Technique for Measurement of Droplet Profiles for Use in Icing Physics Studies 2007-01-3293

Improving numerical simulations of aircraft ice accretion in glaze ice conditions requires empirical data on the static and dynamic behavior of liquid water on rough surfaces. Toward this end, a new non-intrusive technique has been developed that provides instantaneous, full-field measurements of droplet interface heights using laser speckle shift correlations. The technique consists of illuminating a rough surface with coherent laser light to produce a laser speckle field and measuring the deformation of the speckle pattern when a water drop is present on the surface. The pattern's deformation is a function of the droplet interface shape and measuring the deformation provides the data necessary to reconstruct the droplet shape. To accomplish this task, computer algorithms have been developed to identify the irregular droplet perimeter and recover the speckle shift vector field inside. The shift field is used as input to an interface reconstruction method based on a simulated-annealing technique. The shift field associated with a preliminary profile is generated and the mean-squared difference between the guessed interface's shift field and the measured shift field is computed. To improve the guess, the droplet is perturbed to converge upon an interface shape which minimizes the mean-squared error. Tests of the technique show a satisfactory level of functionality and spatial resolution.