High fidelity ray tracing image simulations of transparent fluid objects

Optical images of transparent fluidic objects such as droplets display a pattern of brightness that reflects their three-dimensional shape. Due to optical refraction at the surface or in the body of the object, the brightness pattern has a complicated dependence on the shape of the object and on the details of the illumination. This makes it necessary to perform image simulations to compare the results of CFD simulations to experimental images, but previous image simulations for fluid dynamics showed significant differences from experimental images. We developed ray tracing image simulations and compiled movies that replicated, with high fidelity, brightfield microscopy images of three water microdroplets merging during 30 µs of free fall. The image simulations, which matched experimental images with high fidelity, used objects made by revolving the drop outline from experimental images, or independently computed using CFD. To achieve high fidelity, the simulations must replicate the spatial and angular distribution of illumination rays, and that the optical model must be tested and refined using focusing scans. These accurate image simulations can help verify and refine CFD simulations, improving the accuracy of measurements based on optical images.