A refractive technique for surface flow visualization using image processing

The wake/free surface interaction has become a major research topic in the field of hydrodynamics with special applications in the detection, classification, and tracking of submerged objects. This has led to the development of a simple but effective free surface visualization technique which can be implemented in controlled water tunnel experiments. An experiment was performed to verify the effectiveness and applicability of this method.

Digital images of a spatially varying cosine grid were acquired as seen through the disturbance pattern present on the water surface. Image processing techniques were used to perform phase demodulation of the distorted image by cross- multiplying the data image with a reference image and then low pass filtering the result. Contrast stretching was performed to enhance the subjective quality of the contents in the final image. The resulting product of the process is an image which details the outline, location, and extent of the surface deformation in a grayscale format. Optimal digital filters and spatial cosine grids are determined experimentally for various surface flow conditions.