Development of Digital Image Projection Techniques to Quantify Surface Film/Rivulet Flows

Abstract

() A novel digital fringe projection (DFP) technique is developed to achieve non-intrusive thickness measurements of wind-driven water droplet/rivulet flows in order to quantify the unsteady surface water transport process pertinent to aircraft icing phenomena. The DFP technique is based on the principle of structured light triangulation in a similar manner as a stereo vision system but replacing one of the cameras for stereo imaging with a digital projector. The digital projector is used to project a fringe pattern of known characteristics onto a test object (i.e., a water droplet/rivulet on a test plate for the present study). Due to the 3D shape profile of the test object, the fringe pattern is deformed seen from a perspective different from the projection axis. By comparing the distorted fringe pattern over the test object (i.e., the water droplet/rivulet on the test plate) and a reference fringe pattern on a reference plane (i.e., the test plate only without the water droplet/rivulet), the 3D profile of the test object with respect to the reference plane (i.e., the thickness distribution of the water droplet/rivulet flow) can be retrieved quantitatively and instantaneously. The feasibility and implementation of a DFP system is first demonstrated by measuring the thickness distribution of a small flat-top pyramid over a test plate to evaluate the measurement accuracy level of the DFP system. Then, the DFP system is applied to achieve time-resolved thickness distribution measurements of a droplet/rivulet flow to quantify the transient behavior of the water droplet/rivulet flow driven by boundary layer airflow over a test plate. The dynamic shape change and stumbling runback motion of the wind-driven water droplet/rivulet flow over the test plate are revealed clearly and quantitatively from the DFP measurement results. Such information is highly desirable to elucidate underlying physics to improve our understanding about the surface water transport process pertinent to glaze ice formation and accretion over aircraft wings in atmospheric icing conditions