LCD-projector-based 3D color PTV

Color-based three-dimensional (3D) Particle Tracking Velocimetry (PTV) using a single camera and an liquid crystal display (LCD) projector, which can measure the out-of-plane velocity component from particle color information, is improved for practical use in 3D velocimetry without losing the original simplicity on the optical configuration of planner 2D PTV. To minimize the uncertainties in identifying the representative color of particles, we propose new color identifying algorithms that average hue in polar rather than linear coordinates. The proposed algorithm, a saturation-weighted-averaged hue in polar coordinates, yields smaller standard deviations than conventional algorithms: 1.4% in hue and 2.0% in position detection. Use of a multiple color-cycle gradation light for volumetric illumination of the fluid layers also improves the resolution of the position and velocity measurements in the depth direction. The proposed measurement technique can display the flow patterns of an axisymmetric rotating flow from instantaneous 3D–3C velocity vector fields, as well as the vortex structure of an unsteady flow represented as iso-surfaces of enstrophy density.

► We provide a practical 3D flow measurement technique using simple equipment.
► We used a liquid crystal display projector and a single color camera.
► Multi cycle gradational illumination overcomes difficulties and ambiguities.
► 3D vertical structure of a turbulent flow is visualized.