstract

This work proposes a novel method, to our knowledge, to qualitatively test aerodynamic surfaces by projecting a null screen. The main idea is, by using ray tracing equations in a model with two pinhole systems, to map an ordered predefined array of fringes formed by points on the detection plane, defined as a CMOS sensor, to the projection plane of a commercial projector, passing through the surface under test whose geometry must be known in advance. Once the coordinates on the projection plane are calculated, the pattern is projected onto the aerodynamic surface. The proposed predefined array will be observed by the camera if the surface and the alignment of the optical systems are perfect; otherwise, a distorted array will be detected, indicating misalignment or that the surface has defects that can be observed qualitatively. The mathematical expressions found to perform the direct and inverse ray tracing in 3D are applied to design a null test by projection on a reference plane and on an aerodynamic surface based on the NACA 0012 airfoil.