Laboratory measurements of the rise velocity of frazil ice particles

- Frazil ice particles were observed in a laboratory tank.
- Rise velocities from 0.4 to 13.5 mm/s and diameters from 0.24 to 3.4 mm were observed.
- Theoretical solutions for the rise of a horizontal disc were compared to the measured data.
- A new equation for the rise velocity of a vertically oriented rising disc was derived.
- Particle thicknesses varied from 0.03 to 0.12 mm and aspect ratios from 11 to 71.

The rise velocity of frazil ice particles has proven difficult to measure with any accuracy. As a result the physics of rising frazil ice particles are poorly understood, making it difficult to determine whether skim ice, frazil ice, or both will be present during the early stages of river freeze-up. Past experimental studies have observed particles rising at a wide range of velocities, but without precise measurements of the particle diameter no definitive relationships could be determined. In this study, high-resolution digital images were taken of rising frazil ice particles as they passed between two cross-polarising filters. An image processing algorithm was used to accurately calculate the diameter of each rising particle and the movement of its centroid was then tracked through a series of images to determine the rise velocity based on the total vertical displacement. Rise velocities ranging from 0.40 to 13.47 mm/s for particles with diameters in the range of 0.24 to 3.35 mm were observed. Existing theoretical solutions for the rise of a horizontal disc were compared to the measured data and a new function relating the rise velocity to the diameter of a vertically oriented rising disc was derived. It was found that all of the data could be enveloped by curves corresponding to a vertical disc with an aspect ratio of 10 and a horizontal disc with an aspect ratio of 80. In some cases, the thickness of a particle could also be estimated from the images and thicknesses ranging from 0.03 to 0.12 mm with a mean of 0.07 mm were observed with aspect ratios in the range of 11 to 71. Based on this information it is suggested that the assumption of discs having a constant aspect ratio is inaccurate.